Low thrust rockets are often used in the guidance and control of spacecraft. Impact of a thruster plume on spacecraft surfaces can have serious detrimental effects including disturbance torques, unwanted heating, and contamination of surfaces. Plumes and the resultant impingement phenomena are currently not well understood. Simple engineering models are used conservatively to estimate plume effects. An ongoing collaborative research program at Cornell University and NASA Lewis Research Center seeks to understand these effects. Numerical methods are being developed to characterize plume flows.
The rapidly expanding plume of a thruster firing into vacuum covers several flow regimes - from near continuum at the nozzle exit to collisionless in the far field. This rarefied, nonequilibrium nature requires a non-continuum approach. The direct simulation Monte Carlo (DSMC) method has been used to model these flows. The current research effort aims to validate the DSMC method for use with impingement problems and develop tools to calculate these flows more accurately and efficiently. A three dimensional DSMC code which is numerically efficient and parallelized (MONACO) is being developed in order to consider problems with realistic engineering configurations.
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Impingement flows around various axisymmetric and three dimensional bodies have been considered. One flow that has been simulated is a free jet impinging on a flat plate. Figure 1 shows a schematic of the configuration. A jet of nitrogen is generated by a sonic orifice whose axis is oriented at an angle beta to the surface normal. The chamber pressure of the orifice is low (0.01 bar) so that the jet is rarefied. Experimental measurements of surface properties (pressure, shear stress and heat flux) are available for this configuration.
The jet flow is simulated in three dimensions using an unstructured
(tetrahedral) grid. Figure 2 shows a cutaway
view of the grid used for the parallel flow (beta = 0 degree)
case. Also shown are logarithmic contours of number density on several
planes. Figure 3 shows a comparison between
measured surface properties and simulation results for the beta = 45
degree case. The displayed agreement for pressure and shear stress is
representative of the results for the range of angles from normal
impingement (beta = 90 degrees) to parallel flow.
This work is funded by NASA Lewis Research Center. Computer resources provided by the Cornell Theory Center and the Numerical Aerodynamic Simulation Program.