The Effect of Reflector with Sound-Absorbing Material on Supersonic Jet Noise

This paper describes an experimental work to investigate the effect of a reflector on supersonic jet noise radiated from a convergent-divergent nozzle with a design Mach number 2.0.In the present study,a metal reflector and reflectors made of three different sound-absorbing materials(grass wool and polyurethane foam)were employed,and the reflector size was varied.Acoustic measurement is carried out to obtain the acoustic characteristics such as frequency,amplitude of screech tone and overall sound pressure level(OASPL).A high-quality schlieren optical system is used to visualize the detailed structure of supersonic jet.The results obtained show that the acoustic characteristics of supersonic jet noise are strongly dependent upon the jet pressure ratio and the reflector size.It is also found that the reflector with sound-absorbing material reduces the screech tone amplitude by about 5-13dB and the overall sound pressure levels by about 2-5dB,compared with those of the metal reflector.

Relations between pulsed jet noise and steady jet noise

On the hypothesis that the turbulent flow is practically not affected in character by pulsation of the flow, the formula for sound pressure level of pulsed jet noise is obtained in 90 °direction and 1 meter away from the nozzle. The steady jet noise is , then, a special case when the pulsating frequency is zero. The results agree well with experiments.

The Acoustic Performance of 3D Printed Multiple Jet Nozzles with DifferentConfigurations

This work investigated multiple jet nozzles with various geometrical shape,number of exits,and material on reducing noise radiated from jet flows.Nozzles are categorized in two groups with few and many exit numbers,each with various exit shapes,slot and circular,and geometry.Firstly,nozzles are designed and then fabricated by a 3D printer,Form Labs,Form2USA,with polymeric resin.Also,the nozzle with the most noise reduction made of stainless steel.Noise and air thrust were measured at three air pressure gauges,3,5,7 BAR and directions from nozzle apex,30°,90°,135°.Nozzles with slot exit shape made of both plastic and stainless steel revealed the most noise reduction among all nozzles with few exit numbers,nearly 11–14 dB(A)and 11.5–15 dB(A),respectively.On average,slotted nozzle noise reduction was nearly 5–6 dB(A)more than finned nozzle.However,nozzles with more exit numbers,finned and finned-central exit,illustrated much more noise reduction than nozzles with few exit numbers,by almost 16–18 dB(A),they represented similar sound.All tested nozzles and open pipe demonstrated equal air thrust at each pressure gauges.The nozzles with slotted exit shape,either plastic or stainless steel,can provide reasonable noise reduction in comparison to other configuration with few exit numbers.In contrast,nozzles with more exit numbers demonstrated the most noise reduction.

Numerical Study of Controlling Jet Flow and Noise using Pores on Nozzle Inner Wall

In this paper, the feasibility of controlling the subsonic jet flow and its noise using pores of blind holes added on the nozzle inner wall is explored numerically. These pores are intended to introduce disturbances to the shear layer so as to change the flow mixing. This passive strategy has not been attempted so far. A convergent nozzle with a cylindrical extension is selected as the baseline case. Three nozzles with pores on the inner wall are set up. Validations of the numerical settings are carried out, then the compressible turbulent jets at the exit Math number Mj = 0.6 in the four nozzles are calculated by large eddy simulations （LES）, while the ra-diated sounds are predicted by the FW-H acoustic analogy. The results show that the blind holes have produced some effects on weakening the turbulence intensity in the shear layer. Comparison reveals that both temporal and spatial correlations of the turbulent fluctuations in the modified cases are suppressed to some extent. Meanwhile, the porous nozzles are shown to suppress the pairing of vortices and enhance the flow mixing, and therefore, the development of shear layer and the fragmentation of large scale vortices are accelerated.

Acoustic measurements of models of military style supersonic nozzle jets

Modern military aircraft jet engines are designed with variable-geometry nozzles to provide optimal thrust in different operating conditions, depending on the flight envelope. How- ever, acoustic measurements for such nozzles are scarce, due to the cost involved in making full- scale measurements and the lack of details about the exact geometries of these nozzles. Thus the present effort at Pennsylvania State University （PSU） in partnership with GE Aviation and the NASA Glenn Research Center is aiming to study and characterize the acoustic field produced by supersonic jets issuing from converging-diverging military style nozzles, and to identify and test promising noise reduction techniques. An equally important objective is to develop methodology for using data obtained from small- and moderate-scale experiments to reliably predict the full-scale engine noise. The experimental results presented show reasonable agreement between small-scale and medium-scale jets, as well as between heated jets and heat-simulated ones.

A model-scale test on noise from single-stream nozzle exhaust geometries in static conditions

A model-scale test with single-stream nozzle exhaust geometries was carried out at the anechoic chamber of Beihang University in Beijing, China. The spectral characteristics are investigated, and the effects of the following parametric variations are reported in this paper: impact of nozzle operating conditions on spectra; impact of the presence of a plug; and effectiveness of chevron configurations for noise mitigation. The measurement shows that the change of pressure values has more impact on spectra than the change of temperature values. The spectral change due to pressure is shown at all band numbers for unheated conditions whereas it is more pronounced at highfrequency ranges for heated conditions. An impact of the presence of a plug is also clearly observed.The reduction of noise is moderate up to band number of 35, and becomes more significant at higher band numbers. It is observed that chevron nozzles are more efficient at high pressure and temperature values. It is expected that the quantified analysis will be used to develop an empirical model of single jet noise, which will be the baseline for the development of prediction of noise from turbofan engines of high bypass ratios.

Noise control for high subsonic jet flows by inner wall treatment

Subsonic jet nozzles,commonly used in passenger aircrafts,generate significant noise that travels both downstream and upstream due to large-scale or fine-scale turbulence in the jet plume.To reduce jet noise,a novel wall treatment method,termed the wavy inner wall(WIW),is proposed.With this method,the smooth inner wall near the exit of the nozzle is replaced by treated walls that carry small wavy patterns.Numerical simulations were conducted to investigate the effects of the WIW treatment.Large eddy simulations(LES)were used to predict the unsteady flow field and the far-field noise,followed by the analogy method proposed by Ffowcs Williams and Hawkings.To better understand the mechanism behind the noise reduction achieved by the WIW treatment,the shear-layer instability,radial and azimuthal auto-correlation functions,turbulent kinetic energy,and acoustic source term from the Tam-Auriault(TA)jet-noise model were analyzed.Results indicated that the WIW treatment advances the onset of jet flow instability in the shear-layer,leading to the early breakdown of jet shear-layer and production of different scales of downstream turbulent structures.As a result,the distribution and production of turbulent kinetic energy are affected,and the genera-tion and emission of jet noise are controlled.The WIW treatment enables the control of fine scale turbulence,resulting in the reduction of mid-to high-frequency noise in the far field,while ensuring a low thrust loss.This feature makes the WIW method a promis-ing approach for jet noise control.

Jet Screech Reduction with Perforated Flat Reflector

In the present experimental study, investigations have been carded out to evaluate the performance of the new control technique of jet screech with different perforated flat reflectors. Mainly two types of porous flat reflectors had been used in the experiment. One reflector （reflector-V） designed for placing the reflector surface vertical to the jet axis, when, another type of reflector （reflector-H） designed for placing the reflecting surface horizontal to the jet axis. In both cases the reflectors had been placed at the nozzle （base tube with uniform cross-sectional area） exit. The diameter of the reflector-V was 15D when the diameter of the reflector-H was 10D. The porous area of the reflector-V was 6D and 4.5D for reflector-H where D indicated the diameter of the nozzle exit. The placement of the reflector at the exit of the nozzle reduces the sound pressure at the nozzle exit. Thus the muted sound can not excite the unstable disturbance at the nozzle exit and the loop of the feedback mechanism disappeared, finally, the generation of jet screech be cancelled. The suction space located at the back side of the porous surface of the reflector-V improves the efficiency of the screech control technique. However, in the case of reflector-H, the receptivity process of feedback loop had been controlled by reducing the disturbances at the effective shock fronts as well as at the nozzle exit. The performance of the proposed method was verified with a flat reflector concept and good performance in jet screech suppression has been confirmed in the case of porous reflector.

亚音速湍射流中非线性演化环形模态相干结构的声辐射:两种辐射机理的比较性研究

环形模态(轴对称模态)组成的大尺度相干结构存在于亚音速湍射流中,是射流噪声的重要来源.当相干结构可以用湍流平均流为基本流的不稳定波包来刻画时,其声辐射存在两种机理.其一,相干结构的幅值经历轴向的增长和衰减,形成波数谱的大波数谱尾包含相速度超音速的成分,这些成分可以向远场辐射马赫波,即“广义马赫波辐射”.其二,波包非线性相互作用产生随时间和空间缓慢变化的平均流修正成分,其时空尺度与不稳定波包的包络相近,这些成分可以向远场辐射低频声波,即“波包辐射”.本文通过强非线性临界层理论给出的近场动力学信息,在同一数学框架下研究这两种声辐射的物理过程并考虑相对重要性.在线性演化的极限情形下,广义马赫波辐射存在“超指向性”特征,但这种特征随着非线性的增强趋于消失.非线性作用产生幅值抖动,显著增强广义马赫波辐射,但轻微压制波包辐射.这两种机理对于中等幅值相干结构的声辐射均起重要作用,但辐射指向性呈不同特征:广义马赫波辐射集中在下游方向,而波包辐射在侧向和上游方向更显著.理论预测结果表明,由本理论描述的广义马赫波辐射的指向性与实验测量定性吻合.