Recent results of our quasi-one-dimensional numerical simulation for the performance of an MHD accelerator under chemical and thermal equilibrium conditions are presented. For the present study, the working gas was changed to air-plasma from inert gas of argon plasma, which was studied previously to actualize the application of an MHD accelerator propulsion system. The numerical simulation confirmed the performance of MHD accelerators with segmented Faraday, Hall, and diagonal electrode connections. In order to solve the set of differential equations of magnetohydrodynamics, the MacCormack scheme is employed. The results show several parameters of MHD accelerator performance with air-plasma working gas. The calculation results are compared with previous results to clarify the effect of the difference in working gases.
The laser-driven in-tube accelerator is a unique laser propulsion device, in which a projectile is accelerated in a launch tube with the power of repetitive laser pulses. The impulse is enhanced owing to a confinement effect, and the type and fill pressure of the propellant gas are tunable to the thrust performance. We have conducted operation experiments using a 25-mm-bore launch tube with a laser energy from 2.4 to 3.5 J/pulse at a laser-pulse repetition frequency of up to 70 Hz. The 3-gram projectile has a centerbody; as a parabolic mirror, its base focuses an incident laser beam to a spot on the center axis. Three monoatomic gases (i.e., argon, krypton and xenon) are examined as the propellant. The momentum coupling coefficient is measured from a condition that the time-averaged impulse is balanced with the gravitational force onto the projectile. The measured impulse characteristics are analyzed by referring to scaling relations that are obtained from dimensional analysis and pressure histories measured for the launch tube. In accordance with the dependence of the impulse generation period, the measured impulse is in inverse proportion to the propellant speed of sound. For a fill pressure up to about 100 kPa, the impulse sharply increases as the fill pressure increases, while it becomes saturated for higher pressures. Those performance characteristics are analyzed by estimating the overpressure level and the duration time of the impulse generated by the laser-driven blast wave.
As a feasible welding method in space, the authors previously proposed the space GHTA (Gas Hollow Tungsten Arc) welding process. However, space GHTA welding with a high-frequency device for arc start may cause electromagnetic noise problems for the computer equipment placed on the ISS (International Space Station). Therefore, in this report, welding experiments of space GHTA welding using aluminum alloy with a high-voltage DC device for arc start were carried out at the ISS orbital pressure, 10−5 Pa. It is clear from the experiments using a high-voltage DC device in a high-vacuum condition, that there is a shifting phenomenon in which the spark discharge shifts to either a glow discharge or an arc discharge when starting the arc. Welding projects in space need an arc discharge, so we investigated the effects of welding parameters on the arc formation ratio. As a result, space GHTA welding with a high-voltage DC device can be used for arc start when welding at the ISS orbital pressure.
The pulse detonation engine (PDE) is a propulsion system that generates thrust by repetitive detonation. Performance of the PDE, such as specific impulse, has gained much attention. However, the details of operational conditions related to performance have not been clarified. In this study, to investigate these issues, a hydrogen-air PDE was constructed and the effect of the purge process on multi-cycle operations was studied. The exhaust of the burned gas from the PDE was made by rarefaction waves so that it is expected not to create internal disturbance, such as spiral, in the tube so that the burned gas can be exhausted smoothly. A purge process using air was applied to assist the exhaust process in order to prevent failures in multi-cycle operations. The effects of the purge-volume fraction were investigated. As a result, when a purge-air gas filled the region including the spark plug, detonations occurred in a stable manner. Using these conditions, multi-cycle experiments ranging from 10 to 50 Hz were conducted and profiles of the thrust-wall pressure are discussed in detail.
An ultra-high vacuum diode laser welding system capable of performing a diode laser (DL) welding experiment at the International Space Station orbital pressure of 10−5 Pa was developed for investigating the effect of environmental pressure on DL welding phenomena. A DL welding experiment with a power density of about 100 kW/cm2 was conducted on 304 stainless steel at pressure levels between 105 Pa and 10−5 Pa. Although a laser-induced plasma plume was observed during welding at environmental pressures, 105 Pa and 103 Pa, no laser-induced plasma plume was found in the pressure range from 10 Pa to 10−5 Pa. The mode of the melting process (weld pool shape) under these experimental conditions was a keyhole-type melting mode or a transition-type melting mode at environmental pressures of 105 Pa and 103 Pa. However, the melting mode at pressures lower than 10 Pa changed to a heat-conduction-type melting mode. Although the penetration depth decreased when the environmental pressure was dropped to 103 Pa, it did not change at pressures lower than 10 Pa. We also determined that the prevention technology of metal vapor deposition on optical devices needs to be developed for DL welding technologies used in space.
In this paper, we propose an energy-based nonlinear control for a two-link flexible manipulator. As it is well known, the two-link flexible manipulator is an underactuated system that provides a challenge to control engineers and researchers. Recently, many new control theories and methods are being explored to design controllers for underactuated systems in order to control them more effectively. Among these methods, the energy-based control design method has gained much attention. This method can provide more physical insights in nonlinear control as well as provide a direct candidate for the Lyapunov function, which is very important for proving the stability of closed-loop systems. Especially, the port-controlled Hamiltonian system and generalized canonical transformation have some advantages over the modeling and control design of nonlinear systems. Therefore, we have chosen to study how to control the two-link flexible manipulator via generalized canonical transformation. Both simulation and experimental results are provided to demonstrate the effectiveness of the controllers.
This paper describes an optimal flow control method for compressible laminar/turbulent flows utilizing discrete unsteady aerodynamic sensitivity analysis methods. Unsteady aerodynamic sensitivity codes are developed using a direct differentiation method and an adjoint method, respectively, from two-dimensional unsteady compressible Navier-Stokes equations. Optimal flow controls are conducted by minimizing an unsteady objective function defined at an instant instead of integrating a response for a period of time. Unsteady sensitivity derivatives of the objective function are calculated by the unsteady sensitivity codes, and optimization is conducted utilizing a linear line search method at every physical time step. Several flow control examples of academic interest including circular cylinder vortex shedding control and airfoil shock buffet control show satisfactory results. The present active flow control method utilizing the unsteady sensitivity analysis is robust and problem independent compared to conventional active control methods.
The flow fields around a large apex angle, spiked blunt cone have been analyzed at a hypersonic Mach number through experiments in a free piston-driven shock tunnel, and the results are compared with that of a laminar 2-dimensional axisymmetric unsteady Navier-Stokes solver developed in-house. The model geometry is a 120° apex angle blunt cone equipped with two types of spikes — a disc-tipped spike and a conical-tipped spike. The ratio of total length of the spike to model base diameter was kept at 1.00. The free-stream Mach number and Reynolds number (per unit length) in the free piston shock tunnel were 6.99 and 2.46×106, respectively. After measuring the aerodynamic forces on this model, time resolved visualization of the flow was attempted using a high-speed video camera in order to understand the characteristic features of the high-speed flow over the spiked body and to check if any unsteadiness existed in the flow fields. The experimental results indicate slight shock oscillations near the edge of the cone model when it is equipped with a disc spike, and the shock oscillations appear to be more pronounced when the model is equipped with a conical spike. However, the pulsation flow mode is not observed in the experimental results. The numerical results agree with the experimental results on the unsteadiness of the flow fields. The experimental results obtained in this case also serve as a database for the in-house CFD code validation for such high-speed spiked body flows.
It is necessary to acquire the mechanical properties of advanced plain-weave fabrics as design parameters for application to ultra-light structures. In this study, biaxial tensile experiments of cruciform specimens with an open hole were conducted to evaluate the strength of coated plain-weave fabrics composed of specific high-tensile-strength fibers. Uniaxial tensile tests of strip specimens were also carried out to obtain the fundamental uniaxial properties of the fabrics. The results of these tensile tests show that the open-hole tensile strength of the fabrics under biaxial loading is approximately equal to the strength under uniaxial load irrespective of biaxial load ratio, and warp directional strip specimens exhibit higher strength than weft directional specimens in spite of the same density of yarn in both directions. There is a partial contribution of misalignment of the weft yarn in the membrane to the loss of strength in the weft direction. The results of observations by microscope and single-yarn tensile tests reveal that the strength difference in warp and weft directions is caused by the degradation of the weft yarn by heating in the polymer film coating process coupled with yarn misalignment.