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CFD software: pushing analysis to the limit.

CFD Software: Pushing Analysis To the Limit

In 1991, a host of improved computational fluid dynamics programs will add features like advanced mesh generators, simplified user interfaces, and sophisticated graphics, as engineers tackle tough new applications in product development and manufacturing.

As it enters its second decade, computational fluid dynamics (CFD) software is moving into the mainstream of mechanical engineering. While programs introduced in the early 1980s to model fluid flow and heat transfer were used almost exclusively in aerospace applications, today's codes are tackling increasingly complex real-world problems in the plastics, metals, automotive, computer, and chemical industries.

At the same time, CFD software vendors are beefing up their packages with advanced mesh generators that create better geometric representations of parts, simplified user interfaces, and more sophisticated graphics. Programs are packing in new features that allow them to handle a wider range of CFD problems. And completely new codes are being developed to take advantage of the burgeoning power of PCs and workstations.

Indeed, the major U.S.-based CFD houses have an impressive lineup of product introductions on tap for 1991, including new versions of Fluent from Creare.x Inc. (Hanover, N.H.), Phoenics from Cham of North America Inc. (Huntsville, Ala.), Fidap from Fluid Dynamics International Inc. (Evanston, Ill.), Flowtran from Compuflo Inc. (Charlottesville, Va.), and Flow3D from Flow Science Inc. (Los Alamos, N.M.). Announcements are also being readied by some of the 30 smaller sellers of CFD software based in the United States and Europe.

In many ways, software developers have been hard pressed to keep pace with the demands of mechanical engineers. "People want to solve larger and larger problems," said Jim Rice, executive vice president of research and development at Compuflo. "We're now routinely solving problems with 100,000 nodes on a workstation." Such sizes can easily be reached by assignments like the 3-D analysis of an automobile engine cooling system.

"Running bigger problems is the bottom line in computational fluid dynamics," added Kent Misegades, manager of CFD applications at Cray Research Inc. (Eagan, Minn.). "Problem size in general seems to be approximately doubling every year."

Equally important is a growing requirement by engineers who are not specialists in CFD for software that can be easily used outside of the research lab, as part of the product development cycle. "We're beginning to see a real interest in using CFD for consumer products and manufacturing processes," said Flow Science president C.W. Hirt. Many of the most interesting applications, he said, are "things that are difficult to study experimentally and that haven't been analyzed theoretically to any great depth."

Applications Abound

In plastics manufacturing, for example, CFD has been used to study the injection molding of computer cabinets at Apple Computer Inc. (Cupertino, Calif.) and automobile dashboards at Ford Motor Co. (Dearborn, Mich.). "Companies are using CFD to determine how quickly they can make a part without burning or scorching the plastic," said Misegades.

Specifically, CFD analysis helps to determine how much plastic can be packed into a mold and how much a part will shrink and warp after it is ejected from the mold. A more complex question concerns what happens to the front of molten plastic as it moves through the die. CFD programs model the flow of plastic around obstructions or barriers in the die to see what happens when two fronts hit each other, forming weld and knit lines. Because these lines create structural weak spots as well as blemishes on the material, designers use the results of CFD analysis to help them tailor the gates and runners inside a mold so the weld lines will appear in the most desirable areas.

Indeed, injection molding is an important enough application to boast a number of specialty codes, including C-Flow from Advanced CAE Technology Inc. (Ithaca, N.Y.), Moldflow from Moldflow Ltd. (Kilsyth, Australia), and Poly3D from Rheotek Inc. (Cap Rouge, Quebec, Canada).

In the metals industry, Alcoa Corp. (Pittsburgh) has studied high-production-rate aluminum die casting. In a process similar to injection molding, a mass of molten aluminum is injected into a die and solidifies within a few milliseconds. CFD software predicts how that material flows after it is shot into the die, which determines the quality of the final part.

In their quest for more aerodynamic designs, automakers are using CFD to get a look at the airflow around their cars. Honda has used a code developed in-house to study its new $58,000 Acura NSX sportscar and Nissan has used the technique for its new Cefiro model.

Fluids codes are also providing an internal view of combustion processes. A specialty code called Speed was developed at the mechanical engineering department of the Imperial College of Science, Technology, and Medicine (London), with financial support from automakers Austin-Rover, Fiat, Renault, Volkswagen, and Volvo. The package more accurately models the swirling flow that occurs inside a cylinder head and has been used to create animated films showing the evolution of a flame front following ignition. The program can plot Eulerian scalar fields showing fuel concentration in the unburnt gas after combustion, which can be used to predict engine knock.

In the turbomachinery arena, Northern Research and Engineering Corp. (Woburn, Mass.) is using its home-grown Visiun CFD software in a five-year effort to develop improved centrifugal compressors. The code is being used to gain a better understanding of viscous 3-D flow fields in compressor impeller and diffusion blades. In principle, centrifugal compressors should be able to achieve overall efficiencies of greater than 90 percent. However, a lack of understanding of how to best match impeller and diffuser flow fields has to date prevented such efficiencies from being obtained. In 1992, Northern Research engineers hope to manufacture and test optimum impeller-diffuser combinations designed with information gained from CFD analysis.

Mechanical engineers working on computer printers at Canon and Toshiba have enlisted CFD to improve the designs of ink-jet engines. To blacken paper, one ink-jet technique uses an electrical hot spot to create a vapor bubble that pushes the ink out of the jet. "This seems like a simple problem offhand, but if you do the analysis you find that the vapor bubble will expand, overexpand, recollapse, and reexpand," said Hirt. "It's going through an oscillation that is controlling how the ink jet comes out and when it pinches off."

Here, CFD analysis is particularly useful because the printer's parts are so small that it is almost impossible to take experimental measurements with instruments such as flow-meters. CFD, on the other hand, can simulate the formation, size and shape, and flight of the ink droplets.

In the consumer products sector, even seemingly mundane processes such as filling beer and soda bottles on an assembly line are yielding to CFD analysis. "One of the big problems is how to reduce foaming," explained Hirt. "When there is foam, you have to leave more of a space in the bottle, you've got waste, and the liquid leaks over the top and makes the bottles sticky." The goal is to fill bottles as fast as possible, with a minimum amount of foaming. Complicating the analysis is the nozzle inserted into the top of the bottle to fill it, which partially closes off the top, preventing air from escaping. As a result, if the bottle is filled too quickly, it may explode.

The aerospace industry--the oldest user of CFD applications--is still coming up with new and challenging problems. Because the zero-gravity environment of space is difficult to duplicate on Earth, NASA and the French and Japanese space agencies have used CFD for fuel-sloshing analysis. This determines how liquid moves about the fuel tank, the forces and torques created on the rocket itself, and how to get fuel to stay at one end of the tank so that it can be properly fed to the engine.

At computer manufacturer Digital Equipment Corp. (Maynard, Mass.), CFD has seen service in the design of the cooling system for the Vax 9000 computer line. Initially, DEC engineers planned to use a liquid coolant. With CFD analysis, however, they were able to uncover bottlenecks to airflow inside the cabinet and redesign flow passages. The new setup enabled the computer to be aircooled.

New Features

As CFD vendors ready upgraded programs for release this year, the strongest trend is toward boosting the performance of preprocessing software. "The most important issue right now is mesh generation," said Compuflo's Rice.

A mesh is the geometric representation of the product or part that is to be analyzed by the CFD program. Meshes are generated by the CFD software from CAD models that must be created by the user. "Mesh generation is maybe 80 percent of the job," added Cray's Misegades, "and right now that's the weakest part of CFD." Historically, mesh generators that are included in CFD programs have been lacking in power. While most existing packages contain structured meshes, the goal is to move to the more sophisticated unstructured and adaptive meshes.

"The ideal situation would be to have a geometry description in a CAD data base and have an automatic means of generating a mesh, going through a solution process, quantitatively evaluating the accuracy of that solution, and then automatically refining the mesh," explained Rice. "This is the Holy Grail of CFD."

All the major CFD vendors are currently studying adaptive meshing, which will begin to appear--at least in limited form--in many of the commercial codes by the end of the year.

Chamflow 2000. This summer, Cham of North America will feature adaptive meshing in its new Chamflow 2000 workstation package. An adaptive mesh can better handle complex geometries because it automatically readjusts itself to create finer grids in areas of the model where higher flow gradients occur. The adaptive feature eliminates the need for a program's user to run an initial solution using a coarse grid and then interrupt the program to manually readjust that grid.

"We're trying to reduce the number of times the user has to interact with the code," said Cham of North America vice president Michael Nieburg. The ultimate goal is hands-off operation, where the user sets up the problem once and the code takes over to find the best solution.

Chamflow 2000 will also generate better meshes by using an unstructured grid. In a structured grid, individual cells must be shaped as six-sided brick elements. Though these elements can be compressed, the shape restriction limits the user's ability to refine the grid. An unstructured grid makes it easier to generate a more efficient model by also allowing tetrahedral shapes to be used.

Running Rampant

An unstructured adaptive mesh will be featured in the new Rampant CFD package, which will be shipped to beta test sites by Creare.x Inc. this month and is expected to be ready for market by the fall. It was developed at Creare.x over a two-year period with funding from the NASA Lewis Research Center (Cleveland, Ohio). In iteratively solving a problem, the program refines the mesh by creating additional grid points in areas with intensive flow and erasing unnecessary mesh components in areas where nothing is happening.

The initial release of Rampant will model compressible flows such as those in gas turbine engines and other advanced turbomachinery that operate in the transonic and supersonic flow region. Future versions, possibly ready as soon as year's end, will incorporate solvers to handle incompressible flows. Creare.x said that it will continue to market and improve its flagship CFD code Fluent--indeed several Fluent improvements are planned for 1991.

Phlex and Adapt. Mesh optimization is also at the heart of Phlex and Adapt, the first two CFD codes to be released by The Computation Mechanics Co. Inc. (Austin, Tex.).

Phlex uses h-p meshing, an optimization technique that begins the solution process by arbitrarily selecting an initial mesh size (h) and initial order of approximation (p). An initial solution is calculated so that an estimate of the error can be obtained. Next, an optimization algorithm determines the most efficient way (by changing grid size or the spectral order of that cell) to reduce the error. "It's a very complex data management problem in which you're dynamically changing all the mesh parameters so as to optimize the calculation," said J. Tinsley Oden, CEO and chief scientist at Computation Mechanics Co. and professor of engineering at the University of Texas at Austin.

Released last month, Phlex runs on workstations and is aimed at aerospace and manufacturing applications. The initial version of the program models 3-D incompressible viscous flows. A second release of Phlex that will also handle compressible flows is under development for shipment later this year.

Adapt is a high-end code for modeling very large aerospace problems on a mainframe or supercomputer. Set to be released this spring, Adapt evolved from code Oden's company originally developed for NASA and the Air Force Weapons Laboratory. Adapt uses h-adaptive methods, where the mesh is automatically refined in different areas according to their complexity.

To date, Adapt has been used for analyzing and designing chemical lasers for the Air Force Weapons Lab. At the NASA Lewis Research Center, an early version of the code performed rotor and stator calculations for turbine engines. Computation Mechanics Co. is in the final stages of putting together an Adapt version for the NASA Marshall Space Flight Center (Huntsville, Ala.) that will model incompressible flows in propulsion systems.

First Cut

Automatic meshing techniques that would cut the amount of time users must spend fine-tuning geometries may also be on the way from Fluid Dynamics International. According to company president Michael Engelman, Fidap will incorporate the feature in some form within a year. Fuller adaptive meshing capabilities would follow in later releases.

Creating the CAD model that must be prepared before a mesh can be generated is a time-consuming process, especially for complex parts such as turbine blades or engine exhaust manifolds. "When you're doing 3-D problems in complex geometries, users don't want to spend weeks designing models anymore," said Engelman.

That's why many CFD vendors have forged links with makers of CAD and finite element analysis packages to enable engineers to take models created with popular programs such as Patran from PDA Engineering (Irvine, Calif.), I-Deas and Supertab from Structural Dynamics Research Corp. (Milford, Ohio), and Ansys from Swanson Analysis Systems Inc. (Houston, Pa.). The model can be input directly into the CFD program so that users don't have to recreate new models from scratch. For example, Fluent interfaces with Patran, I-Deas, and Ansys; Easyflow links to Patran and I-Deas; Phoenics works with Patran and Supertab; and Flowtran connects with Ansys. Creare.x's upcoming Rampant release will interface with I-Deas and Ansys.

Flow3D. Other advances that developers are adding to their codes in 1991 can be characterized as incremental improvements designed to expand the range of problems each package can handle. For example, the current version of Flow Science Inc.'s Flow3D is marketed on its ability to do free-surface problems such as bottle sloshing and ink-jet applications. A new release, which will be called Flow3D/91 and is slated to debut this spring, will add the ability to model moving-obstacle flows. Examples of moving-obstacle problems include a hydraulic cylinder closing in a brake, an underwater submarine missile launch where the missile leaves its firing tube and splashes up through the surface of the water, and the decavitation that occurs when a valve is shut off.

Flow3D/91 is also expected to add implicit viscous stresses, which will allow it to handle creep flow, lubrication problems with free surfaces, and coating problems involving thin films on a surface. And it will add more sophisticated handling of unsaturated flow in porous media. This involves heat pipes or any kind of flow where moisture or a liquid is flowing into a material such as a fiber web or sand, but does not completely saturate the material.

Flowtran. New features incorporated into the just-released upgrades of Flowtran and PC-Flowtran from Compuflo Inc. include the ability to model time-varying flows (in addition to the steady-state flow fields they currently handle) and a dynamic memory feature to improve storage efficiency and speed. The Flowtran programs are currently available in separate versions for medium-size problems of roughly 25,000 nodes and large problems requiring 50,000 nodes or more. The upgraded packages automatically and dynamically assign the proper amount of storage space required for any size problem.

Fluent. Creare.x will make its workhorse CFD code more flexible with the release this summer of Fluent version 4, which will combine the separate Fluent and Fluent/BFC packages into a single code. Fluent is currently equipped with physical models that enable it to model combustion, radiation, and dispersed second-phase problems, among others. However, it is limited to Cartesian or cylindrical coordinate geometries. Fluent/BFC uses body-fitted coordinates that enable it to model arbitrary surfaces, but it can only handle fluids and heat transfer, not combustion, radiation, or second-phase situations. Version 4 will combine the capabilities of both existing packages.

Creare.x is also developing a single user interface that it will use across its entire product line. The feature will first appear in the upcoming releases of Fluent version 4, Rampant, and Nekton. The company's plan to continue marketing three different CFD codes typifies an industry trend toward carving out niches where different codes are aimed at different applications. "CFD technology really has not come to the point where you can take one solution scheme and say it's going to solve all kinds of problems," said Creare.x engineer Zahed Sheikh.

Fidap. Fluid Dynamics International is preparing revision 6.0 of Fidap for introduction at its fourth Fidap users conference in April in Evanston, Ill. Fidap 6.0 will be equipped with a new user interface that adheres to the emerging X-Windows and Motif advanced-graphics standards. It will also include an iterative equation solver that will significantly cut the amount of time required to obtain solutions, especially for large problems. In 1990, Fidap broadened the range of applications it could handle with the addition of software to simulate mass transport problems. The feature helps model, for example, the transfer of dopant in crystal growth and chemical vapor deposition.

Workstation Code

For workstation users, Cham of North America Inc. plans this spring to release Chamflow 1000, a package that will run under the Unix operating system. "We're trying to bring CFD computing into a form that is more practical and more usable by everyday engineers," said vice president Michael Nieburg. Chamflow 1000 will contain all the features of Cham's Phoenics CFD program, along with the user interface of its Easyflow PC-based fluids software. (A Unix version of the existing Phoenics package is also currently available.)

The announcement follows on the heels of the recent release of version 2.0 of Easyflow. Easyflow also contains all of the features of Cham's mainframe- and workstation-based Phoenics program, along with a streamlined user interface that simplifies the setting up of analyses on PCs. Simulations with up to 30,000 grid points can easily be run on the small machines, though larger problems are better handled on more powerful hardware. Easyflow can be used as a stand-alone program or in conjunction with Phoenics, where Phoenics performs analysis and Easyflow handles pre- and post-processing.

Cham is also readying a new 3-D grid generator for inclusion in both Phoenics and Easyflow. Previously, Phoenics users have had to use an external package such as Patran or the unpolished tools provided inside Phoenics to generate a 3-D grid.

Though generally classified as an FEA software supplier, Engineering Mechanics Research Corp. (Troy, Mich.) has also entered the CFD market. Recent developments include the FEAP package, which models thermal properties of printed circuit board layouts (see October 1990 ME, page 66). The company is also working to add automatic meshing to its NISA/3D-Fluid CFD code.

Challenges Ahead

Despite the upcoming plethora of product introductions, tough problems remain to be solved. "CFD is still a heck of a difficult application. The uncertainties of using a CFD code haven't disappeared," said Engelman of Fluid Dynamics International. "To get to design engineers and non-CFD specialists, codes have been made easier to use with improved user intefaces and graphics. This is a two-edged sword, however, because if you make it too easy, then you hide some of the difficulties associated with this modeling."

Those uncertainties include questions as to how well fluid codes model the actual physics of liquid and gas motion and interactions, particularly in complex applications. Indeed, experts noted that a CFD program is, in effect, a model of a model, because it is an attempt to encode a numerical approximation of a model that is itself an attempt to mirror a physical process.

"Fluid flows tend to be much more complicated than a structural analysis, because of the distortion of the fluid," said Hirt of Flow Science Inc. "You also get a greater variety of physical phenomena: turbulence, cavitation, multiphase flow, and free surfaces."

Though leading researchers such as Princeton University engineering professor Steven Orszag are working to develop the theoretical foundations of improved turbulence models, today's codes do not do well with so-called high-Reynolds number, highly turbulent flows.

"There are turbulence models, people use them, and they're in most of the codes," said Hirt, "but they really only work for certain limited classes of problems. If you try to go to a more complicated problem, you really need experimental data, and usually you end up having to tweak those models."

As a result, engineers in the field should remain alert for situations where CFD must be carefully applied. "Inexperienced users may turn off some of the options in a CFD package and just assume that the answers are going to be good," said Hirt. "You have to use a lot of judgment and common sense--look at the results and ask whether you understand why something is happening. It's the same in any engineering analysis, but particularly true for CFD."

PHOTO : Real-world applications. Automakers are using fluids codes in their quest for more-aerodynamic shapes. A CFD analysis revealed these low-pressure isosurfaces around the new Nissan Cefiro. Graphics were generated by Cray Research's MPGS visualization software.

PHOTO : Big assignment. Problems such as a full 3-D analysis of an engine cooling system can easily exceed 100,000 nodes on a workstation. Here, temperature contours for a diesel-engine cooling jacket were generated with Compuflo's Flowtran CFD package. The output plot was produced with PDA Engineering's Patran program.

PHOTO : Computer flow. To keep computers cooler, CFD analysis has helped uncover bottlenecks to airflow. Fluid Dynamics International's Fidap code shows flow velocities and pressure distributions inside a Hewlett-Packard 9000 Model 850 computer cabinet.

PHOTO : Far-flung problems. CFD software vendors are working to broaden the range of applications their packages can handle. Here, Cham of North America's Phoenics code was used to analyze the flow inside an experimental stirred reactor.

PHOTO : Precision models. The addition of mesh optimization features is a critical issue for CFD developers. The new Phlex code, which uses the h-p meshing technique, modeled the viscous incompressible flow around a cylinder. An expanded view of the mesh near the cylinder wall is shown.

PHOTO : Breathing better. To determine ventilation patterns inside a "potroom," where aluminum smelting operations are carried out, engineers from Alcan International and Cray Research modeled gas and fume concentrations using Creare.x Inc.'s Fluent code.

Alexander Wolfe Managing Editor
COPYRIGHT 1991 American Society of Mechanical Engineers
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1991 Gale, Cengage Learning. All rights reserved.

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Title Annotation:includes directory of vendors; computational fluid dynamics
Author:Wolfe, Alexander
Publication:Mechanical Engineering-CIME
Date:Jan 1, 1991
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