Effect of Flow Oscillations on Cavity Drag and a Technique for Their Control

Abstract

The phenomenon of cavity flow oscillation is investigated to determine the conditions for onset of periodic oscillations and to understand the relationship between the state of the shear layer and the cavity drag. Experiments have been performed in a water tunnel using a 4" axisymmetric cavity model instrumented with a strip heater on the nose cone and pressure taps in and around the cavity. A complete set of measurements of oscillation phase, amplitude amplification along the flow direction, distribution of shear stress and other momentum flux is obtained by means of a laser Doppler velocimeter. Drag measurements were made by integrating the mean pressure over the solid surfaces of the cavity. Results indicated exponential cavity drag dependence on the length of the cavity. A jump in the cavity drag coefficient is observed as the cavity flow shows a bluff body wake type behavior. An independent estimate of the drag, which is obtained by integration of shear and mean momentum transfer terms over the peripheral area of the cavity, confirms the exponential dependence of drag on the length of the cavity. Results, also reveal that the drag of the cavity in the non-oscillating mode is less than the case if the cavity were replaced by a solid surface. Natural and forced oscillations of the cavity shear layer spanning the gap are studied. The forced oscillations are introduced by a sinusoidally heated thin-film strip which excites the Tollmein-Schlichting waves in the boundary layer upstream of the gap. For a sufficiently large gap, self-sustained periodic oscillations are observed, while for smaller gaps, which do not oscillate naturally, periodic oscillations can be obtained by external forcing through the strip heater. In the latter case resonance is observed whenever the forcing frequency satisfies phase criterion ψ/2π = N, and amplitude exceeds certain threshold levels, but the phenomenon, is non-self-supporting. The drag of the cavity can be increased by one order of magnitude in the non-oscillating case through external forcing. For naturally occurring oscillations, it is possible for two waves to co-exist in the shear layer (natural and forced). Also, it is possible to completely eliminate mode switching by applying external forcing. For the first time a test is performed to cancel or dampen the amplitude of Kelvin-Helmholtz wave in the cavity shear layer. This is done through introducing an external perturbation with the same frequency of the natural component but having a different phase. Reduction by a factor of 2 is obtained in the amplitude of the oscillation.

Files

Additional details