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In a vertical wind tunnel, in order to prevent persons or an experimental model from falling, a protective screen should be installed at the end of the nozzle section. Since the air has the maximum velocity at this section, the pressure drop due to the protective screen will be significantly high. On the other hand, as the protective screen is alternatively exposed to dynamic forces due to free fall of the floating persons or the model, the screen wires will experience fatigue. To prevent this, multi strand cables should be used in the manufacturing of these protective screens. In this research work, using the momentum difference method, drag coefficient for the multi strand cables and circular rods has been measured and compared. For this purpose, a hot wire anemometer (HWA) with a one-dimensional probe has been used. Results show that at Reynolds number in proximity of 2 × 103, the drag coefficient for the multi strand cable exceeds that of a circular rod by 16% and that this amount decreases with further increase in Reynolds number. The trend is such that for Reynolds number of 104, the drag coefficients of the multi strand cable and circular rod are almost equal.

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Airflow over a passenger train has been investigated experimentally and numerically in this research. The experimental model was a 1:26 scale model of a real train including a locomotive with one wagon behind it. A total of 16 pressure tabs for train were employed to measure the air pressure at various points on the model for different air flow velocity. Turbulent, incompressible and 3D model of air flow has been applied in numerical simulation. The numerical results of pressure coefficients were compared with the results obtained by the experimental investigation for the numerical simulation verification. The wagon number affect on the train drag coefficient and air pressure distribution on the symmetry plane of the train have been investigated numerically. The results show that the drag coefficient increases to 1.2336 for a locomotive and 7 wagons behind it but the air flow velocity has not a sensible affect on the drag coefficient. The averaged drag coefficient of each intermediate wagon has been obtained 0.1321.

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Electrohydrodynamic actuator is one of the newest devices in flow control techniques which can delay separation point and reduce the drag coefficient by inducing external momentum to the boundary layer of the flow. In this paper, a 2-D numerical approach was implemented to analyze the presence of electrohydrodynamic actuator on the incompressible, turbulent, steady flow over a NACA 4412 asymmetric airfoil. In this regards, the flow field and aerodynamic characteristics such as the drag and pressure coefficient were evaluated through the variety of attack angles, applied voltages, the location of emitting electrode, and the distance from the upper surface of the airfoil. The numerical results indicate that the drag coefficient with the presence of an electric field decreases with the enhancement of the supplied voltage but increases when the attack angle is augmented. In addition, the location of separation point significantly depends on the position of emitting electrode and the distance between the emitting electrode and the collecting electrode. On the other hand, according to the results, the Electrohydrodynamic effects cause the diminution of the wake region over the airfoil.

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Numerical analysis and simulation of cavitating flows due to appearance and its application in the maritime industry, water turbomachinery, hydrofoils, underwater vehicles, etc. have specific importance. For this reason in this research, the effect of blowing on hydrodynamic behavior of cavitating flows over hydrofoils has been investigated. Jameson's finite volume method and power-law preconditioning method with single-phase cavitation model (Barotropic model) have been used to the analyzing of cavitating flow. The stabilization of solution has been achieved with help of the second and fourth-order dissipation term. Explicit four step Runge-Kutta method has been used to achieve the steady state condition. As regards the cavitation often occurs at high Reynolds number, to facilitate the simulation the inviscid flow equations are considered. For apply the blowing from hydrofoil surface, a jet has been placed on hydrofoil’s upper surface. The parameters of jet location, blowing velocity ratio, blowing angle and width of jet are investigated and simulation has been performed for two different cavitation numbers. The numerical results show that the power-law precondition increases the convergence speed significantly. Blowing reduces the cavity length, lift and pressure drag coefficients compared to no blowing case. Also the increase of blowing velocity ratio, blowing angle and width of jet, decrease the cavity length, lift and pressure drag coefficients.

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present study is done to evaluate the effect of parameters like trip strip installation, free stream velocity, geometery of model nose (SUBBOF nose and DRDC nose) and putting up model in pitch and yaw angle, on drag coefficient. also the effect of stand geometry of an axially symmetric model in wind tunnel on wake flow structure and drag coefficient in zero and ten degree angles of attack was investigated. choosing best distance behind the model for data acquisition in order to calculate drag coefficient under consideration of turbulence effects in one dimension is the other item to investigate in present study. all experiments have been done in an open circuit wind tunnel at university of Yazd and data acquisitions has been done with a one dimensional hot wire. according to calculations installation of trip strip enhanced drag coefficient in all cases. also drag coefficient decreased with increasing free stream velocity. putting up the model in pitch and yaw angle of attack increased drag coefficient. between two nose shapes that examined, the SUBBOF nose shape choosed as suitable nose. a stand with NACA0012-64 geometry and Rod stand were selected as the most appropriate stands for zero and 10 degree angles of attack.

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Flow around a circular cylinder placed in an incompressible uniform stream is investigated via two-dimensional numerical simulation in the present study. Some parts of the cylinder are replaced with moving surfaces, which can control the boundary layer growth. Then, the effects of the moving surfaces locations on the power and drag coefficients are studied at various surface speeds. The flow Reynolds number is varied from 60 to 180. To simulate the fluid flow, the unsteady Navier-Stokes equations are solved by a finite volume pressure-velocity coupling method with second-order accuracy in time and space which is called RK-SIMPLER. In order to validate the present written computer code, some results are compared with previous numerical data, and very good agreement is obtained. The results from this study show that some of these surfaces reduce the drag coefficients and the coefficient of the total power requirements of the system motion. The optimum location and the speed of the surfaces which cause the minimizing the power coefficient are also obtained; By observing the results it is found that in all Reynolds numbers, the minimum power coefficient or in other word, the optimum drag coefficient is occurred at surface angle of 70 deg.

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In this study, the effects of Co-Flow jet and injection temperature on the enhancement of airfoil performance in the compressible flow are investigated numerically. Co-Flow jet is a method of increasing lift to drag ratio and varying the Stall Degree which works via injecting the air from the edge of airfoil and suction from the tail. The much number of studied flow changes from 0.4 to 0.6. Clark-Y airfoil has been chosen for this study because of its application in compressible flow, it is the base airfoil for development of new airfoils. A validation is performed for Clark-Y airfoil by comparing the present numerical result and available experimental data in the literature. Results indicate that the enhancement induced by the Co-Flow jet on the compressible flow is less than one in the incompressible flow. The drag and lift coefficients reduces and increases by increasing the jet momentum coefficient, respectively. Using the Co-Flow Jet increase the stall degree. The maximum of lift decrement and drag increment occurs around the stall degree. Increasing the temperature increases lift coefficient slightly where it seems to be better choice in comparison with increment of Jet momentum coefficient due to ease of operation.

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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%.

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The falling and sedimentation of solid particles in liquids occur in many natural and industrial processes such as water and waste water industries, biotechnologies, environmental engineering, marine engineering, etc. This study represents the results of the experimental study of the falling velocity of steel balls in the water channel for different ball diameters (in the range of 8 to 25mm). The tests are done far from the channel walls. Moreover, as a case study, the wall effect on falling velocity of steel ball (i.e. diameter=12mm) is examined. A high-speed camera is used to determine the coordinate of a falling sphere and estimate the ball velocity and drag coefficients. In addition, a numerical method is used to solve the governing equations in comparison with experimental data. Comparing experimental and numerical results for transient and terminal velocities shows the maximum difference of 12 and 4.5% respectively. Experimental drag coefficients have good agreement with other published data. In addition, falling near the wall leads to a negligible effect on velocity but path diversion is observed.