Experimental Study On The Vortex-Acoustic Coupling Phenomenon In Deep Cavity

When the separation shear flow is coupled with the cavity acoustic standing wave mode in the pipelines,serious noise radiation,structural vibration and even fatigue fracture will occur.This kind of vortex-acoustic coupling phenomenon in deep cavity commonly appears in important engineering applications such as aerospace,electric power,transportation,and national defense military,which seriously affects the operational criterions and service life of industrial equipment,and cannot be neglected in related engineering design process.Undoubtedly,it is necessary to reveal the mechanism of vortex-acoustic coupling phenomenon in deep cavity and corresponding temporal and spatial evolution characteristics through systematic research.This paper aims to systematically study the self-sustained oscillating flow field in deep cavity under vortex-acoustic coupling modulation by advanced flow measurement methods.In our experiments,coaxial deep cavities with right and curved inner corners were taken as research objects,which were commonly used in engineering applications.Firstly,a highprecision particle image velocimetry(PIV)platform was built.In order to eliminate the optical shadow caused by the cross-wall of deep cavity in the PIV experiment,this paper developed the light field enhancement method based on the ray-tracing method and the iterative optimization design of the profile window,by which the characteristics of vortex-acoustic coupling flow field characteristics were perfectly presented.Subsequently,the FPGA real-time signal processing system,combined with the phaselocked PIV method were applied,by which the dynamic phase response relationship between the vortex dynamics and the time-frequency oscillation of acoustic standing wave in deep cavity were successfully obtained.Finally,the cavity acoustic mode theory and Howe's acoustic analogy theory were used to reveal the energy transmission process between the flow field and the acoustic field under vortex-acoustic coupling modulation.The results showed that the proposed light field enhancement method can effectively solve the problem of optical shadow,providing the basis for flow field measurement and vortex-acoustic coupling analysis.When the vortex-acoustic coupling phenomenon occurred,pressure fluctuation in the symmetric deep cavity was enhanced remarkably and the its intensity gradually increased along the depth direction.The intensity of pressure fluctuation at the bottom of the deep cavity was much higher than the dynamic pressure head of main flow.Under the modulation of acoustic standing wave,obvious shear layer oscillation behavior occurred near the opening of the deep cavity,which greatly enhanced the fluctuation intensity of the velocity and vorticity in the region,accompanied by strong periodic vortex shedding process.The shedding vortex near the opening of the deep cavity was transported toward the bottom of the deep cavity in the selfsustained oscillating flow field dominated by the vortex-acoustic coupling phenomenon.And the intensity and spatial scale of the shedding vortex first increased and then decreased during one period of the acoustic standing wave.The apparent dynamic response relationship between vortex dynamics and acoustic standing wave oscillation was directly related to the energy transmission process between the flow field and acoustic field.Influenced by the Coanda effect,the oscillation intensity of shear layer and transport depth of shedding vortex in the deep cavity with curved inner corners were larger than those in the deep cavity with right inner corners,and the vortex-acoustic coupling strength was significantly higher.Thus,special attention need to be paid in the design process of pipelines with deep cavities.