TRPIV Experimental Investigation On Mechanism Of Turbulence Drag Reduction By Micronano Structural Superhydrophobic Surface

The TRPIV(Time-Resolved Particle Image Velocimeter)system was employed to experimentally investigate the multi-scale coherent structures in turbulent boundary layer(TBL) and the drag reduction mechanism of the super hydrophobic surface in wall-bounded turbulence.In this experiment, TBL both over the super hydrophobic and smooth flat plate were measured by TRPIV, and instantaneous time-series of the 2D-2Cvelocity fields at three different Reynolds numbers were recorded with the sampling frequency of 500 Hz.Newton iterative fitting method was performed on the mean velocity profile of the TBL to obtain the skin friction velocity over super hydrophobic and smooth surface respectively.Through program calculation, the logarithm law of the TBL is well matched and viscious inner scale parameters such as the skin friction velocity, skin shear stress,skin friction coefficient, etc were acquired. Compared with the smooth flat plate,it was discovered that the super hydrophobic flat plate has increased the mean velocity in the near-wall region that belows the the logarithm law of the TBL.Which has few influences on the logarithm law of the TBL and the buffer layer.The friction coefficient is used to measure the drag-reduction rate, while at low Reynolds the highest drag-reduction rate of 10.58%was obtained.Moreover, comparison of the drag-reduction rate for three different Reynolds number indicates that with the increase of Reynolds numbers, the drag reduction effect decreased.Using the instantaneous fluctuating longitudinal velocity wavelet coefficients as the characteristic quantity.Wavelet transform was employed to decompose the velocity vector into multi-scale eddy structures respectively.With the the improved quadrant splitting method(ISQM),the spatial topological mode of coherent structures that in different vertical locations can be detected and extracted both in the superhydrophobic and smooth surface.The typical spatial topological modes for fluctuation velocity and vorticity has been obtained during the ejection and sweep process.We discovered that:a)The strength of fluctuation velocity and vorticity in the coherent structures were inhibited by the super hydrophobic flat plate.In this way,the exchanges of the energy has decreased in the ejection and sweep process.b)According to the increasing of the velocity,the integrity of the coherent structures was protected and the exitence of fluctuation was reduced in the TBL of the hydrophobic flat plate.