بررسي نيروي پساي نيم‌مخروط ناقص توخالي با و بدون پوشش زبري

INVESTIGATION OF DRAG FORCE ON HALF TUBULAR FRUSTUM WITH AND WITHOUT ROUGHNESS

تاكنون اندازه‌گيري تجربي نيروهاي آيروديناميكي و حل عددي جريان اطراف اجسام براي مدل‌هاي مختلفي انجام گرفته است، كه آزمايشات تجربي در تونل‌هاي باد و آب صورت مي‌گيرند. اندازه‌گيري نيروي پسا و ضريب پسا براي نيم‌مخروط ناقص تو‌خالي نيز مي‌تواند براي بسياري از كاربرد‌ها از جمله كشتي‌ها و شناور‌ها و پرتابه‌هاي خاص داراي نيروي برد مورد استفاده قرار گيرد. در اين مقاله نيروي پساي نيم‌مخروط ناقص توخالي در دو حالت مختلف ورود جريان از دهانه‌ي بزرگ و دهانه‌ي كوچك در تونل باد مدار باز مكنده مورد بررسي قرار گرفته است و اثرات جهت ورود جريان روي نيروي پسا و ضريب پسا بررسي شده است. اثر زبري نيز روي سطح نيم‌مخروط آزمايش و نتايج آن با مدل بدون پوشش يا صاف مقايسه شده است و در نهايت نتايج به‌دست آمده‌ي جدار صاف با حل عددي مقايسه شده و مشخص شده است كه در چه حالت‌هايي نيروي پسا و ضريب پساي كم‌تري به مدل وارد مي‌شود.

Experimental analysis of aerodynamic forces and the numerical solution of flow around bodies for different shapes have been carried out before, in wind and water tunnels. There are different ways in order to achieve higher velocity in vehicles. One of these methods is using a body with a special form. In this article, the drag force of a half tubular frustum is investigated for two different flow directions; from big diameters in the front inlet and small diameters in the front, in an open-circuit suction wind tunnel, and the effect of flow direction on drag force is investigated. Measurement of drag force and drag coefficient for a half tubular frustum can be used for different applications, like ships and air vessels with lift force. One way of reducing fuel consumption is drag reduction and this would increase velocity in vehicles. On the other hand, reducing drag force reduces initial costs, because smaller and cheaper driving systems are needed and this would be economical in the long-term. In this article, a model with 34.29cm length is used. The big diameter of this model is 10cm and the small diameter is 4cm. Due to growing marine organisms on the outer surface of ships, we have a rough surface, which is a source of problems, such as drag force increasing on the wetted area. The effect of roughness on a half tubular frustum is also investigated and it is obtained, in both conditions of flow inlet, the drag coefficient of the rough model is higher than that of the smooth one. Viscous drag for the condition where flow direction is from the small diameter in front is higher than the condition where the flow direction is from the big diameter in front. When the flow direction is from the big diameter in front, we have separation on the outer surface and under the flat plate of the half tubular frustum. Under this condition, pressure drag is the biggest portion of total drag. In the present study, a computational fluid dynamic (CFD) simulation of this body is performed and commercial CFD software package FLUENT 6.3.26 is used for simulation. From CFD simulation it is obvious that, when flow direction is from a big diameter, the pressure of the big diameter is higher than that of the small diameter side, and the velocity of the big side is less than that of the small side. In contrast to the mentioned condition, if the flow direction is from a small diameter in front, the inlet velocity in the small diameter side is higher than the outlet velocity and we can use this high velocity for different applications, like for ships and air vessels. The result of this paper can even be used for increasing the efficiency of wind turbines. Finally, the results of experimental and numerical solutions are compared and it is determined that there is little difference between them. The reasons for this discrepancy could be formation of vortexes and the wake.