It is difficult to capture the numerous “manufacturing defects” (e.g., steps, gaps, and waviness that result from surface-smoothness requirements) over an entire aircraft. CFD flow-field analysis of simple geometries for benchmark work has been conducted (e.g., on large backward-facing steps). An example is Thangam [6] et al., who described a detailed study of flow past backward steps to understand turbulence modeling (κ-ε). This type of work neither represents the problems associated with the small geometries of excrescence effects nor guarantees accuracy. Another example is Berman’s [9] work on a large rearward-facing step, but it is not representative of the excrescence dimensions.

Assessment of excrescence drag using CFD requires a better understanding of the boundary-layer structure in turbulent flow. Although there is a voluminous literature on CFD code generation and qualitative assessment of the pressure field, no work has been cited in estimating parasitic drag of excrescences. As modern CFD software becomes more capable, it may be possible to predict excrescence drag by simulating real cases of double curvature in compressible flow, with or without shocks or separation.

Reference [11] is a verification of excrescence drag on a flat plate in the absence of a pressure gradient to estimate the excrescence drag on a 2D aerofoil in the pressure gradient. The study of an aerofoil [11] may be seen as a precursor to examining the scope of CFD estimates of excrescence drag in the generic 3D aerofoil configuration.

14.4 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 empen- nage 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

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).

14.4.3 In the Postprocessor (Menu-Driven)

Step 4: The result thus obtained from the solver can now be viewed in the solver; select a display format.

Step 5: Analyze the results.

Step 6: For a new setup, verify and validate the results.

The results can be presented in many ways, such as Cp distribution, pressure contours, streamlines, velocity patterns, CL, CD, L/D, or parameters defined by a user. CFD can depict shock patterns, location, and separation similar to flow visualization with wind tunnels. These results provide insight for aerodynamic designers to improve the design for the best L/D, aerodynamic moments, and compromise shapes to facilitate production, for example.

The results may need adjustments for the iteration that is necessary beginning with Step 2 and/or Step 3.