Vibro-acoustic Analysis Of Sturcture Under Acoustic Enviroment In The Mid-high Frequency Range

One of the important research contents in developing space vehicles is the dynamic response prediction of structure coupled with cavity under noise excitation.Space vehicles suffer terrible noise environment which may reach 0-10000 Hz and 160 dB in working condition.This results in the acoustic fatigue of thin-walled space vehicle structures or the failure of precise instruments in space vehicles,which will lead to disastrous accidents.Accurate and efficient vibro-acoustic response prediction techniques help to the design of space vehicles,and to the formulation of ground test projections.Therefore,the study on accurate and efficient vibro-acoustic response prediction techniques is meaningful to the development of space vehicles.The vibro-acoustic response prediction techniques available at present are just valid in certain frequency bands,therefore,it is impossible to predict broadband vibro-acoustic response via a single technique.Researches on prior modes based methods are launched in this dissertation.The prior modes based methods are developed,and are combined to predict the broadband vibro-acoustic response,which simplifies the pre-and post-processing on results.The main research work and innovative achievements in this dissertation are:(1)Reasearch on the use of dual modal formulation(DMF)for low to mid-frequency vibro-acoustic response prediction.How the modal truncation affects the accuracy of DMF on low to mid-frequency vibro-acoustic response prediction is investigated.Numerical examples are studied,where a panel or a stiffened plate coupled with a cavity.The panel/stiffened plate is excited by determinate/random noise or determinate/random point force.And the vibro-acoustic responses of the coupled systems are predicted via DMF when the cut frequency varies.The DMF results are validated by the results obtained via the direct FEM.Results show that the DMF results gradually convergent to the FEM results when the cut frequency of the modal truncation increases.And the differences between the results obtained via the two methods are negligible when the cut frequency of the modal truncation is higher than 1.5 times the upper frequency limite.(2)Reasearch on the use of modal energy analysis(MODENA)for low to mid-frequency vibro-acoustic response prediction.In order to investigate the errors of the MODENA results due to the approximation on modal input powers,numerical examples are studied on a two-oscillator coupling system,a multi-modal coupling system,a panel/cavity coupling system,and a stiffened plate/cavity coupling system.In the two-oscillator coupling case,a dimensionless coupling quotient is defined,and the coupling strength between the two oscillators is classified as weak,moderate,or strong,according to the coupling quotient with a proposed criterion.In the multi-modal coupling case,the errors mentioned above are negligible if the modes are weakly coupled.But it cannot be neglected if the modes are moderately coupled,in this situation accurate results are obtained via MODENA when the modal input powers are computed exactly.However,large errors occur in the MODENA results when the modes are strongly coupled even though the modal input powers are computed exactly.In the panel/cavity coupling case and the stiffened plate/cavity coupling case,the coupling strength between modes decreases with the increment of modal order in general.Large errors occur in the MODENA results near some eigenfrequencies of low order modes when the modal input powers are approximated.(3)MODENA that deals with spatial correlated pressure loads is proposed,considering the space vehicles suffer spatial correlated noise environment.The applicability of the equivalent rain on the roof loads of the Corcos's turbulent boundary layer(TBL)loads is investigated.Results show that errors due to the equivalent loads are negligible when the frequency is higher than 4 times the acoustic coincidence frequency.A critical correlation strength factor is proposed based on the study on a three-oscillator coupling model.It is found that the errors of the MODENA results are negligible if the correlation strength factor between modal forces is less than the critical value.In a panel/cavity coupling case and a stiffened plate/cavity coupling case,where the panel and the stiffened plate are under a TBL load,only in the very low frequency band the correlation strength factors between modal forces are larger than the critical values.The MODENA approach is proved to be an accurate technique on predicting vibro-acoustic responses of systems under TBL loads.(4)Reasearch on the statistical energy analysis(SEA)for high frequency vibro-acoustic response prediction.A combination of MODENA and the power injection method(PIM),i.e.the so called MODENA-PIM approach is proposed to compute the coupling loss factors between subsystems in SEA.It is compared with other two approaches,i.e.the wave approach and the DMF approach,in a numerical example where a panel coupled to a cavity.The vibrational energies of the panel/cavity coupling system are predicted via SEA,with the coupling loss factors computed via the three approaches mentioned above.Results show that the wave approach is most efficient but least accurate.The DMF approach is more efficient than the MODENA-PIM approach,but it cannot take the non resonant transmission into account,which leads to large errors in large damping systems.The MODENA-PIM approach can compute the coupling loss factors exactly,because the non resonant transmission is considered.The coupling loss factors of a stiffened plate/cavity coupling system are obtained via the DMF approach and the MODENA-PIM approach,then,the high frequency vibro-acoustic responses of the system are predicted by using SEA.(5)Reasearch on the statistical modal energy distribution analysis(SmEdA)and the hybrid SmEdA?SEA approach for mid-frequency vibro-acoustic response prediction.It is discussed that the non resonant transmission is very important in the large damping systems.An extension of the hybrid SmEdA-SEA approach that takes into account the contributions of the non resonant modes is presented,as well the fundamental equations of the approach.Based on SmEdA and SmEdA-SEA,the vibro-acoustic responses of a panel/cavity coupling system and a stiffened plate/cavity coupling system under rain on the roof loads and point force loads are predicted.Results show that the vibro-acoustic responses of large damping system are exactly predicted via the SmEdA approach and the SmEdA-SEA approach that considering the contributions of the non resonant modes.Then,in order to improve efficiency,a measure is taken to simplify the SmEdA-SEA approach which considering the non resonant transmition,that is only modes in certain frequency band are considered to be coupled.Results show that the efficiency of the SmEdA-SEA approach is improved by taking the measure mentioned above,meanwhile,the accuracy of the SmEdA-SEA approach is guaranteed.