Tracking shock movement on the surface of an oscillating, straked delta wing

Abstract

Limit cycle oscillations (LCO), a mechanical nonlinearity similar to flutter, have affected high performance aircraft, like the F-16 and F-18, for years. The phenomena causes a lateral motion of the cockpit that makes performance of typical pilot tasks difficult. To better understand the nature of LCO and why high performance aircraft are typically afflicted, a computational model was developed to compare to wind tunnel tests and investigate the flow field around a straked, semispan delta wing and monitor the changes as the wing was pitched in an oscillatory fashion. The oscillations were intended to mimic LCO. By understanding the flow field around an oscillating wing, the fluid force that acts as the source of the motion could be discerned. The computational investigation focused on tracking shock movement along the top surface of the wing to confirm the presence of shock-induced trailing edge separation, one possible driver of LCO. The surface pressure was shown to be sensitive to the starting trim angle, oscillation frequency and oscillation amplitude. A separation bubble aft of the shock was observed as the source of sharp, localized negative surface pressure coefficient. The large change in surface pressure caused by the separation bubble could be an important phenomena contributing to the onset of LCO.