An Experimental Study Of Axisymmetric Cavity Oscillations In Low Speed Incompressible Flow

This study is the latest increment of an ongoing research effort at the University of Tennessee Space Institute with the goal of accomplishing a greater understanding of cavity instabilities and or making use of the instabilities to amplify their effects on the flows. An experimental procedure was designed to examine the unsteady pressure pulses that occur when an incompressible fluid (water) flows through an axisymmetric cavity. The length to depth ratios (L/D) examined ranged from about 0.69 to about 6.2, and the average flow Reynolds numbers ranged from 0 to 1.2 million.

Results show that large amplitude oscillations are generated in the cavity for specific length to depth ratios for a given flow rate. The peak amplitude of the oscillations was studied to determine the origins of the oscillations. As the length to depth ratio increases the maximum amplitude decreases, but the frequency of the maximum amplitude is greatest at a length to depth ratio of approximately 0.85. As the flow rate decreases, the value of the maximum amplitude and its frequency decreases.

A few possible explanations for the cause of such oscillations are explored. For short length to depth ratios (L/D<0.9), the cavity seems to behave as a Helmholtz Resonator. The cavity has a behavior analogous to Rossiter’s Empirical observations at approximate length to depth ratios greater than 0.9. As the length to depth ratio increases beyond a value of approximately 3.6, the peak oscillations disappear and the cavity does not influence the flow significantly.