Speaker
Description
Introduction
The complexity of the flow field encountered around re-entering vehicles poses significant problems to the design of spacecraft thermal protection systems. One large source of uncertainty is linked to thermochemical non-equilibrium. Ground testing is conducted to generate flows similar to those encountered in flight, replicating the essential features of non-equilibrium flows. One such canonical experimental setup is the shock tube, where a normal shock transiently passes through a straight tube [CB17]. Although subject to several facility related artefacts [CMM23, CSMM22], this setup represents one of the most fundamental fluid mechanical processes, which allows the isolation of thermochemical non-equilibrium from other aspects of complex flowfields. As such, shock tubes provide the opportunity of studying fundamentals of thermochemical reactions for flight-relevant enthalpies. Optical diagnostics provide the ideal vehicle to interrogate these flows with respect to their thermochemical state. As one such technique, laser absorption spectroscopy (LAS) is seeing a rise in use due to the recent improvement in high-speed tunable diode lasers and quantum cascade lasers [SKW+19, GGD+24]. LAS targets the lower state of radiative transitions and can therefore provide absolute number density measurements of low-energy quantum states. If ground states are measured, absolute particle densities of the probed species can be inferred with high accuracy. Scanning absorption spectroscopy techniques utilise a spectrally narrow light source whose central wavelength is changed very rapidly. By scanning over an absorption line in this fashion and recording the transmitted light with a detector, the line profile and absolute absorbance are measured and can be utilised to infer the translational temperature and lower state density. Broad band light sources can be utilised as well, however, the absorption features need to be spectrally resolved by a spectrometer. In the current work, a broad band light source is used, as it provides an instantaneous snapshot of the passing shock wave and does not require tuning of the wavelength during the experiment.
Methodology
Shock tube flows are generated in the Oxford T6 Stalker Tunnel in aluminium shock tube mode [GCM22]. Optical measurements are taken through windows set in the shock tube wall, and acquisition is triggered to record as it passes this location. Flow conditions investigated in this work will consist of a set of velocities between 5.5 km/s and 6.5 km/s [GCM22]. The laser absorption spectroscopy system utilises a bespoke modeless laser based on the work by Ewart [Ewa85]. A pulsed laser source provides a beam with a high degree of collimation that is advantageous for propagation over a long path, accurate steerability through the region of interest and efficient illumination of the spectrometer for recording the absorption spectrum. Laser sources, however, are usually characterized by a spectrum of longitudinal modes with frequency separation related to the laser cavity length. The spectral gaps between modes and their fluctuation in amplitude and frequency leads to difficulties in recording absorption spectra consisting of narrow spectral features. Some molecular absorption lines that may fall between the modes will not be recorded or distorted as a result of the amplitude and frequency fluctuations. The modeless laser, since it operates without a resonant cavity, is free from longitudinal mode-structure and provides an essentially continuous spectrum with spectral noise determined basically by quantum fluctuations in the amplified spontaneous emission from the amplifying medium. The unique advantage of this system is that it provides a tunable centre wavelength with variable bandwidth and a continuous spectrum that eliminates mode noise [SSE91, EAB+05, KE97]. A flashlamp-pumped nanosecond Surelite I-10 Nd:YAG laser is used in its third harmonic mode producing radiation at 355 nm which is passed through a set of lenses to control the beam size. The Nd:YAG fundamental and second harmonic (1064 nm and 532 nm) are dumped in an enclosure outside of the Nd:YAG laser head, with only the third harmonic propagating into the modeless laser system. The third harmonic beam is separated into four beams by a four-faceted prism which are each absorbed by a dye cell at different heights [Ewa85]. The dye cell features a continuous flow of ethanol containing 0.28 g/L of Coumarin dye. The dye produces a spectrally broad output at each of the four pumped locations.
The spontaneous emission from the four pumped strips in the dye cell are amplified as a travelling wave by refection at two totally internally reflecting (TIR) prisms with apexes slightly displaced relative to each other. The dispersing prism selects a band of wavelengths from the fluorescence spectrum of the dye. The orientation of the right-hand TIR prism is used to select a band centred on 452 nm. The output beam is subsequently frequency doubled in a critically phase-matched crystal of BBO (Beta Barium Borate) to 226 nm with a bandwidth (FWHM) of pproximately 2 nm. This ultra-violet beam is separated from the fundamental at 452 nm using a Pellin-Broca prims and directed to the shock tube. Once the beam is produced with the aforementioned spectral properties, it is passed through a system of turning mirrors and relay lenses to the side of the T6 tunnel. At this location it is expanded and collimated by a twolenses system containing a cylindrical and a spherical lens, resulting in a laser sheet. This sheet is aligned with the field of view of a telecentric imaging system. As the shock wave travel through this field of view, the laser is activated and partially absorbed by the nitric oxide in the flow. The laser sheet will be arranged in such a way that it covers the freestream, non-equilibrium region and equilibrium region behind the shock front. The transmitted radiation of the sheet is imaged onto the entrance slit of the spectrometer. Hence, the resulting spectral image corresponds to a spatially resolved image along one dimension. This way, the absorbance can be simultaneously measured at different locations across the shock wave, resulting in a resolution of the non-equilibrium layer. The data acquired in this way will be used to infer NO number densities and excitation temperatures through spectral fitting methods.
Results
The full paper will present the data of the currently ongoing test campaign and will contain setup, calibration and post-processing. Raw absorbance data will be shown, as well as the post-processed properties of nitric oxide in the shock layer. The post-processing will be undertaken by comparing the measured absorbance to a computational model which simulates the absorption through a high temperature gas. This will allow the determination of nitric oxide ground-state densities, as well as vibrational and rotational temperature.
Summary
A flashlamp-pumped nanosecond Surelite I-10 Nd:YAG laser is used in its third harmonic mode producing radiation at 355 nm which is passed through a set of lenses to control the beam size. The Nd:YAG fundamental and second harmonic (1064 nm and 532 nm) are dumped in an enclosure outside of the Nd:YAG laser head, with only the third harmonic propagating into the modeless laser system. The third harmonic beam is separated into
four beams by a four-faceted prism which are each absorbed by a dye cell at different heights [Ewa85]. The dye cell features a continuous flow of ethanol containing 0.28 g.L$^-1$ of Coumarin dye. The dye produces a spectrally broad output at each of the four pumped locations. The spontaneous emission from the four pumped strips in the dye cell are amplified as a travelling wave by refection at two totally internally reflecting (TIR) prisms with apexes slightly displaced relative to each other. The dispersing prism selects a band of wavelengths from the fluorescence spectrum of the dye. The orientation of the
right-hand TIR prism is used to select a band centred on 452 nm. The output beam is subsequently frequency doubled in a critically phase-matched crystal of BBO (Beta Barium Borate) to 226 nm with a bandwidth (FWHM) of approximately 2 nm. This ultra-violet beam is separated from the fundamental at 452 nm using a Pellin-Broca prims and directed to the shock tube. Once the beam is produced with the aforementioned spectral properties, it is passed through a system of turning mirrors and relay lenses to the side of the T6 tunnel. At this location it is expanded and collimated by a twolenses system containing a cylindrical and a spherical lens, resulting in a laser sheet. This sheet is aligned with the field of view of a telecentric imaging system. As the shock wave travel through this field of view, the laser is activated and partially absorbed by the nitric oxide in the flow. The laser sheet will be arranged in such a way that it covers the freestream, non-equilibrium region and equilibrium region behind the shock front. The transmitted radiation of the sheet is imaged onto the entrance slit of the spectrometer. Hence, the resulting spectral image corresponds to a spatially resolved image along one dimension. This way, the absorbance can be simultaneously measured at different locations across the shock wave, resulting in a resolution of the non-equilibrium layer. The data acquired in this way will be used to infer NO number densities and excitation temperatures through spectral fitting methods.