Laser Radiation-Gasdynamic Coupling in the SF,-Air Laminar Boundary Layer

Abstract

PRACTICAL problems arise where the shielding of aerodynamic surfaces from high-power laser beams is of importance. For example, a highly absorbing gas can be injected into the boundary layer of an airplane or missile in order to protect its surface materials from the incoming laser radiation. Initial work demonstrated that SF6 is a very strong absorber u of CO2 laser radiation. Therefore, this work deals solely with the radiative gasdynamic interaction associated with CO2 laser radiation absorption in a SF6-air laminar compressible boundary layer. However, the implicit finite difference technique employed is applicable to any arbitrary laser wavelength provided an absorption coefficient for the associated absorbing gas is known as a function of pressure and temperature. Contents This work extends earlier flat plate results3 to include the stagnation point and downstream region of an axisymmetric body. The explicit numerical scheme used in the flat plate results required initial guesses for the unknown derivatives at the wall (standard shooting point technique) to be correct to within six or seven significant figures before the solution would begin to converge. Therefore, this explicit scheme was replaced with a more efficient implicit numerical scheme4 which does not have these stability difficulties. The boundary-layer equations (laminar, nonlinear, partial differential equations) are solved with mass injection rate of SF6 and incoming CO2 laser radiation. A self-similar solution is employed at the stagnation point to obtain an initial profile as input to the implicit finite difference solution for the nonsimilar region downstream of the stagnation point. The laser beam is of uniform intensity and parallel to the flow direction. As the laser beam is attenuated, the temperature in the boundary layer increases which in turn affects the absorption coefficient of the gas. Therefore, the radiative transport equation is fully coupled to the fluid mechanic boundary-layer equations. In the present analysis, the gas absorbs but does not emit. The radiation that reaches the wall is totally absorbed by the wall and not reflected back into the boundary layer. The translational, rotational, and vibrational energies of the absorbing gas are in thermal equilibrium. Profile variations across the boundary layer are considered in velocity, tern