Pressure Oscillations in Regenerative Liquid Propellant Guns

The present paper deals with pressure oscillations in regenerative liquid propellant guns (RLPG) presenting the state-of-the-art research into sources and control of these combustion instabilities. Pressure oscillations with amplitudes up to 50 % of mean pressure and frequencies up to 60 kHz or more are present in experimental data of RLPGs, especially at medium and large calibers. Amplitudes increase with the volumetric energy density of the liquid propellant and the mass flow rate during injection. Frequency analyses reveal that both acoustic modes and combustion noise are components of the recorded oscillations. Acoustic modes, in particular, have the potential to couple to resonant modes in near-field mechanical structures. A multi-phase, multi-dimensional model investigation at ARL indicates that pressure waves reflected from internal boundaries are amplified as they pass the combustion zone of the highly pressure-sensitive liquid propellant in a localized region near the injector. Experimental data lead to a similar understanding of the amplification of pressure oscillations in RLPGs. Experiments in the United States and in Germany confirm that the pressure oscillations can be mitigated by altering the combustion characteristics of the liquid propellant. Also, techniques that more effectively disperse the liquid propellant jet and thereby decrease the local accumulation of liquid propellant, may reduce pressure oscillations as shown in 30-mm RLPG experiments. Further, energy-absorb-ing chamber walls or liners can serve as broad-band filters and have been shown experimentally to be effective in reducing the amplitude of all frequencies. In addition, physical methods such as cavities and baffles reduce significantly specific acoustic frequencies of the oscillations.     

The nonlinear behavior of surfactant-activated electrorheological suspensions

The nonlinear electrorheological (ER) behavior of nonionic surfactant-activated ER suspensions is investigated. The influence of three nonionic surfactants (Brij®30, GMO, and GTO) on the electrorheological (ER) response of various alumina/silicone oil suspensions shows similar behavior. The prevalent feature common to all formulations is that the yield stress, τ₀, initially increases with surfactant concentration, passes through a maximum, then decreases with surfactant concentration. The nonlinear behavior observed at large surfactant concentrations (i.e., τ₀≈Eⁿ;, where n>2) arises from field-induced phase separation of a surfactant-rich phase as opposed to field-dependent conductivity of a homogeneous continuous phase.     

A novel rheometer design for yield stress fluids

An inexpensive, rapid method for measuring the rheological properties of yield stress fluids is described and tested. The method uses an auger that does not rotate during measurements, and avoids material and instrument‐related difficulties, for example, wall slip and the presence of large particles, associated with yield stress fluids. The method can be used for many types of yield stress fluids, including concentrated lignocellulosic biomass. Sample preparation prior to measurement is minimal, reducing, or eliminating disruption of the sample. We show that measurements using this technique compare well with measurements obtained with a vane rheometer. A variation of the described method is proposed that would make it easier to measure time‐dependent rheological properties. © 2014 American Institute of Chemical Engineers AIChE J, 60: 1523–1528, 2014     

Comparison of two analytical models of blanking and proposal of a new model

With the development of accurate Finite Element models of the punching/blanking process by the present authors and other researchers, important possibilities emerged to model and analyse the process in a research environment. However, Finite Element models are less well suited for use in a production environment, to monitor the process in real time. For real-time monitoring applications, an analytical model could potentially be used to extract relevant information from measurements of force and displacement.In this article, first, two existing analytical models of blanking are compared. One model is based on the assumption of pure shear and the other is a tension model. Secondly, an addition to the shear model is proposed, resulting in a new analytical model, which allows for the modelling of plastic bending during the process. Results of the new model are compared against experimental findings, and show good similarity for the materials tested. Particular attention is paid to the assumptions and the stress–strain systems inherent in the models.     

Fast tuning of external-cavity diode lasers

We report on a method for characterizing the tuning capabilities of small-bandwidth external-cavity diode lasers. The step response of a piezo-driven Littman-Metcalf external-cavity diode laser and the compensation of its optoelectromechanical frequency response is investigated. When an approximately 50-V compensated Gaussian pulse is applied to the piezo element, a detuning of 13.6 GHz is observed. This modification of the laser is useful for several spectroscopical applications and as a tunable seed laser for lidar applications.     

Gun Propulsion Concepts. Part I: Fundamentals

Modern gun propulsion techniques are driven by the desire to increase muzzle kinetic energy as a means to enhance weapon lethality, reduce costs, increase reliability, and improve vulnerability. In recent years, a significant part of research and developmental activities has been focused not only on conventional solid-propellant and novel liquid-propellant gun propulsion techniques but also on advanced concepts, particularly those offering substantial performance increase. High performance can be achieved by pure electric systems(coil or rail gun) as well as by combining chemical propulsion either with gasdynamic principles (light gas gun or ram accelerator) or with electrothermal techniques (hybrid gun). Part I of this paper deals with the fundamentals of projectile acceleration techniques. The objective is to provide the basis for understanding gun technology and to lay the basis for assessing present and future developments.