Optically-based flow diagnostic methods: meeting unique challenges in free-piston shock tunnels A.F.P. Houwing, P.M. Danehy, P.C. Palma, J.S. Fox, S. O’Byrne, M. Cooper, P. Mere Aerophysics and Laser-based Diagnostics Research Laboratory, Department of Physics, Faculty of Science Australian National University, ACT 0200, Australia One of the main motivations behind the development of free-piston shock tunnels has been the need to simu- late, in a ground-based facility, the high enthalpy con- ditions associated with hypersonic flight. An added ad- vantage of these facilities are their suitability for study- ing supersonic mixing and combustion for scramjet re- search. Thus, they are used quite extensively for funda- mental and applied research in the areas of supersonic and hypersonic, external and internal, aerodynamics. In long duration facilities, measurements of flow pa- rameters are often made with probes that are scanned spatially through the flow. However, this approach is unsuitable in pulsed flow facilities, because of their short flow duration. Planar laser-induced fluorescence (PLIF) imaging (Hanson 1992), which yields instanta- neous measurements at a large number of points in a laser-illuminated plane, provides an attractive alterna- tive to such probe-based methods in pulsed facilities, and has the added advantage of being non-intrusive. Techniques based on the PLIF method can accu- rately monitor a variety of gas-phase parameters in- cluding species concentrations, temperature and ve- locity. However, in using such techniques in free- piston shock tunnels, a number of problems are en- countered. One of the major problems is the flow lu- minosity. Metallic contaminants which are eroded from the shock tube walls produce strong emission in high- temperature regions in the flowfield. This background emission can easily swamp fluorescence signals. The effects of this luminosity can be minimised by the use of spectral and temporal filtering, however, at high en- thalpies, contamination of the signal by background emission can not be fully eliminated. It is then nec- essary to make careful measurements of this emission through the acquisition of luminosity images and cor- rect the emission-contaminated PLIF image accord- ingly. Emission from metallic impurities present in the flow imposes a severe limitation, especially at the very high specific flow enthalpies which are important for the study of non-equilibrium phenomena. In spite of this and other difficulties encountered, it has been possible to successfully apply PLIF-based methods to many important flow studies in free-piston shock tun- nels. We will demonstrate this by providing a number of specific examples of PLIF-based flow visualisation, thermometry, velocimetry, and mole-fraction imaging. As an example of PLIF-based flow visualisation, we consider the problem of the establishment of a sepa- rated flow region at the base of a cone in a supersonic flow. Here, the requirement of the visualisation is to show the time-evolution of the main features of the separated flow to determine whether the shock tunnel flow time is sufficiently long-lived to establish a qua- sisteady separated flow region. The limited flow time available for shock tunnel testing makes it essential for any study which aspires to describe the flow in the sep- arated region to ensure that the flow within that region has reached a steady state. The intensity of the laser-induced fluorescence de- pends upon the temperature of the flow and the num- ber density of molecules in the state excited by the laser, as well as a number of other flow parameters. Flow features which cause changes in these quantities, such as shock waves, expansion waves and mixing re- gions, can be visualised using the differing signal inten- sities in these regions. The flow within the separated region has a higher temperature and much lower pressure and velocity than the surrounding inviscid flow. This wide variation of flowfield properties makes the base flow around a cone in a hypersonic freestream a difficult region to im- age. Any visualisation or measurement technique must be capable of detecting very small number densities to provide adequate measurement sensitivity. An impor- tant issue for consideration is which transition is most suitable for the purpose of visualising the flow. Crite- ria for suitability are isolation of the transition, overall LIF signal strength and sufficient difference in LIF sig- nal strength to allow clear differentiation of the differ- ent regions of the flow, in particular the re-circulation zone and the re-attachment shock. To determine which line is most suitable, we use numerical modelling of the flow and PLIF process to select a number of suitable candidates. These are then used in a series of prelimi- nary experiments to determine the most suitable tran- sition before proceeding to more extensive experiments. The success of this approach has been demonstrated by O’Byrne et al. (1998) in their work on imaging the flow in the base region of the cone. We next consider PLIF thermometry. A typical shock tunnel flowfield is a challenging test for accu- rate PLIF thermometry. There are large pressure and temperature gradients which cause large fluorescence signal variations due to high collisional quench rates and changes to the overlap integral. Other problems include laser beam attenuation and hole burning in the laser’s spectral profile, spectral interference from other flow species and fluorescence trapping. Qualita- tive images are often easy to obtain, but inferring quan- titative information from these images is considerably more difficult. However, if these issues are carefully ad- dressed in designing a particular thermometry experi- ment, good experimental data can be obtained. This is illustrated by: Palma et al. (1998), who measured the rotational temperature at the exit of a supersonic nozzle; by Palma et al. (1997), who measured the vi- brational temperature in a shock layer on a hemisphere; and Palma et al. (1998), who measured the rotational temperature profile in a compressible boundary layer on a flat plate. We next consider PLIF velocimetry. A number of difficulties are encountered when searching for a suit- able technique for velocimetry in free-piston shock tunnels. Because the flow is supersonic, physical probes such as hot-wire anemometers are inappropri- ate. Furthermore, imaging methods are preferred to single-point methods such as laser Doppler velocime- try (LDV) because the limited test time of the facil- ity would make it very expensive to map the flow ve- locity. Several laser-based methods have been devel- oped for mapping the velocity in gaseous flows. These include particle image velocimetry, Rayleigh scatter- ing velocimetry, and planar laser-induced fluorescence (PLIF) velocimetry. Particle image velocimetry is 1