In this paper the effects of hydrophobic wall on skin-friction drag in the channel flow are investigated through large eddy simulation on the basis of weaklycompressible flow equations with the MacCormack’s scheme on collocated mesh in the FVM framework. The slip length model is adopted to describe the behavior of the slip velocities in the streamwise and spanwise directions at the interface between the hydrophobic wall and turbulent channel flow. Simulation results are presented by analyzing flow behaviors over hydrophobic wall with the Smagorinky subgrid-scale model and a dynamic model on computational meshes of different resolutions. Comparison and analysis are made on the distributions of timeaveraged velocity, velocity fluctuations, Reynolds stress as well as the skin-friction drag. Excellent agreement between the present study and previous results demonstrates the accuracy of the simple classical second-order scheme in representing turbulent vertox near hydrophobic wall. In addition, the relation of drag reduction efficiency versus time-averaged slip velocity is established. It is also foundthat the decrease of velocity gradient in the close wall region is responsible for the drag reduction. Considering its advantages of high calculation precision and efficiency, the present method has good prospect in its application to practical projects.
An apparatus was specifically developed for micro-friction and adhesion measurements. The force measurement range is 10-2000 μN with a horizontal speed of 10-400 μm/s. The apparatus was tested using a 0.7-mm diameter steel ball as the upper specimen to measure the micro friction and adhesion behaviour of a Si （100） wafer and a TiB2 film. The effects of rest time, speed, and load were studied. The results show that the maximum static and sliding friction forces of both the Si （100） wafer and the TiB2 film increase with the load. At low speeds, the influence of speed on the friction force is significant. The adhesion forces of the Si （100） wafer and the TiB2 film increase with rest time, reaching stable values after about 3000 s. The TiB2 film has significantly less adhesion and micro friction forces than the Si （100） wafer.
