Anxin Liu

The Helmholtz resonance acoustic metamaterial is an effective sound absorber in the field of noise reduction, especially in the low-frequency domain. To overcome the conflict between the number of Helmholtz resonators and the volume of the rear cavity for each chamber with a given front area of single-layer metamaterial, a novel acoustic metamaterial of interlayer parallel connection of multiple Helmholtz resonators (IPC–MHR) is proposed in this study. The developed IPC–MHR consists of several layers, and the Helmholtz resonators among different layers are connected in parallel. The sound absorption property of IPC–MHR is studied by finite element simulation and further optimized by particle swarm optimization algorithm, and it is validated by standing wave tube measurement with the sample fabricated by additive manufacturing. The average sound absorption coefficient in the discrete frequency band [200 Hz, 300 Hz] U [400 Hz, 600 Hz] U [800 Hz, 1250 Hz] is 0.7769 for the IPC–MHR with four layers. Through the optimization of the thickness of each layer, the average sound absorption coefficient in 250–750 Hz is up to 0.8068. Similarly, the optimized IPC–MHR with six layers obtains an average sound absorption coefficient of 0.8454 in 300–950 Hz, which exhibits an excellent sound absorption performance in the low-frequency range with a wide band. The IPC–MHR can be used to suppress obnoxious noise in practical applications.

An acoustic metamaterial absorber of parallel–connection square Helmholtz resonators is proposed in this study, and its sound absorption coefficients are optimized to reduce the noise for the given conditions in the factory. A two–dimensional equivalent simulation model is built to obtain the initial value of parameters and a three–dimensional finite element model is constructed to simulate the sound absorption performance of the metamaterial cell, which aims to improve the research efficiency. The optimal parameters of metamaterial cells are obtained through the particle swarm optimization algorithm, and its effectiveness and accuracy are validated through preparing the experimental sample using 3D printing and measuring the sound absorption coefficient by the standing wave tube detection. The consistency between the experimental data and simulation data verifies feasibility of the proposed optimization method and usefulness of the developed acoustic metamaterial absorber, and the desired sound absorption performances for given conditions are achieved. The experimental results prove that parallel–connection square Helmholtz resonators can achieve an adjustable frequency spectrum for the low frequency noise control by parameter optimization, which is propitious to promote its application in reducing the noise in the factory.

The braking energy can be recovered and recycled by the regenerative braking system, which is significant to improve economics and environmental effect of the hydraulic hybrid vehicle. Influencing factors for the energy recovery rate of regenerative braking system in hydraulic hybrid vehicle were investigated in this study. Based on the theoretical analysis of accumulator and energy recovery rate, modeling of the regenerative braking system and its energy management strategy was conducted in the simulation platform of LMS Imagine Lab AMESim. The simulation results indicated that the influencing factors included braking intensity, initial pressure of the accumulator, and initial braking speed, and the optimal energy recovery rate of 87.61% was achieved when the parameters were 0.4, 19 MPa, and 300 rpm, respectively. Experimental bench was constructed and a series of experiments on energy recovery rate with different parameters were conducted, which aimed to validate the simulation results. It could be found, that with the optimal parameters obtained in the simulation process, the actual energy recovery rate achieved in the experiment was 83.33%, which was almost consistent with the simulation result. The obtained high energy recovery rate would promote the application of regenerative braking system in the hydraulic hybrid vehicle.

The improvement of sound absorption performance of porous metal is a focus of research in the field of noise reduction. Influences of compression ratios on sound absorption performance of a porous nickel-iron (Ni-Fe) alloy were investigated. The samples were compressed with ratios from 10% to 80% at an interval of 10%. Based on the standing wave method, sound absorption coefficients of compressed samples with different thicknesses were obtained. It could be found that with the same compression ratio, sound absorption performance was improved with the increase of thickness. Based on the modified Johnson-Allard model with a correction factor, the sound absorption coefficient of the porous Ni-Fe with a thickness of 20 mm for different compression ratios was derived, whose aim was to quantificationally analyze influences of the compression ratio. The results indicated that the sample with a compression ratio of 70% exhibited optimal sound absorption performance, and its average sound absorption coefficient reached 88.97% in a frequency range of 1000–6000 Hz. Meanwhile, the section morphologies of compressed samples were investigated by a scanning electron microscope, which studied the sound absorption performance by analyzing structures of the porous Ni-Fe samples with different compression ratios. The obtained achievements will promote the application of the porous Ni-Fe alloy in the field of acoustics.