Experimental and analytical investigation of oscillations in flows over cavities

Citation

Sarohia, Virendra (1975) Experimental and analytical investigation of oscillations in flows over cavities. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/WJC1-X942. https://resolver.caltech.edu/CaltechETD:etd-05032007-131245

Abstract

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.

In this study, an analytical and experimental approach has been used to investigate the phenomenon of flow induced oscillations in cavities. Laminar axisymmetric flows over shallow cavities at low subsonic speeds were experimentally investigated using constant temperature hot-wire anemometry. This study comprised the following: study of the effect of the freestream and cavity configuration on onset of cavity oscillations; measurements of cavity shear layer under a wide range of cavity and flow configurations, and the distribution of the phase of the propagating disturbances during both first and second mode of cavity oscillation for a fixed Reynolds number at the upstream corner. Both motion and instant pictures of cavity shear flow, visualized by smoke injection, were obtained. Experiments were also done to investigate the effect of artificial excitation and of mass injection on the onset of cavity oscillations.

The present study indicates that the cavity depth has little effect on oscillations in shallow cavities, except when the depth is of the order of the thickness of the cavity shear flow. For such cavity configurations, measurements indicate a strong stabilizing effect of depth on laminar cavity shear layer. Results of motion pictures and hot-wire surveys of the cavity shear layer show that, close to the downstream cavity corner, large lateral motion of the shear layer occurs, which results in a periodic shedding of vortices at a frequency of cavity oscillations. Mean velocity measurements show growth rates as high as [...] 0.022 where [...] is the shear layer momentum thickness and x is the streamwise coordinate. These are attributed to strong imposed velocity fluctuations on the flow, by the oscillating cavity system.

Phase measurements indicate that the disturbances propagate at a constant phase speed through the cavity shear layer. The wave length of the propagating disturbance bears an approximate integral relation to cavity width, in each mode of cavity oscillation given by [...] where b is the cavity width, [...] the wave length of the propagating disturbance and N is an integer, which takes values 0, 1, 2, ... etc. depending upon the mode of oscillation.

Stability calculations of the measured mean velocity profile were made by numerically integrating the governing equation of motion. These numerical results were used to compute the phase and the integrated amplification of the growing disturbances, through the cavity shear layer. Finally, the mode of cavity oscillation can be predicted for a given cavity flow by studying simultaneously the phase and integrated amplification of various disturbance frequencies through the shear layer and applying the mode relation.

Thesis Files