Effects of Heat Release on Shock-Train Development in COIL Laser Diffuser Systems

North Carolina State University Grant #: FA9451-15-1-0041 'Effects of Heat Release on Shock-Train Development in COIL Laser Diffuser Systems' Abstract COIL laser systems require low pressures for effective operation, and to recover ambient pressure levels, diffuser and ejector technologies are necessary. The design of an effective diffuser system capable of containing the 'shock train' that provides initial pressure recovery involves several considerations. Among these are the need to operate effectively over a range of back pressures and the requirement that the lasing cavity be isolated from the unsteady dynamics of the shock train to improve mixture homogeneity and thereby enhance laser gain. Though nearly laminar-flow conditions may be maintained in the lasing cavity, a transition to turbulence, enhanced by shock-induced amplification of boundary-layer disturbances, will take place in the pressure-recovery section. Local fluctuation intensities, scaling with the shear layer thickness, emerge as a result of turbulent flow, but lower-frequency oscillations also can emerge and can lead to large-scale oscillations of the entire shock-train system. The proposed effort represents a continuation of an on-going study that applies high-resolution large-eddy simulation (LES) and hybrid large-eddy simulation / Reynolds-averaged Navier-Stokes (LES/RANS) techniques to study the unsteady characteristics of shock trains contained within COIL laser diffuser systems. Heat release effects associated with the lasing process itself will be coupled with the LES/RANS method and used to study the influence of heat release on shock-train dynamics.