Scania's new CD7 climatic wind tunnel facility for heavy trucks and buses.
Scania AB has opened the new CD7 climatic wind tunnel test facility, located at the Scania Technical Center in Sodertalje, Sweden. This facility is designed for product development testing of heavy trucks and buses in a range of controllable environments. Having this unique test environment at the main development center enables Scania to test its vehicles in a controlled repeatable environment year round, improving lead times from design to production, producing higher quality and more reliable vehicles, and significantly improves the capability for large vehicle performance research.
This state-of-the-art facility provides environmental conditions from -35[degrees]C to 50[degrees]C with humidity control from 5 to 95 percent. The 13 [m.sup.2] nozzle wind tunnel can produce wind speeds up to 100 km/h. The dynamometer is rated at 800 kW for the rear axle and 150 kW for the front axle, which also has [+ or -]10[degrees] yaw capability.
In addition to the 'standard features' listed above for a climatic wind tunnel, this facility includes several additional capabilities. A Flow Control Device to improve the aerodynamic simulation around the sides of large, long vehicles in the test section was developed and implemented. Acoustic treatments in the circuit and test section plenum were designed to produce low background noise levels suitable for vehicle acoustic tests. This facility is also capable of producing complete simulation of solar, rain, snow, and soiling conditions. Moreover, emissions can be measured under road load conditions and in varying climates with different supplies of fuels.
This paper describes the engineering design of the climatic wind tunnel, important aerodynamic and thermal commissioning results, and performance validation results from initial tests with vehicles.
CITATION: Duell, E., Kharazi, A., Nagle, P., Elofsson, P. et al., "Scania's New CD7 Climatic Wind Tunnel Facility for Heavy Trucks and Buses," SAE Int. J. Passeng. Cars - Mech. Syst. 9(2):2016.
Before the early 1990s, vehicle development engineers used "onroad" testing for vehicle development and design validation. Prototypes and development vehicles were tested on public roads or on specific test tracks. For climatic testing, the geographic locations for the tests needed to provide the required weather conditions (e.g. temperature, humidity, solar load) and terrain (e.g. road incline, altitude, and / or road conditions) for product development. Potential issues with "on-road" testing include dependency on natural weather conditions, traffic conditions for tests on public roads, travel time to get personnel and test vehicles to the remote location, availability of test engineers, and access to the limited number of development vehicles. Additionally, if either the vehicle or the test instrumentation failed during a test, attempting to return to the remote test location may be difficult due to development schedule and prototype availability constraints. Measurement results from on-road tests are also subject to higher variability, due to crosswinds, gusts, and atmospheric turbulence. For climatic testing, variations in temperature, humidity, and solar load throughout a test day can add to the data variability, and therefore to uncertainty with design decisions . Dedicated wind tunnels and other test facilities can overcome many of the issues identified here with "on-road" testing.
Therefore, since the early 1990s, there has been a worldwide automotive industry trend to incorporate test facilities as part of the corporate design center. For the automotive passenger car and truck industry, this trend towards increased vehicle development work in climatic wind tunnels has continued to grow as more test conditions can be simulated in a test facility. Examples of more specific test capabilities include altitude simulation, yaw testing in climatic wind tunnels, and rain, snow and soiling testing. Indeed, some global automotive firms have as many as ten climatic wind tunnels located in their design and development centers.
For the heavy truck and bus industry, there have been relatively few climatic wind tunnels  that have been purpose built with the dimensions needed and the dynamometer capacities required. Scania made the investment in a new climatic wind tunnel that is specifically designed only for heavy trucks and buses to provide this tool for development of their products . Scania will now be less dependent on "on-road" testing and will be able to reduce product development schedules and to provide vehicles with better attributes for fuel consumption, emissions, driver comfort, and operational safety.
CD7 CLIMATIC WIND TUNNEL OVERVIEW
Jacobs was contracted to design and build a climatic wind tunnel for Scania AB in order to test heavy duty trucks and buses. The nozzle exit has dimensions of 3.7 m x 3.5 m (H x W), and a maximum test section velocity of 100 km/h. Detailed aerodynamic and thermal performance and other test capabilities are described below.
The basis for the design of the wind tunnel was developed jointly by Jacobs and Scania over a two year period. The wind tunnel circuit was laid out to have a minimum length and overall size in order to realize a cost effective building and facility. The relatively low maximum wind speed allowed a low contraction ratio which supported the compact size goal. Computational Fluid Dynamics (CFD) was used in the concept development phase to optimize certain wind tunnel component configurations.
The final circuit airline remained close to the circuit defined in the configuration development phase, with minor changes primarily to meet the velocity uniformity, turbulence specification and acoustic requirements. The final overall airline circuit in elevation is shown in Figure 1 and the plan views are in Figure 2. The individual components are pointed out in these figures. Discussions of the individual components follow below.
Development of the Wind Tunnel Circuit Airline
The wind tunnel airline was developed using traditional aerodynamic theory and guidelines and Computational Fluid Dynamics. For standard components such as turning vanes and contraction contours, traditional aerodynamic design theory and guidelines were utilized.
CFD served as a risk reduction tool to ensure that the aerodynamic design of certain wind tunnel components would work as intended. For components where the technical requirements were new and particularly challenging, CFD was used to evaluate different design concepts and the results were used to select the final geometry. Examples where CFD was used to develop new component geometries include the combined Corner 1 and collector configuration and the design and implementation of the Flow Control Devices (FCD).
CFD was also used to determine a velocity correction factor that was implemented into the wind tunnel airspeed control system to account for aerodynamic blockage effects associated with the trucks and buses tested.
CFD also provided freestream pressure coefficient (Cp) reference data (actual road data were not available) from the side of a bus for comparison to wind tunnel generated data. In addition, CFD results from a bus installed in the test section were used to design the size, shape and general location of the Flow Control Devices in the test section so that the pressure data from wind tunnel testing would have better agreement with freestream on-road Cp data.
Flow Control Devices (FCD)
Accurate experimental testing in wind tunnels requires a similar flow field around the vehicles to that of the on-road condition. Providing ideal aerodynamic test conditions for large vehicles like buses and heavy trucks with frontal areas in the range from ~8[m.sup.2] to ~11[m.sup.2] is difficult, particularly when yaw testing is necessary. Following the traditional guidelines for aerodynamic blockage effects [4, 5, 6] in open jet wind tunnels would cause the nozzle exit size to become prohibitively large, from a cost perspective. It should be noted that the primary purposes of the Scania CWT include climatic, powertrain, aerodynamic and acoustic testing. The aerodynamic test objectives included flow field evaluation and soiling testing, and not overall vehicle force measurements. Therefore, the traditional aerodynamic blockage limitations concerning the ratio of vehicle frontal area to nozzle exit size were not strictly applied. Instead, to improve the local aerodynamic simulation with the restrictions of a small nozzle size, an innovative Flow Control Device (FCD) was designed and implemented in the test section.
The FCD system consists of two airfoils that can be positioned around test vehicles to create a local aerodynamic flow field that more closely resembles on-road conditions. One objective of the FCD is to modify the local flow field near the bus engine intake to simulate on-road conditions. The engine intake is located on the side of the bus, at the lower rear corner. The other objective is to modify the local flow field near the driver and passenger doors and windows of trucks at yaw conditions to simulate on-road conditions.
Scania specified the bus and truck geometry that the initial implementation of the FCD was to be designed for. Flexibility in positioning the FCD in the plenum allows the FCD to be used with other vehicles with different dimensions. The ideal position for other vehicle geometries would need to be defined with a CFD analysis process, as outlined below.
Creating a flow field in the wind tunnel that accurately represents the on-road flow near the engine inlet along the rear side of buses is important for development work. However, due to their large frontal area and length, when a bus is installed in a wind tunnel test section, the flow near the engine intake is decelerated, thus affecting the inflow into the engine inlet. To create a flow field in the wind tunnel that is closer to the on-road conditions near the engine intake, a Flow Control Device was designed with CFD tools and analysis.
A detailed geometry of a typical Scania bus was used for the analysis, as shown in Figure 3. For the on-road "free air" CFD analysis, the bus model was inserted into a large rectangular box. The frontal area of the bus was 0.05% of the frontal area of the box to ensure that the free air mesh boundaries had no measurable influence on the aerodynamics around the vehicle. The commercial CFD package STARCCM+ v5.08 was used. Using surface wrapper technology and a polyhedral mesher, a suitable grid resolution was designed. The free air model included over 30 million cells, starting with average 5mm cells at the surface and approaching 400mm at the free air wall boundaries. To accurately model the boundary layers, 12 prism layers with a 1mm layer at the surface and growth of 1.25 was used.
A steady state analysis was used with a standard k-e turbulence model and y+ wall treatment. Rotating tires and moving ground techniques were used to properly simulate the underbody flow field. Along the back of bus at the engine inlet grille location, flow was removed based on manufacturer's available flow rates and then reinjected from under body engine exhaust vents. Figure 4 shows CFD streamlines around the bus model from the free air case.
In the design of the FCD system, CFD was used extensively to determine the location and shape of the components. After the free air CFD analysis was performed, the bus model was placed in the test section plenum, 6 m from the nozzle exit plane. Similar mesh generation guidelines and the same physics were used as for the free air analysis. Figure 5 shows the CFD pressure distribution prediction on the surface of the bus in free air and in the baseline wind tunnel. The higher pressure coefficient along the back of the bus in the baseline wind tunnel demonstrates the need for an FCD system to improve the flow simulation along the backside of the vehicle.
To create a flow field in the wind tunnel that is closer to the on-road conditions near the engine intake, the Flow Control Device developed with CFD was implemented. As shown in Figure 6, the airfoil shape of the FCD provides the necessary accelerated flow near the engine intake and then the flow diffuses downstream of the maximum airfoil thickness location on the FCD. Figure 7 shows a view of the bus in the test section with the FCDs located on each side.
Figure 8 depicts the aerodynamic requirement for the performance of the FCD. The difference in pressure coefficient between the wind tunnel and on-road had to be less than [+ or -]0.05 in the region of the engine inlet (between x = 6000 and 8000mm, where x = 0 is at the center of the front axle). The CFD results from the development phase shows that the FCD successfully changed the pressure distribution in this region. Figure 9 shows measured results during acceptance testing of the wind tunnel with the FCDs in place. The measured data shows that the difference in pressure coefficient relative to the on-road CFD results was within the [+ or -]0.05 limit.
It should be noted that CFD results were used as the reference onroad conditions. The options of full scale wind tunnel tests, scale model wind tunnel tests, and obtaining real on-road data were considered as sources for the reference data. Full scale testing even in the largest European wind tunnel  would result in blockage ratios of 10 - 15%, and significant data corrections would need to be applied. With scale model testing, there were concerns of Reynolds number effects on the pressure distribution along the side of the bus. It was decided not to use on-road tests due to the generally higher uncertainty of pressure coefficient data from repeatability, side winds, and ambient turbulence effects. While these technical concerns could certainly be accounted for, due to cost considerations and the schedule time required to organize full scale tests, it was agreed that the CFD results could be used as acceptable reference pressure data.
The other objective of the FCD system was to modify the local flow field around the truck cab during yaw testing. Due to the large blockage ratios of the truck cabs installed in the wind tunnel, the local flow field near the driver and passenger side doors and windows deviated from the on-road condition, in particular during yaw tests. During the development of soiling and wind-noise properties, it is important to have a correct flow field at the front corners of the truck.
To achieve this goal, a FCD was developed for yaw testing of trucks. The development method used CFD tools and analysis of the proposed configurations. The CFD methodology used for the truck FCD design was similar to that used for the bus analysis, outlined above. The FCD for this application involved re-positioning one of the FCD airfoils used from bus testing to the front of the test section and installing it in a "backward" position (trailing edge at the front).
Figure 10 is an off-axis view showing the FCD relative to the front of a truck cab at yaw. In this position, the FCD behaves less as an airfoil, and more like a flow defector. Figure 11 and Figure 12 highlight the effectiveness of this device in improving the local flow field near the driver side window of the truck. The on-road (or free air) simulations show that the flow remains attached around the front corner of the truck, with a high suction peak (at x = -1.1m in Figure 11) and high negative Cp downstream along the side window. The CFD simulations for the truck at 5 degree yaw with no FCD indicated local separated flow along the side window (center picture of Figure 12) and a reduced suction peak (at x = -1.1m in Figure 11). The CFD simulations with the FCD in "backward" position shows that the FCD airfoil gradually redirects the flow around the front corner of the truck and then forces the flow toward the truck side window. This increases the local suction peak (Figure 11), and eliminates the local flow separation, as shown by the better agreement of the 'Free Air' and 'Backward FCD' simulations in Figure 12.
The implementation of the FCD in the front of the test section is shown in Figure 10. It should be noted that the installed FCD for truck yaw testing offers adjustability in both position and angle. This adjustability can be used to accommodate a range of yaw angles and vehicle configurations. The correct position and angle for the FCD for truck testing at yaw requires some a priori knowledge of the local flow field. This understanding of the flow field can be defined with CFD, for example, and then the FCD can be adjusted to reproduce the correct flow field. When the correct local flow field is produced in the wind tunnel, development work to improve local soiling on the surface or to improve wind noise characteristics can be performed. Thus the FCD is a useful component to enhance the test capabilities of the Scania wind tunnel, within the constraints of a limited nozzle exit size.
CD7 CLIMATIC WIND TUNNEL CIRCUIT COMPONENTS
The climatic wind tunnel airline design was tailored to Scania's vehicle testing and flow quality requirements. The airline design features a combination of successful features from prior Jacobs wind tunnels and new custom solutions. Examples of successful components leveraged from existing wind tunnels include the boundary layer removal system, and acoustic component design. Examples of new custom solutions included the use of CFD for nozzle sizing for large vehicles, especially with yaw testing requirements, and the development of the Flow Control Devices to improve the aerodynamic simulation.
The wind tunnel airline was developed with the guideline of no flow separation in the circuit to keep circuit pressure loss low. Low pressure loss in the airline results in lower fan power requirements, and therefore lower cooling power requirements. The combined effect is reduced operating costs for the facility.
Test Section and Plenum
The 3/4 open jet test section plenum is 20.7 m long from the acoustic lining on the front wall to the beginning of corner 1. The plenum height to the insulated panel ceiling is 8.0 m. The plenum width between the insulated panel side walls is 10.0 m. A 250 mm thick acoustic lining covers the entire front wall, and the ceiling and side walls along the first 12.3 m of the plenum. No avoid issues with direct water and snow impingement, the acoustic treatment does not extend the full length of the test section plenum.
As mentioned above, the plenum is equipped with two Flow Control Devices. The FCDs feature a Clark-Y airfoil design with a 2.7m chord and a 4.48m span. The FCDs are normally stored against the walls of the test section just downstream of the acoustic treatment panels. However, as shown in Figure 7, they can be manually extended away from the walls into the airflow to modify the local flow field near the rear of the bus. With additional manual repositioning, the FCDs can be positioned near the front of a truck to modify the flow field during yaw testing.
The plenum has a solar simulation system installed that is capable of producing between 400 and 1100 W/[m.sup.2] of solar intensity over a 2.6m x 2.3m area with < [+ or -] 10% uniformity variation on a horizontal plane at z = 2.5m above the floor.
The test section also has a 150kW front wheel dynamometer with a 110km/h maximum speed and [+ or -]10[degrees] yaw capability, and an 800kW variable axial position rear wheel dynamometer from MAHA GmbH with a maximum speed of 170 km/h.
Boundary Layer Removal System
To meet the Scania requirements for boundary layer displacement thickness near the front dyno roller in the velocity range from 10 to 100 km/h, a 100 mm high suction scoop boundary layer removal system (BLRS) was selected. The scoop type BLRS removes the boundary layer in a tangential direction with minimal disturbance to the flow outside the area close to the scoop lip. CFD analysis showed that this system produced lower velocity and flow angle disturbances than a system with perpendicular suction through a porous plate. In addition, since the Scania wind tunnel also has acoustic requirements, it was important that the BLRS produced minimum 'self-noise'.
In the climatic wind tunnel community for passenger car and truck testing, there is not yet agreement as to whether boundary layer removal systems (BLRS) are required in CWTs. In the 2005 - 2015 timeframe, numerous new CWTs have been built for automobile testing, some with, and some without, BLRS [2, 8, 9]. For automotive climatic testing, one of the main concerns with proper boundary layer control is to have good thermal simulation at the rear of the vehicle for rear brake cooling, rear differential testing, and battery cooling for electric vehicles . Analysis has shown that with the rough floors in climatic wind tunnels (from dynamometer rolls and tracks), combined with the lack of a moving ground plane (common in aerodynamic wind tunnels), and the long distance between the BLRS in the nozzle exit and the back of a vehicle, the use of a BLRS has a small effect on the boundary layer at the rear of a passenger vehicle.
For the Scania CD7 CWT, the interest in proper boundary layer simulation is at the front of the vehicle, in particular for soiling tests and flow studies at the front of a truck. For these purposes, a BLRS in a climatic wind tunnel does have an effect, as described below in the soiling section and correlation with on-road results.
Collector and Corner 1
The corner 1 inlet measures 8.0 m and 10.0 m and is integral with the plenum. The design analysis showed that the high vehicle blockage ratio and long plenum negated any aerodynamic benefits from a traditional collector. Also, the low maximum velocity of the wind tunnel meant that the aerodynamic losses in the collector and corner 1 area were small relative to other wind tunnel components. Therefore, there is no traditional collector, which also minimized the circuit length.
The Corner 1 outlet is 3.7 m by 8.0 m resulting in a 2.7 to 1 contraction. A total of 7 double circular arc turning vanes are provided to turn the flow. The vanes are aligned at 25 degrees to fit the contracting corner geometry and are biased toward the inside of the corner to ensure there is no flow separation on the inboard part of the corner. An elevation view of the corner is shown in Figure 1. An opening is provided in the center portion of the turning vanes to allow vehicles to enter the test section without needing to move the vanes. This is in the wake area of both trucks and buses, and so the lack of turning vanes does not affect the performance of the wind tunnel.
The effectiveness of the corner 1 turning vane configuration was verified with two Navier Stokes based CFD solutions. The first analysis included a truck cab and the second included a bus installed in the test section. The results indicated that the vanes are effective in turning the flow with either the truck or bus in the test section.
Corner 2 and Fan Inlet Transition
Corner 2 has entrance and exit dimensions of 3.7 m by 6.0 m, and is lined with 150 mm thick acoustic treatment. There are 8 acoustic turning vanes with leading edge extensions that serve as the primary acoustic attenuation of the fan along this noise propagation path.
Fan and Annular Diffuser
The integrated fan and drive system was delivered by the company Clarage. The fan diameter is 3.75 m, with a 2.06 m fan hub. The fan shaft power required for the primary operating point was 403 kW. The outer shell of the fan annular diffuser transitions from a 3.75 m diameter to a 3.75 m square over a 3.1 m length. The tailcone profile was developed to keep a relatively constant effective cone angle and is truncated at a diameter of 420 mm. A numerical analysis indicated that the boundary layer thickness along the tailcone was increasing rapidly at this location. The tailcone was truncated to avoid an unsteady flow separation. The numerical analysis indicated attached flow on both the annular diffuser wall and the tail cone, up to the truncation location.
The back leg diffuser transitions from a 3.75 m square cross-section at the inlet to a 4.9 m square cross-section at the exit, with an area ratio of 1.71. With the diffuser length of 12.1 m, this corresponds to a conical equivalent half-angle of 3.1[degrees]. This combination of length and area ratio has been successfully used in designs employed in previous wind tunnels, and was judged to be a low risk design solution.
The medium angle diffuser just before the heat exchanger transitions to a 7.0 m by 6.6 m cross-section over a 4.0 m distance in two equal lengths segments. This type of increasing area medium angle diffuser follows previous design practice .
The Cu-Al heat exchanger with a loss coefficient, [K.sub.t] = 9.5 is located in a 7.0 m by 6.6 m cross-section area immediately downstream of the medium angle diffuser. The back pressure from the heat exchanger helps to keep the flow attached to the walls in the medium angle diffuser. The maximum air velocity through the heat exchanger is 7.8 m/s.
Acoustic Baffle Section
An acoustic baffle section of the same cross-section dimensions as the heat exchanger section is located immediately downstream. These baffles, along with corner 3 turning vanes, attenuate the fan noise and heat exchanger self-noise along this propagation path. The length of the baffles is 2.0 m. The trailing edges of the baffles were specifically designed to suppress turbulence generation and minimize wind tunnel pressure losses.
All of the acoustic treatment in the wind tunnel consists of bulk mineral wool wrapped in fiberglass cloth to prevent fiber migration, and covered by perforated sheet metal. For applications in climatic wind tunnels, mineral wool based acoustic treatment is strongly recommended over polyurethane foam or melamine foam based products for three reasons: 1) superior fire resistance, 2) no issues with degradation from ultraviolet (UV) light from the solar system in the test section, 3) Excellent longevity and performance in environments with cyclical high humidity and moisture from snow and rain testing. A detailed discussion of acoustic material performance in CWTs can be found in reference . Transitioning from high humidity or rain testing is accomplished by drying out the wind tunnel circuit and acoustic treatment with dry air before operating below freezing temperatures. Based on several years of operational experience, Scania uses a minimum of 24 hours to transition from wet to cold testing. The Scania operations team tries to schedule additional tests during the "dry-out" period before the cold testing to obtain a longer drying interval.
The steam injector section located immediately downstream of the acoustic baffle section provides space for the steam injection pipes which supply steam for humidity control. The 3.75 cm diameter pipes feature trailing edge fairings to suppress turbulence generation.
Corner 3 and Corner 4
The inlet and outlet of both corner 3 and corner 4 measure 7.0 m by 6.6 m. Corner 3 includes 17 double arc acoustic turning vanes and acoustic treatment on the walls as shown in Figure 1 and Figure 2. A mesh screen is attached diagonally across the corner 3 turning vane leading edges to further reduce the turbulence level at the nozzle exit.
Corner 4 includes 36 circular arc plate turning vanes with 6 mm thickness. The turning vanes are standard circular arc plate vanes except for the relatively short chord of 900 mm, resulting in the large number of vanes. The short chord length, along with round leading edges and tapered trailing edges, was selected to suppress turbulence production by minimizing the boundary layer momentum thickness at the vane trailing edge.
Stilling Chamber, Contraction, and Nozzle
The stilling chamber has the same height and width dimensions of corner 4 and contains no flow conditioning elements. The function of the stilling chamber is to provide decay length for the turbulence that is generated by the corner 4 turning vanes and allows for the upstream propagation of the pressure non-uniformity caused by the initial contraction curvature.
The contraction transitions from the stilling chamber dimensions to the contraction exit dimensions of 3.5 m wide by 3.8 m high, with a resulting contraction ratio of 3.5:1. The contraction contour is based on three third order polynomial curve fits with two match points. This contour design has been used previously and provides high flow uniformity at the nozzle exit.
Along one side of the contraction are three doors that allow the horizontal insertion of wings with water spray nozzles, used for rain, snow and soiling tests. The system was provided by Innovag AG, and consists of three wings that are axially located approximately 1/3 of the distance into the contraction, as shown in Figure 13.
The nozzle exit dimensions were determined in the concept development phase. In this phase, extensive 3D Navier-Stokes CFD modeling of the truck cab and bus in the test section were performed and the results were compared to results from on-road (free air) CFD simulations. The purpose of most of these CFD investigations was to define and optimize the nozzle size and thereby the aerodynamic test simulation around the test vehicles. This work included investigations at zero yaw and at yawed conditions for the truck cab. It must be noted that yaw test capability is a very demanding test requirement for a climatic wind tunnel. With information gained from the CFD analysis and interactions with Scania, the nozzle size was increased and the nozzle aspect ratio was changed. It was concluded from this CFD analysis work that a 13[m.sup.2] nozzle size with 3.5m width and 3.7m height would meet the stated test requirements for the listed test vehicles. This nozzle aspect ratio provided flow over the sides and top of the vehicles that could be corrected with a single blockage correction. The 13[m.sup.2] nozzle size was shown to provide very good agreement for Cp distribution on the vehicle surface compared to the on-road (free air) cases studied.
The difference between the contraction exit height and the nozzle exit height allows for a 100 mm boundary layer scoop located at the nozzle exit. The nozzle length of 3.2 m was based on the requirements of the idle city bypass system. The idle city bypass system has two mechanically driven doors located in the sides of the nozzle. When activated, the doors rotate into the nozzle and divert the airflow outward to adjacent idle city bypass plenums, as shown in Figure 2. The airflow is then directed to the sides of the test section, avoiding direct contact with the test article. The airflow through the wind tunnel circuit during idle city operation is sufficient to maintain the heat transfer in the heat exchanger required to maintain test section temperature.
CLIMATIC WIND TUNNEL ACCEPTANCE TEST RESULTS
There were three parts associated with the Scania CD7 Climatic Wind Tunnel commissioning tests.
1. Aero/Thermal tests with an empty test section: These wind tunnel performance tests were conducted with no vehicle or vehicle restraints in the test section. Several of the empty test section tests used the Climatic Wind Tunnel flow survey system installed at the Nozzle Exit Plane (NEP). These tests included the wind speed, temperature and humidity uniformity measurements. Additional tests that were performed in the empty test section but without the use of the flow survey system included turbulence intensity, background noise levels, boundary layer thickness, and functionality of selected sub-systems. The results of these tests are shown in Table 1.
2. Aero/Thermal Tests with a vehicle in the test section: These wind tunnel performance tests were conducted with a vehicle in the test section. These tests included wind speed and temperature stability, temperature transients, and a demonstration of the required wind speed, temperature, and humidity ranges. The results of these tests are shown in Table 2.
3. Sub-system tests: These wind tunnel performance tests were also conducted with a vehicle. These tests included all of the sub-systems including but not limited to: rain system, snow system, soiling system, dynamometer system, solar system and combustion air system.
Aero/Thermal Wind Tunnel Performance
Table 1 and 2 provide a summary of all the Aero/Thermal test results for both empty tunnel and with a vehicle. The following sections focus on specific tests deemed of interest to the vehicle development and wind tunnel testing communities.
Velocity, Temperature, and Humidity Uniformity & Stability
To measure flow uniformity properties, a Velocity and Temperature Uniformity Measurement System (VTUMS) was used, as shown in Figure 14. The VTUMS was able to simultaneously measure both differential pressure and total temperature at 42 points in a flow plane. The system included seven individual flow wings each outfitted with six Pitot-static probes and six 4-wire RTDs. Differential pressure measurements were made with Pitot static probes connected to Scanivalve DSA3217 pressure bricks which were setup with individual pressure references per channel. Temperatures were connected to and measured using the Scania Ipetronik system.
The wind tunnel airspeed control system used the plenum method and included a 10% correction to account for the high aerodynamic blockage ratios. Flow uniformity measurements were made at 40, 65 and 100 km/h with the boundary layer system on. Data was acquired for all measurements at 10Hz for 30 seconds.
Flow uniformity in the Nozzle Exit Plane was calculated as the standard deviation of [DELTA][u.sub.i]/[U.sub.ave] for the total number of points in the plane. The velocity ratio at each point was determined by accounting for both the differential pressure and temperature measured at each point in the grid:
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]
The variable i ranges from 1 to n where n = 42, the total number of points in the measurement grid.
Each of the 42 measurement points was located at the center of a subarea of the measurement grid in the Nozzle Exit Plane and was assumed to represent the flow field in that sub-area. The design intent was to measure the flow uniformity in a grid where the outermost points extended to within ~400 mm of the edges of the nozzle. The sum of the 42 sub-areas was originally intended to cover 90% of the Nozzle Exit Plane area.
Initial measurements along the perimeter points of the 42 point VTUMS flow grid showed higher than expected velocities. After comparing the measured flow velocities with the CFD results from the design, the higher velocities around the perimeter of the measurement area were attributed to the accelerating flow in the contraction. Further investigation showed that the VTUMS flow grid was fabricated larger than intended. The outermost pressure and temperature measurement locations on the VTUMS were actually located 150 mm from the edges of the nozzle, and not ~400 mm as intended.
Once this discrepancy was realized, the final velocity uniformity results were corrected to average the velocities between the internal grid points and edge grid points. These averaged velocities were taken as representative velocities for the originally intended measurement locations in the smaller grid. Velocity uniformity using the 42 actual VTUMS's grid point locations (grid extending to within 150 mm of the edge of the nozzle) was [+ or -]0.95% and the corrected results for the smaller grid (grid extending to within ~400 mm of the edge of the nozzle) resulted in a velocity uniformity = [+ or -]0.63%. The results from the two grids are discussed here to highlight this effect that can be expected in all contraction designs. The CFD results showed that this effect is largest in the Nozzle Exit Plane and the velocity variation decays with downstream distance from the NEP as the flow field settles out.
The temperature uniformity was less affected by the larger measurement grid area than the velocity results. On average across all temperature ranges tested, temperature uniformity was [+ or -]0.27 [degrees]C.
In addition to differential pressure and temperature probes, two of the seven wings were equipped with two Vaisala HMT humidity sensors for a total of four measurement locations. These sensors were located such that each sensor measured one quadrant of the nozzle exit plane. Humidity uniformity was measured as [+ or -]0.84%RH on average across all speeds and humidity levels.
Turbulence measurements were made with an A.A. Lab Systems AN-1003 Thermal Anemometer and x-film probes. The x-film probes were calibrated in-situ using the tunnel airspeed as a reference. Turbulence data was acquired at 20 kHz for 30 seconds and was low-pass filtered at 5 kHz to remove any spurious high frequency noise and high-pass filtered according to a cut-off frequency of [f.sub.cut]= U / 4m which correspond to 2.8, 4.5 and 7 Hz for 40, 65 and 100 km/h, respectively to remove effects of velocity modulations inherent in open jets.
The measurements were made at one point in the flow core on a 1m high floor mounted aerodynamic strut. The strut was centered along the nozzle centerline such that the x-film probe was located at the nozzle exit plane. The resulting longitudinal turbulence intensity was measured as 0.5% at all speeds.
Boundary Layer Control System
Boundary layer profiles were measured with a 16 port total pressure rake screwed to the dynamometer floor. Measurements were made 1m upstream of the dynamometer's front roll centerline (or 5 m downstream of the NEP), and at lateral positions of Y=0 and [+ or -]1.25m. The boundary layer rake's bottom probe was adjusted to be 2mm from the floor and the top probe was 102mm above the floor. The boundary layer rake pressure tubing was connected to a Scanivalve DSA3217 pressure brick. The Pitot-static probe tubing was connected to MKS pressure units, to provide a freestream reference velocity. Data were acquired at 10Hz for 30 seconds and averaged.
The boundary layer measurements were used to determine the displacement thickness. The displacement thickness was calculated using the trapezoidal integration method. Table 1 summarizes the displacement thicknesses at various speeds and lateral 'Y' positions.
The test section floor was kept in standard condition for these tests, without additional sealing or taping. The floor surface in front of the measurements at the Y = +1.25 and -1.25m positions were relatively smooth except a few welded seams. Based on smooth flat plate boundary layer theory, it was expected that the boundary layer at 30 km/h would be thicker than at 90 km/h. The measurements however did not show this trend, which was an unexpected result. This expectation is based on the assumption that the floor is flat and smooth. It is hypothesized that flow disruptions from weld beads on the floor and small gaps around the dynamometer, which act as potential air leaks to the dynamometer basement, caused the measured relationship between velocity and displacement thickness to deviate from theory.
Background Noise Levels
The background noise level measurements were made with a B&K Type 4193 low-frequency microphone mounted 2m above the floor, 1m upstream of the dynamometer front roller, and out of flow at 3.5m from the centerline. A foam ball was installed on the microphone to reduce self-noise from re-circulating flow in the plenum. Acoustic data were acquired at 44 kHz for 30 seconds with a band pass filter of 0.1 Hz - 22.4 kHz on the analog signal.
The acoustic measurement tests were performed in an empty test section. The out-of-flow sound pressure level in the plenum was 65dBA at 90 km/h, meeting the required overall sound pressure level of 76 dBA. Figure 15 shows the measured spectral data at two velocities compared to the Scania specification. The acoustic attenuation in the wind tunnel circuit is effective at suppressing the fan noise at the low and mid frequencies. In the high frequency range, the important acoustic sources are boundary layer noise from the flow in the nozzle and along the test section floor, and the unsteady shear layer flow impinging on the corner 1 turning vanes. The test section floor roughness from dynamometer rolls and tracks, and integrated slots for vehicle restraint attachment points contributes to the boundary layer noise. The attenuation in the wind tunnel circuit does not reduce these test section noise sources.
The test section plenum acoustic lining provides a test environment with low acoustic reflections, with an absorption coefficient, [alpha] > 0.9 for 200 Hz and higher. Figure 15 also shows the effects of acoustic panels covering the control room windows. At this measurement location on the far side of the plenum from the control room, the difference between measurements with and without the acoustic panels on the windows is approximately 0.6 dBA. The difference at locations closer to the windows is expected to be larger, based on similar comparisons made at other wind tunnels.
Plenum Out of Flow Pressure Fluctuations, C[p.sub.rms]
Low frequency pressure fluctuations can develop in an open jet wind tunnel when response modes such as wind tunnel circuit modes or plenum modes coincide at the same frequency with a forcing frequency, such as an edge tone feedback loop, or vortex shedding from the nozzle. [13, 14, 15]. The measured unsteady pressure is normalized by the dynamic pressure, to produce the non-dimensional C[p.sub.rms] term. High levels of C[p.sub.rms] can have significant negative effects on wind tunnel facility components and on aerodynamic and wind noise testing and measurements. The effects of high levels of C[p.sub.rms] for climatic wind tunnel testing have not yet been determined, but it is generally desirable to have low C[p.sub.rms] levels.
The low frequency pressure fluctuation measurements were made with B&K Type 4193 low-frequency microphone, and a B&K low-frequency barrel adapter, UC-0211. The microphone was mounted on a strut 1m above the floor, 5m downstream of the nozzle exit plane and out of the flow. A foam ball was installed on the microphone to reduce self-noise from re-circulating flow in the plenum. A B&K Nexus Type 2690 signal conditioner interfaced to LabVIEW data acquisition software that was used to acquire the data. A band pass filter of 0.1 Hz - 20 Hz was applied to the analog signal.
Data were recorded at 60Hz for 300 seconds and at velocities between 60 km/h and 100 km/h. The boundary layer removal system was turned off. Data were acquired both in an empty test section and with a truck cab installed in the test section. For the empty test section, the measured out-of-flow C[p.sub.rms] ranged from 0.8% to 0.94%. With the truck cab installed, the measured C[p.sub.rms] ranged from 1.78 to 1.84%. This increase with the truck installed is attributed to the microphone being located in the defected shear layer, exposing the microphone to the unsteady shear layer turbulence.
Rain, snow and soiling testing all use the three horizontal wings in the contraction with water spray nozzles.
The initial design flow rate for rain testing was up to 80 liters/min. During the design phase, it was recognized that in order to provide large, heavy rain droplets, a higher flow rate was required, and the final design and performance of the system was a maximum of 200 liters/min. During acceptance testing, a lower rain flow rate of 70 liters / min was defined, because at lower flow rates, the majority of the droplets falls to the floor and do not impact the vehicle.
During commissioning, significant work was performed to determine the proper balance and compromise between flow rates, water supply pressure and droplet size. The delivered rain nozzles generate droplets between .25mm and 1.2mm diameter.
The rain system can be operated at any wind speed. However, the percentage of droplets that reach the test vehicle is dependent on droplet size and wind speed. The rain system functions at any temperature above 0[degrees]C.
Rain uniformity was measured with a 2.6 m x 3 m fixture consisting of 9 small measuring cylinders, located in a vertical plane at the truck front position. During commissioning, significant work was performed with varying flow rates, nozzles, and supply pressure to develop suitable rain uniformity. Typical results showed that most (but not all) of the cylinders collected rain that was in the allowable range of 80% - 120% of the average rain intensity. A recommendation from the commissioning team was to use more measuring cylinders in a tighter grid to get a better representation of the actual rain uniformity.
The system was designed and verified to deliver up to 40 liters/min of snow. The design condition was to produce particle diameters in the range between 0.1 mm and 0.5 mm. Snow production was demonstrated from 30-100 km/h in multiple runs. The higher speeds reduce the seeding (freezing) time for the snow and result in longer "full freezing distances".
Tests were performed to verify that it was possible to make snow between -30[degrees]C and - 12[degrees]C. It should be noted that there is a strong dependency of the settings used for airspeed, water pressure and air pressure on the quality of the snow produced. In some cases where liquid water was present in the front of the test section, powder snow was still created in the rear of the test section after giving the water more time to freeze.
Snow uniformity was measured with a 2.4 m x 3.6 m test area fixture consisting of 12 snow collection plates, located in a vertical plane at the truck front position. During commissioning, significant work was performed with varying settings of wind speed, temperature, RH, water pressure and air pressure to develop suitable snow uniformity. The tests demonstrated a high repeatability for the same settings. The snow system can be used across a wide range, depending on the type of results desired. The system can create all types of snow ranging from dry powder to wet snow and also freezing rain. Figure 16 shows the snow system in operation with a truck installed in the test section.
Conducting soiling tests in the CD7 wind tunnel makes it possible to perform tests with repeatable and stable conditions. Previous on-road testing had been very dependent on weather conditions and the quality of the road surface. For example, it had been difficult to achieve the same level of soiling contamination throughout the day, despite continuous watering of the test track.
There are mainly three areas of concern during soiling tests. These are; 1) contamination of the cab side, 2) visibility through the side glass, and 3) contamination of the mirror glass. The latter two areas have been possible to perform in other climatic wind tunnels for heavy trucks. The possibility to rotate the front wheels in the Scania CWT enables the investigation of truck cab side soiling performance. The front wheel dynamometer allows both wheel rotation, and also yawing up to [+ or -]10 degrees, in order to study soiling performance in crosswind conditions. This feature makes the Scania CD7 CWT unique in Europe for heavy truck applications.
Soiling is achieved by injecting water with a fluorescent dye into the airflow. This can be done in two different ways, depending on what property is to be investigated. When studying the contamination of the side glass and mirror glass, the mix of water and fluorescent dye is injected from 18 nozzles positioned in the three horizontal wings in the contraction. For investigations of the cab side soiling, no water is injected from the nozzles in the wings, instead a "spear" with two nozzles is placed directly in front of the front wheel. The nozzles are aimed at the tire contact patch, and the mix of water and fluorescent dye is transferred to the cab side by the wheel rotation and the airflow. The properties of the injected mixture can be adjusted by controlling the water pressure and the concentration level of the fluorescent dye. During commissioning, it was determined that 0.4% - 0.6% concentration of dye in the mixture was clearly visible. During the tests the light is turned off in the test section, and UV-lamps are used to illuminate the fluorescent dye. Examples of soiling test results are discussed below.
CWT ROAD TO WIND TUNNEL CORRELATION RESULTS
As an important part of the start of regular test operations at the CD7, test procedures were developed and validated in order to ensure that the measurements are not only highly repeatable, but also reflect the real world, "on-road" driving conditions. Two examples of the correlation between measurements in the CD7 CWT and on-road are presented below.
Even though a lot of development can be done by studying relative differences between configurations, good correlation to on-road test data is of great importance. In order to confirm system behavior and to complement worst case (steady state) testing, simulated road load testing is frequently used. Figure 17 and Figure 18 show the correlation between on-road tests and the same road load conditions simulated in the climatic wind tunnel. The on-road test was performed on the road between Motril and Granada in southern Spain.
Figure 17 shows the comparison between the engine coolant temperature for the road test case and the corresponding simulated test in the climatic wind tunnel. As seen in the figure the climatic wind tunnel simulation captures the coolant temperature variation in the on-road test very well. The difference in ambient temperature is also shown in the figure. In this case wind tunnel reference temperature was kept constant and no effort was taken to mimic the small variation in temperature during the on-road test.
Figure 18 shows the temperature at two different locations in the chassis behind the cab. Position 1 shows the air temperature at a location directly behind the drive cab and position 2 shows the surface temperature of the frame in rear part of the truck. Also in this case, the climatic wind tunnel tests show good correlation to the onroad comparison. However, there is a small time shift between the measurements for the temperatures measured at position 1. One reason for this is the different starting conditions between the two tests.
Cab Side Soiling Correlation
During soiling tests in Scania CD7 CWT cameras are mounted in fixed positions in order to take pictures and to record the test. First, the wind speed and corresponding wheel rotation are set. Then, after the flow field has stabilized the injection of the water and fluorescent dye mixture is started. The time needed to reach a state where the contamination of the investigated area has stabilized varies for different trucks, wind speeds, and yaw angles. Currently, all analysis is qualitative, but development of quantitative analysis tools is planned for the near future.
The initial tests in the Scania CD7 CWT for cab side soiling have shown good correlation with previous road tests. Some variation in the absolute location of the contaminated region has been observed, but the trends between different configurations have been very good as shown in Figure 19. The difference in absolute position of the contaminated region could also be due to the use of different trailers or box body between the road test and wind tunnel test. In the road tests, some additional contamination is often observed on the cab side due to splash and spray when driving through deep puddles. This effect will, of course, not be seen in a controlled environment like the climatic wind tunnel.
It has also been observed that the boundary layer removal system has a significant effect on the contamination of the cab side, and that it will influence the position of the contaminated region on the cab side. It is therefore highly recommended to always use the boundary layer removal system in combination with wheel rotation in order to achieve the most realistic boundary conditions as possible. Moreover, good correlation of trends between different configurations in soiling simulations in CFD compared to the test results from the Scania CD7 CWT has also been observed.
The CD7 climatic wind tunnel has opened in Sodertalje Sweden at the Scania base of operations. The wind tunnel includes rain, snow, soiling and acoustic test capabilities, in addition to the standard capabilities for wind (up to 100 km/h), thermal (-35[degrees]C to 50[degrees]C), humidity (5% to 95% RH), solar (400 to 1100 W/[m.sup.2]), and road load (up to 800 kW) simulation. The climatic wind tunnel features an innovative Flow Control Device that can be used to improve the air flow around the sides of buses and also trucks under yaw conditions. The wind tunnel facility has met or exceeded all technical requirements. As a premier facility for simulating real-world driving conditions, Scania test engineers can subject test vehicles to the most demanding weather conditions, thereby reducing product development schedules and improving vehicle performance.
With the ability to reproduce a wide range of worldwide driving conditions in the laboratory, Scania engineers will be less dependent on-road testing. This will allow testing across a range of relevant worldwide climate conditions, regardless of the time of year, and the test results will be more repeatable than on-road tests. Initial correlation comparisons between CD7 wind tunnel test results and on-road results are very favorable. Scania anticipates that testing in the new facility will enable improvements in fuel efficiency, driver comfort, reductions in emissions, and improved product safety due to the testing with snow, rain and soiling conditions.
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[3.] Scania adds Advanced Climate Test Facility to its Technical Centre in Sweden, Scania Press Release, 11 November 2013.
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The authors acknowledge the support of the entire CD7 Scania Wind Tunnel Team who made the project a success. Jacobs Project Management and support included: Steve Arnette, Troy Tarnutzer, Klaus Stocker and Corina de Pierro. From Scania AB, Lars Hult, Erik Soderberg and Richard Sigg made significant contributions during the design, construction, and commissioning work. From Jacobs, Joseph Yen and Bill Martindale made important contributions to the aerodynamic design, including CFD analyses. Ron Lipinski was the Commissioning Manager and was crucial during the on-site start-up and system integration phases.
[alpha] - Acoustic absorption coefficient
BLRS - Boundary Layer Removal System
CFD - Computational Fluid Dynamics
Cp - Pressure Coefficient
C[p.sub.rms] - Dimensionless rms of fluctuating pressure, p'
CWT - Climatic Wind Tunnel
dBA - Decibels, A-weighted
FCD - Flow Control Device
[f.sub.cut] - High-pass filter frequency for turbulence data
[K.sub.t] - Heat Exchanger airside pressure loss coefficient
NEP - Nozzle Exit Plane
SPL - Sound Pressure Level
VTUMS - Velocity & Temperature Uniformity Measurement System
Edward Duell, Amir Kharazi, and Paul Nagle
Per Elofsson, David Soderblom, and Christer Michael Ramden
Table 1. Scania CD7 Climatic Wind Tunnel aero/thermal performance test results for empty test section. Parameter Measured Results @ 100 km/h= 0.62% @ 65 km/h = 0.62% Uniformity @ 40 km/h = 0.63% @ 100 km/h =0.5% Turbulence Level @ 65 km/h =0.5% @ 40 km/h = 0.5% @ 40 km/h 40%RH 50 [degrees]C = 0.85% 95%RH 33 [degrees]C = 0.85% 95%RH 5 [degrees]C=1.0% Humidity Uniformity @ 65 km/h 40%RH 50 [degrees]C = 0.82% 95%RH 33 [degrees]C = 0.82% 95%RH 5 [degrees]C = 0.68% Humidity 10 to 95%RH [approximately equal to] 25min Achievement Time 95 to 10%RH [approximately equal to] 60min @ Y=0 (centerline) Boundary Layer 90km/h=8.8 mm Displacement 30km/h=7.6 mm Thickness (@ 5 m downstream @Y = +1.25 m /-1.25 m of NEP) 90km/h=12.2 mm /14.0 mm 30km/h=9.9 mm /11.6 mm Background Noise SPL = 65 dBA @ 90 km/h Level Table 2. Scania CD7 Climatic Wind Tunnel aero/thermal performance test results with vehicle installed in the test section. Parameter Measured Results Wind Speed Range [U.sub.max]=100 km/h at +20[degrees]C [U.sub.max] = 90 km/h at -35 [degrees]C Wind Speed Ramp 0 to 100 km/h = 32 sec Rate l00 km/h to 10km/h = 17 sec 40 km/h = [+ or -]0.14 km/h on average Wind Speed Stability 65 km/h = [+ or -]0.18 km/h on average 100 km/h=[+ or -]0.23 km/h on average 20 km/h = [+ or -]0.32 [degrees]C on average Temperature 40 km/h = [+ or -]0.35 [degrees]C on average Stability 65 km/h = [+ or -]0.28 [degrees]C on average 100 km/h = [+ or -]0.32 [degrees]C on average Temperature 1 [sigma]([DELTA]T) = + 0.23[degrees]C Stability: Idle City Mode Temperature -20C: 1 [sigma]([DELTA]T) = 0.39[degrees]C +20C: 1 [sigma]([DELTA]T) = 0.16[degrees]C Uniformity +40C: 1 [sigma]([DELTA]T) = 0.26[degrees]C Temperature dT/dt > [+ or -] 29.5 [degrees]C/hour Cool Down Transient dT/dt > [+ or -] 24.2 [degrees]C/hour Warm Up Temperature dT/dt > [+ or -] 25.6 [degrees]C/hour Cool Down dT/dt > [+ or -] 29.2 [degrees]C/hour Warm Up Transient, w/ Vehicle Temperature Range +50 [degrees]C to -35 [degrees]C Humidity Range & 10% RH - 40% RH @ +50 [degrees]C Control (Standard 10% RH - 95% RH @ +33 [degrees]C Operating Mode) 10% RH - 95% RH @ +5 [degrees]C Control (1 [sigma]) = [+ or -]0.49%RH Ave. Humidity Range & 10% RH - 40% RH @ +50 [degrees]C 10% RH - 95% RH @ +35 [degrees]C Control: (Idle City 10% RH - 95% RH @ +5 [degrees]C Mode) Control (la) = [+ or -]0.51%RH Ave. 1.78% < [Cp.sub.rms] < 1.84%, 65 km/h to Out of Flow [Cp.sub.rms] 100 km/h with truck installed 0.80% < [Cp.sub.rms]< 0.94%, 60 km/h to 100 km/h with empty test section Bus Plenum Flow 40 km/h: [C.sub.p] [less than or equal to] 0.05 from Free Air Control System 65 km/h: [C.sub.p] [less than or equal to] 0.05 from Free Air 100 km/h: [C.sub.p] [less than or equal to] 0.05 from Free Air Truck Plenum Flow Control System [C.sub.p] Improvement Demonstrated
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|Author:||Duell, Edward; Kharazi, Amir; Nagle, Paul; Elofsson, Per; Soderblom, David; Ramden, Christer Michael|
|Publication:||SAE International Journal of Passenger Cars - Mechanical Systems|
|Date:||Jun 1, 2016|
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