• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2003.11a
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• pp.92-97
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• 2003

By using a centerbody injection, an effort to reduce shock assoicated noise is made in an underexpanded sonic nozzle with an exit diameter of 10mm. The centerbody or micro nozzle, aligned with the axis of the main jet has an o.d. of 2mm and i.d. of 1.5mm. When measured at 90$^{\circ}$ relative to the main jet the farfield noise spectra showed that the screech tones and broadband shock associated noise can be significantly reduced simply by varying the length of the centerbody and/or mass fraction of the microjet. The maximum reduction in overall sound pressure level (OASPL) was as much as 9 and 4 ㏈ at fully expanded jet Mach numbers Mi of 1.3 and 1.5, respectively, when the length of the centerbody was varied from 0 to 4 main nozzle diameters without blowing. With the aid of the blowing, the maximum reduction in OASPL increased to 12 and 7 ㏈ at M$\sub$j/=1.3 and 1.5, respectively. The impact pressure field in the main jet plume strongly suggested that the reduced periodic pressure distribution in the shear layers and/or centerline is responsible for the reduced screech and broadband shock associated noise. Therefore, the steady blowing by a micro centerbody is a promising technique for shock noise reduction in a supersonic jet.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.35 no.1
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• pp.27-32
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• 2007

this paper the acoustic signature from a supersonic nozzle is measured and compared to the result of a program developed for a gas turbine noise prediction. In order to measure the jet Mach Number, the pressure and temperature at the settling chamber was measured along with pressures from a pitot-tube placed near the exit. The results are also compared to the ones obtained with a shadow graph technique. Jet noise produced by an imperfectly expanded jet contains shock associated noise, which consist of broadband noise and screech tone noise. For subsonic condition, the directivity is dominant to the downstream direction due to turbulence mixing noise. For supersonic conditions, however, the directivity is dominant toward upstream direction due to shock associated noise. The comparison with a jet exhaust noise prediction code shows good agreement at supersonic conditions but needs to be improved at subsonic speeds.

• Journal of Mechanical Science and Technology
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• v.15 no.8
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• pp.1174-1180
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• 2001

Various experimental data, including mixing areas, cross correlation factors, surface flow patterns on nozzle walls, and far field noise spectra, was used to draw a noise control mechanism in a supersonic jet. In the underexpanded case, mixing of the jet air with ambient air was significantly enhanced as presented before, and mixing noise was also dramatically reduced. Screech tones, in the overexpanded case, were effectively suppressed by trailing edge modifications, although mixing enhancement was not noticeable. From mixing and noise performance of nozzles with modified trailing edges, enhancing mixing through streamwise vortices seems an effective way to reduce mixing noise in the underexpanded flow regime. However, screech tones in the overespanded flow regime is well controlled or suppressed by making shock cells and/or spanwise large scale structures irregular and/or less organized by a proper selection of trailing edges. The noise field in the overexpanded flow regime was greatly affected by the symmetricity of the nozzle exit geometry. In the underexpanded flow regime, the effects of the symmetricity of the nozzle exit on mixing were negligible.

• Proceedings of the KSME Conference
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• 2003.11a
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• pp.520-525
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• 2003

Supersonic jet issuing from a nozzle invariably cause high-frequency noises. These consist of three principal components ; the turbulent mixing noise, the broadband shock-associated noise, and the screech tone. In present study, it was experimentally investigated to the effect of nozzle lip thickness on the characteristics of supersonic jet noise. The convergent-divergent nozzle of a design Mach number 2.0 was used in experiment. With three different nozzle-lip thicknesses, the jet pressure ratio was varied in the range between 2.0 and 12.0. Acoustic measurements were conducted by microphones in an anechoic room, and the major structures of the supersonic jets were visualized by a Schlieren optical system to investigate the effect of nozzle lip thickness. The measured results show that the characteristics of supersonic jet noise, such as overall sound pressure level (OASPL) and screech frequency, strongly depend upon the thickness of nozzle-lip.

• Proceedings of the KSME Conference
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• 2004.04a
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• pp.2023-2028
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• 2004

The effects of nozzle-lip thickness on the relationship between screech tone and broadband shock-associated noise were experimentally investigated using a convergent-divergent nozzle with a design Mach number of 2.0. Overall sound pressure levels (OASPL) and noise spectra were obtained at far-field locations. Schlieren optical system was used to visualize the flow-fields of supersonic jets. A baffle plate was installed at the exit of the nozzle and its size was varied to obtain different nozzle-lip thicknesses. Experiment was carried out over a wide range of nozzle pressure ratios from 2.0 and 18.0, which corresponds to over- and under-expanded conditions. The results obtained clearly show that the screech tones are influenced by the nozzle-lip thickness. It is found that the screech tone and its peak amplitude are strongly dependent on whether the jet is over-expanded and under-expanded at the nozzle exit.