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In recent years, many plans have arisen for building constellations in low Earth orbit, some of which have already provided commercial services. The number of satellites that form these constellations will exceed $$10^4$$ 10 4 , and a number of these could become space debris due to accidental failure, leading to serious problems for human activities in space. Laser ablation-induced propulsion achieved by remote irradiation from the service spacecraft has been proposed as one effective method to remove such space debris and it involves effectively generating a propulsion impulse from a laser. Because most of the high-power lasers currently utilized in space applications are Nd:YAG lasers, in this study, we evaluate the characteristics of second harmonic generation (SHG), which can be generated using nonlinear crystals. The momentum coupling coefficient may be dependent on the irradiated laser wave length which has a significant difference in the ablation process such as plasma generation and heating. These effects have been investigated using an impulse measurement instrumental setup with a KD*P nonlinear crystal. As an efficient impulse generation method, the irradiation of the fundamental and SHG beams of the Nd:YAG laser at the same point was found to be more effective than the fundamental beam with the same total energy. SHG does not require additional power-consuming equipment such as additional exciters or amplifiers, but requite temperature-controlled nonlinear crystals, and impulses can be increased with minimal additional power. It turned out to be advantageous for space applications, which will require minimum power operation . Additionally, we found an additional nonlinear laser-induced impulses through the interaction of the beams.

This paper discusses that the unsteady aerodynamic forces caused by vehicle motion in road input have a large influence on the vehicle behaviors. In the present coupling analysis, unsteady aerodynamic forces were obtained by using the response functions for vehicle motion derived in our previous study. By attaching a protrusion on the roof which only measures about 3% of the vehicle height, pitch and heave motions are affected significantly by the change of unsteady aerodynamic effects. Due to the coupling of the unsteady aerodynamic forces and vehicle suspension responses, the vehicle behavior depends on the road input frequency. The pitch motion is enhanced in low-frequency road input (1.6 Hz). Meanwhile, the heave motion is suppressed over 1.2 Hz. Such vehicle behaviors were verified in actual running tests, and were consistent to the subjective evaluation of the drivers feeling. Comparing the unsteady aerodynamic coefficients to the equivalent vehicle specification factors, the aerodynamic inertia caused by pitch motion contributes to 39% of the rear vehicle mass inertia effect in pitch motion. This aerodynamic effect is important in designing suspension specifications of vehicles. These results show that the vehicle stability can be affected even by small detail in vehicle shape due to the unsteady aerodynamic forces.

In this study, the characteristics of side-projected planar shock waves propagating through grid turbulence were investigated using a counter-driver shock tube. At the location of visualization of the shock–turbulence interactions, the range of the shock Mach numbers was $$M_{\mathrm {s}} = 1.01{-}1.15$$ and the representative values of the turbulent Mach numbers $$\widetilde{M_{\mathrm {t}}}$$ were $$ 0.005{-}0.014$$ . The interaction length between the shock wave and grid turbulence was in the range approximately from $$-50$$ to $$300$$ times the integral length scale of the turbulence. For the interaction involving the strongest turbulence with $$\widetilde{M_{\mathrm {t}}} = 0.014$$ , the weakest planar shock wave with $$M_{\mathrm {s}} = 1.01$$ was largely deformed and could not be detected on the projected shadowgraph and schlieren images. The density changes on the grayscale in the projected images of the shock waves weakened and expanded multidimensionally. This undetectable profile of the shock wave could indicate that the shock wave locally lost discontinuous property change profile. The shock waves with $$M_{\mathrm {s}} \ge 1.05$$ did not show the undetectable profile in the projected image. In these cases, the projected thickness of the shock wave increased with an increase in the interaction length. The increase in the projected thickness became larger as the turbulent Mach number increased, and the shock Mach number decreased.

Diaphragm opening processes with a finite time affect the generated shock waves and shock tube flows. This effect is dominant in short and low-pressure shock tubes. In this research, a high-speed visualization of the opening process of a cellophane diaphragm, which is a suitable material for operation in a low-pressure shock tube, is conducted. The self-shaping opening morphology of cellophane diaphragms is observed at the initial stage with an approximate crack propagation speed of 807 m/s, which is greater than the speed of sound of the test gas. The initial opening motion of the petals, which are formed by cracks, is successfully modeled by the rotational equation of motion. Through analysis of the generation and coalescence of compression waves, whose characteristic velocities are calculated from the measured pressure histories, the shock formation distance is reasonably estimated. From the visualized diaphragm opening processes and analysis of coalescence of compression waves, we revealed that the initial stage of the diaphragm opening process plays a dominant role in shock wave formation rather than the entire process.

A high-pressure field is generated in a circular tube by introducing an unsteady jet from its open end. The head of this jet acts as a piston, driving compression waves ahead of it. The peak value of the induced overpressure is evaluated as a solution of a Riemann problem, wherein the jet head is equivalent to a piston head. The jet head of the driver gas, with a filling pressure of 400 kPa, is equivalent to a piston head moving at 160 m/s. This high-pressure generation scheme through the “piston effect” is useful for industrial applications, including filter cleaning in dust collectors, and as an interesting example of unsteady, compressible fluid dynamics.

In this study, an electrostatic thruster was newly developed, in which a diverging magnetic field with a cusp around a cathode was applied by using two solenoid coils. The effects of the magnetic field strength on thrust performance under a similar applied-magnetic-field configuration were investigated. Because of its light weight and lower price, argon was used as a propellant. By increasing the magnetic field strength, the thrust efficiency was improved owing to the suppression of the discharge current while an almost constant thrust was maintained. A specific impulse of 3800 s with thrust efficiency greater than 30% was obtained; the corresponding mass-averaged exhaust velocity exceeded the value by full-potential electrostatic acceleration of singly charged ions. From the thrust performance, ion beam current, and ion energy distribution function that were experimentally measured under the representative operation condition, 32% of the ion beam current and 43% of the total thrust were evaluated as the contribution of doubly charged ions.

Research interest has been growing in recent years in supersonic transport, particularly supersonic propulsion systems. A key component of a commonly studied propulsion system, ramjets, is the air intake. For supersonic propulsion systems a major factor in the overall efficiency is the intake pressure recovery. This refers to the ratio of the average total pressure after the intake to that of the freestream. One phenomenon that can have a large effect on this performance index is flow separation at the inlet. The aim of this work is to examine how pulsed laser energy deposition can be used to improve pressure recovery performance by reducing flow separation at the inlet. This research examines the effects of pulsed laser energy deposition upstream of an intake with an axisymmetric centrebody in a Mach 1.92 indraft wind tunnel. Laser frequency was varied between 1 and 60 kHz with an energy per pulse of 5.6 mJ. Schlieren photography was used to examine the fundamental fluid dynamics while total and static pressure downstream of the intake diffuser were measured to examine the resulting effect on the performance. Schlieren imaging shows that the interaction between the laser generated thermal bubble and the leading edge shock produced by the centrebody results in a significant reduction in separation along the intake cone. Analysis of the schlieren results and the pressure results in tandem illustrate that the average separation location along the length of the centrebody directly correlates to the pressure recovery observed in the intake. At the optimal laser frequency, found for this Mach number to be 10 kHz, the pressure recovery is found to increase by up to 4.7%. When the laser power added to the system is considered, this results in an overall increase in propulsive power of 2.47%. Keywords: Laser energy deposition, Supersonic, Flow dynamics, Intakes, Flow separation

The effects of hollow cathode position on thrust performances of a diverging-magnetic-field electrostatic thruster, which had a diverging magnetic field between a ring anode and a hollow cathode, were investigated. The hollow cathode was located in either a central or an external configuration. Based on the discharge current and direct thrust measurements, electrostatic and magnetic hybrid plasma acceleration was observed in the central cathode operation and even in the external cathode operation especially when magnetic field strength was less than 315 mT. However, the operating points were different: low discharge voltage×large discharge current in the central cathode operation, and high discharge voltage×small discharge current in the external cathode operation. This difference comes from electron-Hall parameter; the estimated electron-Hall parameter was about 50 in the central cathode operation, which was much lower than that of 3000 in the external cathode operation. By combining electromagnetic acceleration, the generated thrust can increase even with lower electron-Hall parameter operation. As a result, similar thrust performances were achieved in both cathode positions. Because of the hybrid plasma acceleration method, thrust density reached 50 N/m^2, which is comparable or even higher than that of the conventional electrostatic thrusters., Published Online:14 Nov 2019

This study experimentally investigated the effects of repetitive laser energy deposition using a supersonic intake model with a central conical compression surface and a 19×19 mm^2 cross-sectional duct in a Mach 1.92 indraft wind tunnel: especially in the subcritical and buzz modes. A single-pulse energy deposition was observed to suppress the flow separation at the compression surface by “sweeping” a shock wave system with the thermal bubble generated by the energy deposition. The duration of the sweeping effect was approximately 160 μs in the subcritical mode and 100–420 μs in the buzz mode. Furthermore, repetitive deposition of laser pulse energies was observed to moderate instabilities in both modes, and it increased the pressure recovery by as much as 8%; also, the occurrence of buzz was delayed, thereby widening the stable, subcritical regime. In both modes, there was a threshold value for the laser pulse repetition frequency fL, thd, which corresponded to the duration of the sweeping effect (e.g., fthd=6 kHz in subcritical mode). Below this frequency, the increase in the pressure was proportional to the repetition frequency; whereas above fthd, the effect per single pulse was reduced., Published Online:26 Oct 2019

In this study, a layer of cellophane, subjected to an air-pressure difference, was ruptured using a piercing needle. Accordingly, petal-like fragmentation was observed in the layer via high-speed imaging. Two types of crack-propagation regimes were subsequently observed experimentally. If a tensile stress lower than 20.6 MPa acted on the cellophane diaphragm, a single crack was generated, whose propagation speed was lower than that under higher-stress conditions. For tensile stresses greater than 23.7 MPa, the crack-propagation speed remained constant at approximately 0.86 km/s, even after altering the device size, pressure, and humidity on the low-pressure side. The number of cracks equidistant from the piercing point was expressed as a linear function of the tensile stress acting on the diaphragm.

An advanced background-oriented schlieren (BOS) method, named as the speckle beam-oriented schlieren technique, was newly developed to measure the distribution of refraction angles in transparent media. A speckle pattern is generated by passing a coherent laser beam through a holographic diffuser, a pinhole, and a lens, generating a collimated background image that is projected directly onto the image sensors. Since the intensity of the background image is maintained at a high level, this method is, in principle, useful for diagnosing fast and/or low signal-to-noise phenomena, such as high-temperature gasses with radiation emission. Moreover, by splitting the background beam into two imaging paths with different focal lengths, the refraction angles can be measured for a schlieren object with uncertain location, and the depth position of the refraction angles can be resolved. This technique was demonstrated by measuring the refraction angle and the depth position distribution in a sonic jet with different injected locations., Published: 08 January 2021 ファイル公開：2022/01/08

A system accompanying a shock wave that exchanges energy and/or momentum with the external environment can enhance the strength of the shock wave or generate a thrust.

In a steady, two-dimensional flow (Chap. 7) and an unsteady, one-dimensional flow (Chap. 8), the flow velocity and thermodynamic properties are expressed as functions of two independent variables by applying Euler’s equation. In those cases, the differential equation is hyperbolic and can be solved by sequentially solving interactions between characteristics with an appropriate boundary or initial conditions. This method, called the method of characteristics, is useful in such simple problems and will be dealt with in this chapter.

By utilizing the characteristics of compressible flow, we can generate high-speed flow, high-pressure, and/or high-temperature states. The design of rocket engine nozzle and that of air intake for an aircraft engine need to be conducted based on the principle of compressible fluid dynamics. In this chapter, we will illustrate representative examples of such devices.

The static characteristics of a gas are described by thermodynamics. Thermodynamics starts with the formulation of experimentally observed phenomena, following which the formulae are related as results of the behavior of a group of gas particles. Compressible fluid dynamics is also based on thermodynamics. The local motion of gas particles is decomposed into that of the center of gravity and that around it. Flow is the motion of the center of gravity, while flow velocity is the velocity of such motion. Pressure, temperature, and other thermodynamic properties are determined from the motion around the center of gravity, that is, the thermal motion. The flow changes when it experiences a force and/or exchange energy. In this chapter, we will derive the relations between gas-particle motions and thermodynamic properties.

Gas and liquid are termed fluid because they can flow with changes to their shape. Most flows occurring in our daily life are not significantly influenced by compressibility. However, in high-speed flows and/or when velocity or pressure rapidly varies, compressibility is significant. The compression and expansion of a fluid, which correspond to variations of its density, lead to the propagation of pressure waves. In compressible fluid dynamics, we study the relationship between pressure-wave propagation and its impact on flow properties. In this chapter, we will learn the basic concepts of pressure-wave propagation through several examples.

Why do we make a small mouth when blowing out candle flames or playing with a mouth whistle? The same question can be applied to a shower, spray, or rocket nozzle, in which high-speed flow is generated by varying the cross-sectional area of a flow passage.

Real flows are usually three dimensional. However, many important flow characteristics appear even in two-dimensional flows. In this chapter, we deal with steady state, two dimensional, supersonic flows. In such flows, compression/shock and expansion waves appear.

The thrust characteristics of electrostatic-magnetic hybrid acceleration have been obtained in the central–cathode electrostatic thruster (CC-EST), which has a diverging magnetic nozzle and injects propellant through an annular slit on the inner surface of a ring anode. The electrostatic and electromagnetic contributions are evaluated by fitting experimentally measured thrust characteristics to formulae derived for steady-state, quasi-neutral plasma flows. The thrust is composed of two terms: one corresponds to the electrostatic acceleration interfaced by the magnetized electrons, the other to electromagnetic acceleration yielding a kinetic energy of swirl motion that is converted to that of axial motion in the diverging magnetic nozzle. The thrust performances of three thrusters having an axisymmetric shape with a hollow cathode on the centre axis and a diverging, applied magnetic field are examined. While only an electromagnetic thrust component is obtained in the conventional applied-field magneto-plasma-dynamics thruster, the other two, CC–ESTs, exhibit comparable, electrostatic and electromagnetic thrust components, and thus improve thrust performance., ファイル公開：2020-11-01

An experimental study was conducted to examine the effects of repetitive energy deposition on the supersonic flow characteristics over a duct system with a central conical compression surface. Boun...

Published online28 October 2018

We evaluate the energy conversion efficiency of an electrical exploding foil accelerator that accelerates a thin dielectric foil (called the flyer) to more than 1 km/s, which is propelled by electrically exploded bridge material. The effective flyer mass ejected from the accelerator is estimated by impulse measurements obtained using a gravity pendulum as well as by time-resolving flyer velocity measurements obtained using a photonic Doppler velocimetry system. For two different bridge sizes (0.2 and 0.4 mm), the flyer velocity and impulse increase with the input energy at the bridge section. The maximum flyer velocity and impulse, that is, 4.0 km/s and 67 µN s, respectively, are obtained by supplying 0.33 J of input energy. Upon increasing the input energy, the effective flyer mass also increases and exceeds the designed-bridge mass for both bridge sizes. The energy conversion efficiency from input electrical energy to flyer kinetic energy is calculated based on the effective flyer mass, velocity, and input energy. Both bridge sizes show comparable efficiencies: 27% and 30% for 0.2 and 0.4 mm bridges, respectively. The efficiency increases with increasing specific input energy at least up to 15 MJ/kg for the 0.4 mm bridge, whereas the efficiency of the 0.2 mm bridge above 30 MJ/kg decreases. This is owing to the excessively high input energy density in the 0.2 mm bridge, which causes the effective flyer mass to increase by including surrounding materials. These results indicate that the specific input energy should be optimized for obtaining maximum efficiency.

The effects of a radio-frequency (RF) power on the ion generation and electrostatic acceleration in a helicon electrostatic thruster were investigated with a constant discharge voltage of 300 V using argon as the working gas at a flow rate either of 0.5 Aeq (Ampere equivalent) or 1.0 Aeq. A RF power that was even smaller than a direct-current (DC) discharge power enhanced the ionization of the working gas, thereby both the ion beam current and energy were increased. However, an excessively high RF power input resulted in their saturation, leading to an unfavorable increase in an ionization cost with doubly charged ion production being accompanied. From the tradeoff between the ion production by the RF power and the electrostatic acceleration made by the direct current discharge power, the thrust efficiency has a maximum value at an optimal RF to DC discharge power ratio of 0.6 – 1.0.

The ablation spot area and impulse characteristics of various polymers were experimentally investigated against burst irradiation of Nd: YLF laser pulses with a pulse repetition frequency of 1 kHz, wavelength of 1047 nm, temporal pulse width of 10 ns, and single-pulse fluence of 6.1 J/cm 2 to 17.1 J/cm 2 . The dependences of ablation area on the pulse energy from 0.72 to 7.48 mJ and the number of pulses from 10 pulses to 1000 pulses were investigated. In order to characterize their impulse performance as a function of fluence, which should not depend on ablation material, an effective ablation spot area was defined as that obtained against aluminum, 1050 A, as the reference material. An impulse that resulted from a single burst of 200 pulses was measured with a torsion-type impulse stand. Various impulse dependences on the fluence, which were not readily predicted from the optical properties of the material without ablation, were obtained. By fitting the experimentally measured impulse performance to Phipps and Sinko's model in the vapor regime, the effective absorption coefficient with laser ablation was evaluated, thereby resulting in three to six orders of magnitude larger than that without ablation. Among the polymers examined using polytetrafluoroethylene (PTFE) as the best volume absorbers, the highest momentum coupling coefficient of 66 μNs/J was obtained with an effective absorption coefficient more than six times smaller than that of the other polymers.

In this paper, applied-field magnetoplasmadynamic thrusters were operated with a discharge current lower than 20 A and an argon propellant flow rate under 4.0 mg/s by using a thermionic electron emission-type hollow cathode. Both axisymmetric and rectangular channel configurations were examined. The characteristic value of the ratio of the applied-field electromagnetic thrust to the self-field electromagnetic thrust, which was a maximum of approximately 50 in previous research, increased to more than 800 in this study, whereas the specific power was in approximately the same range as in previous studies. The trends of thrust and discharge voltage were explained by referring to the “swirl acceleration” model. Such operation could be useful for near-future high-power electric propulsion applications.

The interaction of a planar shock wave with grid generated turbulence finds that at first the turbulence increases the projection thickness of the shock and then at higher intensities fragments the shock.

We investigate interactions between a planar shock wave and homogeneous isotropic turbulence by employing direct numerical simulations. The shock wave at the shock Mach numbers 1.5 and 1.1 propagates through the turbulence without mean velocity. The interactions are studied for the statistical properties of turbulence and pressure increase of the shock wave. Fluid compression due to the shock wave causes velocity fluctuations to increase. This influence is more significant for the velocity component in the shock wave normal direction. The fluid compression in the shock wave normal direction also changes the spatial distribution of the vortical structures, resulting in the amplification of enstrophy. The characteristic length scales of turbulence, such as Taylor microscale, decrease by the interaction. The shock wave propagation in turbulence causes the fluctuation of pressure increase across the shock wave. We also show that the fluctuation of the pressure increase becomes stronger for the larger shock Mach number case.

The shock wave generation in a closed tube using the high-speed jet injection was experimentally investigated. The unsteady behavior of the gas compression by the jet injection was visualized by the schlieren method, and the temporal history of the overpressure was measured by the pressure transducers which flush mounted in a tube. The injected jet can compress the gas in the tube, and the shock wave generation was observed by schlieren visualization and the pressure measurement. The characteristics of the jet and the relation between the cross-sectional area of the jet and the tube had an important role for the shock wave generation.

The shock wave induced by the supersonic flight model and the turbulent flow induced by the free-falling grid were interacted using the time-synchronized launch operating system of the aero-ballistic range. The time-synchronized launch system was successfully done. However, the clear effect of the turbulence on the pressure signature has not been obtained because of the mismatch of the shock Mach number and the induced turbulent Mach number. D-SEND#2, which is similar to the aircraft, scale model was launched with high-precise attitude control by the rectangular-bore core aero-ballistic range, and the near-field pressure around the model was measured. The obtained pressure signature was compared with the numerical simulations, and these pressure signatures agreed well except for the pressure wave originating from the main wing. From these experiments, the experimental capability of the aero-ballistic range was extended.