The nature of the non-equilibrium flow of strongly dissociating nitrogen has been investigated by a series of simulation calculations using non-equilibrium (finite rate) chemical reactions. These were made with the equilibrium flux method (EFM), and the results have been found to compare favourably with experimental results obtained with a free-piston driven shock-tube wind tunnel which was used to obtain interferograms of the flow of pure nitrogen over a blunt-nosed body, 65 mm long at three angles of incidence. No simple relation between the flow with non-equilibrium chemistry and those for frozen or equilibrium chemistry has been found. The problems of relating test flows produced in the shock tunnel to flight conditions are investigated by considering the test flows that might be produced by some ‘ideal equivalent wind tunnels’. It is shown that the degree of frozen dissociation in the test flow in a shock tunnel is not a serious matter, but that the large difference in Mach number between shock tunnel flows and flight conditions may be more important.
Abstract.A particle simulation method, the "relaxation time" simulation method (RTSM), is described. In RTSM the collision phase in standard DSMC is replaced by a procedure whereby some of the particle velocities in each cell at each time step are selected from an equilibrium distribution, while conserving the total energy and momentum in the cell. The remaining velocities in each cell are not changed. The number of velocities to be changed is determined from the local relaxation time, which can be derived from the cell density and temperature and any desired viscosity law. The relaxation time method is a simulation method to solve the BGK equation. RTSM is efficient compared to DSMC, and becomes more so as the collision rate increases, so RTSM appears to be a natural candidate for near continuum flows.
SUMMARYIn the direct simulation Monte-Carlo (DSMC) method for simulating rareÿed gas ows, the velocities of simulator particles that cross a simulation boundary and enter the simulation space are typically generated using the acceptance-rejection procedure that samples the velocities from a truncated theoretical velocity distribution that excludes low and high velocities. This paper analyses an alternative technique, where the velocities of entering particles are obtained by extending the simulation procedures to a region adjacent to the simulation space, and considering the movement of particles generated within that region during the simulation time step. The alternative method may be considered as a form of acceptancerejection procedure, and permits the generation of all possible velocities, although the population of high velocities is depleted with respect to the theoretical distribution. Nevertheless, this is an improvement over the standard acceptance-rejection method. Previous implementations of the alternative method gave a number ux lower than the theoretical number required. Two methods for obtaining the correct number ux are presented. For upstream boundaries in high-speed ows, the alternative method is more computationally e cient than the acceptance-rejection method. However, for downstream boundaries, the alternative method is extremely ine cient. The alternative method, with the correct theoretical number ux, should therefore be used in DSMC computations in favour of the acceptance-rejection method for upstream boundaries in high-speed ows.
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