Koju Hiraki

Counter-Rotating Type Tidal-Stream Power Unit Playing Favorable Features in Various Ocean Circumstances

The authors have invented the unique counter-rotating type tidal-stream power unit, which is composed of the tandem propellers and the double rotational armature type peculiar generator without the traditional stator. In the unit of the downstream type, the front and the rear propellers counter-drive the outer and the inner armatures of the peculiar generator respectively, in keeping the rotational torque counter-balanced between both propellers/armatures.This paper discusses and verifies experimentally the almighty features of the power unit. The axial force acting on the pillar increases naturally with the increase of not only the stream velocity but also the drag of the tandem propellers. Besides, the tandem propellers bring the symmetrical vertical force from side to side though the single propeller brings the force in one direction. The counter-balanced torque makes it possible to moor the power unit with only one cable/wire/rope, and the behavior of the submerged unit was confirmed experimentally.Copyright

Aerodynamic Ground Effect for Transonic Projectiles

The aerodynamic characteristics of an object flying in close proximity to a solid surface are altered by the presence of this boundary. This phenomenon, known as aerodynamic ground effect, has been thoroughly investigated in the subsonic flow regime [1], but to the best of the authors’ knowledge, high-speed applications of this effect with objects flying at transonic speeds have only been briefly considered on a theoretical basis [2]. Furthermore, most of the ground effect literature concentrates on lift-generating objects such as wings and/or aircraft where the presence of the ground alters an existing but inherently asymmetric pressure distribution. A non-lifting body - a projectile - is arguably a simpler case as the ground effect would immediately become obvious through an asymmetry in the originally symmetric pressure distribution. This was clearly shown in an earlier study on projectiles at medium supersonic speeds (M = 2.4). In this case, the pressure distribution around the projectile remained unaffected by the presence of the ground until the reflected bow shock impinged on the projectile. From this point onward, the pressure distribution was drastically changed and led to a non-negligible lift force, an associated pitching moment and a change of the drag (the latter influence was primarily due to a change of base pressure). As soon as the reflected shock did no longer impinge on the projectile or the near wake, the pressure distribution was not affected and the aerodynamic ground effect became negligible and eventually non-existent. For the investigated projectile at M = 2.4 this occurred for clearances h/d > 1, where h is the clearance between the projectile and the ground and d is the projectile diameter. Experiments and numerical simulations established the trends for lift, drag and aerodynamic moment [3].

Particle Damping with Granular Materials for Multi Degree of Freedom System

A particle damper consists of a bed of granular materials moving in cavities within a multi degree-of-freedom (MDOF) structure. This paper deals with the damping effects on forced vibrations of a MDOF structure provided with the vertical particle dampers. In the analysis, the particle bed is assumed to be a single mass, and the collisions between the granules and the cavities are completely inelastic, i.e., all energy dissipation mec hanisms are wrapped into zero coefficient of restitution. To predict the particle damping effect, equations of motion are developed in terms of equivalent single degree-of-freedom (SDOF) system and damper mass with use made of modal approach. In this report, the periodic vibration model comprising sustained contact on or separation of the damper mass from vibrating structure is developed. A digital model is also formulated to simulate the damped motion of the physical system, taking account of all vibration modes. Numerical and experimental studies are made of the damping performance of plural dampers located at selected positions throughout a 3MDOF system. The experimental results confirm numerical prediction that collision between granul es and structures is completely inelastic as the contributi ng mechanism of damping in the vertical vibration. It is found that partic le dampers with properly selected mass ratios and clearances effectively suppress the resonance peaks over a wide frequency range.

Aerodynamic characteristics of compulsively-inflated paraglider for Mars exploration

A compulsively-inflated parafoil was proposed as a favorable way to explore Martian atmosphere. First, the estimation was made on the lift-to-drag ratio, and it revealed that the elimination of the air inlet had an comparable effect on gliding performance to the increased aspect ratio from 3 to 6. Then, the force measurements in a wind tunnel was carried out to validate the effect of the elimination of the air inlet on the gliding ratio. Two kinds of models with the rigid wing and the flexible wing were used, and the results showed that the elimination of the air inlet contributed much to improve the gliding performance. Next, the effect of the internal pressure of the cells on the aerodynamic characteristics were also investigated. The force measurements were made on the flexible wing models in the wind tunnel. As a result, the tendancy was found that the increasing the inner pressure improved the gliding performance.

Balloon-Drop Test for Evaluation of Guiding Performance of Small-Parafoil Flight System

[Abstract] In order to evaluate the terminal-phase guiding performance towards a target of the flight system with a small-size parafoil, the free-flight-test technique was established using a small balloon filled with helium gas. The whole parafoil-flight system weighed approximately 3.2kg. A meteorological balloon was employed to lift up the flight system above the ground. It has a typical diameter of 2.4m, and has the capability to lift up a payload of 8kg, which largely depends on the amount of helium gas inside and a wind condition. The separation of the flight system was made by a wireless command from the ground. The onboard micro-processor unit calculated the angle between the velocity vector and the target vector which indicated the direction from the present position toward the predetermined target. This angle determined which control line should be manipulated. When it was approaching to the target and the height was enough, the manipulated length was determined accordingly to the distance to the target. If it was escaping from the target, the computer attempted to turn the parafoil as sharply as possible. If the height was not sufficient enough to turn the parafoil, the computer directed flare to reduce its speed. The automatic-flight test was conducted from the height of over 100m, and the automatic control was activated soon after the deployment of the parafoil was confirmed. Although the automatic guidance functioned properly as was designed, it flied away from the target once it turned to the target direction. This was largely due to the poor turning response of the parafoil to the left control-line manipulation. After the test the center-of-gravity position of the suspended box was found located slightly to the right, and this might have caused the poor characteristics of the left turn of the parafoil.

Flow instability induced by spiked bodies

In order to investigate the unsteady flow field around a spiked body in supersonic flow, time-resolved color schlieren visualization was applied using a high-speed video camera which could take up to 1 000 000 frames per second at full frame resolution. Conically and spherically tipped spikes of six different lengths could be attached at the center of the model and their effect on the flow unsteadiness was visually observed. The obtained images revealed in great detail the interaction between the incoming free stream flow and the high-pressure region near the model base, which could make its presence known upstream at the tip of the spike by means of displacing the boundary layer on the spike and subsequently inducing a large-scale instability of the flow.

Flow visualization around a rotating body in a wind tunnel

The rotational behavior of capsule-shaped models is investigated in the transonic wind tunnel of JAXA. A special support is developed to allow the model to rotate around the pitch, yaw and roll axes. This 3-DOF free rotational mounting apparatus achieves the least frictional torque from the support and the instruments. Two types of capsule models are prepared, one is drag type (SPH model) and the other is lift type (HTV-R model). The developed mounting apparatus is used in the wind tunnel tests with these capsule models. In a flow of Mach 0.9, the SPH model exhibits oscillations in pitch and yaw, and it rolls half a turn during the test. Similarly, the HTV-R model exhibits pitch and yaw oscillations in a flow of Mach 0.5. Moreover, it rolls multiple times during the test. In order to investigate the flow field around the capsule, the combined technique of color schlieren and surface tufts is applied. This visualization clearly shows the flow reattachment on the back surface of a capsule, which is suspected to induce the rapid rolling motion.

Ground Effect of Transonic and Supersonic Projectiles: Influence of Mach Number and Ground Clearance

The ground effect experienced by transonic/supersonic projectiles of diameter d flying at a distance h above a solid surface is investigated in this study by numerical simulation and wind tunnel experiments. It is verified that the presence of the ground alters the drag force and introduces a lift force and a pitching moment that do not exist when the bullet is in free flight far away from any solid surface. The results indicate that, for the given projectile, the lift force can change sign depending on the non-dimensional ground clearance h/d, but that for Mach numbers of 1.5 and higher, only a positive lift force is observed. Three different force/moment configurations for a projectile in ground effect can be distinguished: a suction force together with a pitch nose-up moment is typically seen at low ground clearances, a lift force and a pitch nose-up moment occur at intermediate values of h/d, and at large ground clearances this changes into a lift force/pitch nose-down moment combination.

Numerical Simulation and Experiments on the Ground Effect of Transonic Projectiles

When a projectile (diameter d, projectile Mach number M p ) flies at transonic speed in the vicinity of a solid surface (separation distance h), its surroundingflowfield undergoes considerable changes

Multi Degree-of-Freedom Technique for Dynamic Characteristics of Atmospheric Entry Capsule

The dynamic characteristics of a model of an atmospheric entry vehicle were investigated in a transonic wind tunnel. The models were allowed to rotate freely in the tunnel and two kinds of the test devices were prepared with different degrees of freedom. The 1-DOF device allowed the model to rotate only around the pitch axis, and the 3-DOF device allowed the model to rotate around the pitch, yaw and roll axes. The dynamic tests were carried out in flows of Mach 0.5 and 0.9. Both the 1-DOF and 3-DOF tests gave the similar results on the damping properties in pitch and yaw axes. The rapid rolling behavior was also recognized in the 3-DOF test which was seemingly related to the energy conversion from the pitch to yaw axis.