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Typical velocity fields and vortical structures around a formula one car, based on experimental investigations using particle image velocimetry.

ABSTRACT

This paper presents typical flow structures around a 60%-scale wind-tunnel model of a Formula One (F1) car, using planar particle image velocimetry (PIV). The customized PIV system is permanently installed in a wind tunnel to help aerodynamicists in the development loop. The PIV results enhance the understanding of the mean velocity field in the two-dimensional plane in some important areas of the car, such as the front-wheel wake and the underfloor flow. These real phenomena obtained in the wind tunnel also help maintain the accuracy of simulations using computational fluid dynamics (CFD) by allowing regular checking of the correlation with the real-world counterpart. This paper first surveys recent literature on unique flow structures around the rotating exposed wheel, mostly that on the isolated wheel, and then gives the background to F1 aerodynamics in the late 2000s. It subsequently describes features of the PIV system that is used in the development loop and discusses improvements to the efficiency of the preparation and operation processes. Typical velocity fields behind the rotating front wheel and the underfloor of two F1 car models, one before and one after a regulation change in 2009, are then analyzed. Finally, important vortical structures around the 2009 car are highlighted from the results obtained in the development loop. These results show the initial effect of the regulation change on the flow structures, and how the 2009 car was developed to overcome the initial deficit in downforce.

CITATION: Nakagawa, M., Kallweit, S., Michaux, F., and Hojo, T., "Typical Velocity Fields and Vortical Structures around a Formula One Car, based on Experimental Investigations using Particle Image Velocimetry," SAE Int. J. Passeng. Cars - Mech. Syst. 9(2):2016.

INTRODUCTION

One of the most challenging tasks in the study of open-wheel Formula One (F1) aerodynamics is to understand actual flow phenomena around the rotating wheel, which is exposed to a high-speed freestream. In particular, the front wheels interact with the wakes of the front wing located upstream, and are sometimes disturbed by turbulence in the wake of the preceding car. In past decades, various studies have conducted experimental and numerical investigations to understand these complex phenomena. This section is a comprehensive survey of the literature of these studies, primarily focusing on wind tunnel testing. It will be shown how quickly research into F1 aerodynamics has increased in complexity to provide solutions that are more practical, owing to the rapid progress of measurement techniques in the 2000s.

Isolated Wheel

The most common investigations are those of the rotating isolated wheel, since the investigations require a relatively simple experimental setup and the single wheel alone involves complex flow phenomena. Since Fackrell and Harvey [1,2] and Fackrell [3] first conducted comprehensive studies of the lift and drag forces computed from measured surface pressure on the rotating isolated full-scale solid wheel in contact with the ground, extensive studies have been conducted both experimentally and numerically, to clarify complex phenomena. In the late 1990s, great efforts were put into developing more accurate pressure measurement techniques; e.g., Hinson [4]. However, an actual flow phenomenon can only be postulated from the surface pressure distribution, leaving open questions about the real flow phenomena. Only classical techniques, such as the use of a pitot probe and hot-wire anemometry, were available to measure the velocity field in the 1990s but these techniques are always subject to directional ambiguity and, importantly, are intrusive. Oil flow and smoke visualization techniques indeed gave useful insights, but only provided quantitative information under limited conditions.

In the 2000s, especially after non-intrusive measurement techniques that employ lasers, such as Laser Doppler anemometry (LDA) and particle image velocimetry (PIV), became available, more accurate and detailed flow structures could be obtained, the level of understanding of flow phenomena around rotating isolated wheels rapidly rose, and the literature became more thorough, as described in the following.

Knowles et al. [5] first presented the velocity field using non-intrusive three-dimensional (3D) velocity measurements around a 40%-scale isolated Champ Car solid wheel, using a long-range LDA system. The measurement was conducted in transverse sections at several downstream positions in the near wake of the wheel. They claimed that the vortex structure in the near wake is different from theoretical models previously proposed by Cogotti [6] and Merker and Berneburg [7], owing to the existence of a support sting and the "hub flow". Saddington et al. [8] subsequently conducted similar measurements around a 50%-scale F1 isolated grooved solid wheel, where they proposed a revised model of the "trailing vortex system" in the near wake.

Mears et al. [9, 10] conducted the first PIV measurement around an isolated 40%-scale non-deformable pneumatic tire/wheel assembly in conjunction with a conventional pressure measurement on the wheel surface, using a radio telemetry system. In that work, they confirmed the existence of the "jetting phenomenon" at the rear of the contact patch from a flow field investigation for the first time, validating the theoretical prediction made by Fackrell [3]. Mears and Dominy [11] later simulated this phenomenon employing computational fluid dynamics (CFD) to show that it can also be predicted numerically, if the computational grids are fine enough to resolve the viscous sublayer in the boundary layer on the wheel surface.

In recent years, many researchers have begun to use a more practical resting rig. Studies have begun to represent the shapes of the contact patch and sidewall more realistically, and rims have detailed interior geometry, such as spokes, a brake disk, and calipers, in conjunction with a brake scoop as used by the F1 teams.

Sprot et al. [12] used a complete assembly of a 50%-scale F1 "solid" wheel to investigate the "through-hub flow pattern" and its effect on the wheel drag, and downstream wheel-wake structure, employing PIV and CFD. They showed that the open and closed configuration of the brake duct inlet scoop affects the wheel drag and the flow field in the wake significantly. They pointed out that the brake duct scoop must therefore be designed so that its "spillage positively interacts with the downstream component of the car". They also investigated the effect of the exit position of the outboard rim cover, which was used by many F1 teams until 2009, on the wheel drag.

Sprot et al. [13] used a similar assembly but with a "deformable wheel" to measure the sidewall profile of the rotating wheel. They showed that "the sidewall shape is highly sensitive to the aerodynamic performance of the wheel", highlighting the importance of having deformable wheels with a proper axle height for the wind tunnel test. The same assembly was used by Emmanoulides [14] to conduct heat transfer analysis using a hot film sensor on the brake disk inside the wheel assembly.

Issakhanian et al. [15] conducted PIV around a 60%-scale F1 wheel assembly with a deformable grooved tire, provided by an F1 team. The test was conducted under the same conditions as experienced by F1 teams to give similar shapes of the contact patch and sidewall. The purpose of their study was to obtain validation data for subsequent CFD work around stationary and rotating wheels conducted by Axerico-Cilies et al. [16,17], and Axerico-Cilies and Iaccarino [18], respectively. The latter study investigated turbulence models by making detailed comparisons with PIV data, to find the best turbulence closure model in unsteady Reynolds-averaged Navier-Stokes (URANS) equations, without employing large eddy simulation (LES), which has a high computational cost although it gives the closest match of flow structures with PIV data.

Flow Interactions between Front Wing and Wheels

The interaction of the front-wing wake and the rotating front wheel has been studied by many researchers in parallel with the above work. These investigations provided insights that are linked to a regulation change in 2009, which involved an increased span of the front wing.

Keller et al. [19] conducted the first study of the interaction of the front wing and rotating isolated wheel both experimentally and numerically. They set up a simplified front corner assembly with a rotating wheel and half of the two-element front wing with different end-plate designs, using a 40%-scale model. They discovered that the wheel drag can be "significantly reduced" by the design of the front-wing endplate, which is attributed to the "strong vortex shed from the bottom edge of the front-wing endplate".

Diasinos and Gatto [20] conducted velocity measurements using 3D LDA. They used a generic 1:7.5 front corner assembly, which consisted of a rotating cylinder with an inverted single-element airfoil placed in front with a flat plate attached as an endplate. They conducted a thorough survey of the velocity fields in transverse sections at several downstream positions in the wheel wake. They showed that that the position of the "primary vortex", which is generated at the wing tip and the top of the endplate, is strongly affected by the angle of attack and span of the front wing.

Van den Berg and Zhang [21] set up a 50%-scale F1 two-element front wing in front of two rotating isolated solid wheels in a wind tunnel. They made various measurements, such as surface pressure, to calculate the aerodynamic loads, oil flow, PIV, and CFD, while varying the ride height. They found that there is a critical ride height, for which wheel drag switches between high and low values. They attributed the critical height to the position of the top vortex generated by the front wing endplate. Heyder-Bruckner [22] extended that study further by conducting an extensive flow field survey using PIV and CFD to support previous conclusions in more detail.

Car Wake

In the late 2000s, many discussions were held to promote overtaking maneuvers and thus increase the entertainment value of F1; e.g., the centerline downwash generating (CDG) wing was proposed as a candidate wing of a new rule for 2008. The drag reduction system (DRS) was officially legalized by the Federation International de l'Automobile (FIA) in 2011 for this purpose. A few studies investigated the flow field in the complete car wake of an F1-car model, although no PIV work has been published in the open literature.

Wilson et al. [23] studied the wake structure behind a 1/6-scale generic F1 car model on a rolling road, using a five-hole pressure probe. According to the results for the wake structure, they invented a "bluff body wake generator" to reproduce a similar wake pattern with a vortex pair in the downstream at a half-car distance. By situating this wake generator in front, they studied the aerodynamic response of the front wing of a following car model, suggesting "the reduction of the strength of the trailing vortex pair" to promote overtaking.

Watts and Watkins [24] conducted a detailed velocity measurement behind a 30%-scale model in a wind tunnel without a rolling road, using a four-hole pressure probe to investigate the effect of the DRS on the wake structure. They drew a similar conclusion, highlighting the dominance of the "counter rotating vortex pair" in the wake, and stating that the "flap must be trimmed to the streamline" to minimize the strength of the vortex pair, making DRS most effective.

Newbon et al [25] recently conducted thorough investigations into the effect of the car wake on the following car using CFD. To find the most sensitive flow features in the wake of the preceding car, they varied fluid dynamic parameters of the inlet plane, which was sampled from the wake of the single-car computation. They found that such an "upwash flow" in the wake actually seemed beneficial in terms of maintaining the overall downforce level as it moved the wake away from the following car, although it is also a main cause of an appreciable rearward-shift in the aerodynamic balance.

Full-Car Model

Knowles et al [26] conducted a wheel wake measurement in the presence of a car body on the rolling road for the first time. They used the same front wheel assembly as used by Saddington [8], but with the installation of a complete 50%-scale car model provided by an F1 team. They conducted 3D LDA measurements on two transversal sections in the near-wake region of the wheel, and compared the measurements with those made behind the isolated wheel. The results showed that the flow separation on the wheels occurs much earlier than that in the isolated case, and the pattern of the trailing vortices seen behind the isolated wheel disappeared. These are the very first results where actual wheel wake structures around a complete car-model were obtained quantitatively in an experiment.

As seen above, a trend of recent research activities has been the rapid shift to more practical applications rather than fundamental research. Apparently, the technical demands of many researchers are growing because of the rapid advance of measurement and simulation technologies. Many findings reported in the relevant literature are beneficial to F1 teams in terms of making measurements that are more precise in a wind tunnel, raising the quality of wind tunnel testing in the recent development of F1 aerodynamics. In fact, many F1 teams are interested in these techniques. For example, Ogawa et al. [27] reported a PIV measurement made around a 50%-scale F1 model and used for development. They measured the separation point around the front wheel to validate their CFD results.

However, to the best of the authors' knowledge, there has been no PIV study of actual aerodynamic phenomena in various areas around a complete F1 car. Therefore, the aim of this paper is to first present such results obtained from detailed experimental investigations of an F1 car model in a wind tunnel, using PIV to show how PIV can be used in the daily development loop, and what can be learned from these investigations [28].

BACKGROUND TO F1 AERODYNAMICS

The main focus of this paper is a discussion of actual flow fields around a recent F1 car, according to PIV results. In particular, the flow phenomena are compared before and after a change in FIA technical regulations [29] made in 2009. This chapter first reviews the background of F1 aerodynamics from 1980s, followed by details of the regulation change in 2009.

Overview of Aerodynamics of a Racing Car

After the importance of aerodynamics to the performance of an open wheel racing car was first highlighted (e.g., Katz [30, 31]), the theoretical background of the principle downforce generating devices on the racing car became well-established by the mid1990s. The history of these earlier works was summarized by Katz [32].

Agathangelou and Gascoyne [33] presented a general overview of the F1 aerodynamics based on the actual data of the F1car at that time. They also highlighted the importance of emerging CFD in terms of advancing the theoretical understanding of complicated flow phenomena around the car. They concluded the report by stating "empirical and numerical methods must coexist... to allow cross validation of investigative tools and progression of the subject".

Computer technology has since evolved quickly; commercial CFD codes have become increasingly powerful, which has improved understanding of the complex flow phenomena around a full car at a more detailed level. Consequently, the actual flow phenomena around a recent F1 car have become more complex, and development has become more rapid than in the 1990s. However, the development process in F1 remains the same; both empirical and numerical approaches are needed as CFD simulation still requires validation with measurements made in a wind tunnel, although the degrees to which each approach is employed depend on the resources available to each team.

The major factors that determine the performance of an F1 car are broken down to the chassis, the aerodynamics, the engine, and the tires. Figure 1 summarizes the effect of each of these factors on lap time improvement analysis presented by Arai [34] based on the data in 2009 season. It is seen that improving aerodynamic efficiency (the downforce to drag ratio) by 0.01, which is a typical gain after a week of wind-tunnel development in F1, is comparable to a weight reduction of 10 kg in terms of the lap time gain. Considering that the car is usually made underweight, as the minimum weight is fixed by a regulation, improving the aerodynamic efficiency is apparently the most effective and possibly the only means to improve car performance during the season. For this reason, research and development of the aerodynamics have been conducted tirelessly throughout the racing season, although the current F1 regulation (introduced for the 2014 season) heavily restricts the amount of inseason wind tunnel testing the teams are allowed to perform. Even the smallest gains are implemented immediately so that the car can be best equipped for the upcoming race events, which take place every couple of weeks during a championship.

Effect of the Regulation Change

The aim of aerodynamic development is to improve key performance indicator (KPI), such as aerodynamic efficiency and stability, by designing a shape of the aerodynamic devices within the geometry specified by the FIA technical regulation (a.k.a., the regulation box), which is subject to change at any time. In fact, there were major regulation changes twice in the first decade of the 2000s, concerning aerodynamics. There are several reasons for changing the rules, such as reducing the cornering speed and promoting overtaking maneuvers. Nevertheless, these changes have always involved reductions in downforce. For example, the total downforce was reduced by 20% initially following the rule change made in 2005, and the subsequent rule change made in 2009 had an even greater impact, reducing the total downforce by 50%. Larsson [35] summarized the initial effect of the latter, according to CFD investigations. However, the recent technology of F1 allows aerodynamicists to quickly recover the loss and finally to exceed the original downforce by the end of the season. The rate of development has increased so dramatically that changes to the regulation now tend to happen more frequently. Figure 2 compares F1 cars before and after the regulation change made in 2009. The appearance of the cars changed appreciably.

The following explains these changes in more detail by highlighting important development areas of F1 cars. Although F1 teams are free to develop their cars according to the F1 regulations, the designs of different teams are similar enough to consider a generic F1 car for each year in the following discussion.

The front wing of F1 cars produced about 25% of the total downforce, as reported by Agathangelou and Gascoyne [33], which is the same level as achieved nowadays even though the level of the total downforce and efficiency are much higher than 1990s. While the front wing efficiently produces a large downforce thanks to the ground effect, the outboard area, which features multiple-element flaps and endplates, also plays an important role. The outboard area controls the flows over the front wheels and other downstream components of the car [19, 20, 21]. The trailing vortices, produced by the top and bottom of the endplates and vertical fences underneath the main plane, called strakes, are important structures, as shown in Figure 3. There is an appreciable difference in these flows before and after the regulation change, since it changed the span of the front wing. Consequently the development strategy had to be completely altered: In 2008, the main focus was on the control of the endplate vortices to have a favorable interaction with the wake of the front wheel. In 2009, as the span of the wing was increased to cover the complete width of the car, the endplate vortices travel outboard. Please note that the front wing span was reduced again by 150 mm in the 2014 regulation, and the authors believe that vortices now travel both inboard and outboard of the front wheels, which could be the reason why the outboard area of the front wing of the most recent F1 car seems to add more complexity to the geometry.

Managing the front-wheel wake has been a constant challenge in F1 aerodynamic development since the 1980s, as the rotating wheels create the largest disturbances acting on the car and can thus reduce the effects of downstream aerodynamic devices. To minimize this potentially detrimental effect, the wheel wake must be well managed using aerodynamic devices on the chassis side. This was done using devices called turning vanes (TV) and bargeboards (BB) situated between the front wheel and the sidepod for the 2008 car as in Figure 3. These devices produce strong downwash flow to feed energized air under the floor, as well as expelling the front-wheel wake outboard so that the underfloor flow can be kept undisturbed. Meanwhile, the front-wheel wake of the 2009 car had to be managed differently, since no such devices existed on the 2009 car, owing to the restricted area in the regulation box. It turns out that the vortices generated by the inboard tips of the front wing flaps, so-called Y250 vortices, became prominent and became a major target of the development; details will be discussed later.

The rear wing of F1 cars generally produced about 25% to 30% of the total drag, as reported by Agathangelou and Gascoyne [33]. Over the course of F1 history, many attempts have been made to reduce undesired drag when the car is traveling down straights, and the DRS was officially legalized in 2010 to promote overtaking. The rear wing has also been subjected to major rule changes; in 2009, for example, the rear wing had to be narrower and taller than that in 2008, as shown in Figure 4. The 2009 rear wing is more isolated in freestream, resulting in a higher downforce generation and stronger interaction with the low beam and diffuser.

The diffuser is the most efficient downforce-generating device, and its dimensions were frequently altered following the regulation changes of 2005 and 2009, restricting the design flexibility. In 2009 in particular, a double-deck diffuser received much attention and was standard in F1 until it was banned in 2011. The diffuser is usually designed to have the maximum allowable expansion such that it generates large negative pressure under the expanded channel in order to collect as much of available mass flow as possible. Several vertical fences are used to prevent stalling, which produces strong longitudinal vortices, also inducing negative pressure. The upwash from the rear end of the car was much steeper in 2009, owing to the combined effect of the taller rear wing and the double-deck diffuser that had a steeper expansion. The outboard shape of the diffuser is also important in preventing disturbances of the rear-wheel wakes from entering inboard.

EXPERIMENTAL APPARATUS

Wind tunnel Facility

The wind tunnels used in this study are owned by Toyota Motorsport GmbH (TMG) in Cologne, Germany. TMG used to be a base factory for the Panasonic Toyota Racing Team, until they withdrew from the F1 championship at the end of 2009. The facility has twin wind tunnels, both of which were operated 24 hours a day, seven days a week at peak time, until the agreement of the Formula One Teams' Association (FOTA, disbanded in 2014) to limit the "wind-on" hours became effective at the beginning of 2009 season.

Test Section

The schematics of the test section are illustrated in Figure 5. Each wind tunnel is a closed-loop (Gottingen) type with a nozzle exit that is 4.1 m wide and 3.7 m tall. The 15-m-long test section can be used with either a slotted-wall or semi-open configuration. The full-belt rolling-road system is equipped with a proper boundary layer treatment in front of it. Aerodynamic loads of the scale model are measured using an external balance on the roof and under-belt load cells just below the wheels. Figure 6 shows that a scale model can be suspended by a strut equipped with a vertical traverse system inside a carbon fairing. One of the wind tunnels is capable of testing a full-scale car, where the downforce can be measured via the under-belt load cells below the wheels, while the drag forces are measured via the load cells mounted inside the support towers of the side struts. The other tunnel is capable of conducting exhaust blowing tests, which were essential for the development of the exhaust blown diffuser in 2010. The detailed specifications are given in Table 1. The maximum wind speed achieved in conjunction with the rolling road is 70 m/s. All the measurements reported in this paper were conducted at a wind speed of 50 m/s, corresponding to a Reynolds number of 5.9 x [10.sup.6] for the length of a 60%-scale 2009 car model.

Scale Model

Initially, a 50%-scale model with a wheel supporting system (wheel arms and towers) was used. This model was eventually replaced by a 60% model with no wheel supporting system. We call the former the "wheel-off" system, as the wheel assemblies are detached from the model, and the latter the "wheel-on" system. Switching to the wheel-on system was a particularly important step forward in minimizing the blockage effect as the model size increased to 60%. This change in size was motivated by the 2009 regulation change that resulted in losses in downforce as mentioned earlier: The model was enlarged to better utilize the measurement capacity, which improved the measurement resolution and correlation with the full-scale phenomena as a result, owing to the higher Reynolds number.

Figure 7 shows the 60% wind tunnel model of the 2008 F1 car. The model dimensions are width of 2.78 m, length of 1.08 m, and height of 0.57 m. Usually, a model consists of more than 500 parts built around a steel skeleton that encloses several essential components: a model motion system, mechanical components such as the active suspension control system that applies preloads to the deformable tire supplied by the tire manufacturer, the front wheel steering system for yaw/steer tests, and pressure sensor modules that measure the surface pressure under the floor and radiators. Different materials are used for the parts that reproduce the actual car shape. The areas on which high aerodynamic loads were expected, such as the front and rear wings, were made from machined aluminum, steel, or carbon. Most of the other parts were made through rapid prototyping with various materials, such as ceramic, resin, and aluminum powder.

PIV System

As described earlier, the task of the aerodynamicists is to improve KPI's during the test session, usually by adjusting and replacing parametrically designed parts on the model to build a huge database. Because of the large number of possible combinations, aerodynamicists used to need to conduct as many wind-tunnel runs as possible to improve the chance of finding new optimizations. Results were analyzed purely using the numbers of KPI, which refer to aerodynamic load measurements. Development was also monitored using these numbers. In other words, experiments were conducted systematically to accomplish the goal of achieving the target KPI numbers, and understanding or confirming the actual flow structure produced by each configuration was usually a secondary task in the wind tunnel. Instead, the work conducted to understand the flow was solely based on CFD simulation.

To a certain extent, this mass-production-style testing procedure functioned well in terms of meeting the KPI target. Consequently, it took some time to realize that there were discrepancies between the results from the wind tunnels and CFD simulation, which required immediate rectification at a certain stage. With this motivation, it was decided to equip both wind tunnels with a PIV system. This system has already been used for several years, not only to validate results of CFD simulation but also to understand the "actual flow phenomena" in the wind tunnel. This section explains how the system was developed to allow use in the development loop.

Overview of the Permanently-Installed System

As the access to the test section from the plenum was limited owing to the test-section structures that hold the side glass, it was not possible to simply install a commercially available PIV system. Instead, a tailor-made system was required. Applying PIV in the wind tunnel with a closed test section has become more common in recent years (e.g., Passmore [36]), but was not common in the early 2000s. When the first demonstration was made in 2003 at TMG, some literature on open test-section facilities was available, such as the study of Cogotti and De Gregorio [37], but to the best of the authors' knowledge, there were no previous examples of PIV systems used in automotive wind tunnels with closed test sections at that time. Thus, a new system had to be built from scratch, through trial and error.

In the initial stage of the feasibility study, a stereoscopic (2D3C) system was used. However, owing to laser reflections from the highly curved model surface of the F1 model, it was difficult to obtain the expected results. These reflections were difficult to treat and the model preparation and data processing at that time were time consuming. Therefore, it was decided to revert to the 2D2C system to minimize any uncertainty in the results and complexity in operation. Although losing a complete out-of-plane velocity component and intrinsic perspective errors (Raffel et al. [38]), it was found that the 2D2C measurement not only gives a better quality of images but also has advantages over the 2D3C system in terms of the times taken for the calibration and post-processing. Moreover, the camera did not need to be traversed when the measurement plane was moved: instead, only the lens focusing was adjusted remotely. Therefore, installation is far simpler and more flexible than that of 2D3C, especially when the surrounding space is limited, as it was in the authors' case. By the end of the 2007 season, both wind tunnels were equipped with 2D2C PIV systems, which proved to be the most efficient solution. Figure 8 shows the typical installation of the PIV system in one of the wind tunnels at TMG.

The original aim of this system was to increase the efficiency of the tests, which is essential for F1 development as discussed above. The system is unique in that all of its components, such as the camera, synchronizer, and traverse system, are controlled by the network server via local-area-network (LAN) cables and the network hub. The remote focus ring on the camera lens is connected to the server machine via an RS232 so that all the adjustments necessary during the measurement can be made remotely on the personal computer in the measurement booth. All the cables are pre-installed behind the test-section walls and connected to the sever machine so that cables do not have to be manually relocated when the measurement area is changed; the user only needs to plug in the camera and set the position of the light sheet optics (LSO) via the light-guiding arm on the traverse system, which is already set up for the region of interest. In this manner, even with a limited number of cameras and laser systems, a variety of areas of the car model can be visualized simply by swapping plugs. The only area where the camera had to be permanently installed was on the test-section roof above the front wheel, owing to the lack of access due to the overhead balance.

Table 2 gives a detailed summary of the specifications of the system. The camera is a PCO.2000 with 4 GB of onboard memory and GigE interfaces. All images are initially stored on the camera and then downloaded after the measurement. A Canon EOS lens is attached to the abovementioned remote-focus ring. The main laser is a Quantel CFR Twins outputting a 200 mJ/pulse, which is sufficient for measurements around the 60%-scale model with a typical viewing area of 450 mm x 450 mm, despite the fact that the laser is attenuated in the light guiding arm and the sidewall, which has an anti-reflection coating that minimizes losses. For PIV measurements of a full-scale car, a Continuum laser with output of a 400-mJ/pulse is available. Diethyl-hexyl-sebacate (DEHS), a material commonly used when conducting PIV in wind tunnels, is adopted for the tracer particles, which were seeded downstream of the test section. The sampling rate was typically 3.3 Hz for the full-plane measurement. Special software was developed to allow the user to operate the system in a push-button style from the data-acquisition personal computer.

It must be remembered that the field of view changes whenever the working distance changes in the moving laser sheet, as the camera is fixed. This means that the field of view is slightly different for each measurement plane, and the resolution varies accordingly. However, this was the most efficient solution for data acquisition.

The above system was introduced so that a snapshot of the velocity fields can be captured easily in important areas around the F1 model, during the development process, whenever it is necessary. The system was tailor-made for each region of interest to achieve this goal. In the following section, setups of the systems are described in detail.

Front-Wheel Wake Measurement

Horizontal Sections

Figure 9 shows the system used to measure the front-wheel wake on horizontal sections at various heights. This system was one of the most frequently used in the initial stage of system development, since this area contains much important information on the wheel wake position and shape. This area is also easily accessible, allowing the front-wheel wake analysis for PIV measurements. As the test section has no space on the ceiling directly above the model, a camera with a lens for which f = 135 mm was mounted horizontally to minimize blockage with a high-reflectance optical mirror placed in front of it, and was covered with a fairing. The mirror was mounted on three-axis servomotors that were remotely controlled, allowing them to be quickly adapted to the configuration of the car. This feature was useful not only when the model was yawed but also, for instance, during the development of models with different wheelbases.

Another unique feature of this system was the LSO assembly, as shown in Figure 9b. When the horizontal laser sheet hit the model, laser reflections were always problematic. As F1 cars have highly curved 3D surfaces, problems arose every time the plane position was changed. This was particularly apparent when the laser reflected off surfaces facing down, such as the bottom part of the front wheel and the undercut sidepod. The reflected light was scattered everywhere, with some of the light scattered onto other areas of the body and some onto the test-section floor, which usually resulted in measurement uncertainties. To prevent these reflections, it is usual to block both sides of the laser sheet so that the sheet does not hit other surfaces. It was troublesome to manually implement this blockage at every plane height, and a special LSO with a flap system was thus designed, where the angles of the two flaps either side of the laser sheet could be controlled via servomotors. By storing the flap angles for every height in advance during the calibration process, the user did not need to make adjustments during the actual measurements. In this way, it was possible to conduct many measurement sequences without time-consuming alterations to deal with the laser reflections.

Vertical Oblique Sections

Oblique sections became of interest after the 2009 regulations forced the inboard end of the front wing flap to have a cliff edge at y = 250 mm full scale. Consequently, Y250 vortices from this area, in conjunction with other vortices from the turning vane, started to play an important role. The two types of vortical structures eventually merged to generate a strong downwash as described earlier. The measurement in horizontal sections in the previous section actually shows traces of the structures, but visualizing longitudinal vortices on a horizontal section is of course not ideal.

A setup for a vertical oblique section measurement was therefore developed, as shown in Figure 10. The laser enters diagonally from the front and the camera is positioned at the rear, perpendicular to the measurement planes. The optical axis is rotated by 30 degrees from the x-axis, and sections are chosen at different positions along this axis. The camera with a f = 100 mm lens and LSO are positioned outside the test section, so that any effects on the aerodynamics of the model are removed, such as vibrations due to the wind and rolling road, minimizing the overall error.

Outboard Measurement on Transverse Sections

Transverse sections are typically used to examine the wake and flow around the wheel housing and side mirrors of commercial automobiles, where all wheels are covered by the body. These sections are frequently used when results from CFD simulation are post-processed, and have also been used in investigating isolated wheel measurements as shown earlier. However, the wheel wake cannot be captured, as related phenomena occur between the front-wheel wake and the sidepod, which is outside of the field of view. Therefore, this section was usually used more for demonstration until 2008. Such measurements were once conducted to show how longitudinal vortices dominate the flow field around the 2008 car model.

However, it was not until the 2009 regulations that measurements using this section became so crucial to development. The front wing was extended to completely cover the front wheels; as a result, the vortices produced by the front-wing endplate traveled downstream outboard of the wheels, whereas previously they had traveled inboard, as described earlier. Consequently, this area became of great interest. Figure 11 shows typical measurement sections outboard of the front-wheel wake. The camera lens used for this study had f =135 mm. The measurement of the most forward plane was barely possible as the working distance was the longest in the current system.

Underfloor Measurement on Transverse Sections

Another frequently used system was designed specifically for underfloor measurements on transverse sections, as the flow in this area contains many small but strong longitudinal vortices created by various upstream devices, such as the turning vanes, bargeboards, and silly plate gurney flaps. This measurement is a representative application that shows the power of applying PIV to show the interaction of vortices.

A typical measurement area and an overview of the system are shown in Figure 12. The camera with the f = 200 mm lens was half-buried in the test-section floor, so that the height of the optical axis matches the center of the field of view, looking under the car. A vertical parallel laser sheet was formed by a special LSO, through the anti-reflection-coated side window of the test section. The traverse system for the LSO was mounted along the sidewall, allowing visualization of the complete underside of the car, except the certain area, as indicated below. The same setup also covers the diffuser area [28].

Figure 13 shows the detailed schematics of the system. Owing to the obstruction of the test-section structures, the system had to be split into two parts in the streamwise direction. The LSO was mounted only on the rear traverse, but a redirection mirror assembly was installed on each traverse system. The parallel laser sheet created by the LSO, facing forward from the rear traverse, was defected into the test section by upstream mirrors. The mirrors were mounted on a motorized rotational stage, allowing the laser sheet angle to be set to any desired angle, which is useful in a yaw test. When the forward part of the car was measured, the mirror on the rearward traverse rotated to flat so that the laser sheet reached the mirror on the forward traverse. The operation of the complex system was made easy using proprietary software, developed by ILA GmbH. The user only needs to enter the measurement coordinates and a yaw angle.

Calibration System

As mentioned earlier, the most important concept of the PIV system is efficient and smooth operation so that its use is accepted during the development loop. In other words, it had to be booted and ready to use as quickly as possible owing to the high-pressure nature of the work conducted in the wind tunnel. When measurements are conducted on a limited number of sections, or the number of test cases is known in advance, the calibration procedure for 2D2C measurements is usually straightforward and can be conducted quickly. However, in the case of systems, like in this case, used for development, it must be possible to make measurements in multiple regions quickly upon request.

Therefore, the camera (not that on the roof) and LSO needed to be set up for frequent removal and reinstallation. Once the camera and LSO had been moved, recalibration was necessary. Moreover, since the switching of planes was done by adjusting the focusing of the lens, instead of traversing the camera, calibration was normally required for each section, since the field of views changed accordingly. However, it would have been a great inconvenience to the aerodynamicists if we had conducted calibration for every section, stopping the wind tunnel every time that the measurement section changed. Therefore, a unique and efficient calibration procedure was developed as follows.

Like in the 2D3C system, target planes with two-dimensional grids were used. Calibration systems for each corresponding measurement section are summarized in Figure 14. The jigs were designed so that they could be easily positioned relative to the reference point on the model in a precise and repeatable manner. The advantage of using a two-dimensional target plane is that any rotation of the camera is automatically corrected by a mapping function, and it is not necessary to make corrections after the post-processing stage. An additional feature is that calibrations for any other position can be calculated using a camera model, by calibrating only two planes (usually the most extreme ones). All the position settings, such as those of the traverse system, and the focusing and aperture of the camera lens, are stored for each plane by the software. Therefore, the user only needs to choose the measurement section in the model's coordinate system. The system then automatically adjusts all the components' positions during the measurement. In this manner, not only the efficiency of the measurement and post-processing but also the repeatability and accuracy of the results improved. This is especially important when comparing the flow fields produced by different aerodynamic devices on the same section in different wind tunnel sessions, for example.

Wheel Wake Measurement for a Full-Scale Car

So far PIV systems and measurement sections have been discussed for a scaled model. The wheel wake around a full-scale car was also measured to obtain the front-wheel wake structure on a horizontal section. For this measurement, two cameras were mounted on the ceiling and a special target plane assembly was made as shown in Figure 15. The forward area of the wheel wake was measured first simultaneously using two cameras, and the cameras were then shifted downstream to measure the rearward area. The mean velocity fields on the four planes were combined using a precise calibration system.

Data acquisition and Post-Processing Procedures

Raw particle images were acquired using data acquisition software (ILA). Typically, 315 image pairs are acquired per case when the images are taken in full resolution owing to the size of the onboard memory of the camera, and the number of pairs was found to be enough to have converging mean data. Each set of pair images was captured in separate instances with the prescribed time separation [DELTA]t in synchronizing the PIV camera and the double-pulse laser. Usually [DELTA]t = 20 [mu]s, except for the underfloor measurement, which uses [DELTA]t = 10 [mu]s, as the longitudinal vortical structure was so strong. The nominal sampling rate was 3.3 Hz for the full-resolution images, owing to the limitation of the data transfer speed of the camera. For the underfloor measurement, the sampling rate increased to 8 Hz, by reducing the region of interest, as it is narrow under the floor.

Acquired pairs of particle images were then post-processed to obtain final vector fields, using VidPIV 4.6 (ILA). The post-processing procedure is summarized as follows. First a mapping function was calculated for each plane using the calibration image of the target plane, and annotation masks were then manually applied in the area, where there were no particles. A pair of raw images was then processed with a standard fast Fourier transform (FFT)-based cross-correlation algorithm. This process was conducted a couple of times, by adaptively reducing the size of the interrogation window. Typically, the initial interrogation window size was chosen as 128 x 128 pixels (sometimes 96 x 96) and was iteratively reduced to 32 x 32 pixels with 50% overlap. Resulting pixel-based vector fields were post-processed with an outlier detection filter. Outliers were then replaced and interpolated using the local median filter. Usually, batch processing tools were used to post-process several cases simultaneously.

The resulting spatial resolution was around 3 mm nominally, depending on the measurement sections. The resolution was set twice as high for the underfloor measurement, by setting the final interrogation window size as 16 x 16 pixels, in order to place enough number of vectors in the limited field of view.

Results and Discussions

After installing the PIV systems and improving the usability as described above, the systems has been used in the wind tunnels at TMG for several years. This chapter shows representative results to demonstrate important flow features unique to F1 cars, and how the flow was affected by the 2009 regulation change. Part 1 gives results for the front-wheel wake and its downstream management, and Part 2 gives results for underfloor flow. The Y250 vortex and outboard vortex structures around the 2009 model are then analyzed, as one of the examples in which a PIV system is used in the development loop.

Part 1: Front-Wheel Wake and its Management

Front-Wheel Wake Structure

Figure 16 shows a typical raw particle image obtained from the measurement in a horizontal section in the front-wheel wake at the height of the wheel center (i.e., z = 175 mm). The contour indicates the mean velocity magnitude, computed from the resulting two velocity components. The small shadow at the top right corner of the raw image shows how the laser sheet was blocked to avoid reflections from the model body.

To compute the mean vector field, 315 post-processed instantaneous vector fields per measured section are averaged. The error in the velocity associated with this number of images actually depends on the regions. The regions where the flow has high fluctuations, the error could be up to the order of 10%. In relatively stable regions, the root mean square (RMS) of the velocity converges and the error in the average is much lower. The number of necessary images was checked and the most practical solution was to take around 300 images. Since the main goal of the typical test was to visualize the size and shape of the wheel wake, this number proved to be sufficient taking into account the shear layer can contain a higher error in flow velocities.

By stacking post-processed mean velocity fields for individual sections at eight different heights, the 3D structure of the front-wheel wake can be visualized and the effects of aerodynamic devices on the front-wheel wake can be clarified. Although the out-of-plane component of the velocity is missing, the characteristic wake structure can be observed well; the bottom part of the wake is defected outboard and the upper part of the flow turns inboard.

Figure 17 compares similar mean velocity contours for the 2009 model at the beginning of development and at the end of the season. The initial wake apparently shows a large velocity deficit and an upright structure. Meanwhile, at the end of development, the wake becomes more twisted and a smaller region of velocity deficit can be observed, similar to that for the 2008 car which is shown in Figure 16. This corresponds to the clearly higher KPI numbers at the end of development.

This is one example of many similar measurements. The PIV results helped clarify that it is better to push the wheel wake outboard in the bottom area and keep it more central or inboard at the top. At the bottom, it is important to keep the low-energy flow away from the underfloor area as thus not to disturb the performance of the floor and the diffuser. Decreasing the velocity deficit of the wheel wake also helps reduce the drag force of the wheel itself. Other examples that support the importance of a twisted wheel wake are given below.

Sensitivity of the Wheel Shape

Other results obtained from measurements around the 2007 full-scale car in the wind tunnel are presented. These measurements were conducted for two different types of tires having different sidewall shapes, as shown in Figure 18. It was found that there is an appreciable difference in the total downforce between the two types of tire, which is comparable to the difference achieved in almost a year of development in the wind tunnel.

Figure 19 shows resulting mean velocity fields for each tire. Note that each plane at one height consists of four planes in which data from individual cameras were post-processed separately, and then combined in one plane. As seen, the wake structure of tire 2 seems more twisted; in particular, the bottom of the wake is strongly swept outboard. This is a convincing example how the wheel wake structure should be managed so as not to affect the performance of the underfloor. Since the wake structure can be altered appreciably by the tire shape, it is important that the model tire shape is as close to the real shape as possible, as mentioned by Sprot [13]. Most of the teams switched to pneumatic deformable model tires supplied by the full-scale tire manufactures in 2005, instead of the solid tires that were previously standard. It is a great challenge to find the optimized profile even now, as the actual wheel shape changes depending on the actual driving conditions.

Y250 Vortex

Assuming the tire shape is fixed, the wheel wake has to be managed somehow. This is achieved using the turning vanes and bargeboards in conjunction with their footplates and canards that are positioned between the front wheel and the chassis. Generating a strong downwash and outwash is also an important goal in this area for the promotion of high-energy flow underneath the car.

A strong downwash and outwash was generated in a complicated manner for the 2008 F1 car using large aerodynamic devices to feed as much mass flow as possible underneath the car. These devices are designed between the front wheel and the inboard chassis in front of the side pod so that the interacting vortical structures from a series of devices are eventually united into a single large stable vortex under the floor. Meanwhile, for the 2009 car, where only smaller devices are allowed in such area, the way such a downwash was generated is completely different, although the basic concept remains the same. Figure 20 presents the results of vorticity fields in several planes from the measurement of vertical oblique sections that show the evidence that the Y250 vortex acts as a downwash generator and the high velocity flows are guided under the car.

Figure 21 compares similar measurements before and after the 2009 regulation change. For the 2008 car, there are two strong vortical flows, generating downwash flows. The top vortex from the turning vane travels outboard downstream and the vortex from the bargeboard travels through the undercut area of the sidepod. On the other hand, in 2009, the nominal vortical structure was the flap vortex. It must be remembered that the turning vane vortices were misaligned with the Y250 vortex. In fact, it was still an early stage of the development when this test was conducted, and thus, these positions were not optimized. Later it will be shown that these turning vane vortices merge with the Y250 vortex. It is also seen that the trailing vortices generated on top of the bargeboards flow through the undercut areas, which effectively seals the underfloor flow to maintain the negative pressure on the floor. This effect can be seen for both cars.

Part 2: Underfloor Flow

Figure 22 shows a typical result of underfloor measurements on the transverse sections for the 2009 car model. The contours show the mean vorticity field at different downstream locations as indicated in Figure 12. There are strong counter-clockwise vortical structures (looking from downstream on the left side of the car) under the floor that are created by the silly plate (or tray) and bargeboard footplate. Additionally, a vortex, which is shed from the bottom of the turning vane situated further upstream, can be observed in the first several planes in the upstream. Although the span-wise position of this vortex seems far from the other vortices under the floor, it strongly affects the other vortices in the downstream, as shown below.

Figure 23 shows similar vorticity fields with and without the turning vane on the 2009 car. Interestingly, there is an appreciable effect on how vortices interact under the floor when the turning vane is removed. The two vortices from the center floor and the bargeboard initially shed similarly, but eventually, their positions become far apart without the turning vane. The two vortices merge further downstream and diffuse quicker than the baseline case. This phenomenon is presumably related to the combined effect of change in the total mass flow provided by the downwash from the top of the turning vane, as well as the sealing effect of the vortex from the bottom of the turning vane. The former possibly causes a weaker vortex, and the latter alters the position of the vortex, which pulls the two vortices apart and outboard. This is a typical example to show that aerodynamic devices upstream are sensitive to the vortical strength and positions under the floor.

Figure 24 compares the underfloor flow structures between the 2008 and 2009 cars. It is obvious that owing to the complicated geometry in front of the sidepod, the 2008 car has smaller vortices, and the vortices quickly dissipate by the time they reach the middle of the floor. Meanwhile, there are few vortices that can be seen under the forward part of the 2009 car, but they merge in the middle of the floor and maintain their strength further downstream. This could also be attributed to the diffuser, which has more expansion than that used in the 2008 car, which can draw more mass flow into it than the 2008 car.

Part 3: Key Vortical Structures around the 2009 Car

In this section, important vortical structures are highlighted for the 2009 car to show how the car was developed to overcome the initial loss in aerodynamic performance due to the regulation change. It is also a good example of how PIV can be used in the development loop.

Figure 25 shows the 2009 car model used during the development of the front wing in low downforce spec. Table 3 summarizes the results of aerodynamic load measurements at two different ride heights. It indicated a strange ride height trend; the downforce was extremely sensitive to the front ride height: the downforce coefficient (Cz) drops by 0.451 when the front ride height was raised by 8.4 mm. Judging from the data, the loss in Cz of front wing (CzFW) itself relative to the total downforce loss seems smaller, and the front aerobalance (FAB) moved rearward, it can be immediately suspected that something happened downstream but in the forward area of the car. However, it is not possible to clearly identify the mechanism of the flow interactions, as there are many aerodynamic devices on the front wing alone. To understand the actual phenomena around the model, it was decided to conduct PIV measurements.

In this investigation, the target vortical structures were the Y250 vortex and front-wing trailing vortices that travel outboard of the front wheel. These phenomena were investigated using the PIV system for vertical oblique sections and for transverse sections, respectively.

Effect on Y250 Vortex

Figure 26 shows the mean vorticity fields on the vertical oblique sections at plane 0 and plane 6 for each ride-height case. Figure 26a for plane 0 clearly shows the Y250 vortex and turning vane top and bottom vortices. It is seen that the position of the vortices at RH 20/60 is lower and more outboard than that at RH 34/60. The velocity field on plane 6 in Figure 26b shows that the clockwise vortices were almost merged on this plane. It is evident that the vortex at RH 34/60 quickly weakens in comparison with that for RH 20/60 for which the vortex retains its strength. Again, the relative position of the main vortex is similar to that in the upstream.

Figure 27 visualizes the magnitude of the standard deviation of each component of velocity vectors on the same plane, showing how the flow fluctuation affects this area. First for RH 20/60, the highly fluctuating area in red is concentrated and relatively isolated from other relatively highly fluctuating areas as shown in light green in the contour plot, which presumably represents the fluctuation of the front-wheel wake. Meanwhile, the highly fluctuating area becomes broader for RH 34/60, and the total fluctuation presumably due to the bottom part of the front-wheel wake is much closer to the car, interacting with the Y250 vortex.

The investigation reveals that the loss in downforce at RH34/60 can be attributed to (i) the Y250 vortex becoming weaker owing to the lower load on the front wing, and thus (ii) the extent of the front wheel displacement being less than that for RH 20/60, meaning that the underfloor flow may be disturbed. Additionally, (iii) owing to the weaker flap vortex, the downwash flow became weaker, which affected the rear downforce performance. By repeating this kind of test on different occasions, it was found that the position and the stability of the Y250 vortex is a good indicator of the aerodynamic performance of the car. The vortex consistently sits lower and more outboard when the performance is better.

Effect of Front Wing Trailing Vortices

Figure 28 shows the mean vorticity fields on the transverse sections at four different downstream locations for each ride-height case, as indicated in Figure 11. Figure 28a presents the results for x = -150 mm just behind the front-wing endplate. The individual vortical structure corresponds to each component in the outboard area of the front-wing positioned upstream. At x = 0 as in Figure 28b, there is the formation of a notable coherent vortical structure, which seems to originate from the footplate vortices for RH 34/60, and the vortex shed from the Benzing becomes weaker, possibly being swallowed by the footplate vortex. This coherent vortical structure persists further downstream. Meanwhile, for RH 20/60, there is not so much space available for the footplate vortex to become organized. The Benzing vortex remains strong and stays more outboard. There is no similar structure for RH 20/60 in the further downstream around the bottom area; presumably the endplate vortex has collapsed already. However, the Benzing vortex still exists and creates flow in the outboard direction.

Figure 29 shows the results of the side component of the velocity on the last plane of Figure 28d. The inboard side-wash at the bottom part of the wheel wake is clearly visible, which supports evidence that the coherent vortical structure in this area is detrimental, hurting the floor performance. Again, we have consistent evidence that when the front wing endplate is ill designed, it tends to draw the wheel wake inboard, which usually has undesired consequences.

SUMMARY

This paper presented typical flow structures nominally around a 60%-scale model of an F1 car, on the basis investigations using a planar particle image velocimetry in wind-tunnels. Using a customized 2D2C PIV system, which is permanently installed in a wind tunnel, many flow phenomena have been investigated. From these experiences, the following conclusions can be drawn about the generic aerodynamic features of F1 cars in late 2000s.

1. Control of the front-wheel wake is essential in improving the aerodynamic performance of the F1 car. The bottom part of the wake must be expelled so that the wake does not interact with the underfloor flows, hurting the floor performance. Front-wheel wake structures are also sensitive to tire shape, as indicated by previous literature on isolated wheels.

2. Creating downwash by means of vortical structures in front of the floor is important in terms of feeding as much mass flow as possible under the floor. This was achieved using turning vanes and bargeboards for the 2008 car before a regulation change, while the Y250 vortex played a primary role as a downwash generator for the 2009 car.

3. Clockwise vortices from the rear view on the left (e.g., top vortices from the turning vane and the Y250 vortex) contribute to downwash generation, whereas the counter-clockwise vortices, such as bottom vortices from the turning vane, affect the positions of the underfloor vortical structures.

4. The positions and steadiness of Y250 vortices are good indicators of the aero performance of the 2009 car after the regulation change. The Y250 vortex must be positioned as low and outboard as possible in a steady manner to achieve good aerodynamic performance.

5. The front-wing endplate and its components shed strong longitudinal vortices. Vortices traveled along the inboard side of the wheel for the 2008 car before the regulation change, and traveled outboard for the 2009 car with a wider front wing. As both types interact with the wheel wake, aerodynamic devices on the outboard area of the front wing are important for the control of the wheel wake downstream.

All the results and system specifications mentioned above were only a part of extensive measurement campaigns. A variety of trials were conducted to investigate flow phenomena around the F1 car until the end of 2009 [28], including improved 2D3C investigations. Finally, it is worth mentioning that the system has since evolved further, with PIV measurements being more integrated in the development map of the wind tunnel now.

REFERENCES

[1.] Fackrell, J. E. and Harvey, J. K., "The Flow Field and Pressure Distribution of an Isolated Road Wheel," Advances in Road Vehicle Aerodynamics. BHRA Fluid Engineering Conference Paper 10, 1973.

[2.] Fackrell, J. E. and Harvey, J. K., "The Aerodynamics of an Isolated Road Wheel," Proceedings of the Second AIAA Symposium of Aerodynamics of Sports and Competition Automobiles, Vol. 16, 1975.

[3.] Fackrell, J. E., "The Aerodynamics of an Isolated Wheel Rotating in Contact with the Ground," PhD Thesis, University of London. 1974.

[4.] Hinson, M., "Measurement of the lift produced by an isolated, rotating formula one wheel using a new pressure measurement system," MSc Thesis, Cranfield University, Cranfield, Bedfordshire, UK, 1999.

[5.] Knowles, R., Saddington, A., and Knowles, K., "On the Near Wake of Rotating, 40%-Scale Champ Car Wheels," SAE Technical Paper 2002-01-3293, 2002, doi:10.4271/2002-01-3293.

[6.] Cogotti, A., "Aerodynamic characteristics of car wheels," Int. J. of Vehicle Design, Technological Advances in Vehicle Design Series, SP3, Impact of Aerodynamics on Vehicle Design, 173-196, 1983.

[7.] Merker, E. and Berneburg, H., "On the simulation of road driving of a passenger car in a wind tunnel using a moving belt and rotating wheels," In Proc. of 3rd Int. Conf. Innovation and Reliability, Florence, Italy, April 8-10, 1992.

[8.] Saddington, A.J., Knowles, R.D., and Knowles, K. "Laser Doppler anemometry measurements in the near-wake of an isolated formula one wheel," Exp. Fluids 42(5): 671-681, 2007.

[9.] Mears, A., Dominy, R., and Sims-Williams, D., "The Air Flow About an Exposed Racing Wheel," SAE Technical Paper 2002-01-3290, 2002, doi:10.4271/2002-01-3290.

[10.] Mears, A., Crossland, S., and Dominy, R., "An Investigation into the Flow-Field About an Exposed Racing Wheel," SAE Technical Paper 2004-01-0446, 2004, doi:10.4271/2004-01-0446.

[11.] Mears, A. and Dominy, R., "Racing Car Wheel Aerodynamics-Comparisons between Experimental and CFD Derived Flow-Field Data," SAE Technical Paper 2004-01-3555, 2004, doi:10.4271/2004-01-3555.

[12.] Sprot, A., Minto, J., Sims-Williams, D., and Dominy, R., "Aerodynamic Investigation on the Effect of Varying Through-Hub Flow on a Formula One Front Wheel Assembly," SAE Int. J. Passeng. Cars - Mech. Syst. 4(1):929-944, 2011, doi:10.4271/2011-01-1431.

[13.] Sprot, A., Sims-Williams, D., and Dominy, R., "The Aerodynamic Characteristics of a Fully Deformable Formula One Wind Tunnel Tyre," SAE Int. J. Passeng. Cars - Mech. Syst. 5(2):1026-1041, 2012, doi:10.4271/2012-01-1166.

[14.] Emmanouilides, K., "Heat Transfer Analysis of a 50% Scale Formula 1 Wheel Assembly," Durham theses, Durham University, 2015, Available at Durham E-Theses Online: http://etheses.dur.ac.uk/11162/.

[15.] Issakhanian, E., Elkins C.J., Lo K.P., and Eaton J.K., "An Experimental Study of the Flow Around a Formula One Racing Car Tire," ASME J. Fluids Eng. 132:071103, 2010.

[16.] Axerio, J., Iaccarino, G., Issakhanian, E., Lo, K. et al., "Computational and Experimental Investigation of the Flow Structure and Vortex Dynamics in the Wake of a Formula 1 Tire," SAE Technical Paper 2009-01-0775, 2009, doi:10.4271/2009-01-0775.

[17.] Axerio-Cilies, J., Issakhanian, E., Jimenez J., and Iaccarino G., "An Aerodynamic Investigation of an Isolated Stationary Formula 1 Wheel Assembly," ASME J. Fluids Eng. 134 (12): 121101-[17pp], doi: 10.1115/1.4005768.

[18.] Axerio-Cilies, J. and Iaccarino, G., "An Aerodynamic Investigation of an Isolated Rotating Formula 1 Wheel Assembly," ASME J. Fluids Eng. 134 (2):121101-[16pp], 2012, doi:10.1115/1.4007890.

[19.] Keller, W.P., Pearse, S.R.G., and Savill, A.M., "Formula 1 car wheel aerodynamics," Sports Engineering 2: 203-212, 1999.

[20.] Diasinos, S. and Gatto A., "Experimental investigation into wing span and angle-of-attack effects on sub-scale race car wing/wheel interaction aerodynamics," Exp. Fluids 45: 537-546, 2008.

[21.] Van Den Berg, M. & Zhang, X. "The aerodynamic interaction between an inverted wing and a rotating wheel," ASME J. Fluids Eng. 131(10): 101104-[13 pp], 2009, doi:10.1115/1.3215942.

[22.] Heyder-Bruckner, Jacques (2011) "The aerodynamics of an inverted wing and a rotating wheel in ground effect. University of Southampton, Engineering Sciences," Doctoral Thesis, University of Southhampton, 205pp.

[23.] Wilson, M., Dominy, R., and Straker, A., "The Aerodynamic Characteristics of a Race Car Wing Operating in a Wake," SAE Int. J. Passeng. Cars - Mech. Syst. 1(1):552-559, 2009, doi:10.4271/2008-01-0658.

[24.] Watts, M. and Watkins, S., "Aerodynamic Structure and Development of Formula 1 Racing Car Wakes," SAE Int. J. Passeng. Cars - Mech. Syst. 7(3):1096-1105, 2014, doi:10.4271/2014-01-0600.

[25.] Newbon, J., Dominy, R., and Sims-Williams, D., "CFD Investigation of the Effect of the Salient Flow Features in the Wake of a Generic Open-Wheel Race Car," SAE Int. J. Passeng. Cars - Mech. Syst. 8(1):217-232, 2015, doi:10.4271/2015-01-1539.

[26.] Knowles, R.D., Saddington, A.J., and Knowles, K. "On the near wake of a Formula One front wheel," Proceedings of the Institution of Mechanical Engineers Part D-Journal of Automobile Engineering 227 (11): 1491-1502, 2013, doi:10.1177/0954407013491903.

[27.] Ogawa A., Yano, S., Mashio S., Takiguchi, T. et al, "Development Methodologies for Formula One Aerodynamics," Honda R&D Technical Review F1 Special (The third Era Activities): 142-151, 2009.

[28.] Nakagawa, M. and Hojo, T., "Typical vortical structures around Formula 1 car, " Proceedings of the 6th International Conference on Vortex Flows and Vortex Models, ICVFM Nagoya 2014, Nov. 2014.

[29.] FIA 2009 Formula One Technical Regulations, 2009, Available from http://www.fia.com/regulations.

[30.] Katz, J., "Investigation of Negative Lifting Surfaces Attached to an Open-Wheel Racing Car Configuration," SAE Technical Paper 850283, 1985, doi:10.4271/850283.

[31.] Katz, J., "Aerodynamic Model for Wing-Generated Down Force on Open-Wheel-Racing-Car Configurations," SAE Technical Paper 860218, 1986, doi:10.4271/860218.

[32.] Katz, J., "Aerodynamics of Race Cars, " Annual Review of Fluid Mechanics, 38: 27-63, 2006, doi:10.1146/annurev. fluid.38.050304.092016.

[33.] Agathangelou B. and Gascoyne M., "Aerodynamic Design Considerations of a Formula 1 Racing Car," SAE Technical Paper 1998-02-23, 1998, doi:10.4271/980399.

[34.] Arai N., "A Development of Toyota TF109," JSAE Paper 20114243, 2011 (in Japanese).

[35.] Larsson T., "2009 Formula One Aerodynamics," EASC 2009, 4th European Automotive Simulation Conference, Munich, Germany, 2009.

[36.] Passmore, M., Spencer, A., Wood, D., Jowsey, L. et al., "The Application of Particle Image Velocimetry in Automotive Aerodynamics.," SAE Technical Paper 2010-01-0120, 2010, doi:10.4271/2010-01-0120.

[37.] Cogotti, A. and De Gregorio, F., "Presentation of Flow Field Investigation by PIV on a Full-Scale Car in the Pininfarina Wind Tunnel," SAE Technical Paper 2000-01-0870, 2000, doi:10.4271/2000-01-0870.

[38.] Raffel, M., Willert, C., Wereley, S., and Kompenhans, J., "Particle Image Velocimetry: A Practical Guide, 2nd Edition," Springer, Berlin, Heidelberg, 57-59, 2007.

CONTACT INFORMATION

Masaki Nakagawa

Vehicle Aerodynamics & Fluid Control Lab

Mechanical Engineering Dept. II

Toyota Central R&D Labs., Inc

41-1 Yokomichi, Nagakute, Aichi 480-1192, Japan

masakin@mosk.tytlabs.co.jp

ACKNOWLEDGMENTS

All the materials in this paper were provided by Toyota Motorsport GmbH in Cologne, Germany. Their full support from the initial feasibility studies to the completion of the flow visualization facility was greatly appreciated. The authors also thank all our former colleagues from the aerodynamics department at TMG, who supported and improved this work.

DEFINITIONS/ABBREVIATIONS

F1 - Formula One

PIV - Particle image velocimetry

LDA - Laser Doppler anemometry

CFD - Computational fluid dynamics

DRS - Drag reduction system

FIA - Federation Internationale de l'Automobile

KPI - Key performance indicator

FW - Front wing

RW - Rear wing

EP - Endplate

FP - Footplate

BB - Bargeboard

TV - Turning vane

TMG - Toyota Motorsport GmbH

2D2(3)C - Two-dimensional and two-(three-) component

LSO - Light sheet optics

RMS - Root mean square

RH - Ride height

Masaki Nakagawa

Toyota Central R&D Labs., Inc.

Stephan Kallweit and Frank Michaux

Intelligent Laser Applications GmbH

Teppei Hojo

Toyota Motor Corporation

Table 1. Specifications of the TMG wind tunnel.

Operation since           March, 2002

Duct size                 11.65 m (H) x 25.0 m (W) x 68.0 m (L)
Duct length               15.0 m (W) x 59.07 m (L) (Total line length
                          148.14 m)
Contraction ratio         6.7:1
Test section size         3.7m(H)x4.1m(W)x 15m(L)
Test section type         Closed (slotted for full scale)
Fan dimensions/power      Coaxial fan (diameter 6.3 m)/2.4 MW
Temperature control       22[+ or -]1[degrees]C
Rolling road dimensions   2.4 m (W) x 7.0 m (L) in steel (MTS)
Wind/Rolling road speed   max 70 m/s (nominally 50 - 60 m/s)
Boundary layer treatment  Double suction at scoop, perforated
                          plate, and
                          tangential blowing (WT1); u/V= 0.98 up to
                          z = 10
                          mm at model nose
Turbulence intensity      Generally between 0.04% and 0.22%
Load cells                Six-component external (Aerotech)

Table 2. Specifications of the PIV system.

Recording method         Double frame/single exposure
Recording medium         14 bit CCD camera (PCO.2000) with 4 GB onboard
                         RAM
Camera resolution        2048x2048 pixels (pixel spacing 7.4
                         [mu]m x 7.4 urn)
Data interface           GigaBit Ethernet
Lens                     Canon EOS Lens,/= 100-200 mm (depends on
                         region
                         of interest)
Typ. fit                 2.8-4
Lens accessory           Remote focusing (EOS) ring with 532 nm
                         band-pass
                         filter (ILA)
Laser                    Double-pulsed Nd: YAG
Pulse energy             200 mJ/pulse (Quantel CFR) for model-scale;
                         400
                         mj/pulse (Continuum) for full-scale
LSO                      Divergent and parallel (for underfloor meas.)
                         (ILA)
Laser related accessory  Laser guide arms
Synchronizer             Network based (TLA)
Seeding particle         DEHS, global seeding method
Seeder                   Laskin-type with electro-magnetic
                         valve (PIVTec)
Typ. region of interest  450 mm * 450 mm
Typ. mag. factor         26.4
Typ. pulse delay         10-20 [mu]s
Typ. frame rate          3.3 Hz typ. in full resolution / 8 Hz when
                         pixel masked
Post-processing method   VidPIV (ILA): FFT-based adaptive
                         cross-correlation
                         with window deformation
Interrogation window     32 x 32 pixels (typ.) /16 x 16 pixels
Typ. net resolution      Nominally 2-3 mm
Typ. valid vectors       > 95%

Table 3. Aerodynamic characteristics for different ride heights
(in full-scale).

RH (Fr/Rr)  FAB [%]     Cz    CzFW
[mmFS]

20/60       49.9      2.514   0.748
34/60       41.7      2.063   0.633
Delta        8.2     -0.451  -0.115
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Author:Nakagawa, Masaki; Kallweit, Stephan; Michaux, Frank; Hojo, Teppei
Publication:SAE International Journal of Passenger Cars - Mechanical Systems
Article Type:Report
Date:Jun 1, 2016
Words:12009
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