Measurements of coherent structures during low-drag hibernation events in turbulent boundary layer flows

Recent experimental investigations of intrinsic low-drag states called "hibernating turbulence" revealed time intervals where the instantaneous streamwise velocity profiles approached the MDR asymptote in channel flow, even with Newtonian fluids. Here we present the detection and characterisation of this hibernating turbulence phenomenon in a turbulent boundary-layer flow. Stereo particle image velocimetry (SPIV) was conducted in a water flume to capture the intrinsic low-drag states associated with hibernating turbulence. During states of hibernating turbulence, the ensemble-averaged wall-shear stress, or skin-friction drag is up to 10% lower than the time-averaged skin-friction drag value. To measure these states, the instantaneous wall-shear stress was measured using a hot-film probe whilst, simultaneously, the three velocity components in the cross-section of the water flume were measured using stereo-PIV system. Conditionally sampling the wall-shear stress data when the instantaneous signal drops 10% below the mean value for a duration of t+ > 200 reveals intervals of hibernating turbulence. 3D coherent structures have been constructed from the stereo PIV data using Taylor hypothesis. These structures show a long low speed steaks passing through the hot-film during the low-drag states. Ensemble averaged coherent structures show pairs of long straight low-high speed streaks together with longitudinal vortices in the hibernation intervals.