Origin of Surface-Shear-Stress Pulses in Turbulent Boundary Layers

摘要

Information related to the large magnitude pulses of surface shear stress that are present in all turbulent boundary layers [1] is sketchy. It has been demonstrated [2] that the pulses are related to the coherent events that make up the turbulent boundary layer. Recent computer simulations [3] related to "vortical" induced transition of laminar-to-turbulent boundary layers predict the occurrence of turbulent "spots" in the outer region of the layer, which in turn produce large pulses in the underlying surface shear stress. Although the transition turbulent spots differ from the coherent structures within the turbulent boundary layer, the process that produces the high-velocity pulses near the surface is expected to be similar. Flow visualization studies [4] for laminar-turbulent transition starting from Tollmien-Schlichting wave instability demonstrate a spectrum of "sweep" pulses first associated with a A vortex and progressing to much stronger sweeps (stage 3) in the final stage of transition. The present experimental study examines the development of the time-dependent surface-shear-stress pulses and/or the near-surface velocities for transition flows with moderate, 1.2-2.7% freestream turbulence levels. A bypass laminar-to-turbulent transition occurs, which is similar to the computer simulation flows.