Study On The Numerical Simulation And Mitigation Measures Of Supersaturated Total Dissolved Gas Generation Based On Turbulent Aerification

The problem of total dissolved gas（TDG）supersaturation caused by the high dam discharge has drawn extensive attention with the increasing dams being built and put into operation.Aerated flow with the large flowrate and high velocity occurs when air entrains and mingles with water during the discharge.Aerated flow carries amounts of bubbles to the bottom of stilling basin or plunge pool,where the bubbles will dissolve under the high pressure,and consequently results in the supersaturated dissolved gas.TDG supersaturation is harmful to the aquatic organism,in particular it may cause fish get the gas bubble disease and increase fish mortality.In recent years,numerical simulation has been widely applied in the field of supersaturated TDG due to its convenience and strong maneuverability.However,the existing supersaturated TDG models lack of the in-depth study of gas concentration downstream the dam caused by air entrainment,which would impede the further study on the generation mechanism of supersaturated TDG.In this study,a new supersaturated TDG numerical model based on turbulent aerification was proposed.This new model considers the distribution of gas concentration due to air entrainment,the bubble movement,the bubble mass transfer,and the TDG transport.This study investigated the characteristics of supersaturated TDG generation and distribution with different energy dissipation patterns using the proposed model.In the light of mechanism analysis,this study also provides the mitigation measures of supersaturated TDG for each energy dissipation pattern.The main works and results of this study are summarized as below:（1）Gas concentration distribution and bubble size will impact the generation of TDG,which is the foundation and difficulty of the supersaturated TDG numerical study.A validated turbulent aerification model which is applied in the fields of breaking wave and bubble flow was introduced to the proposed model.The change of bubble size by mass transfer and pressure was also considered.Gas concentration distribution,bubble velocity filed,and TDG distribution can be simulated by establishing and coupling the bubble momentum equation,the bubble number density equation,the gas concentration continuity equation,the bubble mass transfer equation,and the TDG transport equation.The model was established using the hydraulics condition via the Open FOAM（an open-source platform）with linux and C++.This proposed model provides the foundation of studying the TDG generation mechanism and mitigation measures.（2）This study calibrated and validated the supersaturated TDG model,as well as further analyzed the parameter sensitivity.There is an unknown parameter D₀needs to be calibrated.Four projects（i.e.,hydraulic jump,Tongjiezi Dam,Gongzui Dam,and Dagangshan Dam）were selected to do the calibration.Moreover,the hydrodynamic conditions and TDG distribution of Priest Rapids Dam and Gongzui Dam were validated separately by comparing the simulated results with the experimental or field-observed data.The results show this TDG model performed very well in simulating the gas concentration distribution and TDG distribution downstream the dam.The analysis of parameter sensitivity suggests the impact of D₀on the bubble size in water and the value of D₀need to be decided based on the calibration.（3）This study investigated the characteristics of supersaturated TDG generation and distribution with different energy dissipation patterns（underflow type energy dissipation,roller bucket type energy dissipation,and trajectory bucket type energy dissipation）using the numerical simulation.The simulation results indicate for the underflow type energy dissipation,the entrained bubbles are at the front bottom slab.For the roller bucket type energy dissipation,bubbles appear near the water surface after the hydraulic jump.For the trajectory bucket type energy dissipation,bubble distribution is widely dispersed in the plunge pool.More bubble mass transfer occurs with the increasing interfacial specific surface area under the same and sufficient pressure condition.In addition,compared with bubbles at the water surface,bubbles near the bottom slab will be dissolved in water more easily because of higher effective TDG saturated solubility with larger pressure.In this respect,the pattern of roller bucket type energy dissipation can effectively decrease the TDG level.The last but not the least,for underflow type energy dissipation and trajectory bucket type energy dissipation,the auxiliary weir helps make the mass more fully transfer by reducing bubble velocity and extending bubble residence time.Consequently,the high level TDG will dissipate near the water surface due to the bubble pressure.（4）This study proposed TDG mitigation measures about installing baffle blocks and reducing the inflow TDG level in basin respectively,when taking into account the TDG generation characteristics downstream the dam of underflow type energy dissipation.According to the Tongjiezi Dam simulation results,at the outlet of basin,the cross section mean of TDG was 126.8%and 132.0%with and without installing of baffle blocks in basin,respectively.Water level rises because flow area is reduced when water flows through the baffle blocks.Because of baffle blocks,more bubbles gather near the water surface instead of the bottom slab.Due to the reduced pressure,the bubble mass transfer is insufficient,which can reduce the TDG generation.when the inflow TDG was 129.2%,the cross section mean of TDG was 147.1%.When the inflow TDG was 100%,cross section mean of TDG was 132.0%at the outlet of basin.（5）This study proposed mitigation measures about separate discharge with multiple working spillways when taking into account the TDG generation characteristics below the dam of roller bucket type energy dissipation.For Gongzui Dam,the cross section means of TDG at the area 400 m away from the dam were126.8%、121.4%and 112.5%when the working spillway number were 1,2 and 3,respectively.When dam discharges with multi-spillways,the turbulence become weak,and fewer bubbles are entrained,which can lead to lower TDG level.（6）This study proposed mitigation measures about installing baffle blocks downstream the auxiliary weir and separate discharge when considering the TDG generation characteristic below the dam of trajectory bucket type energy dissipation.For Dagangshan Dam,at the aera 500 m away from the dam,the cross section mean of TDG was 120.9%with installing of baffle blocks downstream the auxiliary weir,about 12.9%less then the result of the case without baffle blocks.The simulation results show that bubbles in bottom layer with high pressure can rise to the water surface with low pressure by the baffle blocks,and TDG generation is reduced.The cross section means of TDG at the area 500 m away from the dam was 120.5%for uniform-spillway case,13.5%less than the result of multi-spillways case（134.0%）.When dam discharges with multi-spillways,due to the weak turbulence,the entrained bubble mass is small,which can reduce the TDG generation.In summary,this study promotes the further development of supersaturated TDG model,as well as provides the theoretical basis on the characteristic of supersaturated TDG generation and distribution with different energy dissipation patterns.In addition,the results provide the future studies with the reference about mitigation measures of supersaturated TDG for each energy dissipation pattern.It has important theory meanings and practical applying value to mitigate the adverse effects of hydropower development on fish and realize sustainable development of hydropower.