Flow Physics of Nanosecond Laser Energy Deposition in a Mach 4 Boundary Layer

Abstract

An investigation into the flow disturbance generated by laser surface ablation in supersonic flow was conducted in the University of Arizona's Indraft Supersonic Wind Tunnel (ISWT) at Mach 4. An Nd:YAG laser deposited 1064~nm, 6~ns pulses at 10~Hz on the surface of a flat plate, 50~mm downstream of the leading edge, with an on-target fluence of $1.78~\mathrm{J/cm^2}$. Low-fluence surface ablation was used to generate flow disturbances without inducing significant surface deformation. High-speed schlieren imaging and surface pressure transducers captured the interaction between the laser ablation event and the Mach 4 flow. The laser deposition produced a complex, transient disturbance consisting of a plasma cloud, a hemispherical shockwave and associated shock boundary layer interaction, and a significant boundary layer perturbation. The boundary layer disturbance consisted of two pressure spikes propagating at the fast and slow acoustic wave speeds, respectively. The second peak broadened as it propagated downstream, with features resembling turbulent spot development. These findings provide a foundation for the development of laser ablation-based flow perturbation for future high-speed boundary layer receptivity and flow control research.