Aerodynamic force acting on a smooth sphere in a uniform flow was investigated near the critical Reynolds number. Drag, lift and side forces acting on the sphere and the pressure distibution on its surface were measured in a wind tunnel. The flow around the sphere was also visualized by a smoke wire method. In the range of the critical Reynolds number, steady lift and side forces generate with a sudden drag reduction, because a laminar-turbulent transition occurs partially around the sphere and the flow becomes asymmetric in respect of the flow direction. When the Reynolds number is slightly larger or smaller than the critical Reynolds number, the flow around the sphere becomes unstable. Irregular and unstable lift and side forces appear, although their time-averaged values are zero. Their trajectories with time show random, bistable or arc-like patterns. In the range of the super critical Reynolds number, a pair of streamwise vortices appears in the wake of the sphere. Since the vortex pair rotates clockwise or counterclockwise randomly around the streamwise axis of the sphere, the direction of the resultant force of lift and side forces rotates at random.
In this paper, instantaneous surface pressure distributions and wake velocities on a NACA0012 airfoil were measured to investigate vortical structures quasi-periodically emitted from a laminar separation bubble near airfoil stall. Reynolds number based on the airfoil chord length was 1.3×105. When the angle of attack is near stall angle, the flow around NACA0012 airfoil oscillates between an attached flow with the laminar separation bubble formed near the leading-edge and a largely separated flow. Strong streamwise velocity disturbances with opposite phases were observed between lower side and upper side of airfoil trailing edge. Flow visualization and pressure measurement showed large scale vortical structures are emitted from laminar separation bubble at the instances when the bubble is broken down and when it is reformed. Results indicated that velocity disturbances observed at wake region have strong relations with vortices emitted from the laminar separation bubble.
In this study, the authors conduced ten firings to investigate a hysteresis characteristics in Axial-Injection End-Burning hybrid rockets under throttling operation. Oxidizer mass flow rate and chamber pressure were throttled by two methods, actuating valves in a fluid circuit consisting of two oxidizer supply lines and a motor controlling. Chamber pressure and oxidizer mass flow rate were measured during each firing. The results show that two types of hysteresis characteristics were observed when throttling operation is repeated. One is a hysteresis with respect to increase and decrease of the oxidizer mass flow rate. Another is a hysteresis for the cycle. It is considered that the former hysteresis has the influence of a chamber pressure response time. In addition, the latter hysteresis is not necessarily observed even in the same chamber pressure region.
Micro vibrations and heat generated by reaction wheels and a refrigerator cause a problem in observing stars or galaxies with high pointing accuracy. Several methods have been proposed for reducing the influence of the vibration. However, these methods are difficult to cut low frequency band off passively. In addition, thermal strain on a primary mirror decreases the pointing accuracy when the heat transmits to a mission part through conventional vibration isolator. A vibration isolator using the flux pinning effect is proposed for solving the problem in this paper. The pinning force can maintain the relative distance and attitude between a type-II superconductor and a magnet without control. The spring and damping characteristics of the pinning force is applied to suppress the vibration effect. A mission part and bus part is not connected mechanically but electromagnetically, therefore it is expected that the influence of the heat transfer can decrease. Results of numerical analysis and experiment are given for discussing the feasibility of the proposed mechanism.