LIGS measurements in the nozzle reservoir of a free-piston shock tunnel

Abstract

Free-piston shock tunnels are ground-based test facilities allowing the simulation of reentry flow conditions in a simple and cost-efficient way. For a better understanding of the processes occurring in a shock tunnel as well as for an optimal comparability of experimental data gained in shock tunnels to numerical simulations, it is highly desirable to have the best possible characterization of the generated test gas flows. This paper describes the final step of the development of a laser-induced grating spectroscopy (LIGS) system capable of measuring the temperature in the nozzle reservoir of a free-piston shock tunnel during tests: the successful adaptation of the measurement system to the shock tunnel. Preliminary measurements were taken with a high-speed camera and a LED lamp in order to investigate the optical transmissibility of the measurement volume during tests. The results helped to successfully measure LIGS signals in shock tube mode and shock tunnel mode in dry air seeded with NO. For the shock tube mode, six successful measurements for a shock Mach number of about 2.35 were taken in total, two of them behind the incoming shock (p≈ 1 MPa, T≈ 600 K) and four after the passing of the reflected shock (p≈ 4 MPa, T≈ 1000 K). For five of the six measurements, the derived temperatures were within a deviation range of 6 % to a reference value calculated from measured shock speed. The uncertainty estimated was less than or equal to 3.5 % for all six measurements. Two LIGS signals from measurements behind the reflected shock in shock tunnel mode were analyzed in detail. One of the signals allowed an unambiguous derivation of the temperature under the conditions of a shock with Mach 2.7 (p≈ 5 MPa, T≈ 1200 K, deviation 0.5 % , uncertainty 4.9 %).