Energy deposition in supersonic flows

A series of four experiments with energy deposition via laser-induced optical breakdown of air, i.e., a laser spark, have been performed. These experiments have demonstrated the possibility of using a laser spark for supersonic flow control. A focused Nd:YAG laser (pulse time of 10 nanoseconds, pulse frequency of 10 Hz, and capable of energy levels up to 600 milli-Joules per pulse) was used to create the energy deposition laser spark. In the first of these experiments, qualitative Rayleigh scattering measurements were performed for energy deposition into quiescent air. A time sequence of images was obtained showing the post breakdown fluid motion created by the laser spark. Likewise, Rayleigh scattering techniques were used in the second experiment to observe the effects of depositing energy upstream of the Mach disk in an under-expanded jet. Two upstream deposition locations were tested, and two time series of images were obtained for each deposition location, respectively. The time sequenced images illustrate qualitatively the interaction of the laser spark induced perturbation with the Mach disk and other jet features. For experiments three and four, energy was deposited upstream of a sphere in Mach 3.45 flow. For experiment three, the energy was deposited one sphere diameter upstream of the front of the sphere. The frontal surface pressure on the sphere was recorded as the laser spark perturbed region interacted with the flow about the sphere. Tests for three different energy levels and two different incident laser beam diameters were completed. Schlieren images were also taken of the flow and correlated in time to the surface pressure measurements. Lastly, in the fourth experiment, energy was deposited upstream of the sphere subjected to an Edney Type IV shock/shock interaction. The surface pressure on the sphere was measured and correlated in time to shadowgraph images taken of the flow interaction. A significant effect on the flow for laser spark energy deposition has been demonstrated for these experimental cases described. It has been demonstrated that the peak surface pressure associated with the Edney IV interaction can be momentarily reduced by the interaction with the thermal spot created by the laser spark.