CAD Integration and automatic meshing reduce CFD analysis times

The simulation of fluid motion, called Computational Fluid Dynamics (CFD), is today a well-established part of the product design and development process. CFD, the numerical solution of the conservation equations linking together fluid velocity, pressure and enthalpy, is a procedure widely used at all levels of industry and academia, to provide more insight into the flow structure within any domain of where fluid flow occurs.

This in turn allows the product engineer or designer to optimise the geometry being studied, with reduced need for laboratory or wind tunnel testing. CFD’s increasing popularity is largely due to significant advances in recent years, both in the software available to perform such analyses and in the hardware on which the programmes are run. A good CFD programme is an extremely useful tool for analysing a wide variety of flows, whether one is interested in the external aerodynamics of a vehicle, the flow, spray and combustion processes in engine applications, or the flow of blood around an artificial heart valve.

A CFD analysis involves three main stages : Pre-processing, Calculation, and Post-processing (analysis of results). Pre-processing, in turn involves two subprocesses: that of meshing, i.e. the spatial discretisation of the geometry, or domain, of interest, and that of providing the solver with boundary conditions, initial conditions and runtime parameters. It is the meshing phase of the analysis which has traditionally taken up most effort, with up to 90% of the total number of man hours spent on a project being dedicated to problem set-up. In recent years, the advent of tetrahedral meshing for CFD programmes has reduced the set-up time considerably, but sometimes at the cost of accuracy and computer run times.

The results produced by such calculations allow the user to gain a better understanding of the process being simulated; frequently he will gain insight into situations where experimental data is not available, or where it is difficult to obtain.

STAR-CD is one of the most widely used CFD codes in industry. It’s developers London-based Computational Dynamics Ltd and New York-based adapco, have been addressing this problem over the last few years. Direct links to CAD programmes such as SolidWorks allow users to create their geometry in a CAD system, and then pass it directly to pro-am, the new integrated mesher, pre-processor and post-processor, developed by adapco as a front end for STAR-CD. Even for CAD systems where no direct interface exists, the user can still import the surface description using a standard format, such as IGES, VDA or STL.

The geometrical flexibility of STAR-CD is achieved through the use of fully unstructured meshes. This means that any shape of element (tetrahedra, hexahedra, polyhedra, prisms and pyramids) can be used in any combination to produce a body-fitted computational mesh for even the most complex geometries. Moreover, the user can arbitrarily put together mesh blocks of different structure to make the job of mesh creation even easier. The mesh can also be created using external packages, to which STAR-CD has interfaces, like ICEM, ANSYS, PATRAN, NASTRAN, IDEAS and GRID-3D.

pro-am, the new mesh generator from adapco, is an easy to use, GUI driven, front end to STAR-CD which allows geometry meshing quickly and automatically, either with all tetrahedral meshes, trimmed meshes, or hybrid meshes. Once the mesh has been created (or imported, should the user already have an in-house mesh generator), pro-am is also used to set the boundary conditions and other run-time parameters necessary to perform the calculation.

Trimmed cell technology, unique to pro-am can be described as follows. The first step is to import the geometry surfaces (from CAD) and to check it for imperfections, such as holes and double surfaces. Once these have been fixed using pro-am tools, the programme creates a subsurface by shrinking the original geometry by a small amount, determined by the user. The subsurface is used to cut a block of hexahedral cells, leaving all cells untouched by the cutting surface as perfect hexahedra, and producing a variety of cell types where the surface cuts the block. These trimmed cells any shape, ranging from tetrahedra to polyhedra (cells with more than 6 faces), depending on the local alignment of the surface to the initial block. The outer cell faces of the trimmed block are then extruded to the original surface, providing a layered mesh in the near-wall region. This is essential for flows where the boundary layer is likely to play an important role in the solution – e.g. where heat transfer or flow separation may occur. The meshes produced in this way tend to be of high quality and are very suitable for CFD – in general over 90% of the cells are perfect hexahedra with no distortion.

These advances, together with developments in the High Performance Computing versions of CFD software (such as STAR-HPC), allow users to experience turnaround times for CFD analyses of the order of days, rather than the weeks, or even months, which would once have been necessary. For example, STAR-HPC allows the user to perform a calculation on several processors, either within a single machine, or distributed across several networked computers running UNIX or Linux