|Alternative Title||A numerical study on the attenuation mechanism of planar shock wave|
|Place of Conferral||北京|
Shock wave is a strong disturbance wave propagating at a supersonic speed, which can propagate through gas, liquid and solid media. Shock waves are usually generated using shock tubes for various experimental studies and are treated as ideal shock waves. However, due to viscous effects and heat conduction of the shock tube wall, the gas flow properties in real shock tube will deviate from the ideal shock tube theory. Since the state of the moving gas is different from the static shock tube wall, an unsteady boundary layer will be induced behind the propagating shock wave, thus the shock wave decelerates gradually and severe shock attenuation occurs in the long time. In particular, this attenuation behavior will be greatly enhanced for smaller scale or low-pressure shock tubes. For the shock tube flow, due to the acceleration of the contact surface and the deceleration of the shock wave, the effective experiment time of zone 2 or 5 in the shock tube will be significantly reduced, and shock attenuation will also greatly decrease the quality of the flow field, which may further lead to shock wave bifurcation, "driving gas contamination" and other adverse effects. Therefore, it is of important to study the physical mechanism of shock wave attenuation, which will in turn help data interpretation and platform optimization of shock tube experiments.
Planar shock wave attenuation is a kind of unsteady motion induced by the static shock tube wall, it involves coupled processes with shock wave discontinuity, unsteady boundary layer, and interaction between shock wave and this induced boundary layer. Although many previous investigations have pointed out that shock wave constantly attenuates when propagating in shock tubes or small channels, the detailed mechanism of this attenuation process has not been clearly discussed yet in the literature, mainly due to lack of research methods, difficulties in microscopic analysis, and large demand for computing resources. This paper divides the study of two-dimensional shock wave attenuation into two steps. First, the flow field evolution of static gas motion above an infinite wall of sudden state changes (that is, one-dimensional compressible Rayleigh problem) is studied through numerical simulations, motion laws and disturbance propagation characteristics of the above compressible gas are investigated. Second, two-dimensional planar shock wave attenuation problem is numerically studied through DSMC method, and detailed mechanism of this attenuation process is investigated, impact factors of shock attenuation degree are discussed as well. The details are as follows.
1）One-dimensional compressible Rayleigh problem is studied through the direct simulation Monte Carlo (DSMC) method and the computational fluid dynamics (CFD) method respectively. Numerical simulations cover the whole flow regime range from free molecular flow to continuous flow regime. Detailed analysis is provided for the development and propagation process of the initial velocity and temperature discontinuity in the normal direction of the wall. Numerical results show that, in the initial stage of wall motion startup (before 10 mean collision times of the undisturbed gas molecules), the CFD method could not give an accurate estimate of both wall surface parameters and disturbance distribution of compressible gas above the wall. In 1000 mean collision times of the undisturbed gas, parameters like heat flux, shear stress on the surface and flow field distribution of the above compressible gas calculated by Roe's FDS scheme are in better agreement with the DSMC results, when compared with the AUSMpw scheme and Van Leer's FVS scheme. Further analysis also suggests that the induced disturbance wave propagates approximately with the velocity magnitude equivalent to the sound velocity, whereas the thickness of the unsteady boundary layer increases nearly proportional to the square root of time. Specifically, this propagation velocity of disturbance wave is closely related to the intensity of initial temperature and velocity discontinuity between the wall and gas, and it will gradually transform to the local sound velocity in the long time; the amplitude of disturbance wave also severely attenuates as time goes by.
2）The DSMC method is employed to study the propagation and attenuation of shock wave in a two-dimensional planar tube. Detailed mechanism analysis is provided for the whole process of shock attenuation, the interaction between wall boundary layer disturbance and shock wave discontinuous structure is described as several different development stages. Impact factors of shock attenuation degree are discussed as well, including shock tube heights and the initial shock Mach numbers. Numerical results show that, similar to the compressible Rayleigh problem, an unsteady boundary layer will develop at the shock tube wall in the downstream of the moving shock wave, and a low-pressure disturbance wave will be induced in the flow field as well, which could be reflected multiple times in the height direction of the shock tube due to the shape limitation and then will be gradually attenuated with time. In the vicinity of the shock wave and boundary layer interaction zone, the normal velocity of the gas rushes to the shock tube wall, and the boundary layer acts as a "leak" taking away a large part of the flow field mass. At the same time, appreciable shock curvature is clearly seen in the downstream direction. Both the shock wave profile and Mach number attenuation curve show periodic oscillation characteristics, and the oscillation frequency is consistent with the reflection frequency of the downstream disturbance wave in the shock tube. Finally, shock wave is found to be more severely attenuated in narrower tube or having larger initial Mach numbers.
|薛晓薇. 平面激波衰减机理的数值模拟研究[D]. 北京. 中国科学院大学,2020.|
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