Study On Structural Vibration Reduction Methods Based On Acoustic Black Hole Structures And Distributed DVAs

Acoustic Black Hole(ABH)structure whose thickness change according to a specific power law,has the characteristic of "focusing" vibration energy to a local area within the structure.Combined with traditional structural vibration suppression methods,this characteristic has the potential to achieve effective structural vibration reduction by adding a small amount of mass in a specific area,so it has attracted extensive attention in recent years.Theoretically,the premise of the ABH structure to achieve the ideal focusing effect is that the characteristic size of the structure in the direction of vibration wave propagation is significantly larger than the wavelength.Due to the limited size of actual engineering structures,the focusing effect of ABH structure in the low-frequency region will be weakened when applied to actual beam-plate members.At the same time,the effectiveness of damping layer decreases with decreasing frequency,which limits the low frequency damping effect of ABH combined with damping layer method.As another classic Absorber for Vibration and noise reduction,Dynamic Vibration absorber(DVA)is able to offer flexible placement,designable tuning frequencies,and targeted control bands.It can be predicted that effective wideband vibration reduction can be achieved by combining the wide-band focusing characteristics of ABH structure with the advantages of DVA in effectively suppressing vibration response at any tuned frequency.Therefore,a structural vibration suppression method combining ABH structure and distributed DVA is proposed in this dissertation.The basic idea is to place one or more DVAs at or near the focus position of one or more ABH structures,thus extend the typical singlefrequency-narrowband DVA vibration suppression measures to multi-frequencies-broadband applications.The key to solve this problem is to study the influence of the limited size of the actual engineering structure on the focusing characteristics of ABH structure in different frequency bands and the coordination design between ABH structure and DVA parameters.Aiming at this key problem,the focusing effect,damping characteristics and engineering applications of ABH-DVA composite structures are systematically studied in this dissertation.The main work and results are as follows:1.Based on the transfer matrix method,the dynamic analytical model of ABH-DVA composite beams with integrated one-dimensional ABH structure is established.The focusing characteristic of ABH beam,the vibration reduction characteristic of ABH-DVA composite beam and the influence of DVA placement on the vibration reduction effect are studied.The concept of focusing ratio of vibration response in specific region was proposed and defined to quantitatively study the influence of ABH characteristic length and frequency variation on focusing effect and focusing region of ABH beam structure.The vibration characteristics of beam with uniform thickness,ABH beam and the ones with DVA are systematically analyzed under different frequencies.The results of the analytical model show that the focusing ratio in the 10 mm region of the ABH beam tip is as high as 0.9 in the frequency range from 600 Hz to6 k Hz.When DVA is tuned at 294 Hz and placed in the focusing region of ABH beam,the total average vibration response of ABH-DVA composite beam in the range of 0-3 k Hz is 19.4 d B lower than that of the uniform thickness beam with DVA.2.Based on the finite element method and the theory of ray acoustics,the focusing characteristic analysis models of the plate with single two-dimensional ABH structure and the plate with multiple distributed two-dimensional ABH structures were established at different frequency bands.Based on the finite element model,the effects of vibration excitation frequency,excitation position,ABH structure characteristic length and plate structure size on focusing characteristics were investigated by using the concept of focusing ratio of vibration response in a specific region.Based on the theory of ray acoustics,the propagation path of acoustic wave in ABH plate was calculated,and the focusing position of acoustic wave was predicted.The results show that the longer the ABH feature length,the better the focusing effect.The larger the plate structure size,the better the stability of focusing action.However,the trend of focusing ratio with frequency has nothing to do with the characteristic length of ABH structure and the plate size.In addition,the theoretical analysis of ray acoustics shows that the focusing position of ABH structure is greatly affected by the incidence angle of acoustic wave,while the excitation frequency has little effect.This result directly verifies the broadband focusing characteristic of ABH structure.3.The finite element vibration analysis model of ABH-DVA composite plate structure is established and verified experimentally.Based on this model,the vibration reduction design method of distributed ABH-DVAs composite plate is further studied,and the influence of the number,additional mass and DVA position of the ABH-DVA composite structure on the broadband vibration reduction effect of the plate structure including the low frequency band is analyzed.The results show that,when designed for low frequency narrow band,the total mass of the composite plate with six ABH-DVAs is reduced by 13.9%,the total vibration amplitude is reduced by 4.7 d B in 60-220 Hz,and the vibration peak attenuation is reduced by 23.8 d B compared with the traditional uniform thickness plate without DVA.When designed for broadband,compared with the uniform thickness plate without DVA,the vibration reduction effect of ABH-DVA composite plate structure in 0-4 k Hz is increased by 42.6 d B.4.According to the vibration reduction design method and theory of ABH-DVA composite structure proposed in this dissertation,a distributed ABH cylinder cover-DVA composite structure was designed to meet the demand of wide frequency vibration reduction and noise reduction of cylinder cover,and its vibration reduction effect was analyzed by finite element model.As the vibration characteristics of the cylinder cover surface directly determine the magnitude of the structure noise radiation,as an application case,this dissertation further analyzes the noise reduction effect of the ABH cylinder cover-DVA combined structure with the boundary element method.The model analysis results show that,compared with the original cylinder cover with DVAs,the vibration response level of ABH cylinder cover and DVA composite structure in 0-4 k Hz is reduced by about 9.1 d B,and the radiated noise power level is reduced by 12.0 d B.