Direct numerical simulation of turbulent flows over superhydrophobic surfaces: pressure characteristics and interfacial robustness

Date: September 27th, 2016 (Tuesday), 16:00 – 17:00
Location: E1 Seminar room (#3213)
Speaker: Jongmin Seo, Ph.D. candidate, Stanford University, USA
Host: 옵토-유체 연성체 상호작용 연구단 / 성형진 교수 (T.3027)

Direct numerical simulation of turbulent flows over superhydrophobic surfaces: pressure characteristics and interfacial robustness

Superhydrophobic surfaces can effectively capture gas pockets within their micro-scale structures when submerged in water. A thin layer of gas pockets forms slippery boundaries for the overlaying liquid flow leading to reduced skin friction. Therefore, superhydrophobic surfaces present opportunities for improving hydrodynamic performance over a wide range of systems such as those in naval applications. In most realistic applications, the flow regime is turbulent. However, in such regimes the key physical phenomenon controlling drag reduction, and stability of gas pockets are not well understood. In this work direct numerical simulations of turbulent channel flows with superhydrophobic walls are used to analyze the kinematics as well as interfacial robustness of superhydrophobic surface at high Reynolds numbers. Consistent with the DNS results, our model predicts that the surface slip length is inversely proportional to the square root of the solid fraction, and directly proportional to the cube root of the pattern size. This study also investigates the validity of the homogenization process in which the patterned geometry is represented by an effective slip length. In addition, the present work addresses the robustness of superhydrophobic surfaces by studying the pressure load fields obtained from DNS data. Effects of stagnation pressure formed by slip flow at the leading edge of geometric textures are mainly investigated. The results show that the larger texture size intensifies the stagnation pressure contribution, while the turbulence contribution is essentially insensitive to texture size when scaled wall units. Based on these results, an upper bound on the texture wavelength is quantified that limits the range of robust operation of superhydrophobic surfaces when exposed to high-speed flows. Furthermore, the current work studies the dynamics associated with the deformability of the air-water interface when superhydrophobic surfaces are exposed to turbulent flows. Our results suggest that the deformability of the interface, characterized by a finite Weber number, contributes to the intensification of the turbulent pressure fluctuations near the surface. We demonstrate that the additional pressure field is due to a capillary wave that is energized by turbulence, and travels upstream. The spatio-temporal characteristics of this pressure are quantified, and analyzed by a semi-analytical model. Finally, this work examines how the randomness of patterns affects the drag reduction effectiveness and interfacial robustness when superhydrophobic surfaces with random textures are in contact with an overlaying turbulent flow

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