Investigating channel flow using wall shear stress signals at transitional Reynolds numbers

摘要

Time-resolved wall shear stress measurements are conducted to investigate channel flow at transitional Reynolds numbers. Constant temperature anemometry (CTA) is employed to measure the instantaneous wall shear stress using glue-on hot films as the sensing probes. Pressure-drop measurements are conducted to calibrate the mean hot-film voltage signals and to ensure that the pressure drop is measured in the so-called "fully-developed" region of the channel, a study of effect of entrance length on the pressure-drop measurements is carried out. Time history and higher order statistics of wall shear stress fluctuations reveal that the flow remains laminar until Re_τ(=u_τh/v)≈ 43 in our channel flow facility, where u_τ, h and u are the friction velocity, channel half-height and kinematic viscosity, respectively. Third and fourth order moments of wall shear stress jump at the onset of transition and increase significantly until they reach maxima at about Re_τ ≈ 48. After this Reynolds number, these two higher order moments start to decrease gradually with increasing Reynolds number and after Re_τ ≈ 73 － 79, any significant dependence of these two moments on Reynolds number disappears. Multiple hot-film measurements, which are located at different spatial locations, are conducted to characterize the large-scale turbulent structures. It is observed that there are structures, at least 7h wide, for Re, between 46.8 and 53.9. Two-point spatial correlations reveal that on average these large structures are angled at approximately 17° for Re_τ = 46.8 and roughly between 32° and 37° for 48.7 < Re_τ < 53.9 relative to the streamwise direction.