Influence Of Different Inflow Conditions On Wave Characteristics Of Underflow Energy Dissipation

During the flood discharge process in reservoirs,it is necessary to implement energy dissipation measures to reduce the energy of the released water and ensure the operational safety of hydraulic structures.Among these measures,bottom flow energy dissipation is widely used due to its high dissipation efficiency and relatively mild atomization issues.For bottom flow energy dissipation,it mainly relies on the forced hydraulic jump to dissipate the energy of the discharged water.In engineering design,the goal is to reduce the kinetic energy of the water after dissipation,maintain a stable flow state,and ensure the safety of downstream channels,riverbanks,and adjacent structures.To alleviate the pressure of energy dissipation,diffuser sections are often considered at the end of the spillway channels.Additionally,drop structures are added to enhance the velocity attenuation of incoming water flow,reducing the velocity near the channel bed.The paper is based on the finite volume method(FVM)and utilizes the RNG k-εturbulence model and Tur VOF free surface tracking method to numerically simulate the energy dissipation of bottom flow under two different inflow conditions at the end of a spillway(with and without a diffuser section).By studying the flow pattern(water surface profile),vertical velocity distribution,pressure variations near the channel bed,and turbulent kinetic energy,as well as the characteristics of the tailwater surge after energy dissipation,including wave frequency,wave height,wave intensity,and wave attenuation,the relevant characteristics of energy dissipation in the tailwater surge under conditions of a small bottom slope and low Froude number were obtained.The main achievements of the paper are as follows:(1)In terms of flow state: With the diffusion section in place,the water flow is somewhat diffused,but the main flow is still near the axis.The flow in the energy dissipation pool is more chaotic and has a significant impact on the tailwater channel.On this basis,with the addition of a drop step,there was almost no change in the degree of turbulence in the flow in the energy dissipation pool,and the tailwater channel still showed frequent wave fluctuations.Without the diffusion section,a sudden expansion type of bottom flow energy dissipation is formed,which leads to increased turbulence and mixing of reverse flow in the energy dissipation pool and intensified shear effects.The outflow from the pool is smoother,and the wave fluctuations in the tailwater channel are significantly improved.On this basis,with the addition of a drop step,there was no significant improvement in the flow state of the energy dissipation pool,and there was little impact on the flow state of the tailwater channel.(2)Regarding the hydraulic characteristics of the energy dissipation pool: Under the conditions of a diffusion section,the vertical velocity distribution within the energy dissipation pool is between the bottom flow and surface flow.The pressure on the bottom plate is relatively high,with a maximum pressure near the outlet of 1.05 MPa.The maximum turbulent kinetic energy within the energy dissipation pool is only 18J/kg,and the diffusion in the area is small.Most of the turbulent kinetic energy carried by the water flows downstream into the tailwater channel,resulting in an energy dissipation rate of only56.72%.The overall energy dissipation effect is not ideal.When a hydraulic jump is introduced,there is no significant change in the vertical velocity distribution within the energy dissipation pool.The maximum pressure on the bottom plate is reduced to 1.03 MPa.The maximum turbulent kinetic energy within the energy dissipation pool increases to 20J/kg,while the diffusion area remains almost unchanged.The energy dissipation rate slightly increases to 58.82%,indicating that the energy is not dissipated effectively.Without the diffusion section,the vertical flow velocity of the incoming water in the energy dissipation pool is mainly concentrated at the bottom.The pressure distribution on the bottom plate is relatively uniform,with the maximum pressure(0.82 MPa)occurring in the latter half of the energy dissipation pool.The maximum turbulent kinetic energy within the pool is 20 J/kg,and the diffusion area is large.The energy dissipation rate is 71.04%,indicating a good energy dissipation effect.When a hydraulic jump is introduced,there is not much change in the flow velocity distribution near the bottom of the energy dissipation pool.The maximum pressure on the bottom plate is 0.83 MPa,and the instantaneous value of turbulent kinetic energy slightly increases along with the diffusion area.(3)In terms of wave characteristics:When the diffusion section is set,with the presence of the diffusion section,the maximum water depth at the wave crest in the tailwater channel reaches 1.89 m,and the wave frequency is approximately 6 cycles per minute.In certain local sections,the phenomenon of water flowing over the sidewall can be observed.When a drop structure is introduced,the water depth at the wave crest in the tailwater channel decreases to 1.80 m,and the frequency decreases to approximately 5cycles per minute.However,the water surface still exhibits noticeable fluctuations.Without the diffusion section,the water depth at the crest of the waves in the tailwater channel continues to decrease to 1.75 m.The decrease in frequency becomes even more significant,with the wave frequency approaching approximately 1.6 cycles per minute.On the other hand,when a drop structure is introduced,the wave crest in the tailwater channel remains at 1.74 m,and the frequency is approximately 1.5 cycles per minute,showing no significant change.In summary,for spillways with small bottom slope and low Froude numbers,setting a diffusion section at the outlet causes the mainstream of the inflowing water to be biased towards the surface.Although the flow velocity near the bottom is not large,the energy dissipation effect is difficult to achieve.The turbulence intensity and wave frequency in the downstream tailwater are more significant,which may endanger the safety of the river channel,bank slopes,and adjacent buildings.Adding a drop step does not change the vertical velocity distribution of the inflowing water in the pool,nor does it improve the hydraulic characteristics,and the tailwater waves remain prominent.Without the diffusion section,the increase of Froude number in the inflowing water in the pool causes the mainstream to be relatively concentrated,and a sudden expansion bottom flow energy dissipation can be formed.The turbulence intensity inside the energy dissipation pool increases,and the energy dissipation effect is improved.The outflowing water flow from the pool is stable,and the surging wave phenomenon is basically eliminated.Similarly,adding a drop step does not improve the energy dissipation effect.In other words,for spillways with small bottom slope and low Froude numbers,it is more advantageous not to set a diffusion section for energy dissipation and tailwater flow.