Estimation of Cavities beneath Plate Structures Using a Microphone: Laboratory Model Tests

The objective of this study is to detect a cavity and estimate its size using sound waves in a laboratory model chamber filled with dry sand. One side of the chamber is covered with an acrylic plate, and a cavity is placed between the plate and sand. Sound waves are generated by impacting the plate with an instrumented hammer, and are measured using a microphone. The measured sound waves are analyzed with four comprehensive analyses including the measured area under the rectified signal envelope (MARSE) energy, flexibility, peak magnitude of wavelet transform, and frequency corresponding to the peak magnitude. The test results show that the accuracy of cavity detection using the MARSE energy is higher for thicker plates, whereas that using flexibility is higher for thinner plates. The accuracies of cavity detection using the peak magnitude of wavelet transform, and frequency corresponding to the peak magnitude are consistently high regardless of the plate thickness. Moreover, the cavity size may be under- or overestimated depending on the plate thickness and the selected analysis method. The average of the cavity sizes estimated by these methods, however, is slightly larger than the actual cavity size regardless of the plate thickness. This study demonstrates that microphones may be effectively used for the identification of a cavity and the estimation of its size.

High-temperature effect of a gas jet of reactants on the front plate

The paper introduces the results of measuring and predicting the heat and force effect of jets of high-temperature reacting mixtures on the oxygen-methane, oxygen-alcohol components when acting on the front plate in the near field of the jet. A high-temperature supersonic gas jet flows out of a model chamber with a Laval nozzle into a medium with atmospheric pressure at a degree of off-design ratio of about unity. In the chamber, ignition and stable combustion of a mixture of selected substances occur, the ratio of these substances providing a stagnation temperature in the range of 1900 ... 3400 K. The pressure distribution function on the front plate obtained in the experiment is used. The proposed model of the high-temperature flow effect on the frontal surface can be used to test software systems and determine the levels of thermal effect during sample tests.

Detection of Cavities Beneath Plate Structure using a Microphone

Failure of plate structures such as pavements can be caused by the occurrence of cavities beneath the structure. In this study, a cavity beneath the plate structure were detected in a laboratory model chamber using a hammer and microphone. Specifically, a chamber was constructed using an acrylic plate and dry soil to simulate the pavement and the subgrade, respectively. A plastic box was placed between the acrylic plate and dry soil to simulate a cavity beneath the pavement. The sound waves generated by impacting the acrylic plate with a hammer were measured using a microphone. The measured area under the rectified signal envelope (MARSE) energy was calculated for the measured sound waves, and the variation in it were analyzed according to the measurement location. The test results show that the signal attenuation was low at the cavity section owing to the conservation of impact energy and that the signal amplitude becomes lower at the soil section owing to the weakened flexural vibration. Therefore, the estimated MARSE energy at the cavity section was larger than that at the soil section. This study demonstrates the effective utilization of microphones for detecting cavities beneath the plate structures.

Investigation on Acoustic Properties of Thruster Chamber with Coaxial Injectors and Plenum Chamber

Based on the URANS equation, a numerical simulation is carried out for acoustic properties of the thruster chamber with coaxial injectors and plenum chamber in a liquid rocket engine. Pressure oscillations with multiacoustic modes are successfully excited in the chamber by using the constant volume bomb method. FFT analysis is applied to obtain the acoustic properties of eigenfrequencies, power amplitudes, and damping rates for each excited acoustic mode. Compared with the acoustic properties in the model chamber with and without an injector as well as with and without the plenum chamber, it can be found that the injector with one open end and one half-open end still can work as a quarter-wave resonator. The power amplitudes of the acoustic mode can be suppressed significantly when its eigenfrequency is close to the tuning frequency of the injector, which is achieved by Cutting down the pressure Peak and Raising up the pressure Trough (CPRT). Compared with the acoustic properties in the model chamber with and without the plenum chamber, it can be found that 1L acoustic pressure oscillation is inhibited completely by the plenum chamber and other acoustic pressure oscillations are also suppressed in a different extent. The injector and plenum chamber have a little effect on the eigenfrequencies and damping rate of each acoustic mode. For multimode pressure oscillation, it is better for tuning frequency of the injector closing to the lower eigenfrequency acoustic mode, which will be effective for suppression of these multiacoustic modes simultaneously.

Investigation of the Arc Characteristics of Switching DC Arcs on Hydrogen Containing Gas Mixtures

<p>The characteristics of switching DC current arcs in hydrogen containing gas mixtures under pressure were investigated using a model chamber. The switching device consists of an electro-mechanic double breaker unit with copper contacts. High-speed imaging and spectroscopy were used to observe and to characterize the switching arc. The experiments indicate how the dynamic interaction of an external magnetic field with a high-pressure discharge causes an elongation and twisting of the arc-channel and consequently a voltage increase. Comparative measurements with and without external magnetic field indicate a strong influence of this factor. Cu and N lines together with strongly broadened H lines were observed.</p>

Unsteady Aspects of Multi-Interacting Swirlers Using POD Analysis

Twin annular premixed swirler known as TAPS by GE has provided an effective solution to generate low NOx combustion by implementing both diffusion and premixed flames. The fuel injector is less susceptible to combustion instabilities owing to its central diffusion flame and eco-friendly due to its radial premixed swirler. To this extent, 2D PIV experiment was conducted on three newly designed triple swirlers to measure their corresponding velocity fields. The swirlers were designed in a way that the inner and intermediate swirlers are axial type, counter-rotating and remained unchanged with three different radial type outer swirlers. The design characteristic swirl numbers of the outer swirlers were 0.5, 1.0 and 1.5, to impose low, medium and intense swirling motion, respectively. The main objective of the current study is to investigate the interaction of the shear layers emerging from the outer swirler with the both inner and intermediate. Air is used as working fluid and experiments were done in confined condition. The study of the mean flow field indicated that the outer swirler dominates the flow field and defines the behavior of the main features such as swirling jet angle, the resultant inner recirculation zone, and the wall stagnation points. Proper Orthogonal Decomposition (POD) analysis showed a wide range of low-frequency modes spread over the entire domain and stated the domination of the outer swirling flow. It was found that an increase in the intensity of the outer swirler contributes to an intensified interaction with the model chamber walls highlighting the significance of the confinement ratio in higher swirl numbers.

Natural convection in a model chamber with the Boussinesq approximation utilization

The paper presents numerical analysis of heat transfer inside a model chamber in time after entering an object of elevated temperature therein. As a result of the temperature difference between the object and the environment in the room, natural convection has occurred. Numerical simulations were performed using ELMER software based on the Finite Element Method. In order to reduce the complexity of the simulation, the Bussinesq approximation was used. The relevant case is similar to Rayleigh-Benard convection, which was starting point to the conducted analysis. Keywords: temperature distribution in the model room, Rayleigh-Benard convection, Bussinesq approximation