Current Status

In his classic review, Chapman [1] (1979) advocated the indispensability of CFD, as computers began showing the promise of overtaking experiments as a princi­pal source of detailed design information. His view is now regarded as an overly optimistic estimate. CFD capabilities complement experiments. Chapman listed the following three reasons for his conclusion: [27]

Figure 14.1. CFD simulation of wing – body drag polar [7]

Chapman showed the chronology of progress as four stages, starting in the late 1960s by solving linear potential flow equations and then reaching a stage where the nonlinear Navier-Stokes equations could be solved.

In a later paper, Chapman [2] reviewed the rapid progress achieved in the 1990s. With a better understanding of turbulence and advances in computer technology in both hardware and software development, researchers successfully generated aero­dynamic results that had been impossible to obtain until then.

A more recent review by Roache [3] demonstrated that considerable progress has been achieved in CFD, but the promise is still far from being fulfilled in esti­mating complete aircraft drag. The AGARD AR256 report [4] is a technical status review of drag prediction and analysis from the CFD perspective. In the report, Schmidt [5] categorically stated that “consistent and accurate prediction of absolute

drag for aircraft configuration is currently beyond CFD reach____________________________ ” Ashill [ 5] was of

the same opinion, stating that the CFD flow modeling was found to be lacking in “certain respects.” Both agreed that the current state of the art in CFD is a useful tool at the conceptual design stages for comparison of shapes and diagnostic pur­poses.

An essential route to establish the robustness of CFD is through the success of the conceptual model code verification and validation. Roache [3] used the seman­tics of “verification” as solving the equations correctly and “validation” as solving the correct equations. The process of benchmarking (i. e., code-to-code comparison) results in the selection of software with the best economic value, although not nec­essarily the best software on the market.

Experimental results are used to validate and calibrate CFD codes. Various degrees of success have been achieved in the case studies. Melnik [5] showed that the CFD status in aircraft drag prediction of a subsonic-jet, transport-type aircraft wing on a simple circular cross-section fuselage had mixed success, as shown in Fig­ure 14.1. Some correlation was achieved after considerable “tweaking” of the results. The results using these methods are not certifiable because of considerable “gray” areas.

Currently, the industry uses CFD as a tool for flow-field analysis wherever it is possible to estimate drag in inviscid flow (e. g., induced drag and wave drag), but it is not used for complete aircraft drag estimation. In the industry, CFD is a general – purpose tool to simulate flow around objects for qualitative studies and diagnostic purposes.

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 (к-є). 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.