Compound Prediction Method For Flood Discharge Atomization Of Jet Flow Based On Physical And Mathematical Models

Recently,with the rapid hydropower development in Western China,a significant number of water conservancy projects with high water head and large flood discharge have been built.Although new type ski-jump energy dissipator can successfully solve the flood discharge and energy dissipation problems of high dam projects characterized by high-water head,large discharge,narrow valley and frequent flood discharge,it brings about greater rainfall intensity and rainfall range.The rainstorm caused by the flood discharge atomization is much heavier than the natural rainfall,which will threaten the safety of dam operation,the stability of the downstream slope and the surrounding ecological environment.Flood discharge atomization is an extremely complex multiphase flow involving many factors,which makes it difficult to predict the intensity distribution of atomization rainfall accurately.Additionally,the nappe shears and mixes with the surrounding air violently in the process of movement to dissipate huge kinetic energy.The sharp aeration and deformation of the nappe makes it a challenge to predict the nappe trajectory accurately.As an important index in ski-jump energy dissipator design,the nappe trajectory has great influence on the layout of the downstream plunge pool,the scouring range of the downstream riverbed and the prediction of intensity distribution of atomization rainfall.Therefore,based on the random splash test,this paper studies the distribution of nappe wind and splashing droplets for different flip bucket under different discharge conditions.Meanwhile,The traditional grid method and SPH method(smooth particle dynamics)are combined to simulate the trajectory of the aerated jet flow.And then,based on the physical model test and numerical simulation method,the stochastic splash mathematical model is improved and applied to the actual project.The influence of flip bucket shape and discharge conditions on the intensity distribution of flood discharge atomization is detailly explored.The main contents are as follows:First of all,it is important to explore the influencing mechanism of flip bucket shape and discharge conditions on the distribution of downstream nappe wind and splash intensity.Therefore,the random splash test is used to compare and analyze the distribution and interaction of the downstream nappe wind and splash intensity of each flip bucket under different discharge conditions.The experimental results demonstrate that the downstream nappe wind is closely related to the hydraulic characteristics of the flip bucket.The downstream nappe wind of the continuous bucket satisfies the normal distribution,and the peak value of nappe wind appears in the axis of spillway.With the increase of the distance between the measuring point and spillway axis,the nappe wind decreased dramatically.While for the tongue-shaped bucket,the peak value of their nappe wind appear in the both sides of the spillway axis,and the distribution of the nappe wind is similar to a hump in the lateral distribution,which is obviously different from that of continuous bucket.With the decrease of discharge,the peak effect of nappe wind weakens gradually.Aiming at the situation of the powerhouse at the Nazixia hydropower station located in the rainfall area of its flood discharge,this paper conducts study on harnessing schemes of flood discharge atomization.Through hydraulic model test,various parameters of different bucket types are compared,including the hydraulic characteristics,downstream riverbed erosion pattern,nappe wind,weight of splashed water.Through optimization,a flip bucket of curved surface attached to the spillway left wall is suggested.For this bucket,its trajectory nappe is deflected rightward away from the spillway axis and far away from the powerhouse.The splash weight near the powerhouse is significantly reduced,only accounting for 3.79% of the original bucket under the check condition.The study aims to provide a reference for flood discharge and energy dissipation design of similar projects,as well as a theoretical basis for the improvement of the subsequent numerical model.Second,it is very difficult to accurately predict the trajectory distances because of the severe aeration and deformation of the prototype jet flow.Both hydraulic model test and traditional grid method can not achieve accurate analyses.Therefore,this paper integrates the traditional grid method with SPH method(smooth particle dynamics)to simulate the trajectory of the aerated jet flow.Based on the Euler-Euler multiphase model,the numerical simulation of the entrainment characteristics of the jet flow is carried out and verified with the experimental results.At the same time,the effects of bubble size,wall roughness height,drag force model and turbulence model on the entrainment characteristics of the jet flow are discussed.Due to the high aeration concentration and the discrete broken droplets patterns of high-speed jet flow,the traditional grid method is difficult to reflect the resistance of droplet group.In this paper,a self-developed SPH method is proposed to simulate the motion characteristics of jet flow.In order to enhance computational efficiency,the SPH method is transplanted to GPU to accelerate the computing speed.The SPH program is verified by a dam break model with high Reynolds number.It is proven that the numerical simulation results can vividly reflect the whole process of splashing,rolling and backward jet flow.At the same time,the SPH method based on Riemann solution,the standard SPH method and the Delta-SPH method are compared.It is found that the SPH method based on Riemann solution has better accuracy and stability even with relatively large particle spacing.Finally,the proposed method which combines Euler-Euler multiphase model and SPH model is verified by the test data of horizontal jet flow with different initial velocity,jet flow with different angle and prototype nappe,respectively.It is shown that the simulated water jet morphology and trajectory distances are in good agreement with the experimental observation.Third,the traditional mathematical model of flood discharge atomization has poor precision in calculating the hydraulic characteristics(the trajectory distance,width,height and velocity of water flow)of the tongue-shaped bucket,curved bucket and other special shaped bucket.Also,the spatial distribution of nappe wind and the correlation between the diameters and the corresponding velocities of the splashing droplet are less studied.These factors mentioned above have great influence on the rainfall intensity and distribution.Based on the physical model test and numerical simulation,the hydraulic characteristics parameters of impinging outer edge of water jet are obtained,and the linear ejection source is divided into multiple segments.Furthermore,the spatial distribution of nappe wind is measured,and a laser raindrop spectrograph is installed to obtain the dimensionless relationship between the droplet diameter and splashing velocity.Then the mathematical model of flood discharge atomization is optimized by combining atomization characteristics test and numerical simulation,and is verified by a splash test.For actual power station,it is difficult to accurately calculate the prototype nappe trajectory because of aeration and air resistance.To obtain more accurate characteristic parameters of atomization,the coupled nappe trajectory model proposed above is used to calculate the prototype nappe trajectory.Then,with the improved stochastic splash numerical model,the influence of bucket type and discharge conditions on the rainfall distribution is explored.By predicting the influence of bucket types on the distribution of rainfall intensity,it is found that under the design conditions,when adopting the asymmetric tongue-shaped bucket,tongue-shaped bucket and continuous bucket,the torrential rainstorm area spread to the left bank to the elevations of 3097.8 m,3096.2 m and3095.5 m,respectively,and the powerhouse is in the torrential rainstorm area.When using the flip bucket with a curved surface attached to the left wall,,the water jet and the peak value of the nappe wind turn away from the powerhouse,and the torrential rainstorm area and the rainstorm area are significantly reduced,only spreading to the left bank to the elevations of 3092.6 m and 3093.0 m,respectively.The powerhouse and the owner camp is no longer in the rainstorm area under different working conditions,which could effectively ensure the safety operation of the powerhouse.