Visual hull based 3D reconstruction of shocks in under-expanded supersonic bevelled jets

Development of schlieren-image based flow diagnostics and analysis for supersonic jet flows

“…Past studies have demonstrated that circular IO nozzles have the capability to simultaneously modify the shock structures and alter the instability characteristics of the shear layer [27,[40][41][42]. In particular, single-bevelled circular nozzles were observed to introduce thrust/jet vectoring and non-diamond shock cells [27,42] that were successful in mitigating jet screech [40] and could offer lower noise pollution in an industrial setting such as cold-spraying applications. The jet shear layer was augmented azimuthally with asymmetric spread rates observed in the two orthogonal planes.…”

“…For the various studies presented in this thesis, all experiments were performed in an open-air blow-down supersonic cold jet facility situated in Temasek Laboratories, National University of Singapore. The same supersonic jet apparatus had been used in several past studies [27,42,110]. Dehumidified compressed air from air reservoirs capable of storing air at a maximum of 69 bars stagnation pressure is channelled through a series of valves and regulators to the settling chamber of the jet, where standard flow straighteners were employed to improve flow quality.…”

“…A new 3D shock reconstruction approach based on the visual hull image processing technique was proposed and demonstrated in an asymmetric free jet application [42] in Chapter 4. As such, it will be useful to utilize this technique to shed more light upon the differences between standoff shocks produced by different supersonic impinging IO jets.…”

“…As the 3D shock reconstruction principle and procedures have been described in great detail in Chapter 4 [42], this section will only highlight the most important features of the reconstruction procedures adopted in the present study. To perform the reconstruction on the standoff shocks, two pieces of information are required -Firstly, the silhouette images of the standoff shock and secondly, the corresponding camera matrices which defines the position and orientation of the cameras capturing the images.…”

“…In accordance to the procedures described in Chapter 4 [42], the image postprocessing mainly consists of binary decisions performed at the location of the standoff shocks, which classifies the image pixels as part of the standoff shock silhouettes or not. For the current study, since the standoff shocks display excellent image intensity contrast with the background, the acceptance criterion is based on a pre-defined image intensity threshold.…”