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27
Flow,
Thermal, Stress - Simulation in STAR-CD V4
Stephen
Ferguson, CD-adapco
STAR-CD V4.02 includes the capability to perform structural analysis calculations using a methodology based upon its industry-leading CFD solver technology, the first time that a comprehensive solution for flow, thermal and stress simulation has been available in a single general-purpose commercial finite-volume code.
In this article we explore some of the benefits that integrated flow, thermal and stress simulation brings, for both the fluid dynamics and structural mechanics communities. While Fluid and Structural Mechanics are both branches of the wider discipline of Continuum Mechanics, in practical application they have traditionally been treated entirely separately, addressed by different groups of engineers, using a different sets of tools for which different numerical simulation methodologies have evolved. For the solution of stress analysis problems the Finite Element Method (FEM) dominates, whereas for fluid dynamics the majority of commercial Computational Fluid Dynamics software is based around the Finite Volume Method (FVM).
Although
most practical problems in fluid dynamics are in some way related
to the interaction between one or more fluids and a solid body,
direct coupling between CFD and structural analysis software has,
until now, remained relatively rare, largely due to the effort involved
in creating models compatible with two disparate software packages
(although as the extensive review of coupling techniques on page
10 demonstrates, CD-adapco have developed a range of methodologies
to make this coupling as simple and as effective as possible).
Most often, the interaction is either neglected all together (if the degree of interaction is relatively small), or accounted for using a mixture of mapping, interpolation and file-transfer, with the results from one type of analysis used to provide the boundary conditions for the other.
CD-adapco
is about to challenge that paradigm with the introduction of a capability
to conduct linear and non-linear structural analysis problems in
STAR-CD V4, using similar FVM solver technology to that which has
led the CFD market for the past 20 years. Although most people automatically
associate structural analysis with the FEM, the FVM that underlies
most CFD software is equally applicable to structural analysis and
– as we shall explore in this article - holds some significant
advantage over traditional techniques.
(Left) Thermal stresses predicted in engine cylinder head. (Right) Temperature contours in engine cylinder head (note the polyhedral mesh).
CD-adapco does not expect that this approach will ever displace the FEM as the principle means of conducting stress analysis, however early experience suggests that the methodology is very attractive for applications that require: conjugate heat transfer and stress analysis; fluid structure interaction; and melting and solidification (where the continuum changes state from solid to liquid or vice versa).
Coupled simulation without the coupling Using STAR-CD V4, both fluid and solid calculations are performed simultaneously on a single computational mesh, created automatically with CD-adapco’s advanced meshing technology. The mesh (which can be constructed from hexahedra, tetrahedra or arbitrary polyhedra) automatically represents the interface between different material regions (whether fluid-solid or solid-solid) using a conformal interface, which means that solution domains are connected implicitly without mapping or interpolation.
As the coupling is performed in the memory of the computer, and not using files passed via the hard-drive, the degree of interaction is much higher than through external coupling; information is interchanged at an inner iteration level, rather than at each external iteration (which typically takes place once per time step), resulting in both increased efficiency and robustness, and the ability to post-process fluxes, forces and displacements at the interface, as the solution progresses. The FVM solver ensures that at each time step all the coupled, non-linear equations are satisfied within the prescribed tolerance; the choice of time step size is guided by accuracy requirements only, since the fullyimplicit formulation of the solution algorithm allows large time steps to be used.
All of this is provided as part of the standard STAR-CD installation, using the regular STAR-CD interface and without requiring the purchase of additional licenses or software.
Problem-free automatic meshing with polyhedra A further advantage of the FVM is that it gives structural analysts access to the improvements in meshing technology recently pioneered by CD-adapco.
Automatic mesh generation is typically more problematic for structural simulation than fluid dynamics. Unlike fluid dynamics problems (which are typically convection dominated), structural mechanics problems are“diffusion” (i.e. stress) dominated, which makes their numerical simulation very sensitive to poor mesh quality. While the FVM can, in principal, be applied to grids of arbitrary polyhedra, when using the FEM analysts are limited to the set of computational elements for which a shape-function has been pre-defined, which in practice means tetrahedra if automatic meshing is required. A single distorted element, among the several thousand used to mesh a typical structural part, can be enough to ensure that a solution cannot be calculated, or worse that the calculated solution is wrong.
CD-adapco
leads the CAE industry in automatic mesh generation, and has invested
heavily in creating a methodology to mesh and solve upon grids of
arbitrary polyhedral cells. These cells (which typically have between
12 and 14 neighbors / faces) have already delivered significant
benefits to the CFD community, and look certain to do the same for
structural analysis, as those relatively low quality tetrahedral
elements that would typically cause a problem in stress analysis
simulation can be avoided by creating polyhedral cells.
Conformal
polyhedral meshes for both the fluid and solid domains can be created
at the click of a button from a CAD surface and, where necessary,
geometries can be automatically de-featured or repaired using CD-adapco’s
unique surface wrapping capability.
Bigger models with memory-efficient,
scalable solver FVM solvers require less memory and are more inherently
scalable than those that use the FEM. This is a major advantage
for engineers performing stress-analysis calculations who, due to
the relatively poor parallel scalability of FEM solvers, have often
been forced to generate a number of highly refined local models
in order to resolve detail in critical areas while keeping the element
count low enough to solve; an approach which introduces a considerable
degree of uncertainty into the calculation, due to difficulty in
accurately prescribing boundary conditions for the local-models.
By
virtue of its parallel scalability, STAR-CD can solve non-linear
stress analysis problems using the level of mesh density that has
become common practice for CFD simulations, which now routinely
use 100 million cells or more, thus allowing engineers to simulate
entire structures rather than use a multitude of local models.
The additional work required to set up a parallel calculation for flow, thermal and stress problems in STAR-CD V4 is minimal; the user need only specify the names of the computers to be used for the simulation and the number of processors required and the software will take care of domain decomposition and job execution automatically.
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