جستجوی مقالات مرتبط با کلیدواژه « طول لغزش » در نشریات گروه « مکانیک »

Superhydrophobic surfaces have gained significant attention as a promising approach for drag reduction of submerged objects. Accurate evaluation and prediction of drag reduction induced by these surfaces require expensive experimental measurements, numerical simulations, or the development of reliable models and correlations. In this paper, a model is proposed for calculating the skin friction coefficient and drag reduction of superhydrophobic flat surfaces. Utilizing previous data on the skin friction coefficient of flat surfaces under no-slip boundary conditions, a model is developed to estimate the skin friction reduction and skin friction coefficient of these surfaces after applying superhydrophobic coatings. The validity of the model is verified by comparing its results with those of computational fluid dynamics (CFD) simulations of flow over a flat plate at different velocities. The results of the model and simulations indicate that for inlet velocities of 1, 5, and 25 m/s and a slip length of 50 μm, drag reductions of 15%, 41%, and 77%, respectively, are expected. Additionally, the skin friction reduction increases with increasing flow Reynolds number. The developed model is validated for flat surfaces and its ability to accurately estimate the skin friction coefficient and drag force of these surfaces is thoroughly examined. However, further investigations are required to assess the model's validity for curved surfaces and variable slip lengths.

The superhydrophobic surfaces have many applications, including skin friction reduction, anti-icing, anti-fouling, and self-cleaning surfaces. Also, with the precise design of these surfaces, it is possible to increase the heat transfer coefficient in the condensation heat transfer. In recent years, a variety of methods have been proposed for the fabrication of the superhydrophobic surfaces, some of which are very complex and not applicable for industrial uses. In this paper, a nanocomposite superhydrophobic coating is produced in a simple and applicable way for large surfaces. Using this method, a superhydrophobic surface with surface structures in multi-scale and with a sliding angle of less than 5 degrees is obtained. After evaluating the specification of superhydrophobic surfaces, slip length measurement of the coating is performed using a fabricated measurement system. It should be noted that the slip length of the superhydrophobic surface is a characteristic feature of these surfaces and always its measurement is associated with challenges. In this research, the slip length of the created coating was measured by use of the proposed measurement system. The results show that the slip lengths of about 40-500 microns can be achieved by use of the proposed measurement system.

Due to low surface energy and hierarchical roughness, fluids on superhydrophobic surfaces are mobile. The slip velocity on these surfaces is formulated using Navier’s slip length. On regular surfaces, slip length is only a few nano-meters. On superhydrophobic surfaces, slip length can be as large as 500 µm. Literature studies usually make the entire surface superhydrophobic which may not be the optimum situation. To find the desirable regions, the problem should be analyzed numerically. Most of the numerical studies are for flat plates. On curved surfaces (e.g. foils), due to the adverse pressure gradient and possibility of separation, analysis is more complicated. Here, the effect of using superhydrophobic surface for a SD7003 hydrofoil is studied numerically and at different Reynolds numbers and slip lengths. The flow pattern is considered laminar, incompressible and isothermal and a hydrofoil made of aluminum with a chord length of 10cm is selected. Results of the shear stress, pressure coefficient and the drag coefficient on the typical boundary condition were compared with the case of slip boundary condition. It was found that by increasing the slip length, the drag coefficient decreases. It was also found that the effectiveness of using superhydrophobic surfaces in decreasing the drag coefficient improves at higher Reynolds numbers. By increasing the Reynolds number from 4.5×〖10〗^4 to 7.5×〖10〗^4 and at the slip length of 50 µm, the drag coefficient reduction increases from 0.7% to 7%.