Time-resolved Particle Image Velocimetry Investigation Of Drag-reduction For Superhydrophobic Surfaces And Coherent Structures In Wall-bounded Turbulent Flows

Drag-reduction mechanism for superhydrophobic surfaces and coherent structures in the wall-bounded turbulent flows are experimentally investigated by the time-resolved particle image velocimetry(TRPIV).Time series of velocity vector fields in TBL over hydrophilic surfaces and superhydrophobic surfaces with different Reynolds numbers were measured in a cyclical water channel by combining large and small field.The mean velocity profiles along wall-normal direction were compared for Re??9000,12000,16000.Drag-reduction rates for superhydrophobic surfaces were 11.6%,10.11%,9.0%.Results indicate that,with the increase of Reynolds number,the drag-reduction rate decreases.The profiles of turbulence intensity and Reynolds stress along wall-normal direction were also compared.Compared with the hydrophilic surfaces,the streamwise Reynolds normal stress,Reynolds shear stress,streamwise turbulence intensity and turbulence intensity for superhydrophobic surfaces were smaller;but the normal turbulence intensity and normal Reynolds stress were greater for y-=30?100.Results indicate that,for the superhydrophobic surfaces,the drag decreases with the streamwise slip and the drag increases with the spanwise slip.Based on the concept of multi-scale spatial locally-averaged structure function,the burst events in the near wall region at different normal positions were detected with the improved quadrant splitting method for Re??12000.Conditional sampling and phase average methods were employed to extract the spatial topologies of streamwise and normal fluctuating velocities,spanwise vorticity,Reynolds shear stress and velocity deformation rate for the ejection and sweep events.The statistical values for ejection events were greater than that for sweep events.Compared with the hydrophilic surfaces,the streamwise fluctuating velocities,spanwise vorticity,Reynolds shear stress and velocity deformation rate for the superhydrophobic surfaces were smaller.Results indicate that the burst strength of superhydrophobic surfaces is weaker.In addition,ejection is far more intense than sweep.The head of hairpin vortex in wall-bounded turbulence was detected with the multi-scale spatial locally-averaged vorticity.Conditional sampling and phase average methods were also introduced to extract the spatial topologies of spanwise vortex head at different normal positions.The features of streamline topologies around spanwise vortex head were analyzed from the saddle-focus dynamical system.Compared with the hydrophilic surfaces,the streamwise turbulence intensity and pitching angle of spanwise vortex for the superhydrophobic surfaces were smaller.The results indicated that,for the superhydrophobic surfaces,the flow is relatively flat,burst strength is weaker,and the development of the hairpin vortex is restrained.