By working directly in CAD, the CFD engineer can easily conduct conjugate heat transfer (CHT) analysis, which calculates fluid flow at the same time as the temperature of the solid part. This allows for more accurate temperature predictions. Shown here is a CHT analysis of a portion of a cylinder head using STAR-CCM+ from CD adapco Group.
This automotive HVAC system CFD simulation used a detailed assembly model from PTC Pro/Engineer Wildfire. The simulation, using CFdesign from Blue Ridge Numerics, is fully associative with the CAD geometry. The designer was able to try out a half-dozen design variations when optimizing the airflow through the HVAC blower and pipe manifold and while monitoring the heat distribution in the blower and across the manifold.
For years, CFD vendors have taken the geometry from computer-aided design (CAD) to build the CFD model for analysis. But, says Thomas Marinaccio, director of CFD Consulting Services for software vendor CD adapco Group in Melville, NY, it's been a one-way transfer. "It's dead," he says, meaning the designer can't modify the geometry once it's inside CFD.
Funny, all the CFD vendors say their products are integrated to CAD. "That can mean several things," explains Judd Kaiser, technical solutions specialist for fluids and meshing at Ansys Canada Ltd (Waterloo, Ont., Canada). Does it provide good data transfer? Does it provide a clean translation of the CAD geometry to the CFD meshing environment? Does the parametric model automatically update when the CFD mesh updates? Does the integration truly provide two-way connectivity? That is, once an optimum solution is understood in CFD, can the designer push changes back into the CAD system?
The key here, continues Kaiser, is that the shared geometry database can be used for finite element analysis (FEA; the structural side of analysis) and CFD (the fluid, thermal, acoustic, vibration, etc., side of analysis). "The key is connecting everything end-to-end and that it's all in the same environment. The minute you do a data transfer, you lose that parametric information. You lose the ability to drive optimization by connecting to those parameters from the analysis end," continues Kaiser.
The latest version of CFX 5.7, Ansys' CFD system, has this capability in two ways. First, the company's Workbench is a user-friendly interface for not just FEA applications, but also for hosting meshing, a CAD interface, and CFD analysis. Of course, this integration is helped in that Ansys actually owns all of those software tools. Second, CFX 5.7 has a feature called "FSI" (fluid structure interaction). This is useful when the results of a fluids analysis changes the shape of a structure, and the structural analysis, in turn, affects the CFD calculation. Case in point: Fuel injectors and anti-lock braking systems, where fluid pressures are so high they actually deflect metal a little bit.
Delinking CFD to expensive desktops
Earlier this year, CD adapco introduced a new CFD product: STAR-CCM+. "CCM" stands for "computational continuum mechanics"; it's sort of CD adapco's moniker for "multi-physics" analysis, and more. (Currently, the product includes analysis for a wide range of external and internal flows, including conjugate heat transfer and porous media modeling; a broad set of turbulence models; and a variety of boundary conditions.) The "and more" comes from a complete rewrite of the CFD package, which is now based on a client-server architecture, Java scripts, and C++.
"With STAR-CCM+, it's like having [a big, expensive workstation] under my desk and I was doing the CFD analysis right here," explains Bill Clark, CD adapco's director of Engineering Software Business in Plymouth, MI. All the CFD can be launched, monitored, and displayed on a lightweight client machine, such as a wireless laptop. (Or a Palm Pilot or a Blackberry, posits Clark and Marinaccio.) This is possible because the client is similar to a web browser that has an operating system-independent, Java-based front-end. All the heavy CFD work, such as mesh generation, solving, and posting, is done on a big number-cruncher somewhere else (the server). The back-end software, the solver itself, is written in C++, an object-oriented programming language.
Moreover, adds Clark, the analysis is fully interactive. An analyst can start a CFD job at work, go home, log in from home, join the server, view how the simulation is going, make an assessment, and then, while the simulation is running, change parameters on-the-fly that might affect the job. Or, a team member can connect and modify the model—all without ever stopping the simulation, which would take the job out of queue and out of the number-crunching environment.
Easier to use, and more to use
Fluent Inc. (Ann Arbor, MI) recently debuted FloWizard, a complement to the company's flagship CFD product called "Fluent." "This is our first attempt at having a tool that's much easier to use," explains Stewart Featherstone, Fluent's Automotive Industry Team sales manager. "The idea here is to open CFD to more people and to compress the design cycle."
FloWizard is a flow simulation software package that prompts non-CFD experts through the process of taking a CAD model through CFD analysis—from CAD file import all the way to HTML output showing flow and heat transfer in-formation, including full-color plots and animations of the flows. Users don't need extensive experience with complex 3D CAD models nor with CAE meshes. In fact, FloWizard alerts the user about suspicious inputs or questionable results. The analysis involves "some very complex physics," continues Featherstone, including 3D, incompressible, steady-state flows; laminar and turbulent flows; and con-vection and conduction heat transfer. It can handle pres-sures, velocities, mass flow rates, volumetric flow rates at inlets and outlets, and walls (with or without internal conduction) and moving belts.
The wizard's ease-of-use comes with some limitations. FloWizard can handle fairly simple designs with simple geometries, such as the flow through a duct or a valve, and some kinds of filtration or heat transfers. However, designers will not be able to use it to analyze, say, engine combustion. For that, there's Fluent's primary CFD product. (As a result, assures Featherstone, CFD analyst experts will still have a job; high-end, very complex fluids analysis still require them.)
Fluent will release v6.2 of its main CFD offering. With this latest version, CFD analysts will be able to dynamically move and deform the CFD mesh. It includes acoustical analysis for rotating surfaces and for broadband noise sources (such as in an A-pillar or along the external leading edges of the passenger cabin). Analysts will find analysis models for automotive ignition (ignition delay in diesel engines and engines knocking), cavitation (such as in a spool valve or a fuel pump), tank sloshing, and even the solar load on cabin interiors.
Blue Ridge Numerics, Inc. (Charlottesville, VA) is offering Solid Motion Module, an ad-in to the company's CFdesign v7.0. The module lets users simulate and optimize devices that rotate or in some way translate motion: pumps, fans, compressors, valves, pistons, etc. The module simulates the geometry in motion and the interactions with the fluids around that geometry. For example, the simulation can show the linear motion of a "translating device," such as a piston moving in and out of a fluid, and how that fluid responds. Likewise, with the circular motion of turbomachinery, both the rotating and non-rotating geometries can be dynamically simulated. Such interactive simulation replaces the "rotating frame" approach, which does not account for neighboring stationary objects.
The module can also simulate a number of thermal conditions. For instance, it can simulate a thin heat source sandwiched within a solid, such as a gasket in a thermostat. The designer need only apply a heat flux directly to a surface at the interface of the two solids, rather than actually design the thin solid in CAD. The module can also simulate the thermostat's heating component as it shuts off when it reaches a target temperature, as well as when it adjusts the heat input to maintain that target temperature. Another thermal tool in the module can simulate Joule (resistive) heating—the heating generated by passing an electric current through metal, such as the connectors in an automotive wiring harness. The simulation would answer the burning question about the harness in use, before it actually happens.
What's it all mean?
In time, designers and engineers will be optimizing designs based on application, not by whether it's a fluid problem or a structural problem, according to Michael Raw, vice president, Global Product Development, Ansys Canada. The CFD analysis will be for improving the overall product performance, durability, and efficiency. "Concurrent engineering, collaborative, all those buzzwords, are housing the benefit of looking at the problem as a whole," notes Raw.