| There are many big rivers containing abundant hydropower in China.In the past few decades,with the rapid economic and social development,a number of large hydropower-plants(HP)have been built in our country.These HPs all feature dam heights and installed capacity ranked top all over the world and thus the problem of flood discharge and energy dissipation is one of the key issues for the safe and stable operation of them.Technically speaking,the main challenge of these problems lies in the high-velocity aerated flow.In hydraulic engineering,the flow is mostly driven by gravity,the aerated high-velocity flow is thus mostly developed from stratified air-water flow(SAWF)associated with the development of the turbulence.Accordingly,the research on high-velocity aerated flow mainly comprises the turbulence of SAWF and aerated flow.At present,the traditional experimental approach has encountered many difficulties such as scale effect and measuring techniques,in the investigation of SAWF.Simultaneously,the rapid development of computing power and computational fluid dynamics(CFD)in the past few decades has made numerical simulation an economically feasible and increasingly irreplaceable method for the study of highvelocity aerated flow.Based on a thorough review of the current research status at home and abroad,an in-depth summary of the microscopic mechanism and simulation theory of air-water two-phase flow is conducted,and then the simulation methods of SAWF and high-velocity aerated flow are addressed.Firstly,the role of the mixture density in the turbulence transport equation of the mixture framework for SAWF is analyzed.It is found that not including mixture density will significantly overestimate the turbulent kinetic energy,turbulent dissipation rate,and turbulent viscosity in the entire domain.The modified solver which introduces mixture density can signif icantly improve the prediction of velocity distribution in the air side and the thickness of the super air layer in the open-system SAWF.Moreover,the improved solver can also accurately calculate the flow velocity and turbulent kinetic energy distribution of water and air in the closed-system SAWF.However,the influence of waves at the interface cannot be easily reflected in the mixture framework.Therefore,the modified solver still slightly overestimates the turbulent kinetic energy at the interface and in the air,returning a lower air-side bottom velocity gradient than that in the experiment.The overestimation is not improved after further refining the mesh.In addition,the performance of the modified mixture solver is also compared with that of the two-fluid solver with interfacial turbulence damping for the modeling of SAWF.It is found comparatively the modified solver has obvious advantages,including clear basis on physical parameters,robustness in numerical calculations,and higher computational efficiency.Moreover,it does not require tuning of any empirical parameter.Secondly,the turbulence transport equation of the per-phase turbulence model in the two-fluid framework is dissected.In doing so,it is found that the influence of the mixture density on the turbulent transport is indirectly reflected by the volume fraction gradient,which leads to the sign inversion of the term related to the mixture density for the light phase.This further results in the overestimation of the turbulent kinetic energy on the air side in stratified circumstances.From this point,a density-based interfacial turbulence damping model for the two-fluid framework is proposed.This model together with the conventional phenomenological turbulence damping model are then tested against three typical SAWF cases.It is found that the density-based interface turbulence damping model performs satisfactorily for all the three test cases.The overestimated turbulence in the original model is mostly corrected.Moreover,the turbulence damping for the lighter phase is more important than the heavier phase,which further confirms that the symmetry damping model is problematic.Similarly,the influence of interface fluctuation on the turbulence near the interface is observed.Furthermore,it is clearly pointed out in this thesis that the influence of waves has been considered in combination with the instrisic mathematical problem of the per-phase turbulence model in previous studies.Therefore,it is suggested that the instrisic problem of the model should be fixed first,and then a specific turbulence correction term should be used to consider the influence of waves on the air-side turbulence.Moreover,it is found that further refining the grid near the air-side interface cannot improve the results substantially.This is because the k-? turbulence model is inherently not suitable for the near-interface flow of the light-phase.In order to correctly simulate the near-interface turb ulence of the lighter phase,it is more important to modify the turbulence model than to improve the mesh resolution.Finally,the models related to aerated flow simulations in FLOW-3D software are summarized and investigated.Based on that,the two-phase flow on steps behind Xshaped flaring gate pier(FGP)under very high unit discharge is simulated.In the test of the bubble size model,it is found that for turbulence induced aerated flow,the bubble size is mainly controlled by the critical capillary number,and its default value 1recommended by the software is basically acceptable;the influence of the initial bubble diameter is insignificant,and the default value 0.001 m can be used directly in the calculation.The drag coefficient and the Richardson-Zaki coefficient multiplier have a great influence on the calculation.It can change the air concentration distribution,the aerated flow depth,and the bubble radius in the upper 20% of the water body.Meanwhile,through the simulation of the two-dimensional air-water two-phase flow on the stepped spillway,it is found that the model in FLOW-3D can roughly reproduce the macroscopic characteristics of the flow.it can also accurately reproduce the development of the turbulent boundary layer in the non-aerated region and the location of the inception point,provided the grid resolution is sufficient.However,the calculated flow velocity and the air concentration still deviate from the experimental values.The magnitude of the calculated bubble size is reasonable.The drag coefficient and Richardson-Zaki coefficient multiplier have little effect on the air concentration results.In the 2D simulation of the uniform aerated flow,it's found that the calculated turbulent kinetic energy of non-aerated open channel flow is reasonable,which is in agreement with the dimensionless distribution curve reported in the literature.The calculated mixture flow velocity is in good agreement with the prototype results reported in the literature under the same roughness.However,the calculated pure water and aerated depths could deviate from the experimental data by ±30% at the highest.In the 3D simulation of flow on steps behind X-shaped FGPs,it is found that FLOW-3D is robust for the simulation of aerated flow in 3D complex structures.Moreover,the flow velocity at the step surface is about 15 m/s,and there are negative pressures near the convex corner of the 11th-45 th steps under multiple opening conditions.The calculated cavitation indices near the steps at steps 21-42 are lower than the threshold value,implying high potential cavitation erosion risk.Further considering the air concentration near the steps,which are larger than the critical value of 7% before the30 th step but lower than 7% between the 30 th to 45 th steps,the possibility of cavitation damage at steps 30-45 is high.This is consistent with the actual damage of the steps.The height of the first step can affect the air concentration along the step to some extent.Further heightening the first step can increase the aeration concentration on the step surface(especially before the 35 th step).Therefore,for existing dams,heightening the first step is an economically feasible measure to increase the concentration of aeration on the step surface. |
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