DES GRID RESOLUTION ISSUES FOR VORTICAL FLOWS ON A DELTA WING AND AN F-18C

An assessment of unstructured grids for use in Detached-Eddy Simulations (DES) of vortical flowfields over two configurations, a 70 degree delta wing and an F-18C are presented. The role of the grid in detached eddy simulations of vortical flowfields, including complex features such as vortex breakdown, is assessed on a delta wing with comparison to wind tunnel data. Adaptive mesh refinement is applied to the delta wing grid to improve the focus region aft of the vortex breakdown where massively separated flow exists and unsteady pressures are generated that could impact the loads on vertical tails of more complex configurations. The adaptively refined mesh is compared to the baseline mesh to determine the advantage of the adaptive mesh refinement approach for vortex breakdown. The focus region grid resolution is then applied to an F-18C in the region of the vortex generat ed from the leading edge extension (LEX). The resulting unsteady tail loads are compared to flight test data from the NASA F-18 HARV database. This paper represents one of the first times adaptive mesh refinement will be applied to a detached eddy simulation of a flight vehicle configuration. INTRODUCTION Many of todays military vehicles exhibit vortex dominated flowfields. At a recent NATO Air Vehicle Technology conference, D. A. Lovell presented a review of “Military Vortices,” where he discussed the declining research budget in this area and the importance of understanding the phenomena. He classified vortex flows into three categories, “those designed into a vehicle to improve performance, those which cannot be avoided and whose adverse affects must be minimized, and those that were not expected to occur.” He gives examples of many of these vortex dominated flowfields: tip vortices on wings having low sweep, leading edge extension vortices from the F-18 and F-16 aircraft, foreplanes on the Rafale, and flow over the MK-82 bomb, to name just a few. He also discusses the fact that governments are relying ever increasingly on the aerospace industry to perform research. Since the aerospace industry concentrates on cruise conditions for optimization of commercial aircraft, these vortical flowfields common in military aircraft are losing their place in research budgets. This is occurring at a time when the three largest US fighter development programs (F/A18E/F, F-22, and F-35) incorporate twin tail configurations and high angle-of-attack maneuvering. The F-18 High Angle of Attack Research Vehicle (HARV; see Fig. 1) has proven to be an excellent source of data for researchers working on high angle of attack flowfields. Extensive flight testing of the HARV has been conducted that provides a rich source of flow visualization, surface pressures, and aeroelastic information. The F-18 utilizes wing leading edge extensions (LEX) to generate * Associate Professor of Aeronautics, AIAA Associate Fellow. # USAF Academy Undergraduate Student, AIAA Student Member. % Distinguished Visiting Professor, AIAA Associate Fellow. & Associate Professor of Aeronautics, AIAA Senior Member. This paper is declared a work of the US government and is not subject to copyright protection in the United States. vortices which enhance the wing lift, and the twin vertical tails are canted to intercept the strong vortex field and increase maneuverability. At large incidence, the LEX vortices breakdown upstream of the vertical tails, resulting in a loss of yaw control power and severe aeroelastic effects. This tail buffet phenomenon was reduced by using extensive flight tests to design a LEX fence. The ultimate goal of computationally modeling the flowfield shown in Fig. 1 would be to accurately simulate the aeroelastic impact of the LEX vortices on the twin vertical tails. The current level of simulation technology, however, has not allowed for accurate prediction of vortex breakdown, and the unsteady flow downstream of breakdown, at flight Reynolds numbers. Because of this, researchers have used simpler geometries, such as slender forebodies and delta wings, to improve their simulation capabilities. Figure 1 : NASA F-18 High Angle of Attack Research Vehicle (HARV). The delta wing vortex breakdown phenomena has been studied extensively since Henri Werlé first photographed it in 1954, during water tunnel tests of a slender delta wing model at Onera. This work was quickly confirmed by Peckham and Atkinson, Elle and Lambourne and Bryer