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Approach to CFD Analyses
CFD analysis requires preprocessing of the geometric model before computation can begin. It consists of creating an acceptable geometry (i. e., 2D or 3D) amenable to analysis (e. g., no hole through which fluid leaks). A preprocessing package comes with its own CAD program to create geometry, specifically suited for a seamless entry to the solver for computation. However, considerable labor can be saved if the aircraft geometry already created in CAD can be used in CFD. This is possible if care has been taken in creating a geometry that is transferable to a CFD preprocessing environment.
The next task is to lay a grid on the geometry in order for CFD to work on small grids/cells until the entire domain is achieved. The surface grids should be laid intelligently to capture details where there are major local geometric variations, typically at the junction of two components and where there is steep curvature. The next task is to generate cells in the application domain that can encompass a large flow-field space around the aircraft body. At the far-field, variation in the flow field between the cells is small and therefore can be made larger. The preprocessor is menu-driven, providing options for various types of grid generation from which to select. Grids must meet the boundary conditions as the physics dictates. Figure 14.2 is a good example of aircraft geometry (simplified by excluding the empennage and the nacelle) with structured grids and a section of the environment to be analyzed. Because it is symmetrical on the vertical plane, only half of the aircraft needs to be analyzed – the other half is the mirror image.
Figure 14.3 is another example of 3D meshing on a complete aircraft with the nacelle included. After grid generation in the preprocessor, the model is then
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Figure 14.2. Wing-fuselage geometry with meshing on the surface and in the space [12] |
introduced into the flow solver, which is also menu-driven. The options in the solver are specific and a user must know which to apply. The solver then computes the results: The runtime depends on the geometry, type of grid, and solver options, as well as the computing power.
The results can be examined in many different ways in a postprocessor; the most important on an aircraft body include the Cp distribution, temperature distribution, streamlines, and velocity vectors. The Cp and temperature distributions are shown in grades of color representing bands of ranges. Figure 14.4a is a gray-scale
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Figure 14.3. Grid mesh in 3D [14] |
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(a) Pressure Distribution (b) Streamline Patterns at High a Figure 14.4. CFD postprocessor visualization [14 and 15] |
version of the color distribution and Figure 14.4b depicts the flow-field streamlines. The results also can be obtained numerically in tabular form. It is now understood that readers must have the background information and be familiar with the CFD software package. For newly initiated readers, this instruction should be conducted with supervision.
Following is a summary of the approach to CFD.
14.4.1 In the Preprocessor (Menu-Driven)
Step 1: Create the 3D geometry of an aircraft.
Step 2: Generate the grid on the body surface and in the application domain; match it to the boundary conditions.
14.4.2 In the Flow Solver (Menu-Driven)
Step 3: Bring the preprocessed geometry into the solver; set the boundary and initial conditions; make appropriate choices for the solver; run the solver; check results and refine (i. e., iterate) if necessary (including the grid pattern).
