Drag Reduction Research Over Superhydrophobic Surface In Turbulent Boundary Layer

The advancement of science and technology is driving the speed of vehicles,ships,and airplanes to become faster and faster.It is necessary to reduce friction.According to whether it is necessary to inject energy into the fluid,the control methods can be divided into active control and passive control.Active control has high drag reduction efficiency,but at the same time,its high design cost,high energy consumption,complex system,etc.Passive control has the advantages of simple operation and low cost.This paper use superhydrophobic surface which belong to passive control as the object to study the drag reduction mechanism in the turbulent boundary layer.Two micrograted superhydrophobic surfaces were used in the experiment.The control group was a smooth surface.The friction coefficient of the superhydrophobic surface and the smooth surface was measured with the help of a force balance and the drag reduction rate was calculated.It was found that the superhydrophobic surface at Reynolds number Re _L=6.2×10⁵,the maxmum drag reduction is 33.3%.The Particle Image Velocimetry(PIV)technology is used to measure and calculate the turbulence statistics and turbulence structures in the flow field of the superhydrophobic surface and the smooth surface.It is found that the Reynolds shear stress and the flow RMS velocity of the superhydrophobic surface are significantly smaller than the smooth surface near the wall.At y⁺?20,the buffer layer,the energy production term P of the superhydrophobic surface and smooth surface reaches its peak,and the P of the superhydrophobic surface is significantly lower than that of the smooth surface.When used the?_ci and?₂ criterion to extract the average vortex strength in the two surface,it is found that the vortex strength of the superhydrophobic surface is obviously weaker than the smooth surface near the wall.When the threshold is selected as 2,the turbulence burst frequency of superhydrophobic surface is reduced by 47.6%at y⁺=20 and 48.6%at y⁺=30.The skew factor S_u of the superhydrophobic wall is larger than the smooth wall in the region of 20<y⁺<40,and the superhydrophobic surface is shifted to the right in the region,while the smooth wall is shifted to the left,which means that the high-speed fluid on the smooth wall is more than the superhydrophobic walls,while high speed fluids near the wall are often accompanied by sweep,which is the main source of friction near the wall.Logarithmic scaling changes in drag reducing turbulent boundary layer over a micrograte superhydrophobic surface was derived.Using the Micro-PIV technology found that when Re_?0=511,the slip length l_x⁺=3.5,then the slip length and drag reduction rate were compared with the theoretical formula,and it was found that the relationship between the slip length of the micrograted superhydrophobic surface and the drag reduction rate is closer to the theoretical calculation value when l_z⁺=0.