Hibernating Turbulence in Boundary Layer Flows
Abstract
Experiments are presented to characterise low-drag turbulence events, or so-called intervals of hibernating turbulence, which last for a prolonged duration in a zero-pressure gradient flat-plate turbulent boundary-layer flow over a friction Reynolds number range of Re? = 335 to 880 in either a wind tunnel or water flume. The presented experimental data is acquired at a friction Reynolds number several times higher than all previous investigations on hibernating turbulence in channel flows. The spatiotemporal low-drag intermittencies are identified by applying conditional sampling techniques to either simultaneously acquired wall-shear stress and pointwise streamwise and wall-normal Laser Doppler Velocimetry (LDV) data, or simultaneously acquired wall-shear stress and cross-stream stereoscopic particle image velocimetry (SPIV) data. The hibernating turbulence events occur randomly in space and in time in the turbulent boundary-layer flow, with these intrinsic events generating local skin-friction drag values up to 75% lower than the time-averaged mean skin friction. However, these hibernating turbulence events are rare, typically being found in a single location less than 10% of the time depending on the conditional sampling criteria. It is shown that ensemble-averaged streamwise velocity during intervals of hibernating turbulence fall close to the Maximum Drag Reduction Asymptote (MDR) asymptote at wall-normal distance of y+ < 40, in excellent agreement with existing experimental and Direct Numerical Simulation (DNS) data on hibernating turbulence in transitional channel flows. Further from the wall, the conditionally sampled streamwise velocity data follows a gradient similar to the classical Prandtl-von Krmn log-law, upshifted by the scaling with the low-drag friction velocity. This suggests, from an ensemble-averaged point of view, that the wall-turbulence during these intervals of low drag loses communication with the wall beyond y+ > 40.
Document Details
- Document Type
- Technical Report
- Publication Date
- Oct 23, 2023
- Accession Number
- AD1217762
Entities
People
- Richard Whalley
Organizations
- Newcastle University