Analysis of shear rate effects on drag reduction in turbulent channel flow with superhydrophobic wall

We examined the shear rate effect on drag reduction of superhydrophobic surfaces with different slip lengths. For this purpose, turbulent channel flow was considered at the friction Reynolds numbers of Reτ = 180, 395, 500. By using Navier＇s slip condition it is shown that increasing shear rate leads to the greater reduction in drag force and also more reduction occurs in larger slip length. Based on the results, more than 25% drag reduction happens at a friction Reynolds number of Reτ= 500 for slip length of 1 ×10 5 m. The simulation results suggest that reduction in drag force occurs because slip condition reduces the Reynolds stresses, also weakens vorticity filed and the near-wall coherent structures, and therefore turbulence production is decreased.     

DRAG REDUCTION IN A TURBULENT CHANNEL FLOW WITH HYDRO- PHOBIC WALL

This paper investigates a theoretical prediction of friction drag reduction in turbulent channel flow which is achieved by using superhydrophobic surfaces. The effect of the hydrophobic surface is considered to be a slip boundary condition on the wall, and this new boundary condition is added to Large Eddy Simulation (LES) equations. The predicted drag reduction at Reτ=180 is approximately 30%, which concurs with results obtained from Direct Numerical Simulation (DNS). An important implication of the present finding is that the near-wall turbulence structures are modified with streamwise slip velocity. In addition, a noticeable effect on the turbulence structure occurs when the slip length is greater than a certain value.     

Tribological properties improvement of conventionally-cast Al-8.5Fe-1.3V-1.7Si alloy by multi-pass friction stir processing

The effect of multi-pass friction stir processing (FSP) on the tribological properties of conventionally-cast Al-8.5Fe-1.3V-1.7Si (FVS0812) alloy was investigated. The pin-on-disk dry sliding wear tests were conducted at room temperature under the applied pressures of 0.25, 0.50, and 0.75 MPa. The results showed that FSP substantially refined and improved the distribution of coarse θ-Al₁₃Fe₄ platelets and α-Al₁₂(Fe,V)₃Si intermetallics in the microstructure of alloys and eliminated the intermetallic-related defects. Consequently, the mechanical properties of the alloys, especially their ductility, were improved, which enhanced the stability of the protective tribolayer formed on their worn surfaces. According to the wear test results, the FSPed samples showed improved tribological properties especially at the higher applied pressures. For instance, at the applied pressure of 0.75 MPa, the wear rate and average friction coefficient of four-pass FSPed sample were lower than those of the base as-cast sample by 97% and 52%, respectively. SEM examination of the worn surfaces and wear debris also demonstrated that the wear mechanism changed from severe delamination/abrasion and microcracking of the tribolayer in the as-cast samples to mild delamination/abrasion and minor plastic wear in the FSPed samples.