In aerothermodynamics, the fundamental equations (e.g. Momentum, Energy, & Continuity) determine the quantities of interest with regards to measurements. This includes velocity, temperature, pressure, etc. Additionally, measurements of the positions of features such as shockwaves are important. A variety of techniques exist for measuring these quantities, but laser and optical techniques provide minimally-intrusive measurements that reduce the potential for disturbing the flow environment. After-all, the most ideal measurement is is also the least intrusive. Furthermore, these techniques can be readily applied to nearly any flow condition.
One of our primary efforts in this area is the transition of existing techniques from the optical bench-top, to the T&E scale aerospace testing facilities. Typically, optical signals have intensities proportional to the density in the probe region. However, the isentropic expansion of gases through a nozzle is necessary to achieve supersonic flow and this process usually results in significant reduction of gas density in the freestream. Furthermore, scattering or emission based techniques have an inverse squared dependence. This means that if your signal is generated at the center of your tunnel and the collection optics are outside of the tunnel, you will have significant signal loss if the tunnel is large. These are some of the challenges we are attempted to address by developing methodologies and new techniques suitable for aerospace testing platforms. Our various tunnel scales provide nice stepping-stones for this process.
Schlieren is one of the oldest flow visualization techniques that makes use of optical principles. Collimated (i.e. light with parallel rays) is passed through a region of interest and is focused down to a point afterwards. At that focal point, a knife edge is placed. Any of the collimated light that passes through a region with changing density will deflect at a slight angle due to a local change in index of refraction (see Gladstone-Dale relation). This for example occurs for shockwaves. The deflected rays focus a point slightly different from the collimated light due to the angle associated with the deflection. Hence, the some of the light with be deflected over the knife edge, and some light will be deflected and blocked by the knife edge. This gives rise to bright and dark spots in an image, and shows regions of density change. Note too that density changes are related to temperature changes through the ideal gas law, and so thermal effects can be detected in schlieren too.
We utilize schlieren in each of our facilities. We have the capability to do continuous 400 kHz schlieren imaging thanks to custom pulsed LEDS. While schlieren is typically considered a qualitative diagnostic, we utilize the advances in data analysis to extract quantitative info. This includes using frequency analysis, modal analysis techniques (e.g. SPOD, DMD, etc.), optical flow, and feature tracking. We are also exploring the use of machine learning techniques for data processing.