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A Case for Application-Specific CFD Meshing
Posted Thu April 11, 2002 @11:32AM
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Grid Generation A main challenge facing CFD analysis today is the efficient computation of engineering data within the constraints of the design environment. Requirements of the design environment are fast turn-around time and software tools which match the designer's domain. To a large extent, CFD meshing restricts the adaption of CFD by design engineers due to the fact that it is the most labor-intensive task in the CFD process.

In order to extend the benefits of CFD to product engineers, CFD meshing must be enhanced through specialized meshing applications which automate the mesh generation process and replace CFD jargon with terminology familiar to designers. In this article I will describe application-specific CFD meshing and a method for creating such applications.

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A number of commercial CFD vendors have recognized the need for specialized geometry-specific (vertical) applications and responded with new programs (Fluent's MixSim and Airpak, AEA Technology's CFX-TurboGrid and CFX-ProMixus, and CD adapco's EZ-Turbo and EZ-ICE are examples). These applications all share a common trait -- the specialization of pre- and post-processing for the task at hand. The flow solver is left largely untouched from the general version.

The advantages of a vertical meshing applications over more generalized approaches are:

  • automation - only a narrow range of geometries and topologies need to be considered
  • simplification - all CFD jargon is replaced with design terminology familiar to the user

One method of creating vertical CFD meshing applications, the method I will discuss here, is to take an existing full-featured general CFD mesher and write an application-specific wrapper for the particular market. In this example, the general CFD mesher Gridgen, from Pointwise, was used to create two application-specific meshers.

In order to write a vertical meshing application effectively, the core features of the general mesher must be readily accessible. The designers of Gridgen have done this by exposing its core functionality in a scripting language called Glyph. Glyph is based on TCL which has a large user base, is portable and extensible, and has a companion graphical toolkit (TK). The low level Glyph implementation provides the basis for the vertical applications demonstrated in the following sections.

Re-Entry Vehicle CFD Mesher

One application which can benefit from customized meshing is the calculation of two-dimensional hypersonic flow over re-entry vehicle shapes. Re-entry shapes are changed many times during space vehicle design to accomodate changing crew or payload space requirements. Because this type of calculation is performed over and over, it is ideal for a vertical application. The simple geometry can be easily generated from design parameters and the grid topology is also very simple. The image below shows the re-entry vehicle CFD meshing application.

Re-entry Vehicle App
A customized application for 2D meshing of a re-entry vehicle.

Notice that the application simplifies the mesh generation process to the specification of a few design parameters. CFD-centric jargon is removed in favor of design terminology such as Shelf Length and Shoulder Radius. The grid topology (in this case a simple quadrilateral mesh) is pre-determined by experienced CFD analysts. If desired, the designer can adjust the grid density, however, the default values are adequate for most geometries.

After setting the design parameters, the design engineer presses the Generate button to start the meshing algorithm. The geometry is constructed from the parametric information, the grid is defined and optimized with an elliptic smoother, and the grid files are generated. The mesh is completed in less than one minute.

Axial Turbine Blade CFD Mesher

Another application which can benefit from customized meshing is the calculation of three-dimensional flow through an axial turbine blade row. The design of axial turbines is performed in an iterative manner by proposing a blade shape based on a desired mean flow field, analyzing the flow field generated by the proposed blade, then making adjustments to the blade shape and re-analyzing, and so on.

Most turbine designers utilize a blade design program which describes the blade parametrically and exports the blade shape as a series of blade profiles (cross-sections) from hub to shroud. Because the blade geometry is always similiar, the geometry can be easily generated and a suitable grid topology can be pre-defined. The image below shows the axial turbine blade CFD meshing application.

Axial Turbine Blade App
A customized application for 3D meshing of an axial turbine blade.

In this case, the information is reduced to its lowest form -- the blade to blade view. Because most of the information can be determined from the blade shape definition coming from the design code (the periodic surface for instance), the designer is presented with a minimal list of input, namely the blade count and the grid dimension and distribution parameters. Furthermore, the grid dimension and distribution have been tuned for these types of geometries so they are rarely in need of adjustment.

After the designer makes any input changes and presses the OK button, the blade design data is imported from the design files, the hub, shroud and periodic surfaces are defined, a multi-block hexahedral mesh is created and optimized, a number of grid quality checks are performed generating a report, and the flow solver native grid file is exported. This process is completed in a matter of minutes.

These two cases are examples of what is possible with application-specific CFD meshing. Other areas for application specific CFD meshing include wing-fuselage, underwater vehicle, HVAC, automotive, chemical process, and electronics cooling geometries.

As CFD analysis matures, the trend toward vertical applications aimed at design engineers will increase and, as a consequence, CFD analysis will become more popular in the design environment.

Nick Wyman is a CFD professional with more than 7 years of experience in commercial CFD application.

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