The Test Methods And Their Applications Of The Correlation In Spatial Acoustic Field

In this dissertation,strong motivations are dedicated to the evaluation of the underwater acoustic properties of coating materials,hydrodynamic excitation source identification at high speeds,and the corresponding experiment techniques for large scale models at lake trial tests.Accordingly,by utilizing the spatial correlation of 1-D,2-D and 3-D acoustic field,novel experimental methods are proposed in the following aspects,namely,the acoustic impedance and reactance of underwater acoustic coating materials,frequency-wavenumber domain of turbulence boundary layer fluctuations,and some practical considerations in shallow water.Firstly,the conventional experiment technique on acoustic impedance coefficients for acoustic coating materials is improved,in which more practical acoustic considerations are taken into account.A novel testing method,together with a delicate equipment,and new rigid backing mass are designed for testing in lower frequency range,which make it possible to measure the acoustic impedance for small acoustic coating material models ranging from 800 Hz to 5 k Hz in pressurized environments.The acoustic impedance coefficients of typical acoustic coating material are therefore measured in water-filled acoustic tube.Furthermore,the acoustic impedance coefficients are used to predict the acoustic absorption and insulation of the acoustic coating materials.In comparison with the testing results,the acoustic absorption prediction error is less than 0.1,and the acoustic insulation prediction error is less than 2 d B.In response to the difficulties in the implementation of rigid backings for acoustic impedance testing below 800 Hz,the acoustic reactance testing method using soft backing is set up so that the design of soft backing in pressurized environment,as well as the vibration measurement on the surface of acoustic coating material is solved.As a result,the acoustic reactance coefficients of acoustic coating material from 200 Hz to 2 k Hz are measured in pressurized water-filled tube.The acoustic reactance coefficients of steel panel model and typical acoustic coating material model are used to predict the acoustic absorption and insulation of model,which shows that the predicted insulation of steel panel model error is 2 d B less than the theoretical results.The acoustic impedance and reactance test technique provide efficient method and data to evaluate the noise reduction and target strength reduction of acoustic coating material.Secondly,frequency spectrum and frequency-wavenumber spectrum test method are developed for non-equilibrium turbulent boundary layer fluctuations on surface with non-zero pressure gradient.The anechoic wind tunnel is designed and constructed to provide high SNR for wall pressure measurement.Line array with 21 MEMS microphones are deployed to measure the non-equilibrium turbulent boundary layer fluctuations and the corresponding frequency-wavenumber spectrum using wing section models with variable pressure gradient.The normalized mathematical model of non-equilibrium turbulent boundary layer fluctuations frequency spectrum are deduced through fitted data with respect to Reynolds number and pressure gradient.In addition,the low frequency turbulent boundary layer fluctuation pressure increments at the joint area of the bulge and main body,and the area around the openings are measured,which are utilized to obtain the regression curve of increment region and the quantification law.The turbulent boundary layer fluctuation pressure frequency spectrum and frequency-wavenumber spectrum are measured using flexible microphone array on large scale vehicle model,the distribution law of turbulent boundary layer fluctuation pressure at windward and leeward in drift conditions.The non-equilibrium turbulent boundary layer fluctuation pressure measurement technique provides efficient excitation source coefficients for calculating the hydrodynamic excited structure vibration and noise.Last but not least,the acoustic field of point source in 3-D parallel space is analyzed using normal-mode method,and the acoustic field and sound spreading law of shallow water with multi-point source are also addressed.The experimental results show that there are strong fluctuation distribution,low frequency cut-off effect and dipole effect in shallow water,the former of which brings the source level measurement results with large bias,while the latter,i.e.,low frequency cut-off phenomenon reduces the SNR,which is not applicable to the low frequency noise measurement.Accordingly,the receiving response of hydrophone array in free-field are analyzed,while the noise measurement method for middle and high frequency span based on 1-D array is developed.The test results show that,hydrophone array has obvious directivity and background noise reduction effect,as a result,the SNR above 500 Hz is increased with 10~15 d B.Moreover,the hydrophone array could reduce the effect of fluctuation distribution of acoustic field,the acoustic field measurement bias reduced from 17 d B to 4 d B.The inverse matrix model is used to reconstruct acoustic source in bounded water space.Using normal mode method and mirror source method,the effect of multi source reconstruction by varied arrays,source and hydrophone distribution,coefficients selection are verified: the reconstruction precision depends on the distance between source and hydrophone,which implies smaller distance bring smaller reconstruction bias;besides,there are corner frequencies in reconstruction results.Below the corner frequency,reconstruction results are much larger than source power,which implies that it is inefficient to reduce the bias.On the contrary,above the corner frequency,reconstruction results are much smaller than source power so that increasing the hydrophone number and the size of hydrophone array could reduce the reconstruction bias effectively.Furthermore,increasing the water depth could reduce the corner frequency and reconstruction bias effectively.