TRPIV Experimental Research Of Drag Reduction Mechanism In Wall-bounded Turbulence Over Superhydrophobic Surfaces

The wall-bounded turbulence coherent structure is the main source of wall frictional resistance and how to reduce the wall frictional resistance is of great significance in practical engineering applications.Based on the angle of the coherent structure,the time-resolved particle image velocimetry(TRPIV)is used to measure the wall-bounded turbulence of smooth and superhydrophobic surfaces with the velocity of 0.165 m/s and the drag reduction mechanism of the superhydrophobic surface is studied by contrast method.Firstly,the inner scale dimensionless average velocity profile and the turbulence profile of the two surfaces are compared.The results show that the superhydrophobic surface appears as a thickening of the buffer layer,and the logarithmic region is shifted outward.The entire inner dimensionless of the average velocity profile shifts slightly upward,which is consistent with the typical characteristics of wall-bounded turbulence drag reduction.In addition,the superhydrophobic surface mainly inhibits the streamwise pulsation of the fluid particles,and has little effect on the normal pulsation.The method of two-point correlation coefficient in space is used to extract the coherent structure of the two surfaces.It is found that the superhydrophobic surface can not only reduce the spatial scale of small-scale coherent structures,but also reduce the spatial scale of large-scale coherent structures.In addition,the super-hydrophobic surface can reduce the scale of the correlation coefficient between the streamwise and normal pulsation,which can reflect that the super-hydrophobic surface can weaken the burst event in wall turbulence.In view of the important role of the hairpin vortex and vortex package structure in the wall turbulence,and can be extracted with the linear stochastic estimation method.It is found that the superhydrophobic surface can change the organization of the vortex package structure,so that the streamwise pulsation of the low velocity fluid below is weakened.It's benefit to reduce the wall frictional resistance.Finally,the eigen-orthogonal decomposition of the velocity field under two surface conditions is performed.It was found that the low-order modal of the superhydrophobic surface contained less energy than the hydrophilic surface,while the higher-order modal of the superhydrophobic surface contained slightly more energy than the hydrophilic surface.It is indicated that the superhydrophobic surface can change the energy distribution contained in the coherent structure in the wall turbulent flow.That is,the superhydrophobic surface can tilt the energy in the wall turbulence to the structure of the small energy scale,which is also beneficial to the reduction of the wall frictional resistance.