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An experimental study was conducted to investigate and compare the effects of beveled and double-beveled circular nozzles on supersonic jet screech at a nozzle pressure ratio of 5. In particular, near- and far-field microphone measurements and schlieren visualizations were utilized to look into selected acoustic and flow features associated with jet screech radiation. Results show that beveled nozzles eliminate jet screech by producing asymmetric shock structures and instability waves that are mismatched in phase and amplitude. In contrast, double-beveled nozzles produce symmetric shock structures and amplify screech intensity, even when jet mixing effects have been significantly enhanced. It is further observed that amplified jet screeches produced by double-beveled nozzles are highly unsteady and undergo nonperiodic and stochastic temporal variations. Last but not least, double-beveled nozzles also significantly impact screech peak noise locations and confer different changes along different measurement planes. The present study demonstrates that not all beveled-type nozzles are able to mitigate jet screech, with nonoptimal designs amplifying it instead. [ABSTRACT FROM AUTHOR]

Detailed near- and far-field acoustic measurements were conducted for two circular stepped nozzles with 30°deg and 60°deg design inclinations at over- and perfectly-expanded supersonic jet flow conditions and compared to those for a circular nonstepped nozzle. Far-field acoustic results show that stepped nozzles play an insignificant role in altering noise emissions at perfectly expanded condition. At an over-expanded condition, however, the longer stepped nozzle produces significant noise reductions at the sideline and upstream quadrants, while the shorter stepped nozzle does not. Noise spectra analysis and Schlieren visualizations show that noise reduction can be primarily attributed to mitigations in the broadband shock-associated noise (BSAN), due to the ability of the longer stepped nozzle in suppressing shock strengths at downstream region. Near-field acoustic measurements reveal that the source region, as well as the intensity of turbulent and shock noises, are highly sensitive to the stepped nozzle configuration. Furthermore, BSAN seems to be eliminated by the longer stepped nozzle in near-field region due to the shock structure modifications. [ABSTRACT FROM AUTHOR]

An experimental investigation into noise reduction of supersonic jets through nozzle trailing-edge modifications was conducted, whereby far-field acoustic measurements were captured for two different stepped nozzles under two distinct under-expanded conditions. When compared to a baseline nozzle, results show that stepped nozzles lead to significant noise reductions at certain polar and azimuthal angles. In particular, a maximum noise reduction of 6 dB is observed for the longest stepped nozzle at a nozzle-pressure-ratio of 4 and 0° azimuthal angle. Spectral analysis shows that the noise reduction is mainly due to reduction in broadband shock associated noise and elimination of jet screech phenomenon. Abrupt changes in nozzle lip lengths of the stepped nozzles appear to disrupt acoustic feedback loop, thus resulting in screech cessation. Qualitative schlieren imaging and quantitative schlieren measurements were subsequently performed to correlate the shock structures and density gradient fields with the resulting noise components. Unlike those produced by the baseline nozzle, shock structures generated by the stepped nozzles are highly irregular and the jet plumes undergo discernible deflections. Lastly, the reduction in broadband shock associated noise is related to the lower shock strengths, as demonstrated by the density gradient profiles. [ABSTRACT FROM AUTHOR]

Highlights • Novel shock structure reconstruction technique based on schlieren and visual hull. • Prior knowledge of global velocity or density field not required for reconstruction. • Achieved high resolution and 2.5% average error in reconstructed shock structures. • Successful reconstruction of axisymmetric and asymmetric shock structures. • Relative to current methods, proposed technique is fast, robust and easy to apply. Abstract Three-dimensional shock structures produced by Mach 1.45 supersonic bevelled jets were digitally reconstructed based on schlieren photography and a voxel-based visual hull technique. By taking advantage of the strong edge features commonly found in schlieren images of shock waves, the proposed technique demonstrates the possibility of performing shock wave reconstruction in supersonic jet applications without prior knowledge of the global density or velocity field. Semi-synthetic camera parameters were introduced as a method to circumvent camera calibration issues faced in the reconstruction procedure. This is key to achieving accurate and high-resolution reconstructed shock waves for both axisymmetric and asymmetric test cases with an average of 2.5% error when validated against raw schlieren images. When applied to bevelled jets with non-uniform nozzle exit geometries, an additional assumption was made to address the problem of schlieren line-of-sight blockage by the non-conventional nozzle, and reconstruction errors were found to be larger near regions of poorer shock wave contrast. Current results indicate that the technique is robust and fast during image calibration and processing, with accuracy of reconstructed shock waves in both conventional and non-conventional nozzles strongly dependent on shock wave contrast. Compared to existing techniques that can be used to reconstruct 3D shock structures, the proposed technique has the advantage of being totally non-intrusive as compared to point or particle-based measurements, requires significantly less computation than tomographic methods, offers high resolution reconstruction even with limited camera resolution and projected schlieren views, and is easy and cost effective to implement. [ABSTRACT FROM AUTHOR]

Moderately under-expanded jets issuing from a circular baseline and two bevelled circular nozzles impinging upon a perpendicular flat plate were experimentally studied. The effects of nozzle-pressure-ratio and separation distance variations on the standoff shock formations were investigated with schlieren visualizations and a visual hull based three-dimensional (3D) shock reconstruction technique to provide deeper insights into their 3D features. Across all flow configurations arising from the different combinations of these parameters, results indicated that the bevelled nozzles are effective in introducing asymmetry to the standoff shock geometries. Depending on the exact flow configuration, standoff shock locations may also undergo significant upstream displacements. In particular, the single-bevelled nozzle produces highly unsteady standoff shocks with asymmetric oscillation amplitudes along both side of the nozzle lip regions. Changes to the standoff shock key characteristics were observed to be sensitive towards the jet shock structures and reflection point modified by the bevelled nozzle exits. In particular, the strength and relative position of the reflection point are identified as the major contributing factors influencing the upstream static pressure distribution of the standoff shock, hence leading to the observed changes in the standoff shock behaviour. [ABSTRACT FROM AUTHOR]