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In the present paper, a two dimensional numerical analysis of the dynamic stall phenomenon associated with unsteady flow around the NACA 0012 airfoil at low Reynolds number (Re ≈ 130000) is studied. For this purpose, a thin blade with height of 0.005 chord length was placed vertically on the airfoil to control the bursting of the laminar leading edge separation bubble. The numerical simulation of flow is based on discretization of convective flaxes of the turbulent unsteady Navier-stokes equations by second-order Roe’s scheme and an explicit finite volume method in a moving coordinate system. Because of the importance of the time dependent parameters in the solution, the second-order time accurate is applied by dual time stepping approach. Three oscillating patterns with different frequencies and angular amplitudes were used to study the dynamic stall phenomenon. In order to validate the operation of computer code, some results for static and dynamic stall are compared with experimental data. The results of this study showed that the burst control blade had the acceptable effects on the dynamic stall control; so that these effects were increased while the oscillation frequency was raised. The best result occurs in 5 deg angular amplitude and reduced frequency of 0.15; so that the lift stall reduced 50% and there was not any obvious stall in drag coefficient.

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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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Improvement of shooting accuracy with air gun pellets is very important in sport competitions which is always questioned by shooting enthusiasts. In this study, the performance of a transonic spherical projectile as an air gun pellet with 4.5 mm-caliber under a mechanism known as Hop-up is numerically examined. The motion of this projectile is assumed in four degrees of freedom including three translational motions and one transverse rotational motion. Hop-up mechanism is resulted in a rotational motion of spherical projectile, so a Magnus Force is generated which prevents the altitude loss of the projectile. The Navier-Stokes equations are solved in compressible non-stationary turbulent conditions with equations of the pellet motion in a coupled form and in a moving computational grid by a computer program. The present numerical simulation is based on “Roe” scheme with second-order accuracy using a finite volume method and because of the importance of time dependent parameters, second-order time accurate was applied. To validate the computer program operation, the results were compared to valid experimental data. The results obtained from these studies showed that proper rotation of the projectile for a certain distance prevents its height drop when hits the target. A relation was also obtained between the target location, shooting kinetic energy and proper angular velocity which can neutralize the projectile altitude loss at arbitrary distavces. It is also demonstrated that by increasing the angular velocity, the vortex shedding onset is accelerated and the projectile momentum is decreased.

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One of the important issues in shooting by air guns is to select the appropriate projectile for different distances of the target. In this paper, the performance of four samples of air gun projectiles (pellets) is studied. The motion of these projectiles is assumed in four degrees of freedom including three translational motions and one rotational motion. The considered projectiles have three calibers of 4.5, 5.5 and 6.35 mm, and four different types, namely flat nose, sharp nose, round nose and spherical. In order to numerical simulation of the problem, after these projectiles have been modeled geometrically, the 3-D compressible turbulent Navier-Stokes equations and dynamic equations of the projectiles motion are solved in a coupled form and in a moving computational grid. The numerical simulation is based on “Roe” scheme with second-order accuracy in space and time using a finite volume method. To validate the computer program operation, the results are compared to valid experimental data. Computed results describe the trajectory, velocity variations and altitude loss of the projectiles with time and location. Comparison of the projectiles performance including the trajectory, velocity variations and altitude loss indicate that the round nose projectile has the best performance in long distances compared to the other samples and the flat nose projectile has a great performance in short distances, while it has a weak behavior in long distances. Additionally, effect of nose shape on the performance of the sharp and round nose projectiles is investigated and the optimum nose shapes are obtained.