Going with the flow

Software for computational fluid dynamics has come a long way. Dave Wilson compares and contrasts a selection of the CFD packages available.

Computational Fluid Dynamics (CFD) is the analysis of fluid flow, heat transfer and related phenomena using advanced software and numerical techniques. CFD has become an integral part of the engineering design and analysis environment of many companies because of its ability to predict the performance of new designs or processes before they are ever manufactured or implemented. The savings to be made are obvious: fewer iterations to the final design; shorter lead times, and fewer expensive prototypes to produce. On top of this, CFD also encourages innovation through providing a cost-effective means of testing novel designs and concepts that would otherwise be too expensive and risky to investigate.

According to AEA, CFD works by dividing the region of interest, the inside of a mixing vessel or the air around the outside of a car for example, into a large number of cells or control volumes (the mesh or grid). In each of these cells, the partial differential equations describing the fluid flow (the Navier-Stokes equations) are rewritten as algebraic equations that relate the pressure, velocity, temperature and other variables, such as species concentrations, to the values in the neighbouring cells.

These equations are then solved numerically yielding a complete picture of the flow.

These techniques date back to the early 1970s, and the first commercial CFD software became available in the early 1980s.

THREE DECADES ON

Since then, CFD has come a long way – geometric flexibility has increased to the point where there are now very few geometries too complex to be represented accurately; models have been developed for physical phenomena such as turbulence, multiphase flow, chemical reactions and radiative heat transfer; and the usability of software has increased greatly, with powerful pre- and post-processors.

Some CFD software can now take the designer through the entire modelling process: from geometry definition to computation to analysis of the results. Starting with a description of a fluid system, a designer can build a computer model quickly and easily, predict the flow behaviour using the solver technology best suited to his problem and obtain the information required for critical design decisions.

Over the past few years, many new CFD vendors have entered the marketplace. Some offer general purpose solutions, while others are tailored to meet specific applications needs. But software packages for fluid flow and heat transfer analysis come in many forms. At the very least, these packages differ greatly in their physical approximations and numerical solution techniques, which makes the selection of a suitable package a challenging proposition.

Developers at Fluent claim that the accuracy of a CFD prediction depends on the quality of the mesh that is used. Unfortunately, designers can not usually predict ahead of time where they will need to concentrate the mesh. One solution to this problem is dynamic, solution-based mesh adaption. This lets the designer change the mesh in response to the flow prediction. Dynamic adaption of the mesh, based on the flow prediction, allows designers to efficiently capture details of the flow that other CFD codes may miss. With dynamic adaption, the mesh is refined only where required to improve the solution, and you do not have to restart the calculation. You get greater accuracy with a minimum of computational effort.

Fluent’s latest product, announced in August, is Icepak 2.2, electronics cooling design software that allows users to set up, solve and analyse solutions for cooling electronics in computer and data processing products. Icepak allows an engineer to build a computer model of a product, then virtually prototype it, testing it under real-world conditions.

The new version of Icepak has tetrahedral meshing capabilities and a greatly expanded tool kit filled with new object models. Behind Icepak is a new solver engine that uses the finite-volume method (FVM) solver from Fluent’s general-purpose CFD software, Fluent/UNS. This core solver methodology debuted in Icepak version 2.1 and results in most problems being solved seven to eight times faster than before.

AEA Technology’s CFX-5, a fluid engineering software tool which can be used to simulate many types of industrial flow and heat transfer processes, uses an unstructured tetrahedral mesh, for which the engineer only need specify the geometry and surface grid. The solver then automatically performs the volume meshing, using a very fast Delaunay algorithm. This speeds up pre-processing enormously and produces high quality meshes which in turn ensure faster convergence.

FEWER ITERATIONS

CFX-5’s coupled algorithm solves the pressure and momentum conservation equations simultaneously. This technique provides a more robust and reliable solver for which far fewer iterations are needed. Typically, CFX-5 requires only a few dozen iterations to converge, where a segregated solver would need hundreds or thousands. Furthermore, CPU times for CFX-5 scale much better with mesh size than for the SIMPLE algorithm. For the large meshes often required in real engineering simulations, this approach leads to significant reductions in run times and faster project completion.

Flow-3D from Flow Science is a general purpose CFD software package capable of simulating a wide variety of fluid flows. While the company’s speciality is the simulation of free surface flows, designers can also use the package to model confined flows. The software includes a variety of features which enable engineers to model acoustic waves, cavitation, the solidification and shrinkage of metals, flow through porous media, surface tension, and wall adhesion.

Flow Science points out that methods that employ finite-element or `body-fitted co-ordinates’ require the generation of a solution grid that conforms to the geometry of the flow region. It is a non-trivial task to generate these grids with acceptable element sizes and shapes for accurate numerical approximations. In complicated cases, this type of grid generation may consume days or even weeks of effort. Some programs attempt to eliminate this generation problem by using only rectangular grid elements, but then they must contend with `stair-step’ boundaries that alter flow and heat-transfer properties. Flow Science claims that its software solves both problems using easy-to-generate rectangular grids in which geometric features are smoothly embedded using the Favor (fractional area/volume) method. A simple and powerful solids modeller is packaged with Flow-3D or users may choose to import geometric data from a CAD program.

For its part, Blue Ridge Numerics designs and develops software for modelling real world fluid flow and heat transfer problems. The company has developed a finite element based algorithm that, it claims, delivers accurate results with less computational effort than other available solutions.

The product, CFDesign, is a general-purpose, multi-functional fluid flow and heat transfer analysis software tool. It is a Navier-Stokes solver capable of solving problems that range from the laminar regime all the way to high-speed supersonic flow. Because CFDesign uses a finite element mesh, the free mesh capabilities available in a number of popular CAD packages can be used, so that only mesh sizes need to be entered, and the computer does the work.

Numeritec’s Concert, a CFD software environment for geometry modelling, grid generation, fluid flow solution, and data visualisation, is somewhat unique in that all the methodologies are in one program. What is more, it is also offered on the Internet, so engineers can use it even from inside a web browser. Concert handles both unstructured as well as structured grids, and has a wealth of physical models for solving fluid flow, heat transfer and chemistry problems. It offers push-button automatic grid generation for all 2-D plus some 3-D geometry configurations.

COMPLETE PACKAGE

PowerFLOW from Exa is suitable for a range of applications including transient and steady-state flows. It is a complete software package that includes setup, simulation, multiprocessing support, and post-processing for internal and external flow simulations. PowerFLOW uses a fully automatic grid generation that provides time-accurate results from realistic transient behaviour simulations. PowerFLOW claims to be the first fluid flow analysis product to enable quiet external flow designs. This capability results from analysing surface pressure fluctuations for noise source identification, using the same model used for aerodynamic simulation.

Exa’s Digital Fluids method contrasts with traditional CFD tools, with Exa claiming that these older methods yield answers that are not sufficiently accurate for real world design problems because they rely upon approximations to differential equations run on digital computers. In addition, these methods are costly to apply, time consuming to compute, and require highly skilled practitioners to use. Instead, the Exa software models the microscopic physics of the particles that compose the macroscopic fluid.

PowerFLOW’s performance can scale between 23 and 503, depending on the number of parallel processors used to run the software. Its CAD interface enables the use of existing engineering models.

There are many more CFD vendors with products on offer. Some are listed in the table (right). For the designer looking for a very comprehensive list of product information, there is no better place to start than at CFD Online – an online centre for Computational Fluid Dynamics. The Web site, which can be reached at www.cfd-online.com, provides an extensive vendor listing, a jobs database as well as an online forum.

Pointwise is a supplier of grid generation software and services centred around the `Gridgen’ software product. Gridgen acts as a software bridge that connects CAD software and analysis software such as CFD and FEA. Gridgen itself is an interactive, graphically oriented software system for the generation of 3D grids for engineering analysis. After the designer has generated a grid and a solution, he can visualise the results using Fieldview, a CFD post-processor. Fieldview (from Intelligent Light) is an interactive data visualisation package designed to assist an engineer in investigating features of complex CFD data sets. Fieldview itself is compatible with the leading CFD flow solvers and can handle virtually any kind of mesh, from structured to multi-block, to unstructured. Shawn Strande of the San Diego Supercomputer Center used Pointwise’s Gridgen to generate a grid for a human artery that was defined by magnetic resonance and angiography data. Mr Strande is working with Dr Larry Frank of the Magnetic Resonance Center at University of California San Diego Medical School to perform CFD analyses of blood flow.

{{INFORMATION:AEA Technology Tel: 01235 433813AeroSoft Tel: +1 540 231 6125Blue Ridge Numerics Tel: +1 804 977 2764Century Dynamics Tel: +1 510 552 1600CFD Research Tel: +1 256 726 4800Exa Tel: +1 781 686 8500Flow Science Tel: + 1 505 662 2636Fluent Europe Tel: 01142 818888Numeritec Tel: +1 603 889 2800Pointwise Tel: +1 817 377 2807}}