Particle-Image Velocimetry Investigation of an Oscillating Turbulent Channel Flow

Particle-Image Velocimetry is used to study the cyclic modulation of the wall shear stress and turbulence properties of an oscillating channel flow. The PIV instrument employed here utilizes a dynamically adjusted delay between the laser pulses to accommodate the wide variations in velocity encountered in the oscillating flow. Both high- and low-magnification digital PIV recordings are obtained to reveal the near-wall boundary layer structure and wall shear stress, as well as the full-field turbulence throughout the channel. We present wall-shear-stress and global turbulence data for Stokes-thickness Reynolds numbers of Reδ = 1220, 2033, and 2875. The results reveal a fully developed turbulent state, relaminarization, and an explosive transition back to turbulence. The flow is examined in detail for the case at Reδ = 1220, where instantaneous PIV realizations at low magnification reveal the structure of the flow during relaminarization and transition back to turbulence. High-magnification PIV results are used to reveal the phase modulation of the mean velocity profiles in the viscous sublayer and logarithmic layers through the half cycle and quantitative profiles of in-plane Reynolds stresses and turbulence production are presented. To our knowledge, this is the first PIV investigation of this canonical unsteady turbulent channel flow and these results represent a needed contribution to the limited turbulence data which exists for unsteady wall flows.