Modeling of ultraviolet radiation in steady and transient high-altitude plume flows

Ultraviolet radiation from hydrazine spacecraft thruster plumes interacting with ambient atomic oxygen is modeled for low-Earth-orbit conditions. Two numerical techniques that employ the direct simulation Monte Carlo method are applied for the first time to the modeling of space plume radiation transient and three-dimensional flows. These efficient procedures allow one to analyze the effect of atomic oxygen penetration of the thruster plume, which is a key factor in modeling rarefied space plume radiation. The overlay technique is used to model the transient flow evolution during the first several seconds after motor ignition. Good agreement between modeling and experiment are obtained before 1-s motor burn time. The sensitivity of the plume radiation to the molecular total collision model is analyzed using the overlay technique, and the radiation spatial distribution was found to be strongly dependent on the temperature exponent of the coefficient of viscosity. Three-dimensional computations are conducted for different angles between the plume axis and the freestream directions, and the radiation maps for OH(A) and NH(A) are presented. Significant difference between OH(A) and NH(A) radiation fields as a function of the angle of attack is shown.