SIMULATION OF TAIL BUFFET USING DELTA WING-VERTICAL TAIL CONFIGURATION Osama A. Kandil*, Hamdy A. Kandil**, and Steven J. Massey*** Old Dominion University, Norfolk, VA 23529 ABSTRACT Computational simulation of the vertical tail buf- fet problem is accomplished using a delta wing- vertical tail configuration. Flow conditions are se- lected such that the wing primary-vortex cores expe- rience vortex breakdown and the resulting flow in- teracts with the vertical tail. This multidisciplinary problem is solved successively using three sets of equations for the fluid flow, aeroelastic deflections and grid displacements. For the fluid dynamics part, the unsteady, compressible, full Navier-Stokes equa- tions are solved accurately in time using an im- plicit, upwind, flux-difference splitting, finite-volume scheme. For the aeroelastic part, the aeroelastic equa- tion for bending vibrations is solved accurately in time using the Galerkin method and the four-stage Runge-Kutta scheme. The grid for the fluid dynamics computations is updated every few time steps using a third set of interpolation equations. The computa- tional application includes a delta wing of aspect ra- tio 1 and a rectangular vertical tail of aspect ratio 2, which is placed at 0.5 root-chord length downstream of the wing trailing edge. The wing angle of attack is 35" and the flow Mach number and Reynolds number are 0.4 and 10,000; respectively. INTRODUCTION Recently, the design of modern fighter aircraft has been focused on the high angle of attack maneuver- ability at high loading conditions, and the interest in the tail buffet problem is again renewed. For these fighters, the ability to fly and maneuver at high angles of attack is of prime importance. This capability is achieved, for example in the F/A-18 fighter, through the combination of the leading-edge extension (LEX) of the delta wing and the use of vertical tails. The LEX maintains lift at high angles of attack by gener- ating a pair of vortices that trail aft over the top of the Professor, Eminent Scholar and Chairman of Aerospace Engineering Dept., Associate Fellow AIAA. .. Research Associate, Aerospace Engineering Dept., Member AIAA. .** Graduate Research Assistant, Aerospace Engineering Dept., Member AIAA. Copyright O 1993 by Osama Kandil. Published by The American Institute of Aeronautics and Astronautics, Inc. with permission. aircraft. The vortex entrains air over the vertical tails to maintain stability of the aircraft. This combination of LEX and vertical tails leads to the aircraft excel- lent high angle of attack performance. However, at some flight conditions, the vortices emananting from the highly-swept LEX of the delta wing breakdown before reaching the vertical tails which get bathed in a wake of highly-turbulent, swirling flow. The vortex-breakdown flow produces severe buffet on the vertical tails and has led to their premature fatigue failure. Buffeting of the vertical tails of the F/A-18 fits into this category. During flight operations of this airplane large vibrations of the vertical tails have been observed. Sellers et al.' conducted some three-component velocity surveys for a YF-17 model (a configura- tion similar to the F-18) at low speeds. Their re- sults clearly show that at 25" angle of attack the vortex produced by the LEX experiences breakdown and that there are large fluctuations in the velocity in the vicinity of the vertical tails. They measured rms fluctuations as high as 40% of the freestream stream velocity. Erickson, et a1.2 presented a wind tunnel investigation of the F/A-18 aircraft. The investiga- tion focused mainly on the measurements of steady forces and pressures on the LEX and laser light sheet measurements of the vortex structure. Some water tunnel studies conducted by wentz3 using an F-18 model also showed that the vortex produced by the LEX of the wing breaks down ahead of the vertical tails at angles of attack of 25" and higher. If these flows contain substantial energy at frequencies corre- sponding to the lower modes of vibration of the tail structure, significant structural response can result. Another wind tunnel investigation of buffeting is published by Lee and ~ r o w n ~ . The buffeting on the vertical fin of a rigid 6% model of the F/A-18 has been investigated. Unsteady pressure measurements on the vertical fin were conducted and the vortex flow structure behind the fin was studied. The study was carried out with LEX fences on and off to conclude that the LEX fence has a small influence on the steady balance measurements such as lift and pitching moment.