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# Accuracy of non-resonant laser-induced thermal acoustics (LITA) in a convergent–divergent nozzle flow

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Non-resonant laser-induced thermal acoustics (LITA) was applied to measure Mach number, temperature and turbulence level along the centerline of a transonic nozzle flow. The accuracy of the measurement results was systematically studied regarding misalignment of the interrogation beam and frequency analysis of the LITA signals. 2D steady-state Reynolds-averaged Navier–Stokes (RANS) simulations were performed for reference. The simulations were conducted using ANSYS CFX 18 employing the shear-stress transport turbulence model. Post-processing of the LITA signals is performed by applying a discrete Fourier transformation (DFT) to determine the beat frequencies. It is shown that the systematical error of the DFT, which depends on the number of oscillations, signal chirp, and damping rate, is less than $$1.5\%$$ for our experiments resulting in an average error of $$1.9\%$$ for Mach number. Further, the maximum calibration error is investigated for a worst-case scenario involving maximum in situ readjustment of the interrogation beam within the limits of constructive interference. It is shown that the signal intensity becomes zero if the interrogation angle is altered by $$2\%$$. This, together with the accuracy of frequency analysis, results in an error of about $$5.4\%$$ for temperature throughout the nozzle. Comparison with numerical results shows good agreement within the error bars.
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J. Richter, J. Mayer, B. Weigand
"Accuracy of non-resonant laser-induced thermal acoustics (LITA) in a convergentdivergent nozzle
flow,"
Applied Physics B (2018) 124:19
https://rdcu.be/bf1l2
... Besides influence from inherent shot-to-shot variation, experimental scatter observed can further be affected by non-ideal temporal variation of postreflected shock conditions, estimated less than 2% for temperature T 5 at test time instant according to Peterson and Hanson [8]. Similar findings were reported by Richter et al. [46] who observed a negative frequency chirp, i.e., temporally decreasing signal frequency, that is indicative of gradually decreasing sound speed and temperature in the post-reflected shock regime, even for tailored interface conditions. ...
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