Investigations On Prototype Observation And Mechanisms Of Low Frequency Noise Induced By High Dam Flood Discharge

The issues of layout and security operation of high dam during flood discharge and energy dissipation have received sufficient attention from the hydraulic engineering field in recent decades,since a majority of high dams were projected and built in China with the features of high water head,big flood-discharge capacity,deep narrow valley and large flood-discharge power.The downstream scouring,cavitation erosion,structure vibration and atomization induced by flood discharge of high dam have been widely studied and achieved abundant technological achievements,while few studies have been done on low frequency noise(air pulsation lower than 10 Hz,LFN)so far.However,some environmental hazards from LFN was found occurring during flood discharge period surrounding Xiangjiaba,Jinping First Stage,Xiluodu,Huangjinping Hydropwer Station and so on recently,such as the sustained vibration of roller shutter doors on some shops and windows of residential buildings in downstream towns.Aiming at studying the mechanisms and the key influencing factors of LFN induced by flood discharge and energy dissipation of high dam,systematic researches about the flow-induced vortex sound generation mechanisms,the on-site LFN's distribution and propagation laws,the LFN's generation mechanism and prediction during energy dissipation by hydraulic jump and the LFN's multi-sources mechanisms during energy dissipation by ski-jump have carried out in this paper with the methods of theoretical analysis of hydrodynamic acoustic,LFN prototype observation,two-phase turbulent numerical simulation,acoustic numerical simulation,etc.The main results are as follows:Firstly,study on vortex sound theory and generation mechanisms in flood discharge and energy dissipation of high dam.The phenomena of LFN induced by flood discharge of high dam belong to research category of hydrodynamic acoustic.Based on hydrodynamic acoustic,vortex sound theory is first applied to the mechanism research of flow-induced LFN.Vortex sound equation is solved and analyzed theoretically through Green function and matched asymptotic expansions.The result indicates flow-induced acoustic pressure is directly related to the sizes,variations and motion of vortexes,and only the region with time-varying vorticity is the effective acoustic source region.Vortex sound generation mechanisms of flood discharge and energy dissipation of high dam are put forward based upon vortex sound theory.Secondly,analysis and research on prototype observation and propagation laws of induced LFN.The distribution regularities and time and frequency domain characters of the induced LFN along the downstream are analyzed.The relationship between LFN amplitude and flow regime is studied.The similarities and differences about LFN characteristic between energy dissipation by hydraulic jump and ski-jump are compared.LFN's propagation and attenuation laws are researched by means of acoustic absorption mechanism in atmosphere.The correlations between wall pressure fluctuations and LFN are investigated according to prototype observation and hydraulic model test in the view of hydraulics.Thirdly,study on generation mechanism and prediction model of LFN induced by energy dissipation of hydraulic jump of high dam.In terms of the CFD numerical simulation results of prototype discharge conditions of Xiangjiaba Hydropower Station,the flow field characteristic is analyzed,and Q-criterion is utilized to visualize the vortical structures in energy dissipation area.The vorticity fluctuation characteristics and vortex sound generation mechanisms of hydraulic jump are studied in detail.The mathematical prediction model of LFN intensity for energy dissipation by hydraulic jump is established through vortex sound theory,turbulent model and prototype observation.The results indicate,the vorticity fluctuation data from the strong shear layers around the high-velocity submerged jets in the hydraulic jump have larger RMS values where vorticity fluctuation data are highly correlated to on-site LFN data,and the strong shear layers are the main acoustic source regions of LFN.In addition,the predicted and observed LFN intensity coincides well.Finally,study on the multi-sources mechanisms of LFN induced by energy dissipation of ski-jump of high dam.As the generation mechanism of LFN induced by overflow water curtain vibration can't completely be used in our country's high-dam hydraulic engineering with ski-jump type,the multi-sources mechanisms are proposed according to the prototype observation results of LFN of Jinping First Stage Hydropower Station.The multi-sources mechanisms contain the sources of LFN from submerged jets in plunge pool and nappe-cavity coupled oscillation.Turbulent numerical simulation and vortex sound theory are carried out to study the mechanism from submerged jets in plunge pool.Acoustic numerical simulation is carried out to study the mechanism from nappe-cavity coupled oscillation.The multi-sources influence analysis is made based on prototype observation results.The results manifest the existence of multi-sources mechanisms.The influence degree from two sound sources is almost same,and nappe-cavity coupled oscillation can induce LFN with wider spectrum energy.In a word,to flood discharge and energy dissipation whether by hydraulic jump or by ski-jump of high dam,vortexes generate and gather in the strong shear layers of flow field where are the main acoustic source regions of LFN.In addition,the LFN energy from nappe-cavity coupled oscillation should also be taken into account in practical engineering.The studies in this paper provide theoretical foundation,assessment measure and prediction method for environmental hazards of LFN induced by flood discharge of high dam.