Characterizing Laser-Generated Hot Spots for Receptivity Studies

Abstract

Simple, well-characterized experimental perturbations are needed to validate CFD simulations of supersonic and hypersonic boundary-layer receptivity. A laser-generated hot-spot perturbation has been characterized and modeled, based on experiments in the Purdue University Mach-4 quiet-flow Ludwieg tube. The model provides a detailed density profile and time evolution for the perturbation, which consists of a hot spot and the surrounding weak shock. The model is validated by comparing simulated optical path differences to measurements from a highsensitivity, high-bandwidth laser differential interferometer. CFD simulations are compared with receptivity experiments on a hemisphere to illustrate the usefulness of the perturbation model.

Observations of hypervelocity boundary-layer instability

“…Frequencies in the range of the anticipated second-mode values were observed with the LDI, and were consistent with hot-wire anemometry, although the SNR was poor, and the frequency response appeared to be limited to less than 200 kHz. Salyer, Collicott & Schneider (2006) used an LDI to make measurements in front of and behind the bow shock on a hemisphere for a hot spot generated by laser breakdown in a Mach 4 flow. Because the interest was in receptivity, all measurements were made outside the boundary layer.…”

Oscillations in a Forward-Facing Cavity Measured Using Laser-Differential Interferometry in a Hypersonic Quiet Tunnel

“…Among the advantages of the LDI over other nonintrusive techniques are its high sensitivity, good spatial resolution, and large bandwidth [14][15][16][17][18]. All of these make it a fine instrument to analyze the flow in the PQFLT.…”

“…Salyer characterized the perturbation profile across the shock and compared it with the LDI results. The laser system used to generate the laser perturbation in the work done by Salyer [15][16][17][18], Schmisseur [21], and Ladoon [22,23] could not be readily resurrected for this project, so it could not be used to test the performance of the LDI.…”

“…This was an upgrade from the home-built photoreceivers that Salyer used for his research in the PQFLT [15][16][17][18]. Recall that the function of this receiver is to subtract the photocurrent generated by the two photodiodes.…”

“…Smeets and George developed the first version of the LDI in 1973, with which they were able to resolve optical path length differences as small as λ/30, 000 over bandwidths between 100 Hz and 10 MHz [7]. Salyer developed a feedback stabilized version of the LDI at Purdue University able to resolve minimum path length differences of λ/13, 000 at a bandwidth of 6 MHz [17,18]. Azzazy and O'Hare also developed optical and nonintrusive methods to make similar measurements [19,20].…”