Numerical simulations of the flow in the draft tube of a Francis turbine are carried out in order to elucidate the effects of tangential velocity on flow stability.Influence of the location of the maximum tangential v...Numerical simulations of the flow in the draft tube of a Francis turbine are carried out in order to elucidate the effects of tangential velocity on flow stability.Influence of the location of the maximum tangential velocity is explored considering the equality of the total energy at the inlet of the draft tube.It is found that the amplitude of the pressure fluctuation decreases when the location of the maximum of the tangential velocity moves from the centre to the wall on the cross section.Thus,the stability of the flow in the draft tube increases with the moving of the location of the maximum tangential velocity.However,the relative hydraulic loss increases and the recovery coefficient of the draft tube decreases slightly.展开更多
For Francis turbines,frequent operations under extremely low load conditions result in significant noise and pressure fluctuation issues.These issues may cause vibration and fatigue damage to the unit,accompanied by d...For Francis turbines,frequent operations under extremely low load conditions result in significant noise and pressure fluctuation issues.These issues may cause vibration and fatigue damage to the unit,accompanied by difficulties in connecting to the grid and reductions in the power generation efficiency of renewable energy.However,there is limited research on the relationship between pressure fluctuations and the induced noise of Francis turbines during extreme operations.In the present study,an acoustic numerical simulation based on the Ffowcs Williams-Hawkings equation and large eddy simulation is used to analyze the acoustic performances of Francis turbines.In the current study,for evaluating the acoustic characteristics under such terrible conditions,the results of variable flow rate and guide vane opening conditions are compared.Results indicated that Francis turbine noise is mostly due to pressure fluctuations brought on by rotor-stator interference and corkscrew-shaped vortices.The blade passing frequency(BPF)of 130.00 Hz and the low frequency of 0.33 f_(n)(where f_(n)denotes the rotating frequency)are the key factors affecting pressure and noise fluctuations.The influence of low frequency is reduced as the flow rate rises,whereas the influence of BPF gradually increases.Besides,the hydrodynamic noise of Francis turbines is primarily low-frequency,with discrete and broad-band features.The rotating noise with distinct peak values and the turbulence noise produced by large-scale vortices(corkscrew-shaped vortices)make up the majority of low-frequency noise.Therefore,reducing pressure fluctuations is a key strategy for lowering flow-induced noise radiation.展开更多
Tight formation flight,as a significant way for fixed-wing unmanned aerial vehicle(UAV)to execute missions,generates synergistic aerodynamic effects that significantly influence themotion decision-making and control o...Tight formation flight,as a significant way for fixed-wing unmanned aerial vehicle(UAV)to execute missions,generates synergistic aerodynamic effects that significantly influence themotion decision-making and control ofUAVs.In aerial refueling missions,this is manifested as complex aerodynamic effects such as vortices affecting the path planning of the refueling UAV.This paper proposes a path-planning method for fixed-wing UAVs to conduct aerial refueling under the constraints of synergistic aerodynamics.Firstly,an environment constraint model for vortex distribution is obtained from aerodynamic experimental data of the refueling formation.Subsequently,by utilizing the differential flatness property of fixed-wing UAVs,the nonlinear system states and control variables are mapped to linear functions of flat outputs.This allows the establishment of segment constraints for the path,enabling the use of a key-point heuristic algorithm in the flat output space to generate the aerial refueling flight path.Furthermore,a flat output minimum snap algorithm is applied for multi-constraint optimization of the flight path,resulting in a smooth and feasible optimal path.Simulation experiments demonstrate the effectiveness and advancement of the proposed path-planning method under the influence of vortices.展开更多
基金supported by the open fund of State Key Laboratory of Hydroscience and Engineer of Tsinghua University(No.sklhse-2013-E-02)the Special Major Project of Science and Technology of Zhejiang province(No.2013C 01139)
文摘Numerical simulations of the flow in the draft tube of a Francis turbine are carried out in order to elucidate the effects of tangential velocity on flow stability.Influence of the location of the maximum tangential velocity is explored considering the equality of the total energy at the inlet of the draft tube.It is found that the amplitude of the pressure fluctuation decreases when the location of the maximum of the tangential velocity moves from the centre to the wall on the cross section.Thus,the stability of the flow in the draft tube increases with the moving of the location of the maximum tangential velocity.However,the relative hydraulic loss increases and the recovery coefficient of the draft tube decreases slightly.
基金supported by the National Natural Science Foundation of China(Grant No.52179087).
文摘For Francis turbines,frequent operations under extremely low load conditions result in significant noise and pressure fluctuation issues.These issues may cause vibration and fatigue damage to the unit,accompanied by difficulties in connecting to the grid and reductions in the power generation efficiency of renewable energy.However,there is limited research on the relationship between pressure fluctuations and the induced noise of Francis turbines during extreme operations.In the present study,an acoustic numerical simulation based on the Ffowcs Williams-Hawkings equation and large eddy simulation is used to analyze the acoustic performances of Francis turbines.In the current study,for evaluating the acoustic characteristics under such terrible conditions,the results of variable flow rate and guide vane opening conditions are compared.Results indicated that Francis turbine noise is mostly due to pressure fluctuations brought on by rotor-stator interference and corkscrew-shaped vortices.The blade passing frequency(BPF)of 130.00 Hz and the low frequency of 0.33 f_(n)(where f_(n)denotes the rotating frequency)are the key factors affecting pressure and noise fluctuations.The influence of low frequency is reduced as the flow rate rises,whereas the influence of BPF gradually increases.Besides,the hydrodynamic noise of Francis turbines is primarily low-frequency,with discrete and broad-band features.The rotating noise with distinct peak values and the turbulence noise produced by large-scale vortices(corkscrew-shaped vortices)make up the majority of low-frequency noise.Therefore,reducing pressure fluctuations is a key strategy for lowering flow-induced noise radiation.
基金supported in part by Natural Science Foundation of Sichuan Province(2024NSFSC0181)Aviation Science Foundation of China(201901080001)in part by Fundamental Research Funds for China Central Universities(ZYGX2014J098).
文摘Tight formation flight,as a significant way for fixed-wing unmanned aerial vehicle(UAV)to execute missions,generates synergistic aerodynamic effects that significantly influence themotion decision-making and control ofUAVs.In aerial refueling missions,this is manifested as complex aerodynamic effects such as vortices affecting the path planning of the refueling UAV.This paper proposes a path-planning method for fixed-wing UAVs to conduct aerial refueling under the constraints of synergistic aerodynamics.Firstly,an environment constraint model for vortex distribution is obtained from aerodynamic experimental data of the refueling formation.Subsequently,by utilizing the differential flatness property of fixed-wing UAVs,the nonlinear system states and control variables are mapped to linear functions of flat outputs.This allows the establishment of segment constraints for the path,enabling the use of a key-point heuristic algorithm in the flat output space to generate the aerial refueling flight path.Furthermore,a flat output minimum snap algorithm is applied for multi-constraint optimization of the flight path,resulting in a smooth and feasible optimal path.Simulation experiments demonstrate the effectiveness and advancement of the proposed path-planning method under the influence of vortices.
