Experimental Investigation Of Drag Reduction Mechanism In Turbulent Boundary Layer Over The Superhydrophobic-Riblets Surface With Anisotropic Structures

The superhydrophobic surface is a recently developed technique for efficient passive drag reduction in turbulent boundary layer,which is the current hot topic in the current research of drag reduction in turbulent boundary.In this paper,we use the femtosecond laser to etch streamwise micro-grooves on an aluminum alloy plate surface and then fluorinate it to obtain a more stable and stronger anisotropic superhydrophobic surface.After machined millimeter flow serrated grooves,the surface is then treated as described above to form a superhydrophobic-riblets surface with anisotropic structures with better drag reduction effect.In this paper,we use experimental measurements such like TR-PIV（Time-resolved Particle Tracking Velocimetry）,MTR-PIV（Moving Time-Resolved Particle Image Velocimetry）,and Tomo-TRPIV（Tomographic Time-resolved Particle Image Velocimetry）combined with the basic theory of turbulence to study the drag reduction mechanism of anisotropic superhydrophobic surface and superhydrophobic-riblets surface.Elaborate high temporal resolution TR-PIV measurements were performed on the near-wall region over an anisotropic superhydrophobic surface,and the average velocity profile of the viscous sublayer was obtained by using single-pixel ensemble correlation of the particle images.The drag reduction rate of 17.6%,the streamwise slip velocity of 0.0119 m/s and the streamwise slip length of 90.8μm were obtained by linear fitting the velocity profile of the viscous sublayer.The Reynolds stress at the buffer layer was obtained using the narrow rectangular window correlation technique.By comparing with the previous results of isotropic superhydrophobic surface,it is found that the anisotropic superhydrophobic surface reduce the gain on quasi streamwise vortices by suppressing spanwise slip,which in turn affects burst events in the near-wall region through a self-sustaining mechanism,resulting in superior drag reduction.SPOD（Spectral Proper Orthogonal Decomposition）and SPDMD（Sparsity-Promoting Dynamic Mode Decomposition）are used to downscale the flow field,it is found that the（super）large-scale structures at low frequencies over the superhydrophobic surface become"upright".Using spatial-temporal correlation and spatial two-point correlation,it is found that the slip effect over the superhydrophobic surface on the coherent structure makes the lower wall-normal layer structure develop faster downstream,eventually leading to a change in the structure morphology.We use MTR-PIV to track the coherent structures over the anisotropic superhydrophobic surface up to 500 mm length and capture the"younger packet"in the log-law region and the"older packet"in the log-law and wake regions,respectively.The overall vortex strength of the"younger packet"over the superhydrophobic surface and the intensity of the burst events driven by it are weaker and the duration of the hairpin packet is shorter.The newly generated hairpin vortices in the near wall upstream of the"older packet"over the superhydrophobic surface are not strong enough to drive a large amount of low-speed fluid lift,which is also shown as weaker instantaneous Reynolds shear stress.In the y/δ₉₉<0.3 region,the Reynolds stress over the superhydrophobic-riblets surface present a lower value than that over the superhydrophobic surface,which exhibits better drag reduction effect.By both fitting the velocity profile in the log-law region and estimating the total shear stress,the drag reduction effect of the superhydrophobic-riblets surface is improved by about 20%.The quadrant splitting results show that the superhydrophobic-riblets surface is more effective in suppressing the burst events.Further analysis of the Pre-multiplication energy spectrum reveals that the（super）large-scale structure energy at low wavenumbers over the superhydrophobic and superhydrophobic-riblets surface is weakened at y/δ₉₉<0.3 while the large-scale structure energy over the superhydrophobic-riblets surface is enhanced at y/δ₉₉>0.3,which is correlated with the plateau of its Reynolds stress profiles at the high wall-normal position.Further using POD（Proper Orthogonal Decomposition）to scale decomposition and reconstruction of the flow field based on the threshold of 50%POD energy,it is found that the superhydrophobic-riblets surface has weakened the strength of both large/small scale structures at y/δ₉₉<0.3,while large-scale structures are significantly enhanced aty/δ₉₉>0.3.Based on the spatial two-point correlation and linear stochastic estimates,a relatively larger hairpin packet structure is found here.The 3D results by Tomo-TRPIV present a larger spanwise spacing in the streamwise-spanwise plane of velocity structures over the superhydrophobic and superhydrophobic-riblets surfaces.The three-dimensional hairpin vortex is obtained by using linear stochastic estimation with the eject event as the phase center.The vortex leg spacing of the hairpin vortex is found to be larger over the superhydrophobic and superhydrophobic-riblets surfaces,which provides experimental evidence for previous theoretical models of coherent structures over the superhydrophobic surface.