Takehisa Takaishi

Application of porous material to reduce aerodynamic sound from bluff bodies

Aerodynamic sound derived from bluff bodies can be considerably reduced by flow control. In this paper, the authors propose a new method in which porous material covers a body surface as one of the flow control methods. From wind tunnel tests on flows around a bare cylinder and a cylinder with porous material, it has been clarified that the application of porous materials is effective in reducing aerodynamic sound. Correlation between aerodynamic sound and aerodynamic force fluctuation, and a surface pressure distribution of cylinders are measured to investigate a mechanism of aerodynamic sound reduction. As a result, the correlation between aerodynamic sound and aerodynamic force fluctuation exists in the flow around the bare cylinder and disappears in the flow around the cylinder with porous material. Moreover, the aerodynamic force fluctuation of the cylinder with porous material is less than that of the bare cylinder. The surface pressure distribution of the cylinder with porous material is quite different from that of the bare cylinder. These facts indicate that aerodynamic sound is reduced by suppressing the motion of vortices because aerodynamic sound is induced by the unstable motion of vortices. In addition, an instantaneous flow field in the wake of the cylinder is measured by application of the PIV technique. Vortices that are shed alternately from the bare cylinder disappear by application of porous material, and the region of zero velocity spreads widely behind the cylinder with porous material. Shear layers between the stationary region and the uniform flow become thin and stable. These results suggest that porous material mainly affects the flow field adjacent to bluff bodies and reduces aerodynamic sound by depriving momentum of the wake and suppressing the unsteady motion of vortices.

Method of evaluating dipole sound source in a finite computational domain

Numerical prediction of dipole sound based on Lighthill–Curle’s equation gives little information on the structure of sound sources. On the other hand, a hybrid method that combines the large eddy simulation (LES) and the compact Green’s function proposed by Howe provides detailed information on the vortices in the flow that most contribute to the generation of sound. However, when the dipole sound is evaluated from the momentum change in fluid inside a finite computational domain, the result does not in general agree with the sound evaluated from the fluctuating pressure on the body surface because contribution from vortices outside the computational domain is not taken into account. In this study, the balance of momentum in a finite computational domain is considered strictly, and the effect of outer vortices is replaced with contribution from inner properties by using an imaginary velocity potential φi. This process avoids sudden termination of Lighthill’s stress tensor at the outer boundary and extrac...

Aerodynamic Noise Reduction of a Pantograph by Shape-Smoothing of Panhead and Its Support and by the Surface Covering with Porous Material

To reduce aerodynamic noise generated by a pantograph, which is one of the dominant noise sources of high-speed trains, the authors proposed some noise reduction techniques, that is, shape-optimization of a panhead, relaxation of aerodynamic interference between panhead and articulated frame and surface covering with porous material. To evaluate total noise reduction effect of them, wind tunnel tests were performed with a prototype pantograph to which these techniques were applied. The test results show that noise level of the prototype pantograph is lower than that of the currently-used pantograph by about 4 dB. Furthermore, it was also confirmed that the prototype pantograph has enough aerodynamic stability against change of attack angle.

Wind Tunnel Tests on the Control of Aeroacoustic Noise from High Speed Train

In this study, we propose techniques for reducing the noise from gaps between Shinkansen cars based on the results of noise source localization in wind tunnel testing. In order to obtain the accurate noise source distributions, the microphone array is installed near the train model. The influence of the shear layer around the main flow on the directivity of the microphone array is clarified so that the microphone array should be set in the shear layer rather than the outside of the flow. Analysis of the noise source localization reveals the principal noise sources around the gap, which suggests efficient approaches to the noise reduction. Firstly, we found that the noise level of the gap section with rounded edge can be effectively reduced by approximately 7 dB compared with that of the case with current condition. We also confirmed qualitatively the effect of noise reduction techniques for the gap section by the field test.

Effects of Periodic Holes on the Suppression of Aeroacoustic Noise From a Pantograph Horn

Three types of pantograph horn model; simple cylinder, a cylinder with periodic holes and a cylinder with a continuous slit, are tested in a low noise wind tunnel to compare their characteristics of aerodynamic noise and flow fields in the wake. Formation of strong vortices of alternate sign that have large structure in the spanwise direction is suppressed due to the flow through holes or the slit. The cylinder with the continuos slit is proved to reduce the noise sufficiently, but an unstable flow through the slit seems to produce distinct noise. Since formation of strong vortices is mainly suppressed due to momentum injection through holes or the slit, periodic holes have little effect on collapsing the spanwise structure of vortices, but they contribute to making the flow around the horn stable. The shape of holes should be optimized to avoid strong acoustic resonance.Copyright

Visualization of Dipole Noise Source around High Speed Pantograph

パンタグラフの舟体は新幹線の主要な空力音源の1つであり，その低騒音化が強く要求されている．ただし，高速域でパンタグラフが良好な集電性能を維持するためには舟体の揚力特性を適正に保つ必要があるため，空力音低減は容易ではない．そこで著者らは，舟体の空力音発生メカニズムを明示することによって低騒音化設計を支援することを目指し，流れ場のCFD解析とHoweの渦音理論に基づく音響解析手法を組合せた手法を用いて，舟体から発生する空力音の遠方場における評価と舟体まわりの二重極音源分布の可視化を行った．また，検証のための風洞実験も実施した．これらの結果，二重極音の評価結果が実験結果と定性的に一致することを示した．また，二重極音源分布の可視化結果から，対象とした舟体では剥離領域の大きさや再付着の有無が二重極音の発生に大きく寄与していることを明らかにした．

Airframe noise measurements on JAXA Jet Flying Test Bed “Hisho” using a phased microphone array:

In 2015, the Japan Aerospace Exploration Agency launched the Flight demonstration of QUiet technology to Reduce nOise from High-lift configurations project to verify by flight demonstration the feasibility of practical noise-reducing aircraft modification concepts. In order to serve as a baseline for comparison before modification, airframe noise sources of the JAXA Jet Flying Test Bed “Hisho” were measured with a 30 m diameter array of 195 microphones mounted on a wooden platform built temporary beside the runway of Noto Satoyama Airport in Japan. A classical Delay and Sum in the time domain beamforming algorithm was adapted for the present study, with weight factors introduced to improve the low-frequency resolution and autocorrelations eliminated to suppress wind noise at high frequencies. In the landing configuration at idle thrust, the main landing gear, nose landing gear, and side edges of the six extended flap panels were found to be the dominant “Hisho” airframe noise sources. Deconvolution by the DAMAS and CLEAN-SC algorithms provided clearer positions of these sound sources at low frequencies. Integration of acoustical maps agreed well with the sound pressure level measured by a microphone placed at the center of the microphone array and gave detailed information about the contribution of each noise source.

Recent Studies on Aerodynamic Noise Reduction at RTRI

As the maximum speed of high-speed trains increases, the effect of aerodynamic noise on the sound level at the wayside of the track becomes important. When the surface of bluff bodies such as pantographs is covered with porous materials, the aerodynamic noise generated by unsteady motion of vortices is significantly reduced. Experimental evaluation techniques of instantaneous flow fields using time-resolved PIV enable prediction of sound in the far field based on the theory of vortex sound. Aerodynamic noise emitted from a partial model of a pantograph is predicted numerically by coupling the calculation of unsteady flow with the evaluation of acoustical behaviour. The simulation succeeds in giving detailed information on the structure of aerodynamic sound sources.

Method of Evaluating Dipole Sound in a Finite Computational Domain

Numerical prediction of dipole sound based on Lighthill-Curles equation gives little information on the structure of sound sources. On the other hand, a hybrid method that combines the large eddy simulation (LES) and the compact Greens function proposed by Howe provides detailed information on the vortices in the flow that most contribute to the generation of sound. However, when the dipole sound is evaluated from the momentum change in fluid inside a finite computational domain, the result does not in general agree with the sound evaluated from the fluctuating pressure on the body surface because contribution from vortices outside the computational domain is not taken into account. In this study, the balance of momentum in a finite computational domain is considered strictly, and the effect of outer vortices is replaced with contribution from inner properties by using an imaginary velocity potential ψi.This method allows quantitative prediction of dipole sound and also gives accurate information about sound sources.