Numerical Investigation On Effectively Reducing Noise On The Inside Surfaces Of The Noise Barrier Under Train Induced Wind And Cross Wind

With the increase of running speed,aerodynamic noise becomes the main noise source of high-speed train.Aerodynamic noise of high-speed train is broadband noise distributed at all heights of the train body.The noise energy is mainly concentrated in frequencies of 500 ~ 4000 Hz.Noise of medium and high frequencies has studied and achieved good results.The low frequencies noise between 20 ~ 250 Hz has the characteristics of acoustic wavelength,slow attenuation and strong diffraction ability.Noise barrier,as one of the most important methods for noise isolation,can effectively block the propagation of noise to some extent.However,studies on the noise reduction of low-frequency aerodynamic noise are relatively few.Aiming at the aerodynamic noise problem on the inner surface of the noise barrier,in this thesis,The high-speed train and bridge model were set up based on fluid dynamics software Fluent,and a vertical sound barrier with the height of 2.15 was established in the numerical model.Ignoring the four level sub sound source,all the numerical analysis are based on Lighthill’s acoustic theory.adopting two-equation RANS simulation method,the FW-H acoustic analogy,and the Farassat’s time domain integral expression of the thickness and load noise,aerodynamic noise characteristics and effective noise reduction area distribution on the inner surface of the sound barrier are studied.Meanwhile,the influence of cross wind on the aerodynamic noise is focused.and the aerodynamic noise characteristics and the effective noise reduction area distribution of the high speed train under the cross wind and train induced wind running along different track of the bridge deck are calculated and analyzed.The main research contents include the following points:1.A two-dimensional noise barrier bridge calculation model was established.The CFD numerical method was used to simulate the unsteady flow field,and the aerodynamic sound pressure of each acoustic monitoring point is calculated simultaneously.The total sound pressure level and spectrum distribution of the noise were calculated by FFT transformation.The research shows that: the backpressure gradient and turbulence effect in the cavity area were enhanced by the barrier.Comparing to the absence of barriers,aerodynamic noise on the windward side is significantly increased inside the cavity area.The aerodynamic noise on the windward side of the sound barrier in the bridge deck cavity area is obviously larger than that without the noise barrier,the total sound pressure level increases by 7.7dB,the minimum increase is 3.3dB;However,the leeward side reduces obviously without the sound barrier than with the noise barrier,the total sound pressure level is reduced by 6.7dB,and the minimum 1.7dB is reduced;The cavity area under the train is one of the main sources of aerodynamic noise,and the maximum level difference appears in the cavity bottom area.2.A two dimensional noise barrier train bridge model was established.The influence of the total weighted sound pressure level of the cross wind and the position of the train running in the cavity area was considered.The research shows that the total noise pressure level and the low frequency aerodynamic noise increase with the increase of cross wind.The low frequency aerodynamic noise of the train running on the far-track side is greater than that of the near-track side.The two-dimensional cavity characteristics of the bridge is changed by the location of train.The windward and leeward sides of the train show different vortex return characteristics.Under the same crosswind,the aerodynamic noise level of the windward side of the train is always higher than that of the leeward side,when the train is located on the far-track side,the total sound pressure level of the internal noise on the inside surface of the leeward side noise barrier is significantly higher than that of the train running near-track side.The noise energy of the train running along the inner surface of the sound barrier near-track side and the far-track side is mainly distributed in the range of 3.5m and 5.5m from the top of the track.3.A three dimensional sound barrier train bridge model is established considering the combined effect of train induced wind and cross wind.The multiple reference frame method is used to calculate the transient solution of high-speed train running on a railway bridge.The research indicates that the aerodynamic noise of high-speed train has broadband characteristics,while the low frequency aerodynamic noise(<200Hz)enhances the sound pressure level of aerodynamic noise in near and far-field of high-speed train,Moreover,the noise frequency increases with the increase of vehicle speed.The total sound pressure level of the noise radiation along the longitudinal head and tail region of the train is larger than that of the central region.The vertical noise reveals obvious acoustic directivity along the vertical height of the barrier,and the aerodynamic noise decreases with the increase of the distance from the top of the track.Based on the influence of train position on sound field distribution,the amplitude gradient of the low frequency and high frequency segments of the train is higher than that on the far side.The maximum difference of sound pressure level between the windward side and the middle side of the train head area is 14.8dB and 4.1dB respectively.4.The height of effective noise reduction region of aerodynamic noise on the inside surfaces of high-speed train noise barriers is proposed.The aerodynamic noise of the 2.15 m vertical noise barrier at the high speed section of the railway bridge deck is considerably lower than that of the top track 1.5m.Taking the train speed 200km/h as an example,between 1.5m and 2.5m from the top of the track,the maximum pressure difference between the windward and leeward side of the train running on the near rail side reaches the maximum value of 7.3dB and 9.2dB respectively,while the maximum total acoustic pressure level difference on the far side of the train is also found between the measured points,corresponding to 4.8dB and 8.6dB respectively.The analysis displays that the area below the 1.5m of the sound barrier can be used as an effective area for noise reduction design.