Advancements and Practical Applications of Molecular Tagging Velocimetry in Hypersonic Flows

Jordan Matthew Fisher, Purdue University

Abstract

Hypersonic flows consist of harsh environments where chemistry effects are relevant and low speed assumptions such as the ideal gas law and the continuum hypothesis begin to break down. Because of these processes, computer models do a poor job of predicting behavior of vehicles in hypersonic flight. High fidelity ground test measurements are necessary to anchor and extrapolate CFD simulations so that flight vehicle designs can continue to improve. Due to the harsh conditions and complexities of test facilities, implementing experimental measurements can prove challenging. Molecular tagging methods such as Femtosecond Laser Electronic Excitation Tagging (FLEET) are attractive for use in hypersonic ground test facilities for many reasons. They are generally considered non-intrusive, since they require no physical probes or seed particles to be placed in the flow. This both keeps the facility safe from damage and minimizes the disturbance imparted on the flow field by the measurement. Since the tracer is comprised of molecules already present in the flow, the measurement is reliable and can track velocities over a wide dynamic range. The optical arrangement for FLEET is rather simple, requiring only a focused laser beam and a camera to capture the signal. The method can even be applied as a one-sided measurement requiring only one direction of optical access. The current state-of-the-art for the FLEET method is point-wise measurements made at 1 kHz with a commercially available laser system. The basis for this thesis is to identify and address current limitations in the implementation of FLEET to relevant flow facilities in terms of the useful aerodynamic information that can be extracted. Fundamental advances to the spatial extent and temporal resolution of FLEET are investigated, and novel applied measurements in high speed flow facilities are presented. Considerations of the precision, spatial resolution and ability to implement fundamental advances to harsh and more complex environments are discussed. A custom-built burst-mode femtosecond laser system is used to enable FLEET measurements at 1 MHz, an improvement of three orders of magnitude in measurement rate. New optical arrangements including microlens arrays and holographic beamsplitters are developed to allow multi-dimensional grids to be tracked to instantaneously measure velocity gradients. Shock wave and shear measurements in a supersonic bladeless turbine and boundary layer measurements on a Mach 6 cone-cylinder-flare are demonstrated. Additionally, an adapted method, Femtosecond Laser Activation and Sensing of Hydroxyl (FLASH) is developed and applied to measure velocity in reacting environments such as a Rotating Detonation Engine (RDE). These innovations provide a path forward for improving the spatiotemproal fidelity of velocity measurements and extending the capability for investigation high-speed reacting and non-reacting flows in hypersonic ground test facilities.

Degree

Ph.D.

Advisors

Paniagua, Purdue University.

Subject Area

Optics

COinS