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To provide further understanding of the low to mid frequency tonal noise characteristics of the leading-edge slat in the processes of take-off and landing, both experimental and numerical investigations are carried out on a two-dimensional high-lift configuration, with a stowed flap at varying incoming flow speeds (from 30 m/s to 60 m/s). The intensity variation of the low to mid frequency tonal noise is found to be closely related to the selection of the main mode of self-excited oscillation within the leading-edge slat cove. With increasing incoming flow speed, the main mode of the self-excited oscillation switches to a higher one. Meanwhile, the ratio of the self-excited oscillation period to the vortex shedding period is introduced to explain further the relationship between the main mode and incoming flow speed. As the incoming flow speed increases from 30 m/s to 60 m/s, the self-excited oscillation period is basically constant due to the flow similarity, whereas, the vortex shedding frequency is gradually increasing. The characteristic frequency of the shedding vortex near the cusp is found to be approximately proportional to 3/2 of the incoming flow speed power law. [ABSTRACT FROM AUTHOR]

Highlights • The helical-cable technique is to reduce cylinder noise and tandem cylinder noise. • The method can suppress the vortex shedding process and noise radiation efficiency. • Control on the upstream cylinder is the key for the tandem cylinder noise reduction. Abstract Cylinder noise can be remarkably reduced when the surface is helically wrapped with cables along the cylinder span. In this paper, aeroacoustic and aerodynamic experiments are conducted to reveal the noise reduction mechanism of the helical-cable technique. With the application of helical cables on the single circular cylinder, three-dimensional flow is promoted into the cylinder near wake, leading to the suppression of two-dimensional vortex shedding process and flow fluctuations. Besides, the three-dimensional wake also destroys the in-phase relationship of flow fluctuations along the cylinder span, leading to the reduction of spanwise correlation length and noise radiation efficiency. These two effects together contribute to the cylinder noise reduction. Furthermore, this helical-cable technique is extended to tandem cylinders and the results show that it can also reduce the tandem cylinder noise effectively. Especially for the moderate and large spacing tandem cylinder configurations, the key of the noise reduction is the control on the upstream cylinder. [ABSTRACT FROM AUTHOR]

Aerodynamic and aeroacoustic experiments of 30P30N three-element high-lift configuration are conducted in D5 aeroacoustic wind tunnel at Beihang University. The test section is composed of Kevlar and perforated plate which can measure aerodynamic features with less jet shear layer interference and aeroacoustic characteristics with good sound transmission simultaneously. Aiming at the research of high-lift configurations during the aircraft approach and landing, Mach number and angle of attack vary from 0.09 to 0.17 and 3 ° to 8 ° , respectively. The measured surface pressure distributions in D5 agree well with the results of JAXA-LWT2 and computational fluid dynamics predictions. Consistent with previous investigations, the far-field noise spectra obtained by 1/2 in. free-field microphones nearly decrease with a slope proportional to f - 2 in a wide frequency range and scale with Ma 4.5 . Moreover, all mid-frequency peaks are observed in the range of 1 < St < 5 with one maximum within St = 1 - 3 . The tonal frequencies and noise spectral shape seem less sensitive to the tested angles of attack, except that the tonal intensity decreases with increasing angles of attack. Noise source maps for 1/3 octave band obtained from data processed by conventional beamforming with the far-field noise spectra indicate that the slat cove and slat-main element region is the main noise source that can radiate narrow-band peaks. [ABSTRACT FROM AUTHOR]

This paper introduces the continuous wavelet transform method to analyze different multiple tonal phenomena of the high-lift configurations, aiming at revealing the time-frequency information of these multiple tones simultaneously. Two types of multiple tones can be respectively observed in the far-field noise spectra at landing and cruise configurations, but their inherent physical mechanisms are different. Two wavelet functions are applied in this paper. The Morlet function as a complex wavelet function returns both amplitude and phase information for capturing oscillatory behaviors, so that it can clearly distinguish the mode switching mechanism and the amplitude modulation mechanism of different multiple tonal phenomena. While the Mexihat function as a real wavelet function returns only amplitude information, so that it is usually to isolate peaks or discontinuities of signals and cannot reveal the mechanisms of different multiple tonal phenomena. In summary, the Morlet function is more appropriate to analyze the periodical oscillatory acoustic tonal signals from multiple tonal phenomena. [ABSTRACT FROM AUTHOR]

This paper presents a parametric study on the far-field noise control effect of a circular cylinder with helical cables wrapping along cylinder span, aiming at finding an optimal operation condition for this treatment. Experimental results reveal that the noise control efficiency of this treatment is highly dependent on the non-dimensional thread pitch and cable density. The noise control efficiency gradually increases with the non-dimensional thread pitch and cable density increasing when both values are smaller than critical values. Further increasing the values of the two parameters, the control efficiency cannot increase any more. By choosing the optimal parameters of helical cables, both tonal and broadband noise can be remarkably suppressed, and the overall sound pressure level can be reduced by as large as 17 dB. [ABSTRACT FROM AUTHOR]

The D5 aeroacoustic wind tunnel at Beihang University is a newly commissioned small-scale closed-circuit wind tunnel with low turbulence intensity and low background noise. The wind tunnel is built to study both aerodynamic and aeroacoustic performance of aircraft components or scaled models. The wind tunnel has two types of test sections, the closed type test section is used for aerodynamic tests while the open type test section is mainly used for aeroacoustic experiments. Both types of test section are 1 m in height and 1 m in width, and the maximum wind velocity in the test section can be up to 80 m/s. An anechoic chamber is built surrounding the test section to provide the non-reflecting condition. This paper provides an overview of design criteria and performance of the small-scale wind tunnel. The layout of the wind tunnel and some critical design treatments to improve aerodynamic and acoustic performance are discussed in detail. Some experiments are conducted to verify the performance of D5 wind tunnel, results confirm that the turbulence intensity is less than 0.08% in the core of test section and the background noise is comparable with other aeroacoustic wind tunnels. A scaled simplified nose landing gear model is also measured as a benchmark test, results reveal that noise radiated from the model is adequately higher than the background noise for a wide frequency range and remarkably consistent with other results from literatures. [ABSTRACT FROM AUTHOR]

• Aeroacoustic experiment and wavelet analysis are conductedon the 30P30N model. • Time-frequency characteristicsof differentslat and flap configurations. • Multiple tonesof flap noiseis satisfied withamplitude modulation phenomenon. • Multiple tones of slat noise is satisfied withmode switching phenomenon. High-lift device installed on modern large aircraft is a major noise source during the approach and landing phases. For high-lift device, slat and flap parameters such as gap, overlap and deflection angle have strong effect on aerodynamic and aeroacoustic characteristics. However, the research about time–frequency characteristics of high-lift device noise affected by slat and flap parameters is lacking. This paper presents an investigation on 30P30N model to reveal the effect of different slat and flap geometrical configurations on the aeroacoustic time–frequency characteristics with experimental measurement and wavelet analysis. The results show that the multiple tones at small flap and slat deflection angles are mainly generated from the flap gap region. The time–frequency characteristic of the tones is satisfied with the amplitude modulation phenomenon, namely that the acoustic energy concentrates on the primary tone and the other tones are generated from the modulation process of the primary tone. While the multiple tones at large slat deflection angles are generated from the slat cove and slat gap regions. The time–frequency characteristic of the tones is satisfied with the mode switching phenomenon, namely that the dominant noise energy temporally switches from one mode to another in time and these multiple tones cannot coexist independent of one another. [ABSTRACT FROM AUTHOR]