چكيده به لاتين
It is well known that the exerted force on a flat plate and the pressure drop in channels are related to wall shear stress. Also, in axisymmetric bodies much of the drag (about 80%) which is applied on the body is due to the wall shear stress. Fabrication of the superhydrophobic surfaces in the large-scale for frictional drag reduction is the main goal of this research. Presence of micron-sized structures on the surface is necessary part of the superhydrophobic fabrication. The important role of these structures in the static contact angle and sliding angle as the main characteristic of the superhydrophobic surfaces has been shown previously. In addition, the stability of the surface in front of the flow situation (shear stress and hydrostatic pressure) is mainly stems from these structures. Besides of the stability and complicated behaviors of the superhydrophobic surfaces there is a lack of simple and large scalable strategy in the fabrication of these surfaces. This issue restricts usage of the superhydrophobic surfaces for practical applications such as drag reduction of the underwater vehicles. In the present study, after evaluation of different methods, two fabrication strategies are presented which they are meeting the requirements of the research: Fiber-based surfaces and composite coatings. Our experiments are shown that using the presented methods it is possible to fabricate superhydrophobic surfaces with static contact angle greater than 160º and sliding angle lower than 5º. The minimum stability of the surfaces in front of the water is around 7 days. Also, drag reduction of the fiber-based surfaces in the laminar channel flow are investigated. The results show that using these surfaces it is possible to achieve drag reduction of 13% to 43% at the wall shear stress of 1.5 Pa to 25 Pa. The slip lengths are 10 µm to 50 µm in the wall shear stress range. The hydrostatic pressures which are exerted on the surfaces are in the range of 5 to 15 m. Duration of the experiments is specially 30 min to 3 h, and 6 h in some experiments.
Investigation of the superhydrophobic drag reduction mechanism is another part of the research. Large eddy simulation and Navier slip model are used to investigate the effects of slip on the fully developed turbulent channel flow friction reduction at frictional Reynolds number of 180, 395, and 500. Results show that decrease in the velocity fluctuations, Reynolds stress and surface friction are more affected by increasing flow shear stress. Also, due to slip the streamwise component of the vorticity field are weakened. In addition to Navier slip model, direct simulation of superhydrophobic surface structures on the turbulent quantities is another approach to investigate superhydrophobic drag reduction mechanism. So, to detect changes in the turbulent structures, we compared the no-slip and micro-structured superhydrophobic results. The comparisons were made for mean flow quantities, such as mean velocity profiles, drag reduction, slip velocity, and turbulence statistics. Further, spatial correlation coefficients, integral length scales, streamwise vortical structures, and streak instability were analyzed to explore the changes in the turbulent structures. The results show that the peak of the RMS of the fluctuating components and Reynolds stress are decreased and moved toward the wall. The outward motion of the lifted low-speed streaks is restricted to the lower wall layers, and the region of maximum production of streamwise vortices is shifted to the micro-structured wall. Comparison of coherent structures show that the density of near-wall coherent structures is reduced and the location of their maximum existence is shifted to the bottom wall. The results indicate that increased streak strength do not result in stronger generation of turbulence, while the Reynolds shear stress and streamwise vorticity are weakened. The quadrant analysis of Reynolds stress show that the stronger increase of the outward motion of high-speed fluid (Q1) and the inward motion of low-speed fluid (Q3), in comparison with ejection (Q2) and sweep (Q4), result in more viscous dissipation of turbulent kinetic energy and decrease the turbulence production. The correlation coefficients indicate that the spanwise distance of lifted low-speed streaks is increased. Comparisons of the results of Navier slip model and direct simulation of the surface structures shows that using Navier slip model the turbulent flow alternations could not be extract exactly. Hence, special consideration should be made if precise information of the turbulent flow alternations is required.
Keywords: Superhydrophobic Surfaces; Silanization, Electrostatic Coating, Drag Reduction, Coherent Structures
