Pulsed Laser Imaging of Flows, Flames and Plumes


Advanced Laser Diagnostics of Supersonic Combusting Flows

Tim McIntyre, Halina Rubinsztein-Dunlop

Research in the Laser Diagnostics Laboratory focuses on developing and applying advanced laser diagnostic techniques to hypersonic flow. In particular, the group is part of the Centre for Hypersonics which has a range of experimental wind tunnels that are unique in the world. Experimental and numerical studies are conducted of flows relevant to orbital re-entry and to supersonic combustion.

Running from 2004-2008, a research project, funded by the Australian Research Council, has been conducted on Mach 10 hydrogen fuelled scramjet development. A scramjet (supersonic combustion ramjet) is a proposed propulsion system for future hypersonic vehicles. It functions by mixing air entering the front of the engine with on-board fuel to generate thrust at extremely high speeds. Engines of this type have previously being studied at the Centre for Hypersonics through shock tunnel testing and a rocket-launched flight test. A significant difficulty with the development of these engines is the lack of understanding of the physical processes during the mixing and combustion stages of the gas flow. Shock tunnel testing in the Department of Mechanical Engineering’s T4 facility focuses on improving our knowledge of the physics and chemistry involved, leading to a better engine design.

Laser diagnostics of gas flows rely on the interaction of light with the atoms or molecules in the flow to generate some observable effect. Common interactions include absorption, refraction, and non-linear methods. Honours projects will look at two techniques for studying scramjet flow. Planar laser-induced fluorescence involves absorption of the laser light by a molecule such as OH or NO and subsequent re-emission in all directions. This fluorescence is captured on a camera and can be processed to obtain relative species and temperature information. This technique is to be implemented on the T4 Shock Tunnel. The second technique is holographic interferometry. In this method the phase of the laser light passing through the gas is retarded due to the density variations in the flow. The phase is recorded interferometrically and can be used to quantify the density, electron concentration and/or species concentration. This method is to be implemented in the X3 Expansion Tube.

Interferometric measurements of ionizing super-orbital flows

Tim McIntyre, Halina Rubinsztein-Dunlop

Spacecraft on interplanetary flights travel at such extremely high velocities that, upon entering a planetary atmosphere, generate a high temperature reacting gas flow. The physics and chemistry of the gas flow have important consequences for the trajectory of the vehicle and its thermal protection. A current area of research at the Centre for Hypersonics are so-called super-orbital flows in which ground-based facilities are used to generate such high velocity conditions to allow their study. The purpose of this honours project is to use a laser-based diagnostic technique, holographic interferometry, to investigate these flows and to provide results for comparison with numerical simulations.

Holographic interferometry relies on the change of the phases of a beam of light as it passes through a gas flow. At the conditions generated in the facility used here, this phase is influenced by both the heavy particles (atoms and ions) in the flow as well as the electrons. To allow the measurement of both of these quantities, a two-wavelength holographic interferometry technique has been developed that relies on recording two interferograms for each flow. This technique has been successfully implemented in our tunnel on two-dimensional bodies to provide high quality measurements not otherwise obtained.

The focus of this project will be on the study of axisymmetric bodies (spheres, cones etc) in the tunnel. In a two-dimensional flow (cylinders, wedges etc), the refractivity of the gas is considered constant across the flow leading to a fairly simple analysis to obtain density. In axisymmetric flows, the refractivity continually varies. Thus an extra step must be performed to obtain flow density as a function of radius. The resulting measurements will be compared with numerical simulations performed as part of an agreement with the NASA Langley Research Centre.