Ground Structure – Impinging Jet Interaction

Impinging jets, such as those created by the propulsion systems on a short take-off/vertical landing (STOVL) aircraft in hover, can generate large steady and unsteady loads onto the ground or aircraft carrier deck. Under certain exhaust conditions and heights, powerful resonance occurs that concentrates a significant portion of the unsteady pressure power in one or several narrow frequency bands. These oscillations may interact with the natural structural modes of the ground structure, potentially leading to increased structural fatigue or catastrophic failure. On the other hand, a compliant ground structure may interact with imposed flow oscillation so that these oscillations are changed, stabilizing the flow field and reducing the emitted acoustic noise which presents a significant hazard to nearby personnel. In the present study, a Mach 1.5 jet at overexpanded, ideally expanded and under-expanded supply pressures has been impinged onto a ground plane with various levels of compliance from a range of distances. The study investigates the time-averaged and unsteady response of the panel using digital image correlation (DIC), the flowfield using highspeed shadowgraph and the acoustics above and below the flexible ground plane using microphones. The ground plane has been designed to exhibit significant deflection and some static dynamics at the expected kHz-range flow oscillations. Acoustic measurements reveal both screech and impingement tones over a range of heights and expansion conditions. DIC analysis shows the panel deformation under jet excitation exhibits mode shapes similar to its natural vibration modes. For over-expanded mode of operation, multiple screech tones appear for thinner compliance panel suggesting strong shock interaction. These tones are alleviated for design pressure operation..