Study of the imposition of bulk flow pulsations on plane channel flow at moderate stokes numbers

Abstract

A channel with rectangular cross section, 40 : 1 aspect ratio (height is 0.0127 m), and 4.27 m test section length (336 channel heights) was used to study the effects of imposed pulsations on laminar, transitional, and turbulent flow phenomena. Periodic velocity variations are produced in the test section by using a single rotating vane located in the flow downstream of the test section. Details of the imposed pulsation and phase-averaging techniques are given, along with data that show how the pulsating velocity varies in the test section, including changes to the shapes, amplitudes, and phase shifts of velocity waveforms at different channel locations. Flows with Reynolds numbers (Re) ranging from 1100 to 3700 are studied at Stokes numbers (St) of 4.08 and 5.79 and Strouhal numbers of 0.033 to 0.121 by imposing pulsations at 1 Hz and at 2 Hz. Pulsations at St = 4.08 have peak-to-peak amplitudes 1.4–16% of the local mean velocity, and at 5.79, peak-to-peak amplitudes are 4.5–25% of the local mean velocity. With these amplitudes and frequencies of imposed pulsation, time-averaged velocity profiles both within and outside of the Stokes layer are unaffected by the imposed pulsations for the entire range of Reynolds numbers studied. Phase-averaged traces show sinusoidal behavior across the channel height at Stokes numbers of 4.08 and 5.79 when flow is laminar (Re = 1100), transitional (Re = 1580), and turbulent (Re = 2550 and 3660). The sinusoidal nature of the waveforms is evident from spectra of voltage fluctuations from hot-wire sensors, which show principal pulsation amplitudes that are from one to three orders of magnitude greater than the amplitudes of harmonic peaks. Phase-averaged velocities are significantly nonsinusoidal at higher transitional Reynolds numbers near 2010, particularly for St = 4.08. These deviations coincide with significant changes in the amplitudes and phase lags of phase-averaged velocities compared to results at other Reynolds numbers where measurements were obtained.