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UBC Theses and Dissertations

An investigation of the effects of periodic wake disturbances on flat-plate boundary layers Yip, Ronald S. K.


Flat plate turbulent boundary layers disturbed by periodic moving wakes have been observed in an experimental rig mounted in a low speed wind tunnel. The wakes are produced periodically by cylinders traversing in front of the leading edge of a flat plate on which the boundary layers are measured. This is to simulate the unsteady flow pattern generated by upstream blades on the downstream blade boundary layer in an axial flow turbomachine. Both the time-averaged and ensemble-averaged data are taken from the free stream and boundary layer at different flow conditions. Free stream steady and unsteady wakes are compared and found to be similar to each other. The wake disturbance in the free stream is a function of time and distance from the cylinder. The periodic disturbance in the inner half of the boundary layer lags behind that in the free stream. This phase lag is due to the lower convection velocity near the solid surface. Similar to a steady wake, the velocity defect of an unsteady wake is higher in boundary layer than in free stream. This results in the maximum velocity defect amplitude in the inner half of the boundary layer. Phase lag and amplitude ratio profiles of the boundary layers are plotted and found to be similar to data obtained from axial flow turbomachines. Phase-averaged velocity and turbulence intensity profiles at different phase angles between two successive wakes are shown in a series of transparencies.

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