Research On Ultrasonic Standing Wave Acoustic Field Characteristics Using LDV Measurement Method

Acoustic standing wave levitation,as a non-contact suspension technology with good lateral stability,is widely used in special materials preparation,biochemical analysis,small object transmission,spatial stereoscopic imaging and other scientific research.The acoustic pressure within the levitation cavity is the basis of this technique.The non-linear phenomena,such as jump phenomenon,hysteresis phenomenon and harmonic generation in the levitation cavity,are the main factors that limit the optimization of the current transducers and even affect suspension effect of the levitated object.In this paper,the method of measuring the static acoustic field within the levitation cavity by using the laser Doppler vibrometer(LDV)and acoustic field reconstruction are studied.The partial nature of the nonlinear phenomenon in the levitation cavity is studied by this measurement method.First of all,for the emitter or reflector surface is flat,the mechanism of acoustic levitation is analyzed,and the vibration velocity of the media within the levitation cavity,acoustic pressure distribution equation and acoustic levitation force of the levitated objects are deduced.The principle of using LDV to measure the acoustic pressure distribution within the levitation cavity is proposed and the related formulas are provided.Two kinds of acoustic field reconstruction algorithm are provided,one is Hankel Fourier(HF)algorithm and the other is the filtered back projection(FBP)algorithm.The former is fit to reconstruction of the axisymmetric acoustic field,while the latter matches reconstruction of the non-axisymmetric acoustic field.Next,based on plane wave assumption,the simulated acoustic pressure distribution within the levitation cavity and the simulated levitation cavity length corresponding to different levitation modes are acquired by COMSOL multiphysics.For axisymmetric acoustic field,combined with COMSOL-simulated acoustic pressure,the COMSOLMATLAB(C-M)co-simulation is employed to acquire the simulated LDV velocity output(?sLDV).Then,the HF algorithm is applied to achieve acoustic field reconstruction.For non-axisymmetric acoustic field,Radon transform is used to process the COMSOLsimulated acoustic pressure aiming at obtaining ?sLDV.Hereafter,FBP algorithm is employed to reconstruct the acoustic field within the levitation cavity.The reconstruction error of the HF algorithm and the FBP algorithm are analyzed respectively under the simulation condition.Finally,the feed structure of the reflector in the levitator is improved and the acoustic field measurement platform is established.The relationship between vibration velocity of the emitter and voltage is calibrated by using LDV.The levitation cavity length for different levitation modes are measured by LDV for both axisymmetric and non-axisymmetric levitation cavity.At the third levitation mode,the acoustic field on the typical cross section for both axisymmetric and non-axisymmetric levitation cavity are measured.Hereafter,the experimental data are processed by the HF algorithm and the FBP algorithm.The reconstruction results are compared with the simulated results.For the axisymmetric acoustic field,the feasibility of using LDV to measure the acoustic field in the resonator is analyzed.The effects of vibration amplitude of the emitter and levitation cavity structure(reflection surface cross section / surface radius,axis deviation)on jump phenomenon,hysteresis phenomenon and harmonic generation are studied.