Effects of 1, 2, 3 and 4 Hz imposed bulk flow oscillations on transition in a straight channel with 40 to 1 aspect ratio

Abstract

A channel with 40 to 1 aspect ratio and rectangular cross-section is used to study the effects of imposed oscillations on transition from laminar to turbulent flow. Oscillations are imposed using a single rotating vane located in the flow downstream of the test section. Flows with Reynolds numbers ranging from 1100 to 3580 and Strouhal numbers from 0.0211 to 0.2418 are studied. For all cases, time-averaged velocity profiles are unaffected by imposed oscillations. Imposed oscillations have a destabilizing effect on the flow near the edge of the Stokes layer. Turbulence intensity magnitudes with imposed oscillations show that transition to turbulence begins at lower Reynolds numbers and extends over a wider range of Reynolds numbers than when no imposed oscillations are present. In addition, higher levels of intermittency are present with imposed oscillations for Reynolds numbers 1900 to 2200 at yd=0.90. At Reynolds numbers from 1450 to 1800, frequency spectra evidence high intensity intermittent turbulent fluctuations, followed and preceded by quiescent flow, both with and without imposed oscillations. A center mode of secondary instability is evidenced by high values of normalized longitudinal turbulence intensity measured in the channel center when no oscillations are imposed on the flow. Imposed oscillations are found to suppress this center mode of secondary instability, as evidenced by reductions in longitudinal turbulence intensity values near the channel center.