Norio Arai

Application of porous material to reduce aerodynamic sound from bluff bodies

Aerodynamic sound derived from bluff bodies can be considerably reduced by flow control. In this paper, the authors propose a new method in which porous material covers a body surface as one of the flow control methods. From wind tunnel tests on flows around a bare cylinder and a cylinder with porous material, it has been clarified that the application of porous materials is effective in reducing aerodynamic sound. Correlation between aerodynamic sound and aerodynamic force fluctuation, and a surface pressure distribution of cylinders are measured to investigate a mechanism of aerodynamic sound reduction. As a result, the correlation between aerodynamic sound and aerodynamic force fluctuation exists in the flow around the bare cylinder and disappears in the flow around the cylinder with porous material. Moreover, the aerodynamic force fluctuation of the cylinder with porous material is less than that of the bare cylinder. The surface pressure distribution of the cylinder with porous material is quite different from that of the bare cylinder. These facts indicate that aerodynamic sound is reduced by suppressing the motion of vortices because aerodynamic sound is induced by the unstable motion of vortices. In addition, an instantaneous flow field in the wake of the cylinder is measured by application of the PIV technique. Vortices that are shed alternately from the bare cylinder disappear by application of porous material, and the region of zero velocity spreads widely behind the cylinder with porous material. Shear layers between the stationary region and the uniform flow become thin and stable. These results suggest that porous material mainly affects the flow field adjacent to bluff bodies and reduces aerodynamic sound by depriving momentum of the wake and suppressing the unsteady motion of vortices.

Numerical Simulation of Flow Around a Train

It was recognized that the vibration amplitude of the tail car is greater, especially in the tunnel, than that of the other cars in a high-speed train, in which an aerodynamic force has some effects. However, few aerodynamical studies have been conducted and little knowledge has been gained. To clarify the aerodynamical effect on the train, a three-dimensional unsteady Navier-Stokes simulation was carried out. Unsteady flow separations on the rear nose, which cause fluctuations of the yawing moment of the tail car, were successfully obtained by the simulation. In the tunnel section, it is proved that the tunnel wall makes the flow separation asymmetric and that the expansion of the effective flow area along the rear nose causes a greater pressure fluctuation. We also proposed a modified shape of rear nose, which suppresses the flow separation and reduces the yawing moment fluctuation.

Development of vertical takeoff and landing vehicle with flapping wings

Flying beings by flapping wings show a hovering flight without stall because they can exploit vortices for their flight. The mechanism of the flapping wing flight makes clear, gradually. Therefore, it is considered that a flapping wing vehicle can be developed based on the mechanism with high efficiency rather than fixed-wing aircraft and rotary-wing aircraft in some situation. However, the flapping motion of flying beings is too complicated to reproduce completely. In this paper, flowfields around simplified flapping motion for the real flapping vehicle are investigated using CFD to make clear the relationship between vortices and unsteady fluid forces. Moreover, several motion parameters to develop the vertical takeoff and landing (VTOL) aircraft with flapping motion are discussed based on the actual equipment.

Fluid-Structure Interaction of Circular Cylinders With Elastic Surface

This contribution presents the numerical study of Fluid Structure Interaction (FSI) problems and discusses the oscillatory characteristics of the elastic bodies and flowfield around circular cylinders. This paper deals with the motion of the elastic body and the flowfield using computational fluid dynamics (CFD) in two-dimensional and three-dimensional simulations. The governing equations are the continuity equation and incompressible Navier-Stokes equations. These equations are solved by MAC (Marker and cell) method by using Poisson equation for pressure component and momentum equations for velocity components. The convective terms of momentum equations are discretized by the third-order upwind Kawamura-Kuwahara scheme. All of the discretized equations are solved by the Successive over-relaxation (SOR) method. The equation of motion consists of mass-spring-damper system and it is solved by the 4th order Runge-Kutta method. The objective is to investigate the influence of the elastic surfaces with respect to the vibration characteristics of cylinders in unsteady flows. As a result, it is obtained that due to passive deformation of elastic surface for single cylinder the drag coefficient increases in both 2D and 3D cases. It is noticed that the effect of elastic surface in 2D case is stronger compared to 3D case. In the case of double cylinders, the elastic surface affects on vibration characteristics of upstream and downstream cylinders, and it is significant on downstream cylinder.Copyright

Three-dimensional Motion Analysis of a Free Descent Parachute-like Body

Parachutes are used for various purposes and in wide speed range as one of the deceleration systems, because of the small storage space, the light weight and excellent dragto-weight ratio. The equilibrium of the gravity force and the drag force determines the velocity in real free descent stage, in which the descent velocity can be changed with time, obviously. Therefore, the free descent parachute should be focused in the present study. The flowfield is solved by incompressible Navier-Stokes equations, and the motion is solved by the equation of translational motion. These equations are solved simultaneously based on weak coupling algorithm. As a result, characteristic flow phenomena are confirmed in two dimension and three dimension. Nomenclature ax = X component of the acceleration of the parachute ay = Y component of the acceleration of the parachute Cl = force coefficient in the x direction (Lift coefficient) Cd = force coefficient in the y direction (Drag coefficient) Fx = X component of the force acting on the parachute Fy = Y component of the force acting on the parachute g = acceleration of gravity m = mass of the parachute p = pressure of fluid t = time u = velocity vector of fluid ug = velocity vector of computational grid ρ = density of fluid � = viscosity of fluid