Acoustic measurements of supersonic jet flows

• Main Author: Wei, Xiaofeng
• Other Authors: New Tze How, Daniel
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Mechanical engineering; Engineering::Aeronautical engineering
• Online Access: https://hdl.handle.net/10356/145568
• Institution: Nanyang Technological University

There is an increasing interest in studying supersonic jet noise due to the immense noise level and poor understanding of the generation mechanisms. In many earlier studies, some nonuniform nozzle geometries were proposed to attain noise reductions in supersonic jets. However, the effectiveness and efficiency of noise reduction vary with the nozzle geometries and the fundamental mechanisms responsible for noise reduction are still unclear due to limitations of experimental measurement techniques for supersonic jet flows. Thus the present effort aims to study the noise radiated from different nonuniform nozzles to provide an experimental database for detailed characterizations. An equally important goal is to study acoustic properties in the near-field region to better understand the effect of nozzle exits on the generation of different noise components. In the present study, six lip-modified nozzles were designed and their detailed acoustic fields are compared to that of the circular baseline nozzle. Two different stepped nozzles were firstly tested at over- and perfectly- expanded conditions. The far-field noise spectrum analyses show that the stepped exits play an insignificant role in altering noise emissions at perfectly-expanded condition. At 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. To explore the physics responsible for the observed noise reduction, detailed near-field acoustic measurements were conducted to locate the sources of different noise components. The location, extent and intensity of large-scale turbulent noise are slightly modified by stepped nozzles at different azimuthal angles. More importantly, no prominent behaviour of broadband shock noise appears to exist in the near-field of the longer stepped nozzle. Subsequently, cross-correlation technique was employed to study the effects of nozzle geometry on the spatial structure of acoustic field. Baseline and stepped nozzles lead to large coherence noise sources at far downstream regions and the noise coherence region appears to be enlarged by the stepped nozzles, especially for the longer stepped nozzle. An experimental investigation for the stepped nozzles was also conducted at under-expanded conditions. When compared to the circular baseline nozzle, far-field acoustic measurements show that stepped nozzles lead to significant noise reductions at certain polar and azimuthal angles. Detailed spectral analysis shows that the noise reduction is mainly due to reduction in broadband shock associated noise and elimination of jet screech phenomenon. Subsequently, qualitative schlieren technique was employed to visualize the resulting shock structures and the results show that shock cell structures within the jets are asymmetrically modified by stepped nozzles. Quantitative schlieren measurements were performed to correlate the shock structures and density gradient fields with the resulting noise components. The results also show that the reduction in broadband shock associated noise is related to the lower shock strengths, as demonstrated by the density gradient profiles. Additionally, acoustic fields of bevelled, double-bevelled and stepped nozzles were compared using detailed far-field acoustic measurements at different jet expansion conditions. At perfectly-expanded condition, the lip-modified nozzles lead insignificant effect on noise emission, while they generate prominent noise reduction at forward quadrant for imperfectly expanded jets. Particually, the S60 nozzle produces the highest noise reduction of 5.5dB and 6dB at conditions of NPR=2.8 and 4, respectively. At under-expanded condition of NPR=5, the B60 is the optimal nozzle configuration where it generates the maximum noise reduction of 9dB at ϕ=90° and θ=180°. It should be mentioned that the studied nozzles occasionally generate noise tradeoff at shallow polar angles, i.e. ϕ≤50°, where they produce comparable or slightly higher noise level to the baseline nozzle. Finally, the effects of nozzle trailing shape on screech radiation are also investigated through acoustic measurements at an under-expanded condition of NPR=5. Bevelled nozzles eliminate jet screech by producing asymmetric shock structures which results in instability waves within the jet column, as well as mismatches in phase and amplitude. Additionally, double-bevelled nozzles produce symmetric shock structures and amplified screech intensity, even when jet-mixing effects have been significantly enhanced. To study the screech dynamics, wavelet transform was employed to analyze the near-field microphone signals. Jet screech and harmonics of different nozzles are highly unsteady and undergo non-periodic and stochastic temporal variations. Subsequently, near-field acoustic measurements were conducted to locate the screech sources. Double-bevelled nozzles have significant impact upon screech intensity levels and peak noise locations, as well as conferring different changes along different measurement planes.