Numerical Analysis of a Dual Polarization Mode-Locked Laser with a Quarter Wave Plate

Dynamical behaviors and frequency characteristics of an active mode-locked laser with a quarter wave plate (QWP) are numerically studied by using a set pf vectorial laser equation. Like a polarization self-modulated laser, a frequency shift of half the cavity mode spacing exists between the eigen-modes in the two neutral axes of QWP. Within the active medium, the symmetric gain and cavity structure maintain the pulse's circular polarization with left-hand and right-hand in turn for each round trip. Once the left-hand or right-hand circularly polarized pulse passes through QWP, its polarization is linear and the polarized direction is in one of the directions of i45o with respect to the neutral axes of QWP. The output components in the directions of i45" from the mirror close to QWP are all linearly polarized with a period of twice the round-trip time

Instability Criterion of One-Dimensional Detonation Wave with Three-Step Chain Branching Reaction Model

One-dimensional detonation waves are simulated with the three-step chain branching reaction model, and the instability criterion is studied. The ratio of the induction zone length and the reaction zone length may be used to decide the instability, and the detonation becomes unstable with the high ratio. However, the ratio is not invariable with different heat release values. The critical ratio, corresponding to the transition from the stable detonation to the unstable detonation, has a negative correlation with the heat release. An empirical relation of the Chapman-Jouguet Mach number and the length ratio is proposed as the instability criterion

Application of short time energy analysis in monitoring the stability of arc sound signal

This paper employed the short time energy of arc sound signals to online quantitatively describe the stability of arc sound signal. At first the signal can be preprocessed by wavelet packet filtering and then detailed information of the short time energy of the signal was obtained using hamming window. After statistical analyzed the short time energy the energy distribution possibility and cumulative distribution function of the signal can be collected. Then a proposed stability evaluation criterion was employed to quantitatively describe the stability of arc sound signal. Relative experimental data showed that more stable signal corresponded lager value of the criterion. The proposed method which combined the short time energy and statistical analysis was supported by many actual experiments. This contribution can benefit the quantitative evaluation of the arc welding process and instructed the future parameters optimization to obtain welding products with high quality. (C) 2017 Elsevier Ltd. All rights reserved

Effects of Mach number on turbulent separation behaviours induced by blunt fin

An experimental study of the interaction between shock wave and turbulent boundary layer induced by blunt fin has been carried out at M-infinity = 7.8 using oil flow visualization and simultaneous measurements of fluctuating wall pressure and heat transfer. This paper presents the effects of Mach number on turbulent separation behaviours induced by blunt fin

Laboratory study on cracks in saturated sands

It has been reported([1]) that when a loosely packed column of saturated sand in a vertical cylindrical container is shock loaded axially by dropping to the floor, large horizontal cracks initiate, grow and eventually fade away in the sand as it settles under gravity. This paper shows that a similar phenomenon can also be observed when shock loading is replaced by forcing water to percolate upward through the sand column. It is believed that our result sheds further light on the physics of formation of these cracks

A Microscopic Analysis of Premixed Hydrogen-Oxygen Auto-ignition

Combustion is a complex problem involving multi-stage chemical reactions and multi-scale physics. The microscopic process of chemical reactions has obvious stochastic character and may bring important influence to combustion phenomena both locally and globally. In this study, we employ a stochastic simulation algorithm (SSA) to simulate the microscopic hydrogen-oxygen auto-ignition process. Statistical result of SSA calculation shows that fluctuation is controlled by both temperature and microscopic volume. The non-homogeneous distribution of radicals caused by local fluctuation may cause the difference between average microscopic quantities and their macroscopic counterparts due to some radical-radical reactions

Optimum Jet-to-Plate Spacing of Inline Impingement Heat Transfer for Different Crossflow Schemes

A nine-by-nine jet array impinging on a flat plate at Reynolds numbers from 15,000 to 35,000 has been studied by the transient liquid crystal method. The spacing between the impingement plate and target plate is adjusted to be 1, 2, 3, 4, and 5 jet diameters. The effect of jet-to-plate spacing has been investigated for three jet-induced crossflow schemes, referred as minimum, medium, and maximum crossflow, correspondingly. The local air jet temperature is measured at several positions on the impingement plate to account for an appropriate reference temperature of the heat transfer coefficient. The jet-to-plate spacing, H/d - 3, is found to be better than the others for all the crossflow schemes. Jet-to-plate spacings H/d = 1 and H/d = 2 result in a sudden decrease in the stagnation zone. The large jet-to-plate spacings H/d = 4 and H/d = 5 could not provide higher heat transfer performance with higher crossflow

Reconsideration On The Role Of The Specific Heat Ratio In Arrhenius Law Applications

Arrhenius law implicates that only those molecules which possess the internal energy greater than the activation energy E-a can react. However, the internal energy will not be proportional to the gas temperature if the specific heat ratio gamma and the gas constant R vary during chemical reaction processes. The varying gamma may affect significantly the chemical reaction rate calculated with the Arrhenius law under the constant gamma assumption, which has been widely accepted in detonation and combustion simulations for many years. In this paper, the roles of variable gamma and R in Arrhenius law applications are reconsidered, and their effects on the chemical reaction rate are demonstrated by simulating one-dimensional C-J and two-dimensional cellular detonations. A new overall one-step detonation model with variable gamma and R is proposed to improve the Arrhenius law. Numerical experiments demonstrate that this improved Arrhenius law works well in predicting detonation phenomena with the numerical results being in good agreement with experimental data

Intermittency of near-bottom turbulence in tidal flow on a shallow shelf

The higher-order structure functions of vertical velocity fluctuations (transverse structure functions (TSF)) were employed to study the characteristics of turbulence intermittency in a reversing tidal flow on a 19 m deep shallow shelf of the East China Sea. Measurements from a downward-looking, bottom-mounted Acoustic Doppler Velocimeter, positioned 0.45 m above the seafloor, which spanned two semidiurnal tidal cycles, were analyzed. A classical lognormal single-parameter (mu) model for intermittency and the universal multifractal approach (specifically, the two-parameter (C-1 and alpha) log-Levy model) were employed to analyze the TSF exponent xi(q) in tidally driven turbulent boundary layer and to estimate mu, C-1, and alpha. During the energetic flooding tidal phases, the parameters of intermittency models approached the mean values of (mu) over tilde approximate to 0.24, (C) over tilde (1) approximate to 0.15, and (alpha) over tilde approximate to 1.5, which are accepted as the universal values for fully developed turbulence at high Reynolds numbers. With the decrease of advection velocity, mu and C-1 increased up to mu approximate to 0.5-0.6 and C-1 approximate to 0.25-0.35, but a decreased to about 1.4. The results explain the reported disparities between the smaller "universal" values of intermittency parameters mu and C-1 (mostly measured in laboratory and atmospheric high Reynolds number flows) and those (mu = 0.4-0.5) reported for oceanic stratified turbulence in the pycnocline, which is associated with relatively low local Reynolds numbers R-lambda w. The scaling exponents xi(2) of the second-order TSF, relative to the third-order structure function, was also found to be a decreasing function of R-lambda w, approaching the classical value of 2/3 only at very high R-lambda w. A larger departure from the universal turbulent regime at lower Reynolds numbers could be attributed to the higher anisotropy and associated intermittency of underdeveloped turbulence.U.S. Office of Naval Research [N00014-05-1-0245]; Spanish Ministry of Education and Science [FIS2008-03608]; Major State Program of China for Basic Research [2006CB400602]; Catalan Institute for Water Research (ICRA

Large-Eddy Simulation of Time Evolution and Instability of Highly Underexpanded Sonic Jets

High-pressure jet injection into quiescent air is a challenging fluid dynamics problem in the field of aerospace engineering. Although plenty of experimental, theoretical, and numerical studies have been conducted to explore this flow, there is a dearth of literature detailing the flow evolution and instability characteristics, which is vital to the mixing enhancement design and jet noise reduction. In this paper, a density-based solver for compressible supersonic flow, astroFoam, is developed based on the OpenFOAM library. Large-eddy simulations of highly underexpanded jets with nozzle pressure ratios from 5.60 to 11.21 at a Reynolds number around 10(5) are carried out with a highresolution grid. A grid-convergence study has been conducted to confirm the fidelity of the large-eddy simulation results. The large-eddy simulation results have also been validated against available literature data in terms of the time-averaged near-field properties of underexpanded jets. The turbulent transition processes are revealed based on the instantaneous flow features and are quantitatively resolved according to the jet penetration and maximum width. The vorticity analysis is conducted to understand the turbulent transition mechanism, and it is found that the vortex stretching term plays a leading role on the distortion of the vortex rings in the near field of the jets. The dominant instability modes of jets, visualized by helicity, are quantitatively revealed based on the spectrum and relative phase of pressure fluctuation. The single helical modes corresponding to a phase angle close to +/- 180 deg with the 1 + 1 helices are dominant for nozzle pressure ratios of 5.60 and 7.47, whereas the complex and multiple helices for the other two higher nozzle pressure ratios are due to the superposition of the single and double helical modes. In addition, the performance of the coarse mesh and different subgrid-scale models on capturing the dominant instability characteristics in large-eddy simulation of underexpanded jets is investigated