Keywords: Laser gain, Doppler Broadening, Lorentzian Broadening, lineshape, computational fluid dynamics, flow unsteadiness.
IntroductionThe lineshape of the I 2 P 1/2 2 P 3/2 transition provides a means to ascertain a variety of useful information regarding the performance of the chemical oxygen-iodine laser (COIL). The value at the center of the lineshape, commonly referred to as the 'line center,' is proportional to the laser amplification on the I 2 P 1/2 2 P 3/2 transition. The infinite integral of the lineshape is proportional to the number density of the ground and excited states of atomic iodine in the gas, indicating the degree of I 2 dissociation. And the width of the lineshape indicates the amount of broadening of the transition, both due to collisional and Doppler shift effects. As the Doppler shift is proportional to velocity, the width of the transition can be used to estimate the degree of random molecular motion in the gas, expressed in macroscopic terms as temperature. A Doppler shift to the frequencies in the transition can also occur through the straight-line, bulk motion of the gas, and this can be used to examine the velocity field of the gas. However, the flow may experience rotation through the presence of eddies carried within the gas, and these too may contribute to the Doppler shift of the lineshape frequencies. Given that eddies by virtue of their positive and negative velocity components can induce positive and negative Doppler shift, the widening of the lineshape is similar to thermal motion which also includes positive and negative velocities. Thus, when interpreting transition lineshapes, if some account is not made for both thermal and rotational motion, the effect of either physical process will be over-estimated.The work discussed here is oriented toward examining the interplay between the gas dynamics and the lineshape of the I 2 P 1/2 2 P 3/2 transition, and in turn determine the ramifications for the use of spectroscopic lineshape based diagnostics and interpretation of their data. These efforts in turn are directly linked to efforts improve the understanding of the physical processes underlying chemical lasers, as excursions outside the traditional operational parameter space become increasingly necessary.
Problem and MethodologyThe objective of this work is to generate a greater physical understanding of the physical processes underlying COILs through use of the models employed in simulation of these devices. This objective is achieved through 3-D, time accurate numerical simulation of the COIL flowfield using the Navier-Stokes equations coupled to species continuity equations. Previous work by Madden and Miller 1 presented computational data for COIL simulation indicating that the flowfield should be marked by substantial flow unsteadiness, as documented in the experiments of Fric and Roshko 2 for incompressible, air-air jet-in-crossflows at Reynolds numbers similar to those found in COILs. Building on the earlier results, this work further elucidates the flow unsteadiness,...