Experimental and numerical investigations are performed and compared for the flow of nitrogen in a small nozzle and in the near field of the plume resulting from expansion into near-vacuum conditions. The experimental data obtained were in the form of pressure measurements using a pitot tube, in the nozzle-exit plane and near field of the plume. Since the flow regimes vary from continuum, at the nozzle throat, to rarefied, in the plume, two different numerical studies are undertaken: the first employs a continuum approach in solving the Navier-Stokes equations, and the second employs a stochastic particle approach through the use of the direct simulation Monte Carlo (DSMC) method. Comparison of the experimental data and the numerical results at the nozzle exit reveals that the DSMC technique provides the more accurate description of the expanding flow. It is discovered that the DSMC solutions are quite sensitive to the model employed to simulate the interaction between the gas and the nozzle-wall surface. It is concluded that the simple fully diffuse model is quite satisfactory for the present application. The study provides the strongest evidence to date that the DSMC technique predicts accurately the flow characteristics of low-density expanding flows.Nomenclature D e = diameter at nozzle exit D p = diameter of probe D t = diameter at nozzle throat M x -Mach number ahead of a shock P x = static pressure ahead of a shock P 0 = total pressure P oy = total pressure behind a shock P x = static pressure ahead of a shock Re p -probe Reynolds number T 0 -total temperature r ref = reference temperature T w = temperature of nozzle wall T x = static temperature ahead of a shock T y = static temperature behind a shock U 0 = thermal velocity at total temperature t/oo = freestream velocity 7 = ratio of specific heats jLt ref = viscosity given by reference temperature H y = viscosity given by temperature behind a shock a?= viscosity temperature exponent Poo = freestream density

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The direct simulation Monte Carlo method was applied to the analysis of low-density nitrogen plumes exhausting from a small converging-diverging nozzle into finite ambient pressures. Two cases were considered that simulated actual test conditions in a vacuum facility. The numerical simulations readily captured the complicated flow structure of the overexpanded plumes adjusting to the finite ambient pressures, including Mach disks and barrelshaped shocks. The numerical simulations compared well to experimental data of Rothe.

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