Relationship between superhydrophobic surface area and drag reduction effect in turbulent boundary layer flow

Abstract

In this study, we compared the results of two conditions: one in which a superhydrophobic coating was applied over a wide area, and the other in which it was applied in part, taking into account the size of the longitudinal vortex, and investigated how the turbulent structure flowing over the superhydrophobic surface changed and contributed to frictional resistance, focusing on vorticity. As a result, in the Slip condition and Partial slip condition, the slope of the average velocity in the main flow direction near the wall was mitigated, and a slip phenomenon occurred. In addition, the Slip condition was observed at high values from any wall position, and a reduction in frictional resistance was confirmed. In the Slip condition and Partial slip condition, an increase in frictional resistance was confirmed at low Re_θ, and a reduction in frictional resistance was confirmed at high Re_θ. This means that the relative magnitudes of the effects of friction drag reduction due to flow direction slip and friction drag increase due to span direction slip change with Re_θ, and that when Re_θ increases, the superhydrophobic surface becomes relatively larger in relation to the longitudinal vortex, and the effect of friction drag reduction becomes dominant. Using the experimental data for each condition, we solved the FIK identity equation developed using vorticity, and confirmed that the effect of friction drag reduction that occurs when a superhydrophobic surface is installed is due to the reduction in vorticity fluctuations.