In search of low drag events in Newtonian turbulent channel flow at low Reynolds number

Recent Direct Numerical Simulations (DNS) and experiments in turbulent channel flow have found intermittent low drag behaviour in Newtonian fluid flows, between friction Reynolds numbers 70 and 100, whose mean streamwise velocity profile approaches the Virk Maximum Drag Reduction (MDR) asymptote; a profile which is more routinely associated with turbulent flows containing a dragreducing additive. These intervals of low drag have been termed “hibernating turbulence” and in this experimental investigation, further verification of the hibernating turbulence phenomena is investigated in a channel flow facility using Newtonian fluids. Simultaneous measurements of instantaneous streamwise and wall-normal velocities, and wall shear stress are conducted using Laser Doppler Velocimetry (LDV) and Hot-film Anemometry (HFA), respectively. Experiments are conducted at friction Reynolds number of 70 for 10 different wall-normal locations. The effect of varying the criteria for a hibernating event on the ensemble-averaged wall shear stress during hibernation is investigated. Velocity data obtained during the low drag events are conditionally-averaged and normalized using two different values of mean wall shears stress: long time-averaged wall shear stress and conditionally-averaged wall shear stress. It is observed that the conditionally averaged streamwise velocity when normalized with the conditionally-averaged wall shear stress approaches Virk MDR asymptote closer to the wall. There is found to be a decrease in the conditionally-averaged Reynolds shear stress (RSS) near the wall when the normalization is based on long time-averaged wall shear stress.