Dust Eruptions

<p>We present a new model for the fragmentation of dust beds in laboratory shock tube experiments. The model successfully explains the formation of layers in the bed using mass and momentum conservation. Our model includes the effect of wall friction, inherent cohesion, and gravitational overburden. We find that the pressure changes caused by the expansion wave take time to penetrate into the bed, while simultaneously increasing in magnitude. By the time the pressure difference is large enough to overcome wall friction, the overburden and the intrinsic cohesion of the bed, it has penetrated ~8-15 bead diameters into the bed, thus causing a layer of dust to be lifted off. We have found the dependence of layer size upon bead diameter and found a good match to experiment. We have also predicted the dependence of layer size and fragmentation time on bead density.</p>

Dust Eruptions

<p>We present a new model for the fragmentation of dust beds in laboratory shock tube experiments. The model successfully explains the formation of layers in the bed using mass and momentum conservation. Our model includes the effect of wall friction, inherent cohesion, and gravitational overburden. We find that the pressure changes caused by the expansion wave take time to penetrate into the bed, while simultaneously increasing in magnitude. By the time the pressure difference is large enough to overcome wall friction, the overburden and the intrinsic cohesion of the bed, it has penetrated ~8-15 bead diameters into the bed, thus causing a layer of dust to be lifted off. We have found the dependence of layer size upon bead diameter and found a good match to experiment. We have also predicted the dependence of layer size and fragmentation time on bead density.</p>

Effects of Cowl-Induced Expansion on the Wave Complex Induced by Oblique Detonation Wave Reflection

Oblique detonation wave (ODW) reflection on the upper wall leads to a sophisticated wave complex, whose stability is critical to the application of oblique detonation engines. The unstable wave complex characterized with a continuous moving Mach stem has been observed, but the corresponding re-stability adjusting method is still unclear so far. In this study, the cowl-induced expansion wave based on the model with an upper-side expansion wall is introduced, and the ODW dynamics have been analyzed using the reactive Euler equations with a two-step induction–reaction kinetic model. With the addition of a cowl-induced expansion wave, the re-stabilized Mach stem has been distinguished. This re-stability is determined by the weakened secondary reflection wave of lower wall, while the final location of Mach stem is not sensitive to the position of the expansion corner. The re-stabilized ODW structure is also basically irrelevant to the expansion angle, while it may shift to unstable due to the merging of subsonic zones. Transient phenomena for the unstable state have been also discussed, clarifying fine wave structures further.

Internal flow mechanism in a supersonic expander-rotor

This article proposes a numerical investigation into the internal flow structure in the supersonic expander-rotor (SER). In order to reveal internal flow mechanism, the significant influencing factors in the flow structure are identified, and the solutions to improving the integrated performance of the SER are developed. According to the numerical results, the wave structure of the expansion wave and the oblique shock wave is what characterizes the flow in the mainstream region of the SER. In addition, the expansion wave and the oblique shock wave impose control on the pattern of static pressure distribution in the 3-D channel and then the 3-D flow structure. The formation and breakdown of the tip leakage vortex are the main form that the motion of vortex takes in the SER. The concentration, recirculation, and separation of the boundary layer; the low energy fluid mixing with mainstream; and the interaction between the oblique shock waves and the boundary layer are the crucial motion tracing near the endwall. Compared with the traditional turbines, the flow structures in the tip region of the SER are relatively simpler; the essential motion tracing is the airflow near the leading edge of the strake wall moving from the PS through the tip gap to the SS as a result of the transverse pressure difference.

Numerical investigation of the pressure and friction resistance of a high-speed subway train based on an overset mesh method

The aerodynamic resistance induced by a high-speed metro train entering and operating in a tunnel with a speed of 120 km/h is simulated using a three-dimensional, compressible turbulence model. An overset mesh method is adopted to solve the moving boundary problem, and the flow field around the train is simulated with a k-omega SST turbulence model to obtain accurate stress values at the walls. The selected model is verified with experimental and numerical data from the literature. Then, numerical simulations are performed to analyse the formation mechanisms of the pressure and friction drags. The aerodynamic drag basically stabilizes after the expansion wave passes the train head. The results reveal that the variations in friction resistance are related to the direction of the Mach wave, and this trend differs from that observed for pressure resistance. Mach wave influences the velocity where it passes, and further influences the friction drag. Result shows that friction drag of the train increases when encounter with compression wave propagated from the front or expansion wave propagated from the back, and decreases otherwise. The effects of the blockage ratio on the maximum and average pressure and friction resistance values of each train section are evaluated based on fitting functions. The predicted aerodynamic drag varies with the blockage ratio and for each train section, and the results are summarized and compared with predictions and experimental data from the literature. The variations in tunnel resistance and the overall trend are in good agreement with the previous results. Therefore, the findings presented in this study may provide a reference for the design of high-speed subway tunnels.