An Assessment of CFD Effectiveness for Vortex Flow Simulation to Meet Preliminary Design Needs

1.0 Introduction The low-speed flight and transonic maneuvering characteristics of combat air vehicles designed for efficient supersonic flight are significantly affected by the presence of free vortices. At moderate-to-high angles of attack, the flow invariably separates from the leading edges of the swept slender wings, as well as from the forebodies of the air vehicles, and rolls up to form free vortices (see Figure 1). The design of military vehicles is heavily driven by the need to simultaneously improve performance and affordability. In order to meet this need, increasing emphasis is being placed on using Modeling & Simulation environments employing the Integrated Product & Process Development (IPPD) concept. The primary focus is on expeditiously providing design teams with high-fidelity data needed to make more informed decisions in the preliminary design stage. Extensive aerodynamic data are needed to support combat air vehicle design. Force and moment data are used to evaluate performance and handling qualities; surface pressures provide inputs for structural design; and flow-field data facilitate system integration. Continuing advances in computational fluid dynamics (CFD) provide an attractive means of generating the desired data in a manner that is responsive to the needs of the preliminary design efforts. The responsiveness is readily characterized as timely delivery of quality data at low cost. Lockheed Martin (LM) and National Aeronautics & Space Administration-Langley Research Center (NASA-LaRC) conducted several joint as well as separate studies in the 1990s. The studies were aimed at assessing the effectiveness of the state-of-the-art CFD methods (solving Euler and Navier-Stokes equations) in producing aerodynamic data for preliminary design of combat air vehicles. The principal focus was on flight conditions where the flow is dominated by free vortices. In the present context, effectiveness is defined as the ability to meet the desires and expectations of the design teams. It is expressed as a product of two factors: quality and acceptance. Accuracy and credibility of results are the quality factors, and timeliness and affordability of the process of generating those results are the acceptance factors. CFD methods for vortex-flow simulation can be broadly categorized into lower-order methods (based on potential-flow equations), inviscid Euler methods, and viscous Navier-Stokes (N-S) methods. The lower-order methods rate high in acceptance factors because of rapid turnaround and low levels of labor and computer resources. But their rating for quality factors is quite low because their simplified physics model does not allow capturing nonlinear aerodynamic effects such as transonic compressibility. By virtue of the proper model of flow physics, viscous N-S methods alleviate the deficiencies of the lower-order methods.