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Designing vehicles for efficient hypersonic flight is a complex engineering challenge that requires designers to use high-fidelity computational models for reacting, high-enthalpy gas flows. These advanced models must be validated with hypersonic experiments, typically conducted in ground-test facilities. The effectiveness of a hypersonic experiment is limited by the quantity and quality of measurements that can be collected. Key parameters for validating and improving computational models include the spatiotemporally-localized chemical composition, temperature (or multiple thermally non-equilibrated temperatures), density, and velocity of the hypersonic test gas. Therefore, developing advanced sensors capable of accurately measuring these parameters in hypersonic experiments is essential for validating computational methods and designing efficient hypersonic vehicles.
This dissertation describes the development of a series of sensors based on laser absorption spectroscopy and their deployment in several hypersonic ground-test facilities to (1) characterize the gas conditions produced by these facilities and (2) generate measurements that support the validation and improvement of hypersonic computational models. Additionally, a novel in-situ sensor architecture for laser absorption was developed to enhance the sensors’ ability to extract quantitative information from hypersonic test gases.
The primary diagnostic technique employed in this dissertation is near-infrared tunable diode laser absorption spectroscopy (TDLAS). TDLAS sensors utilize robust, low-cost, off-the-shelf optical equipment and can collect quantitative, calibration-free, and temporally-resolved measurements of crucial parameters in hypersonic gases. Additional sensors based on mid-infrared TDLAS and optical emission spectroscopy were used to complement the near-infrared TDLAS sensors. Thisdissertation presents three separate studies detailing the deployment of TDLAS-based sensors in three distinct hypersonic ground-test facilities.
In the first study (Chapter 3), a TDLAS-based sensor was developed and deployed in the Hypervelocity Expansion Tube (HET) at the California Institute of Technology (Caltech). The sensor was used to characterize the hypersonic freestream conditions generated by HET and to provide a dataset supporting improvements in computational modeling of the facility. This sensor targeted the D1 spectroscopic transition of atomic potassium near 770 nm wavelength and employed rapidscanning techniques to measure the flow velocity and translational temperature of the test gas at 200 kHz, enabling inferences of total enthalpy. Multiple iterations of the sensor progressively reduced experimental uncertainty. Several HET conditions were characterized, with gas temperatures ranging from 900 to 1600 K, flow velocities from 3.3 to 4.4 km/s, total enthalpies from 7 to 10 MJ/kg, and test-gas compositions relevant to hypersonic flight on both Earth and Mars. The measurements showed discrepancies with computational models of the facility, prompting ongoing efforts to improve those models and enhance understanding of HET’s operations. A set of mid-infrared (near 4.2 µm) carbon dioxide-targeting TDLAS sensors validated select measurements collected by the potassium-targeting sensor.
In the second study (Chapter 4), a pair of TDLAS-based sensors was utilized in the T5 freepiston shock tunnel, also at Caltech. These sensors targeted the populations of two electronicallyexcited energy states of atomic oxygen and atomic nitrogen, the 5S2 (777 nm) and the 4P5/2 (868 nm), respectively. The TDLAS sensors were deployed behind a Mach stem created by a symmetric, opposing pair of wedge test articles. Measurements were collected under conditions where T5 generated Mach 5 air with a total enthalpy of approximately 16 MJ/kg. The targeted 5S2 electronic state population of atomic oxygen was detected during the facility test time, and these measurements were compared to preliminary computations from the US3D software.