The flow field induced by the ablation plume in the presence of background gas is simulated numerically. The study of plume flow that occurs in laser ablation is important for it can yield information on ablation process itself and the properties of end product for which the ablation is carried out. Unsteady compressible axisymmetric Navier-Stokes equations govern the plume flow. The major challenge involved, even in this simplified model of plume dynamics, is twofold: (i) the time scale of simulation spans six orders of magnitude, from nanosecond to millisecond, and (ii) the high nonlinearity of governing equations because of high pressure, temperature and injection velocity of plume. A computational model is developed that can account for the entire range of time scale and high nonlinearity. This model is a combination of numerical methods and includes multi-time step and multi-size grid technique. The uniqueness of model lies in choosing the combination of numerical methods and handling multi-size grid interface in a conservative way. The combination of numerical methods is decided after comparing the results of few numerical methods for a single plume.
The plume dynamics for single plume is explained with the help of proposed post-processing model based on vorticity dynamics. The model not only helps in understanding the expansion dynamics of plume but also provides quantitative comparison amongst numerical methods. The validity of nano-to-micro second range viscous and inviscid models of plume dynamics is discussed by means of evaluation of source terms in the vorticity transport equation.
The role of turbulence is evaluated by millisecond-scale modeling of plume expanding in surrounding furnace gas with imposed turbulent gust. The results for multiple plumes typical for real life ablation are presented and discussed. Shielding of laser beam by previously ejected plume in multiple laser hits is important because it changes energy deposition of incident laser pulse at the target surface and in turn influences the ablation dynamics and amount of material removed. To account for this shielding effect, shielding models are developed and implemented. The quantity of ablated mass due to the shielding effect is evaluated.
Ionization of carbon plume and its impact on plume dynamics and shielding is studied. An iterative procedure is developed to determine the local equilibrium temperature affected by ionization. It is shown that though shielding due to the presence of ionized particles in carbon plume is small, the effect of ionization on plume dynamics can be considerable. Shielding effect is calculated for laser pulses with different time interval between pulses. The effect of high temperature and low density of plume are controversary and cause shielding behavior to be non-monotonic with pulse number. It is shown that the non-monotonic dependence of the delivered laser energy with the pulse number and the difference in shielding characteristics between planar and axisymmetric formulations increase with the time duration between two consecutive pulses.
The developed numerical methodology is employed to study the heat transfer modulation between the Thermal Protection Shield (TPS) and the gas flow occurring because of ejection of under-expanded pyrolysis gases through the cracks in the TPS in hypersonic flight. The simulations are performed for an axisymmetric bluff body flying at Mach 7. The influence of the geometry of the TPS on heat transfer pattern is studied for two representative shapes. The results are presented for three different flight altitudes (low-ground level, moderate-20km and high-30km). At the low altitude the plume pressure is lower than the pressure behind the detached front shock wave and the plume propagates slowly along the wall surface. At high and moderate altitudes, the plume path and consequently, convective heat transfer between the TPS and the plume depends on the plume interaction with the bow shock wave. The effect of viscosity for the plume injection conditions and free stream Mach number considered is found to be negligible at simulated altitudes. However, the effect of initial pressure of pyrolysis gas on the plume dynamics is significant. The presence of the blast wave associated with under-expanded plume alters the heat transfer and increases mixing. Finally, the enhanced heat transfer caused by the emergence of multiple pyrolysis plumes is investigated.