Compressible fluid dynamics and shock waves

This book offers comprehensive coverage of compressible flow phenomena and their applications, and is intended for undergraduate/graduate students, practicing professionals, and researchers interested in the topic. Thanks to the clear explanations provided of a wide range of basic principles, the equations and formulas presented here can be understood with only a basic grasp of mathematics. The book particularly focuses on shock waves, offering a unique approach to the derivation of shock wave relations from conservation relations in fluids together with a contact surface, slip line or surface; in addition, the thrust of a rocket engine and that of an air-breathing engine are also formulated. Furthermore, the book covers important fundamentals of various aspects of physical fluid dynamics and engineering, including one-dimensional unsteady flows, and two-dimensional flows, in which oblique shock waves and Prandtl-Meyer expansion can be observed

Laser Propulsion Standardization Issues

It is a relevant issue in the research on laser propulsion that experimental results are treated seriously and that meaningful scientific comparison is possible between groups using different equipment and measurement techniques. However, critical aspects of experimental measurements are sparsely addressed in the literature. In addition, few studies so far have the benefit of independent confirmation by other laser propulsion groups. In this paper, we recommend several approaches towards standardization of published laser propulsion experiments. Such standards are particularly important for the measurement of laser ablation pulse energy, laser spot area, imparted impulse or thrust, and mass removal during ablation. Related examples are presented from experiences of an actual scientific cooperation between NU and DLR. On the basis of a given standardization, researchers may better understand and contribute their findings more clearly in the future, and compare those findings confidently with those already published in the laser propulsion literature. Relevant ISO standards are analyzed, and revised formats are recommended for application to laser propulsion studies

Frequency characteristics and dynamical behaviors of self-modulation in vertical-cavity surface-emitting lasers

Abstract The frequency characteristics of a VCSEL with a quarter-wave plate (QWP) and an external reflector are investigated with the translation matrix of the vectorial field. Two series of eigenmode with a shift of half the free spectrum range are linearly polarized, respectively, along the neutral axes of QWP. We also numerically explore the polarization self-modulation phenomenon by using a vectorial laser equation and considering the inhomogeneous broadening of the gain medium. If the external cavity is so short that the shift is bigger than the homogeneous broadening, two stable longitudinal modes oscillate, respectively, on the neutral axes of QWP because they consume different carriers. With a long external cavity, the competition of the modes for the common carriers causes the intensity fluctuation of the modes with a period of one round-trip time of the external cavity

CO2 Laser Ablation Area Scaling and Redeposition on Flat Polyoxymethylene Targets

One of the remaining subjects of interest for laser ablation propulsion study is whether special benefits or challenges exist when applying a particularly large or small laser spot area to a target. This subject is of high importance for topics including laser removal of space debris, micropropulsion, and design of laser propulsion vehicles. Analysis of spot area-dependent effects is complex since ablation phenomena differ between atmosphere and vacuum conditions. Progress has also been impeded by the difficulty of setting control parameters (particularly fluence) constant while the spot area is adjusted. It is virtually impossible for one group to address small- and large-area effects using a single high-power laser system. Recent collaborative experiments using 100-J class and 10-J class CO2 lasers have advanced the understanding of laser propulsion area scaling. Experiments were conducted below the threshold for plasma formation. The dependence of various laser propulsion parameters on the laser spot area has been investigated within areas covering approximately 0.5-50 cm2 on the target

Update on CO2 Laser Ablation of Polyoxymethylene

Polyoxymethylene (POM) propellants have been studied since the 1970's, and perhaps represent the most promising match of a propellant to the CO2 laser for laser propulsion studies. Applications range from ground-launch of laser propulsion vehicles at atmospheric pressure to spacebased laser ablation propulsion microthrusters. In this paper we broadly update the state of understanding of CO2 laser ablation of POM based on new experiments conducted in Japan and Germany, with a focus on the basic physics of ablation of flat POM targets. Measurements using 10 J-class and 100 J-class lasers are compared to previous literature results for ablation of POM. Emphasis is placed on the influence of control parameters on ablation, especially the incident laser fluence and ambient pressure. New results highlight the influence of the ambient pressure on ablation physics from the vaporization threshold to the plasma regime, and clarify the role of the fluence in determining ablation behavior in air and vacuum environments. Imparted impulse and ablated mass were measured at the target using piezoelectric force sensors, impulse pendulums, and scientific balances. The new experimental investigations cover orders of magnitude in fluence (10E-2 to 10E3 J/cm2) and ambient pressure (10E-3 to 10E5 Pa)