| 1. |
- Lim, H. D., et al.
(author)
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Short-time proper orthogonal decomposition of time-resolved schlieren images for transient jet screech characterization
- 2020
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In: Aerospace Science and Technology. - : Elsevier BV. - 1270-9638 .- 1626-3219. ; 107
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Journal article (peer-reviewed)abstract
- Short-time Proper Orthogonal Decomposition (POD) is proposed as an image-based technique to study the transient jet screech characteristics of moderately under-expanded supersonic jets emanating from a circular baseline and two bevelled nozzles. Time-resolved schlieren imaging of turbulent flow structures were performed with an ultrahigh-speed schlieren setup. Short-time POD was performed by systematically sampling image-series with a short time delay, performing PODs and applying spectral analyses on the first POD mode coefficients, and plotting the peak frequencies from the resulting PSDs into a peak frequency-occurrence count histogram. The results are in good agreement with the near-field noise spectra and wavelet transform analysis of the microphone measurements, which revealed intermittent jet screech occurrences at St=0.25 for both baseline and 30 degrees bevelled jets, while none was detected for the 60 degrees bevelled jet. In particular, the occurrence counts of the frequency bins is proposed as a suitable parameter to characterize the intermittent nature of jet screech, with the frequency bin revealing the jet screech frequency if present. The present study demonstrates the advantage of short-time POD analysis on time-resolved schlieren images over traditional image-based POD methods, which includes computational gains from parallelization, the ability to handle much larger datasets and revealing insights into a transient flow and noise phenomenon.
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| 2. |
- Mariani, Raffaello, et al.
(author)
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A comparative study on the use of calibrated and rainbow schlieren techniques in axisymmetric supersonic jets
- 2019
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In: Flow Measurement and Instrumentation. - : Elsevier BV. - 0955-5986 .- 1873-6998. ; 66, s. 218-228
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Journal article (peer-reviewed)abstract
- Detailed experimental comparisons had been conducted between calibrated and rainbow schlieren on perfectly-and under-expanded axisymmetric supersonic jets through a modified Z-type schlieren system. The techniques were implemented by using a weak lens in the field-of-view to provide calibration information for the extraction of quantitative density gradients from the experimental schlieren images. Sixth-order polynomial curve fits were obtained for both calibrated and rainbow schlieren respectively. The effects of light inhomogeneity caused by the mirrors and system diaphragm aperture had been evaluated for the colour images and results indicate that averaging the background hue is an acceptable approach for minimizing light variations with less than 2% experimental error. Density gradients as calculated via Abel transform have also been evaluated to validate the two different set-ups. Additionally, experimental results have been compared to validated numerical results and they show that calibrated schlieren is able to predict density gradients within 2% of the numerical results. This is significantly more superior to rainbow schlieren, where errors in the estimated density gradients are clOser to 20%. It is shown here that rainbow schlieren results are more adversely impacted by the system diaphragm aperture, especially for vertical light cut-off configuration. This is partly due to the loss of sensitivity of the schlieren system, as well as potential light diffusion caused by the filter.
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| 3. |
- Mariani, Raffaello, Assistant Prof. 1981-, et al.
(author)
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On the application of non-standard rainbow schlieren technique upon supersonic jets
- 2020
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In: Journal of Visualization. - : Springer Nature. - 1343-8875 .- 1875-8975. ; 23:3, s. 383-393
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Journal article (peer-reviewed)abstract
- Abstract: A quantitative rainbow schlieren study was conducted on an over-expanded jet at nozzle pressure ratio of 2.8, based on two different schlieren set-ups: the standard z-type and a single-mirror schlieren set-up. The technique used a single, weak focal-length lens placed in the field of view of the system to provide the calibration information required for the extraction of the quantitative data. In the case of the single-mirror set-up, the calibration image required further post-processing procedures to take into account the double refraction experienced by the light. Density gradients were calculated using Abel transform and compared to validated reference data. Results indicate that the single-mirror set-up is able to improve prediction of the density gradient field as compared to the standard z-type schlieren, due to its inherent property of higher sensitivity. The study has shown that the single-mirror set-up performs on average better than the standard z-type system, yielding an overall averaged error of ± 20%, with localized values as low as ± 5% where the shock cell structure is clearly defined, with respect to the validated reference data. At the same time, both systems perform poorly in regions where the flow structure displays poor image contrast. Graphic abstract: [Figure not available: see fulltext.].
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