Early Stage Of Acceleration Research Articles

Flame front dynamics, shape and structure on acceleration and deflagration-to-detonation transition

Developing a new generation of rocket engines based on detonation principles needs methods which ensure stability of operation including the onset of detonation wave after mild ignition. This aspect is crucial for ensuring safety of such engines. In the present paper the transition processes associated with flame acceleration and the onset of a detonation mode are under investigation. High-speed flame front photography has been performed during the deflagration of a premixed gas mixture in a long smooth tube. The evolution of flame front structure and shape has been determined, starting from the moment of deflagration initiation and ending with the occurrence of a local explosion leading to the formation of a detonation wave. Four characteristic flame propagation stages have been identified: the initial flame acceleration stage, followed by the apparent velocity deceleration stage, followed by the nearly constant propagation velocity stage, and finally the second acceleration stage, during which detonation is formed. The changes in process dynamics with a change in the initial pressure have been demonstrated. The behavior of flame front at the early stage of acceleration has been identified. The experimental findings are compared with the theoretical models available in the literature. At the initial stage of the accelerated propagation of flame extended along the channel walls — until deceleration — the velocity of the leading tip of the front is effectively described by the exponential relationship proposed by C. Clanet and G. Searby (1996). The most interesting and poorly studied, in the authors' opinion, stage of the deflagration-to-detonation transition process, the intensive repeated acceleration stage, during which flame shape changes abruptly, has been described in detail. The laminar burning velocity of the stoichiometric acetylene–oxygen mixture diluted with either nitrogen or argon at a reduced initial pressure (8–21 kPa) has been identified.

The influence of gas pore pressure in dense granular flows: numerical simulations versus experiments and implications for pyroclastic density currents

We investigate the influence of gas pore pressure in granular flows through numerical simulations on horizontal and low-angle inclined surfaces. We present a two-phase formulation that allows a description of dam-break experiments considering high-aspect-ratio collapsing columns and depth-dependent variations of flow properties. The model is confirmed by comparing its results with data from analogue experiments. The results suggest that a constant, effective pore pressure diffusion coefficient can be determined in order to reproduce reasonably well the dynamics of the studied dam-break experiments, with values of the diffusion coefficient consistent with experimental estimates from defluidizing static columns. The discrepancies between simulations performed using different effective pore pressure diffusion coefficients are mainly observed during the early acceleration stage, while the final deceleration rate, once pore pressure has been dissipated, is similar in all the studied numerical experiments. However, these short-lasting discrepancies in the acceleration stage can be manifested in large differences in the resulting run-out distance. We also analyze the pore pressure at different distances along the channel. Although our model is not able to simulate the under-pressure phase generated by the sliding head of the flows in experiments and measured beneath the flow-substrate interface, the spatio-temporal characteristics of the subsequent over-pressure phase are compatible with experimental data. Additionally, we studied the deposition dynamics of the granular material, showing that the timescale of deposition is much smaller than that of the granular flow, while the time of the deposition onset varies as a function of the distance from the reservoir, being strongly controlled by the surface slope angle. The simulations reveal that an increment of the surface slope angle from 0° to 10° is able to increase significantly the flow run-out distance (by a factor between 2.05 and 2.25, depending on the fluidization conditions). This has major implications for pyroclastic density currents, which typically propagate at such gentle slope angles.

Early acceleration of electrons and protons at the nonrelativistic quasiparallel shocks with different obliquity angles

The early acceleration of protons and electrons in the nonrelativistic collisionless shocks with three obliquities are investigated through 1D particle-in-cell simulations. In the simulations, the charged particles possessing a velocity of 0.2 c flow towards a reflecting boundary, and the shocks with a sonic Mach number of 13.4 and an Alfven Mach number of 16.5 in the downstream shock frame are generated. In these quasi-parallel shocks with the obliquity angles θ = 15°, 30° and 45°, some of the protons and the electrons can be injected into the acceleration processes, and their downstream spectra in the momentum space show a power law tail at a time of , where ωpe is the electron plasma frequency. Moreover, the charged particles reflected at the shock excite magnetic waves upstream of the shock. The shock drift acceleration is more prominent with a larger obliquity angle for the shocks, but the accelerated particles diffuse parallel to the shock propagation direction more easily to participate in the diffusive shock acceleration. In the early acceleration stage, more energetic protons and electrons appear in the downstream of the shock for θ = 15° compared with the other two obliquities. Moreover, in the upstream region, the spectrum of the accelerated electrons is the hardest for θnB = 45° among the three obliquities, whereas the proton spectra for θnB = 15° and 45° are similar as a result of the competition of the effectiveness of the shock drift acceleration and the diffusive shock acceleration.

Usage of internal magnetic fields to study the early hydration process of cement paste by MGSE method

Internal magnetic field gradients, arising within the porous media due to susceptibility differences at the interfaces of solid and liquid as well as due to the contained magnetic impurities, can be employed by the method of modulated gradient spin echo to get insight into the velocity autocorrelation spectrum of liquid confined in the porous structure. New theoretical treatment of spin interaction with the radio-frequency field and the simultaneously applied static non-uniform magnetic field provides the formula that match well with the measurement of restricted diffusion of water in pores of cement paste. Its fitting to the experimental data gives the changes in the mean size of capillary pores, the spin relaxation and the magnitude of mean internal magnetic field gradients during the induction period and early acceleration stage of hydration processes at different temperatures.

Hydrodynamic instability of elastic-plastic solid plates at the early stage of acceleration.

A model is presented for the linear Rayleigh-Taylor instability taking place at the early stage of acceleration of an elastic-plastic solid, when the shock wave is still running into the solid and is driven by a time varying pressure on the interface. When the the shock is formed sufficiently close to the interface, this stage is considered to follow a previous initial phase controlled by the Ritchmyer-Meshkov instability that settles new initial conditions. The model reproduces the behavior of the instability observed in former numerical simulation results and provides a relatively simpler physical picture than the currently existing one for this stage of the instability evolution.

Hypothalamic histamine H1 receptor-AMPK signaling time-dependently mediates olanzapine-induced hyperphagia and weight gain in female rats

Although second-generation antipsychotics induce severe weight gain and obesity, there is a lack of detailed knowledge about the progressive development of antipsychotic-induced obesity. This study examined the hypothalamic histamine H1 receptor and AMP-activated protein kinase (H1R-AMPK) signaling at three distinctive stages of olanzapine-induced weight gain (day 1-12: early acceleration, day 13-28: middle new equilibrium, and day 29-36: late heavy weight maintenance). At the early acceleration stage, the rats were hyperphagic with an underlying mechanism of olanzapine-increased H1R mRNA expression and AMPK phosphorylation (pAMPK), in which pAMPK levels positively correlated with H1R mRNA expression and food intake. At the middle stage, when the rats were no longer hyperphagic, the changes in H1R-AMPK signaling vanished. At the late stage, olanzapine increased H1R mRNA expression but decreased pAMPK which were positively and negatively correlated with weight gain, respectively. These data suggest a time-dependent change of H1R-AMPK signaling, where olanzapine activates AMPK by blocking the H1Rs and causing hyperphagia in the acute phase. The chronic blockade of H1R may contribute to the late stage of olanzapine-induced heavy weight maintenance. However, pAMPK was no longer elevated and actually decreased. This indicates that AMPK acts as an energy sensor and negatively responds to the positive energy balance induced by olanzapine. Furthermore, we showed that an H1R agonist, 2-(3-trifluoromethylphenyl) histamine, can significantly inhibit olanzapine-induced hyperphagia and AMPK activation in the mediobasal hypothalamus in a dose dependent manner. Therefore, lowering H1R-AMPK signaling is an effective treatment for the olanzapine-induced hyperphagia associated with the development of obesity.

Orbit bump by DC magnets and halo collimation for the RCS extraction

The beam loss during the single turn extraction from a Rapid Cycling Synchrotron (RCS) with high beam power is of important concern. The extraction kickers are usually designed to have exigent total strength to avoid the beam loss. This will increase the construction cost or reduce the kickers’ availability during operation. This paper introduces a method employing DC bump magnets and beam collimation during the early acceleration stage in order to reduce the requirement to the extraction kickers and the beam loss at the extraction. The orbit bump at the extraction septum produced by small DC magnets will collapse during the acceleration, and this will lower the requirement of the orbit separation by the kickers. At the same time, the similar orbit bump at the transverse collimators will allow the beam cleaning in the early acceleration stage and result in much smaller beam emittance at the extraction. The combined effect gives the low beam loss extraction with significantly lower kicker strength. The different ways of applying the method in the China Spallation Neutron Source are also presented.

How the lack of visuomotor feedback affects even the early stages of goal-directed pointing movements.

Pointing movements made with a hidden cursor from the center of gaze to a stationary, visible target overshot the actual target location. The systematic error decreased when the final cursor location from the previous trial was shown, which likely led to the creation of an internal sensorimotor model of movement. However, the putative model had a short memory, and could not substitute for on-line visuomotor feedback on subsequent trials. Contrary to common belief, the effect of a lack of visuomotor feedback was seen even in the early acceleration stage of the movement trajectory. Unchecked in the absence of visual monitoring, the acceleration stage of the movement lasted longer, as was evidenced by the significantly larger value of the peak cursor speed. Moreover, the speed peaked much later in the course of the movement. Speed declined more rapidly thereafter. Consequently, the delayed deceleration stage lasted far less than the acceleration stage. In the absence of visual feedback, the shift rightward in time of the peak speed position (PSP) in relation to total movement duration and other changes in the trajectory imply that visual feedback must play a significant role in determining when acceleration ceases (d V/d t=0), and argue against the traditional notion that visuomotor feedback is unavailable until the later stages of movement. Moreover, our data suggest that non-visual modalities, e.g., proprioception, may be too slow to make up for the absence of vision.

Preliminary experimental results of a novel induction type railgun

This paper describes the concept of an induction type railgun and the results of preliminary experiments with such a railgun. The induction type railgun may surmount the barrier which has been created by secondary arcs in the operations of plasma armature railguns. The induction type railgun is operated as a conventional railgun during the early stage of acceleration and is designed to accelerate a projectile by the plasma armature which is enhanced by the induction from a set of external current loops during the late stage. The external current loops are configured just outside of the two rails at the middle segment of the railgun bore, and are shorter than the railgun. The induced voltage generated by the external current loops is applied to the circuit consisting of the plasma armature, the secondary arc and the two rails between the two arcs. The induced voltage increases the plasma armature current, and simultaneously reduces the secondary arc current. The authors have performed preliminary experiments of the induction type railgun with a relatively low energy stored in a capacitor bank. An increase in the plasma armature current and projectile velocity have been observed in their preliminary experiments.