2. CAD to CFD via VR and PARSOL

[Chapter 2 of the lecture CAD to SFT.
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2.1 Transferring the geometrical data from CAD to CFD, via STL, DXF or IGES files

2.2 An aeronautical example: the 3-part airfoil, with PARSOL and fine-grid embedding

2.3 A test of PARSOL; the turn-around duct

2.4 Flow around an automobile, with PARSOL and fine-grid embedding; and other PARSOL examples

2.5 Concluding remarks about CAD to CFD

2.1 Transferring the geometrical data

(a) The data-format question

CAD packages are used for defining the shapes and sizes of the objects of which the fluid-flow or solid-stress performance is studied.

The definitions can be expressed in various formats, of which IGES, DXF and STL are examples.

Here the STL (ie STereo-Lithographic) format is considered. It describes solid bodies by defining the locations of their surfaces, these surfaces being made up of an array of approximating triangles, each of which shares its edges with (only) one other.

There exist translator programs which can effect IGES-to-STL, DXF- to-STL and similar conversions. CADfix, from FEGS Ltd, is one.

(b) Import into a "virtual-reality" interface

The STL format is a convenient one for representing objects visually in "virtual-reality"-type data-input interfaces for CFD codes.

Such interfaces can immediately accept and display the objects which the CAD users have created; and they provide their own users with the further abilities:-

  1. to change the sizes, numbers, positions and orientations of those objects;
  2. to specify what materials they are to be made of (if that has not already been done), what their surface roughnesses are, and in what fluids they are immersed;
  3. to add CFD-type boundary conditions such as temperatures, velocities and pressures at various places; and
  4. to provide solution-influencing information regarding the grid-fineness distribution, accuracy tolerances, etc.

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Fig. 2.1-1 a CAD-generated object after transfer into PHOENICS-VR.

Click here for an example of DXF-to-VR conversion

Click here for a description of the DXF-to-VR converter program

Click here for an IGES-to-VR example

(c) Export to the CFD equation-solving package

Once all the data have been inserted, and the user is satisfied that the problem specified is the one which he wants to solve, all that should be necessary is to click on the EXIT button; then the data should be exported to the equation solver; and, after the requisite "number-crunching" time, the results should be returned to the VR Interface.

The "should be" implies the condition "if the user is not a specialist in computational fluid dynamics, but simply wants to get the results of the computations, as quickly as possible, in a form which he can understand".

This condition is frequently satisfied; and it will be almost universally so in the future, as the CAD-to-CFD traffic increases.

Fig. 2.1-2 shows some of the results of the flow simulation corresponding to the data-input specification of Fig. 2.1-1. The VR-viewer is capable of showing vectors, streamlines, contours and iso-surfaces.

Fig. 2.1-2 The same object in the VR-viewer

2.2 An aeronautical example: the 3-part airfoil

(a) Description of the problem

Creating computational grids to fit bodies with curved surfaces is one of the tedious tasks of conventional CFD; and it wastes the time of highly-paid specialists.

In order to show that it is often unnecessary, a two-dimensional example will be shown, in which a three-part airfoil is represented in a cartesian grid possessing three levels of fineness.

The grid-refinement is easily effected by way of mouse-clicks, and keyboard-entered refinement ratios, in the VR-editor operation.

The first of the following two pictures shows the airfoil itself; and the second shows a close-up of part of it, and of the associated grid.

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Fig. 2.2-1 The three-part airfoil

Fig. 2.2-2 The three-part airfoil;close-up

(b) Handling the obliquely-cut cells via PARSOL

There was a time at which inaccuracies of solution were generated in those cells of the cartesian grid which were cut obliquely by the surface of immersed solids.

Taking extra care about the formulation of the equations relating to such cells has however removed the inaccuracies. Relevant references are Yang et al, 1997 a,b,c; and PHOENICS has its own version of the technique, called PARSOL (standing for PARtial SOLid).

When appropriately implemented, computer codes which employ such techniques can provide solutions of the fluid-flow equations of a quality which is equal to those which employ body-fitted grids.

Because of their superior ease of use, such codes make travel along the CAD-to-CFD road especially smooth.

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The next four pictures show results for the three-part airfoil.

Fig. 2.2.3a Contours of velocity.

Fig. 2.2.3b a closer look

Fig. 2.2.3c and another

Fig. 2.2.4 Contours of pressure.

2.3 A test of PARSOL

As a further demonstration of the accuracy which the cut-cell techniqe can provide, the next two pictures show the results of a study of the inviscid flow in a "turn-around" duct.

The first picture shows the grid, which is rather coarse.

Fig. 2.3-1 The cartesian grid

The second shows the computed pressure distribution, albeit first with an early version of the VR-Viewer, which could not do reflect the cut cells properly. How much depends on the Viewer is shown by a supplementary picture.
This distribution should be perfectly symmetrical; and the one shown is very nearly so.

Fig. 2.3-2a The pressure distribution (with early Viewer)

Fig. 2.3-2b The pressure distribution (with improved Viewer)

The present author knows of no code employing a body-fitted grid with a comparable number of cells which can procure superior symmetry.

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2.4 Flow around an automobile

As a final example, a few pictures are shown from a study, which employed the techniques just described, of the flow around the automobile body specified as a benchmark problem for the 1996 WUA Conference.

These pictures illustrate:-

No body-fitted coordinate code presented results at the conference which were better than these.

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Fig. 2.4-1 the automobile in the VR-viewer

Fig. 2.4-2 the automatically-created grid, side view

Fig. 2.4-3 the automatically-created grid, end view

Fig. 2.4-4 comparison with the experimental data

Click here for more examples of PARSOL

2.5 Concluding remarks about CAD to CFD

The foregoing arguments and examples, while not being conclusive, lend plausibility to the following suggestions:

A competent user of CAD packages who also understands fluid and heat flow from a practical viewpoint, can reasonably expect to become a fluid- and heat-flow predictor after very little acquaintance with the relevant software,

The STL format, being common to the CAD, Virtual Reality and CFD packages, is worth bringing into greater prominence. The good accuracy obtained with cartesian grids, combined with fine-grid embedding and the "cut-cell" technique, may render the more expensive body-fitted-coordinate formulation unnecessary.

When combined with simultaneous solid-stress analysis, to be described in the next section, a very considerable advance in the designer's powers will have been achieved.
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