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Hot-wire measurements of passive control for coherent structure in the turbulent boundary layer of a groove-riblet surface

Mechanisms for drag reduction in the turbulent boundary layer over a V-grooved riblet surface were studied,which is very important technology to ships and pipeline transportation.Using the hot-wire technique,the experimental study was made on drag reduction mechanism and passive control of multi-scale coherent structure of turbulence boundary layer on riblet plate.From the angle of control of multi-scale coherent structure of the turbulence,the drag reduction mechanism was analyzed.Constant temperature anemometry was used to study possible mechanisms of drag reduction.Wind tunnel time sequences of longitudinal velocity components across the turbulent boundary layer over both the riblet surface and a smooth surface were precisely measured by an IFA300 constant temperature anemometer and parallel-double-hot-wire-sensor probe with resolution higher than the Kolmogorov scale.The bursting and sweeping processes of coherent structure were identified by wavelet coefficients of the longitudinal fluctuating velocity component.Phased-average waveforms of the longitudinal fluctuation velocity component,as well as the Reynolds stress component of the coherent structure's bursts in the turbulent boundary layer were extracted by the conditional sampling technique.The statistical characteristics of coherent structure,such as burst intensity,phase-averaged waveforms over the turbulent boundary layer of the riblet surface and the smooth surface were compared to clarify the effect of drag reduction over the V-grooved riblet surface.