Abstract. A numerical simulation was performed for the process of formation of single Mach reflection on a wedge by soiving a BGK type kinetic equation for the reduced disl;ribution function with a finite difference ,;cheme. The calculations were carried out for a shock Math number 2.75 and wedge angle 250 in a monatomic gas, which corresponds to the conditions of single Mach reflection in the, classical yon Neumann theory. The calculations were performed for both diffuse and specular reflection of molecules at the wall surface. It is concluded that the diffuse reflection of molecules at the wall surface or the existence of the viscous or thermal layer is an essential factor for a nonstationary process at the initial stage of Mach reflection. Furthermore, the numerical results for diffuse reflection are found to simulate the experimental results very well, such as a transient process from regular reflection to Math reflection along with shock propagation.
An
easy, effective, and reversible strategy for tuning the Krafft
temperature (K
T) of selenium-containing
ionic surfactants, with head groups ranging in nature from anionic
to amphoteric, has been achieved for the first time via the redox
chemistry of selenium. After oxidation with H2O2, the selenide group was converted to a more hydrophilic selenoxide
group. This made the oxidized forms of the surfactants more water-soluble,
which results in a marked reduction in the K
T. In contrast, the hydrophilic selenoxide was restored to
its reduced form of selenide via reduction reaction, which allowed
the K
T value to return to its initial
value. By alternating the oxidization and reduction treatments, the K
T of the selenium-containing surfactants in
this work could be reversibly switched over 5–10 cycles without
causing obvious adverse distortions.
Abstract. The direct simulation Monte Carlo (DSMC)method is applied to simulation of nonstationary Mach reflection of strong shock waves. Normally the DSMC method is very time consuming in solving unsteady flow field problems especially for high Mach numbers, because of the necessity of iterative calculations to eliminate the inherent statistical fluctuation caused by a finite sample size. A centrail weighted smoothing technique is introduced to process the DSMC results, so that the iteration time can be significantly reduced. In spite of some relaxations of the shock wave structure, the. smoothing technique is verified to be useful to estima te the flow fields qualitatively and even quantitatively by using a relatively small sample size. The comparison between the present approach and the kineticmodel approach (Xu et al. 1991a(Xu et al. , 1991b on the application to unsteady rarefied flow fields was also carried out.
Abstract. Effects of viscosity and vibrational nonequilibrium on the profile of a weak, spherical N-wave in air are experimentally and numerically studied. Weak blast waves were generated, in a quiescent air dome, by spark discharges and exploding wires and observed by high frequency response microphones over 40 meters. Some similarity relationships were obtained from the blast wave experiments. For observed N-waves having less than 100 Pa peak overpressure, the peak overpres- Similar trends were also found for the negative phase of the blast wave. Numerical simulations were carried out to compare with the blast wave data. The Navier-Stokes equations for spherical symmetric flows were solved by coupling with a relaxation equation for vibrational excitation of oxygen using the random choice method (RCM) adapted to supercomputing with an operator splitting technique. The resultant N-wave profiles are in good agreement with the experimental results. The numerical results clearly indicate that the wave-easing process due to the dispersive effect of vibrational relaxation plays a dominant role in determining the rise time of the N-wave.
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