A study of unsteady wake-boundary layer interaction in turbomachines

The relative motions of rotor and stator blade rows in a turbomachine generate periodically unsteady flow subject to traveling wake perturbations. To better understand the attendant wake-boundary layer interactions, the study used an experimental twodimensional physical model and a new calculation procedure. In the range of Reynolds number 0.144 x 10⁵ to 1.44 x 10⁵, measurements of instantaneous boundary layer velocity and wall shear stress were made on a tripped flat plate with a hot wire probe and a glue-on probe. Data was reduced through the process of ensemble averaging. For the reduced frequencies tested (ω = 0.33 to 9.33), skin friction coefficient was found to be frequency dependent and to have values up to twice that of the unperturbed boundary layers. Due to the nonlinear interactions with the boundary layer, the traveling discrete frequency wakes increased velocity profile fullness and increased random turbulent fluctuations. Unlike the response to stationary-wave-like external disturbances, for the traveling wake fluctuations the boundary layers exhibited increasing negative velocity phase shifts from 0° in the free stream to values of order 360° at the wall. Amplitude overshoots occurred close to the wall and undershoots were observed in the outer region of the profile. Calculated results of steady and unsteady velocity profiles and skin friction compare reasonably with experiment. Although the agreement in phase shift was rather poor, in both experiment and calculation the negative phase shift aspect of the traveling-wave fluctuations has been captured.Applied Science, Faculty ofMechanical Engineering, Department ofGraduat