Pressure fluctuation may cause high amplitude of vibration of double-suction centrifugal pumps, but the impact of impeller stagger angles is still not well understood. In this paper, pressure fluctuation experiments a...Pressure fluctuation may cause high amplitude of vibration of double-suction centrifugal pumps, but the impact of impeller stagger angles is still not well understood. In this paper, pressure fluctuation experiments are carried out for five impeller configurations with different stagger angles by using the same test rig system. Results show that the stagger angles exert negligible effects on the characteristics of head and efficiency. The distributions of pressure fluctuations are relatively uniform along the suction chamber wall, and the maximum pressure fluctuation amplitude is reached near the suction inlet tongue region. The pressure fluctuation characteristics are affected largely by impeller rotation, whose dominant frequencies include impeller rotation frequency and its harmonic frequencies, and half blade passage frequency. The stagger angle exerts a small effect on the pressure fluctuations in the suction chamber while a great effect on the pressure fluctuation in volute casing, especially on the aspect of decreasing the amplitude on blade passage frequency. Among the tested cases, the distribution of pressure fluctuations in the volute becomes more uniform than the other impeller configurations and the level of pressure fluctuation may be reduced by up to 50% when the impeller stagger angle is close to 24° or 360°.The impeller structure pattern needs to be taken into consideration during the design period, and the halfway staggered impeller is strongly recommended.展开更多
In order to investigate the complex flow characteristics inside an unshrouded centrifugal impeller with splitter blades at off-design conditions,and analyze its influence on pump operation stability,a numerical simula...In order to investigate the complex flow characteristics inside an unshrouded centrifugal impeller with splitter blades at off-design conditions,and analyze its influence on pump operation stability,a numerical simulation study was carried on using the curvature-corrected SST-CC turbulence model;the head and efficiency accorded with experimental results.The pressure fluctuation,unsteady radial force and velocity were analyzed quantitatively and the numerical results indicate this:the peak to peak value of pressure fluctuation in the impeller channel gradually increases in the flow direction and at 0.49Qn,the slope of peak to peak value to normalized impeller channel behind the splitter blade is 8.57 times greater than that before the splitter blade.The greater the flow rate deviates from the design condition,the larger the peak to peak value of the pressure fluctuation and radial force;in particular at 0.27Qn,the maximum radial force is 194.29%greater than that of the design condition.When the operating flow rate is smaller than 0.83Qn,the stall occurs and the stall vortex could block the impeller discharge;as the flow rate decreases further,the pressure amplitude at rotational frequency gradually increases in the impeller channel and the prevailing frequency changes from the blade passage frequency(BPF)to the rotating stall frequency in the diffuser.The tip leakage vortex(TLV)is generated in the tip region and rotated move downstream in the impeller flow channel,and the backflows appear on the blade suction side and in the tip and the tongue regions;the smaller the flow rate,the more serious the TLV and backflow phenomenon.The rotating stall causes uneven flow in the impeller channel,increasing the pressure fluctuation and the radial force,and resulting in an imbalance of the impeller rotation.展开更多
The fact that the staggered impeller of a double-suction centrifugal pump can effectively suppress pressure fluctuations has been proved by engineering practice,but the flow mechanism behind it is still not fully unde...The fact that the staggered impeller of a double-suction centrifugal pump can effectively suppress pressure fluctuations has been proved by engineering practice,but the flow mechanism behind it is still not fully understood.In this study,numerical simulations with a proof experiment were conducted,and the vortex dynamics analyses were performed using the newly developed rigid vorticity(Liutex)theory.The following valuable results are obtained:(1)In terms of the intuitive vortex structure,each blade of the impeller induces a trailing vortex rope with a strong rigid vorticity,which gradually evolves inside the volute casing with the rotation of the impeller.The trailing vortex ropes of the symmetric impeller are symmetrically distributed,while those of the staggered impeller present a staggered distribution,and the latter corresponds to a relatively lower rigid vorticity.(2)In terms of the correlation between the vortex and the pressure,the high rigid vorticity zone corresponds to the low-pressure zone.For a fixed point in the volute casing,there is a major“falling-rising”fluctuation in pressure as the symmetric vortex ropes transit it simultaneously,and a minor“falling-rising”fluctuation in pressure as the staggered vortex ropes transit it successively,corresponding to a lower peak-to-peak value of the pressure fluctuations.(3)In terms of the relation between the vortex and the velocity,the vortex ropes induced by the left and right impellers are counter-rotating and develop along the radial direction.This pattern results in high-speed zones at the middle part of the cross-section of the volute casing,both in the streamwise and radial directions,and contributes to velocity fluctuations due to the evolving vortex rope.However,the staggered distribution of vortex ropes can weaken the coupling of vortex pairs,thereby causing lower velocity and pressure pulsations,but can make the main frequency twice that of the symmetric impeller.This study enriches our physical knowledge by revealing the vortex dynamics mechanism of the staggered impeller of a double-suction centrifugal pump to suppress pressure fluctuations.展开更多
The vortex-induced vibration may lead to a premature failure of hydraulic mechanical systems,especially under the resonance condition in the torsional mode.To predict the structural fatigue life,a careful consideratio...The vortex-induced vibration may lead to a premature failure of hydraulic mechanical systems,especially under the resonance condition in the torsional mode.To predict the structural fatigue life,a careful consideration of the dynamic response to the hydraulic excitations is essential in the design phase.This study focuses on the numerical investigation of the relationship between the flow velocity,the added mass and the hydrodynamic damping,particularly,with respect to a Donaldson-type hydrofoil,vibrating in the first torsional mode.A two-way fluid-structure interaction(FSI)method is used to predict above two parameters.The flow velocity is in the range of 0 m/s-20m/s.To evaluate the hydrodynamic damping ratio,an identification method is proposed,based on a modified version of the logarithmic decay method.The relative deviations of the simulated natural frequencies and hydrodynamic damping ratios as compared with the experimental data for the first torsional modes,are within 8.1%and 16.6%,respectively.The analysis results show that the added mass coefficient for the first torsional mode is in the range of 1.59-1.86,and is around 44%of that for the first bending mode.The trends of the boundary layer thickness and the wake width against the reduced velocity are found to be opposite to that of the hydrodynamic damping ratio.The theoretical equation for predicting the hydrodynamic damping ratio is modified,which is shown to be more reliable due to its consideration of the velocity independent hydrodynamic damping phase.展开更多
The presence of a rigid wall causes a microjet of the cavitation bubble to collapse to move toward the wall,while a free surface does the opposite.When a rigid wall surface is combined with a free surface,it may affec...The presence of a rigid wall causes a microjet of the cavitation bubble to collapse to move toward the wall,while a free surface does the opposite.When a rigid wall surface is combined with a free surface,it may affect the direction of the microjet.The motive of this study is to find out the influence of the dynamic behavior of a laser-induced bubble near the rigid wall with a gas-containing hole.Evolutions of the bubble at different distances from the gas-containing hole in the horizontal and vertical directions were recorded by a high-speed camera(2.3×10^(5) fps).When the bubble collapse near the boundary,the bubble will produce two situations:away from or toward the boundary.It focuses on the direction of the bubble,the oscillation period of a bubble,and reflection angle,and quantitative analysis of the results.It was found that the boundary not only changes the morphologic of the bubble and the overall direction of movement but also affects the oscillation period.In addition,it can control the deflection of the bubble.展开更多
Rotating separation flow(RSF)in hydraulic machinery is characterized by the large flow separations and complex vortical structures induced by the effects of strong rotation,large curvature and multiple wall surfaces,a...Rotating separation flow(RSF)in hydraulic machinery is characterized by the large flow separations and complex vortical structures induced by the effects of strong rotation,large curvature and multiple wall surfaces,and conducting efficient engineering computation and putting forward effective control strategy for the RSF are important topics in the inner flow theory.To meet these engineering requirements,the studies on computational method and control strategy of the RSF are conducted in this paper.In terms of the computational method,the time-scale-driven(TSD)hybrid unsteady Reynolds-averaged Navier-Stokes/large eddy simulation(URANS/LES)modelling strategy is clarified,and an adaptive TSD hybrid model is established based on the RSF characteristics in hydraulic machinery,thereby avoiding the problem of non-monotonic grid convergence and improving the robustness.Besides,a novel vortex-feature-driven idea suitable for the RSF is further developed inspired by it.In terms of the control strategy,the secondary flow generation mechanism in a rotor domain is revealed,and the relationship between natural secondary flows and blade loading distributions is grasped.On the basis of it,an active control strategy with general significance is proposed,and a general alternate loading technique(GALT)is established.Both aspects can provide generalized paradigms with expandable potential,which are of benefit to the efficient computation and effective control of the RSF in hydraulic machinery.展开更多
The collapse of the cavitation bubble near the rigid wall emits shock waves and creates micro-jet,causing cavitation damage and operation instability of the hydraulic machinery.In this paper,the millimeter-scale bubbl...The collapse of the cavitation bubble near the rigid wall emits shock waves and creates micro-jet,causing cavitation damage and operation instability of the hydraulic machinery.In this paper,the millimeter-scale bubble near the rigid wall was investigated experimentally and numerically with the help of a laser photogrammetry system with nanosecond-micron space-time resolution and the open source package OpenFOAM-2212.The morphological characteristics of the bubble during its growth phase,collapse phase and rebound phase were observed by experiment and numerical simulation,and characteristics of the accompanying phenomena including the shock wave propagation and micro-jet evolution were well elucidated.The numerical results agree well with the experimental data.The bubble starts from a tiny small size with high internal pressure and expands into a sphere with a radius of 1.07 mm forγ=d/R_(max)=1.78.The bubble collapses into a heart shape and moves towards to the rigid wall during its collapse phase,resulting in a higher pressure load for the rigid wall in the second collapse.The maximum pressure of the shock wave of the first bubble collapse phase reaches 5.4 MPa,and the velocity of the micro-jet reaches approximately 100 m/s.This study enriches the existing experimental and numerical results of the dynamics of the near-wall cavitation bubble.展开更多
As a controllable alternative to cavitation collapse-induced shock waves,numerous cavitation studies on laser-induced breakdown have been carried out in hydromechanics.When the laser focusing region is not spherical,t...As a controllable alternative to cavitation collapse-induced shock waves,numerous cavitation studies on laser-induced breakdown have been carried out in hydromechanics.When the laser focusing region is not spherical,the shock waves caused by laser breakdown also exhibit non-spherical symmetry propagation.Recently,some researchers have proposed the linear superposition theory based on the far field measurement data to explain this asymmetry,assuming that it is essentially the linear superposition of multiple wave fronts caused by multiple points of laser-induced breakdown that leads to the asymmetric propagation of shock waves.In this study,measurements of shock wave propagation processes with different breakdown energies are carried out based on a nanosecond resolution photogrammetry system,and the propagation velocities of shock waves in different directions are directly measured using a double exposure technique on a single frame.In the experiment,the velocity of the shock wave at the beginning of the breakdown was measured up to nearly 4000 m/s.The early shock wave front was ellipsoidal,and the propagation velocity in the laser incident direction was generally slower than that in the perpendicular direction,decaying to the speed of sound in water within 1000 ns after the breakdown,and the wave front gradually approached to a circle.The variability of the shock wave front pressure ratio in the laser propagation direction and the vertical direction implies that the linear superposition theory applicable to the far field is not applicable to the near field.There may be more complex mechanism for the near-field shock wave propagation process.展开更多
In the present study, an analytical concept model is built, using a two-phase model for the sediment transport in a pure acceleration-skewed oscillatory sheet flow. The analytical model is based on the asymmetric wave...In the present study, an analytical concept model is built, using a two-phase model for the sediment transport in a pure acceleration-skewed oscillatory sheet flow. The analytical model is based on the asymmetric wave theory, the irregular boundary layer theory and the exponential concentration distribution theory, to be used for analyzing the phase lag and the boundary layer development related to the acceleration skewness. The two-phase model is applied for the calculations of the instantaneous erosion depth, the sediment flux, the boundary layer thickness and the sediment transport rate, as well as the differences between the positive acceleration stage and the negative acceleration stage caused by the acceleration skewness, as very important in the net current and sediment transport. The effects of the sediment diameter and the phase lag are explained by a comparison with the instantaneous type empirical formula, as is closely related to the acceleration skewness. With the analytical concept model and the two-phase model, the generation of the net sediment transport in the pure acceleration-skewed flows is clearly explained. The phase lag effect is important for the instantaneous sediment transport in the pure acceleration-skewed flow, whereas the boundary layer development difference between the positive acceleration stage and the negative acceleration stage plays a major role in the determination of the net sediment transport.展开更多
The internal flow in an axial flow rotating machinery is affected by the rotating characteristics, often accompanied by a strong rotating separation under small flow conditions. At present, the very large eddy simulat...The internal flow in an axial flow rotating machinery is affected by the rotating characteristics, often accompanied by a strong rotating separation under small flow conditions. At present, the very large eddy simulation (VLES) model commonly used for the separation flow simulation still has certain limitations in simulating such rotating separation flow: (1) The Reynolds stress level is overestimated in the near-wall region. (2) The influence of the rotating effect cannot be effectively considered. The above two limitations affect the simulation accuracy of the VLES model for the rotating separation flow under small flow conditions in the axial flow rotating machinery. The objective of this paper is to provide a new hybrid unsteady Reynolds average Navier-Stokes/large eddy simulation (URANS/LES) model suitable for the simulation of the rotating separation flow in an axial flow rotating machinery. Compared with the original VLES method, the modifications are as follows: (1) A Reynolds stress damping function in the near-wall region is introduced to reduce the overestimation of the Reynolds stress caused by the near-wall Reynolds average Navier-Stokes (RANS) behavior of the VLES model. (2) A control function driven by the vortex is introduced to reflect the influence of the rotating effect. Three typical cases are used to verify the calculation accuracy of the modified model. It is shown that the modified model can capture more turbulent vortices based on the URANS grids, and the prediction accuracy of the rotating separation flow is effectively improved. Compared with the original VLES model, the modified model can accurately predict the head change in the hump region of the axial flow pump.展开更多
A novel Omega(Ω)-driven dynamic partially-averaged Navier-Stokes(PANS)model is proposed in this paper.The ratio of the modeled-to-total turbulent kinetic energies fk is dynamically adjusted by the rigid vorticity rat...A novel Omega(Ω)-driven dynamic partially-averaged Navier-Stokes(PANS)model is proposed in this paper.The ratio of the modeled-to-total turbulent kinetic energies fk is dynamically adjusted by the rigid vorticity ratio(the ratio of the rigid vorticity to the total vorticity),the key parameter of the Ω vortex identification method.Three classical flow cases with rotation and curvature are used to test the model.The results show that the turbulent viscosity is effectively adjusted by the new dynamic fk and the LES-like mode is activated,which can help the revelation of more turbulence information and improve the prediction accuracy.The new PANS model does not contain any explicit dependency on the grid size and enjoys good adaptability to the flow fields,and can be used for efficient engineering computations of the turbulent flows in the hydraulic machinery.展开更多
An experimental study to investigate various features of subsurface vortices and their evolutions inside a closed pump intake under different pressure conditions at free surface,and to analyze the influence of a pipe ...An experimental study to investigate various features of subsurface vortices and their evolutions inside a closed pump intake under different pressure conditions at free surface,and to analyze the influence of a pipe bell with different mean axial velocities at the same water level on flow patterns is presented.For all cases,three different flow regimes are closely observed:various side-wall attached,back-wall attached and submerged vortices structures,and their evolutions are subject to the free-surface pressure for a critical flow rate,the number and strength of all these vortex rise as the absolute value of the pressure increases for a higher flow rate,for a lower flow rate,whether the side-wall vortices occur or not is related closely to the pressure magnitude,and there are no vortices for lower absolute pressure values,the number and the strength of the side-wall vortices increase as the absolute pressure values increase.Besides,based on statistical analyses,the side-wall vortex probability on the right side along the stream-wise direction is much higher than that on the opposite side for all operation cases,particularly single vortex probability,and the back-wall vortex probabilities are quite higher with values of more than 90%,meanwhile,the multi-vortices probability is much higher than that of the single-vortex.The captured vortices have been qualitatively classified into different“vortex zones”:no vortex zones and vortex zones including transition,single-and multi-vortex.The Reynolds number and the absolute value pressure are the most important parameters which influence the existence and the number of these vortices,and the increase of the two parameters reduces the no vortex transition zone and causes bigger difference in the back-wall and side-wall vortex zones.展开更多
The cavitation bubble precipitation refers to the formation process of the spherical cavities,known as cavitation bubbles,as the ambient pressure of water decreases.In the fields of hydraulic machinery,the saturated v...The cavitation bubble precipitation refers to the formation process of the spherical cavities,known as cavitation bubbles,as the ambient pressure of water decreases.In the fields of hydraulic machinery,the saturated vapor pressure of clean water is often used as the pressure threshold for cavitation occurrence.However,the engineering practice has demonstrated that,the incipient cavitation pressure may be significantly higher than the saturated vapor pressure,especially in sand-laden water conditions.Therefore,to determine a reasonable cavitation pressure threshold and ensure the accurate cavitation flow simulations and effective assessment of cavitation risks for hydraulic machinery operating in sand-laden water conditions,an experimental investigation is conducted.First,a high-precision experimental setup based on the vacuum pump,high-frequency pressure sensor and high-speed camera is constructed.This setup allows for the continuous pressure reduction in water,acquisition of high-precision pressure data and tracking of the entire cavitation bubble precipitation process.Second,based on the experiments in clean water conditions,the relationship between the cavitation bubble precipitation degree and pressure is established,and two key states of incipient cavitation and boiling cavitation are defined.Third,based on the experiments in sand-laden water conditions,it is observed that the numerous cavitation nuclei on sand surfaces make both the incipient and boiling cavitation pressure in sand-laden water higher than those in clean water.The quantitative relationship between the sand concentration and diameter,and the cavitation pressure is established,providing a more reasonable cavitation pressure threshold.This investigation enhances the understanding of cavitation bubble precipitation in sand-laden water and supports the development of more accurate cavitation models for hydraulic machinery operating in sand-laden water conditions.展开更多
An elastic model that combines the dead-end and the orifice at the downstream end is proposed in order to study the hydraulic transients during the rapid filling in small-scale pipelines with submerged outlet.In this ...An elastic model that combines the dead-end and the orifice at the downstream end is proposed in order to study the hydraulic transients during the rapid filling in small-scale pipelines with submerged outlet.In this model,the unsteady water flow is solved by the method of characteristics,the water-air interface is traced by a shock fitting method,and the air phase is controlled by the polytropic law.The model is validated by a filling test in a pipeline with orifice.Then the proposed model is applied to the filling transient analysis in a small-scale pipeline with submerged outlet.Finally,the effects of the submerged depth of the downstream reservoir on the filling transients are analyzed.This model provides a reasonable approximation of the submerged outlet boundary condition in small-scale pipelines.展开更多
基金Supported by National Natural Science Foundation of China (Grant Nos.51621061,51139007,51409247)National Science and Technology Support Project of China(Grant No.2015BAD20B01)
文摘Pressure fluctuation may cause high amplitude of vibration of double-suction centrifugal pumps, but the impact of impeller stagger angles is still not well understood. In this paper, pressure fluctuation experiments are carried out for five impeller configurations with different stagger angles by using the same test rig system. Results show that the stagger angles exert negligible effects on the characteristics of head and efficiency. The distributions of pressure fluctuations are relatively uniform along the suction chamber wall, and the maximum pressure fluctuation amplitude is reached near the suction inlet tongue region. The pressure fluctuation characteristics are affected largely by impeller rotation, whose dominant frequencies include impeller rotation frequency and its harmonic frequencies, and half blade passage frequency. The stagger angle exerts a small effect on the pressure fluctuations in the suction chamber while a great effect on the pressure fluctuation in volute casing, especially on the aspect of decreasing the amplitude on blade passage frequency. Among the tested cases, the distribution of pressure fluctuations in the volute becomes more uniform than the other impeller configurations and the level of pressure fluctuation may be reduced by up to 50% when the impeller stagger angle is close to 24° or 360°.The impeller structure pattern needs to be taken into consideration during the design period, and the halfway staggered impeller is strongly recommended.
基金National Natural Science Foundation of China(No.51836010,51879266 and 51839001)the Beijing Municipal Science and Technology Project(No.Z181100005518013).
文摘In order to investigate the complex flow characteristics inside an unshrouded centrifugal impeller with splitter blades at off-design conditions,and analyze its influence on pump operation stability,a numerical simulation study was carried on using the curvature-corrected SST-CC turbulence model;the head and efficiency accorded with experimental results.The pressure fluctuation,unsteady radial force and velocity were analyzed quantitatively and the numerical results indicate this:the peak to peak value of pressure fluctuation in the impeller channel gradually increases in the flow direction and at 0.49Qn,the slope of peak to peak value to normalized impeller channel behind the splitter blade is 8.57 times greater than that before the splitter blade.The greater the flow rate deviates from the design condition,the larger the peak to peak value of the pressure fluctuation and radial force;in particular at 0.27Qn,the maximum radial force is 194.29%greater than that of the design condition.When the operating flow rate is smaller than 0.83Qn,the stall occurs and the stall vortex could block the impeller discharge;as the flow rate decreases further,the pressure amplitude at rotational frequency gradually increases in the impeller channel and the prevailing frequency changes from the blade passage frequency(BPF)to the rotating stall frequency in the diffuser.The tip leakage vortex(TLV)is generated in the tip region and rotated move downstream in the impeller flow channel,and the backflows appear on the blade suction side and in the tip and the tongue regions;the smaller the flow rate,the more serious the TLV and backflow phenomenon.The rotating stall causes uneven flow in the impeller channel,increasing the pressure fluctuation and the radial force,and resulting in an imbalance of the impeller rotation.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.U22A20238,52209117).
文摘The fact that the staggered impeller of a double-suction centrifugal pump can effectively suppress pressure fluctuations has been proved by engineering practice,but the flow mechanism behind it is still not fully understood.In this study,numerical simulations with a proof experiment were conducted,and the vortex dynamics analyses were performed using the newly developed rigid vorticity(Liutex)theory.The following valuable results are obtained:(1)In terms of the intuitive vortex structure,each blade of the impeller induces a trailing vortex rope with a strong rigid vorticity,which gradually evolves inside the volute casing with the rotation of the impeller.The trailing vortex ropes of the symmetric impeller are symmetrically distributed,while those of the staggered impeller present a staggered distribution,and the latter corresponds to a relatively lower rigid vorticity.(2)In terms of the correlation between the vortex and the pressure,the high rigid vorticity zone corresponds to the low-pressure zone.For a fixed point in the volute casing,there is a major“falling-rising”fluctuation in pressure as the symmetric vortex ropes transit it simultaneously,and a minor“falling-rising”fluctuation in pressure as the staggered vortex ropes transit it successively,corresponding to a lower peak-to-peak value of the pressure fluctuations.(3)In terms of the relation between the vortex and the velocity,the vortex ropes induced by the left and right impellers are counter-rotating and develop along the radial direction.This pattern results in high-speed zones at the middle part of the cross-section of the volute casing,both in the streamwise and radial directions,and contributes to velocity fluctuations due to the evolving vortex rope.However,the staggered distribution of vortex ropes can weaken the coupling of vortex pairs,thereby causing lower velocity and pressure pulsations,but can make the main frequency twice that of the symmetric impeller.This study enriches our physical knowledge by revealing the vortex dynamics mechanism of the staggered impeller of a double-suction centrifugal pump to suppress pressure fluctuations.
基金the National Natural Science Foundation of China(Grant Nos.51836010,51879266 and 51839001)the National Key Research and Development Program of China(Grant No.2017YFC0403206)+1 种基金the Beijing Municipal Science and Technology Project(Grant No.Z181100005518013)the Chinese Universities Scientific Fund(Grant No.2019TC040).
文摘The vortex-induced vibration may lead to a premature failure of hydraulic mechanical systems,especially under the resonance condition in the torsional mode.To predict the structural fatigue life,a careful consideration of the dynamic response to the hydraulic excitations is essential in the design phase.This study focuses on the numerical investigation of the relationship between the flow velocity,the added mass and the hydrodynamic damping,particularly,with respect to a Donaldson-type hydrofoil,vibrating in the first torsional mode.A two-way fluid-structure interaction(FSI)method is used to predict above two parameters.The flow velocity is in the range of 0 m/s-20m/s.To evaluate the hydrodynamic damping ratio,an identification method is proposed,based on a modified version of the logarithmic decay method.The relative deviations of the simulated natural frequencies and hydrodynamic damping ratios as compared with the experimental data for the first torsional modes,are within 8.1%and 16.6%,respectively.The analysis results show that the added mass coefficient for the first torsional mode is in the range of 1.59-1.86,and is around 44%of that for the first bending mode.The trends of the boundary layer thickness and the wake width against the reduced velocity are found to be opposite to that of the hydrodynamic damping ratio.The theoretical equation for predicting the hydrodynamic damping ratio is modified,which is shown to be more reliable due to its consideration of the velocity independent hydrodynamic damping phase.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.91852101,91952301,52179092 and 52179081).
文摘The presence of a rigid wall causes a microjet of the cavitation bubble to collapse to move toward the wall,while a free surface does the opposite.When a rigid wall surface is combined with a free surface,it may affect the direction of the microjet.The motive of this study is to find out the influence of the dynamic behavior of a laser-induced bubble near the rigid wall with a gas-containing hole.Evolutions of the bubble at different distances from the gas-containing hole in the horizontal and vertical directions were recorded by a high-speed camera(2.3×10^(5) fps).When the bubble collapse near the boundary,the bubble will produce two situations:away from or toward the boundary.It focuses on the direction of the bubble,the oscillation period of a bubble,and reflection angle,and quantitative analysis of the results.It was found that the boundary not only changes the morphologic of the bubble and the overall direction of movement but also affects the oscillation period.In addition,it can control the deflection of the bubble.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.51836010,U22A20238 and 52209117)the China Postdoctoral Science Foundation(Grant No.2021M703516).
文摘Rotating separation flow(RSF)in hydraulic machinery is characterized by the large flow separations and complex vortical structures induced by the effects of strong rotation,large curvature and multiple wall surfaces,and conducting efficient engineering computation and putting forward effective control strategy for the RSF are important topics in the inner flow theory.To meet these engineering requirements,the studies on computational method and control strategy of the RSF are conducted in this paper.In terms of the computational method,the time-scale-driven(TSD)hybrid unsteady Reynolds-averaged Navier-Stokes/large eddy simulation(URANS/LES)modelling strategy is clarified,and an adaptive TSD hybrid model is established based on the RSF characteristics in hydraulic machinery,thereby avoiding the problem of non-monotonic grid convergence and improving the robustness.Besides,a novel vortex-feature-driven idea suitable for the RSF is further developed inspired by it.In terms of the control strategy,the secondary flow generation mechanism in a rotor domain is revealed,and the relationship between natural secondary flows and blade loading distributions is grasped.On the basis of it,an active control strategy with general significance is proposed,and a general alternate loading technique(GALT)is established.Both aspects can provide generalized paradigms with expandable potential,which are of benefit to the efficient computation and effective control of the RSF in hydraulic machinery.
基金supported by the National Natural Science Foundation of China(Grant Nos.52179092,U22A20238,52009136)supported by the 2115 Talent Development Program of China Agricultural University.
文摘The collapse of the cavitation bubble near the rigid wall emits shock waves and creates micro-jet,causing cavitation damage and operation instability of the hydraulic machinery.In this paper,the millimeter-scale bubble near the rigid wall was investigated experimentally and numerically with the help of a laser photogrammetry system with nanosecond-micron space-time resolution and the open source package OpenFOAM-2212.The morphological characteristics of the bubble during its growth phase,collapse phase and rebound phase were observed by experiment and numerical simulation,and characteristics of the accompanying phenomena including the shock wave propagation and micro-jet evolution were well elucidated.The numerical results agree well with the experimental data.The bubble starts from a tiny small size with high internal pressure and expands into a sphere with a radius of 1.07 mm forγ=d/R_(max)=1.78.The bubble collapses into a heart shape and moves towards to the rigid wall during its collapse phase,resulting in a higher pressure load for the rigid wall in the second collapse.The maximum pressure of the shock wave of the first bubble collapse phase reaches 5.4 MPa,and the velocity of the micro-jet reaches approximately 100 m/s.This study enriches the existing experimental and numerical results of the dynamics of the near-wall cavitation bubble.
基金supported by the National Natural Science Foundation of China(Grant Nos.91852101,91952301 and 52179081).
文摘As a controllable alternative to cavitation collapse-induced shock waves,numerous cavitation studies on laser-induced breakdown have been carried out in hydromechanics.When the laser focusing region is not spherical,the shock waves caused by laser breakdown also exhibit non-spherical symmetry propagation.Recently,some researchers have proposed the linear superposition theory based on the far field measurement data to explain this asymmetry,assuming that it is essentially the linear superposition of multiple wave fronts caused by multiple points of laser-induced breakdown that leads to the asymmetric propagation of shock waves.In this study,measurements of shock wave propagation processes with different breakdown energies are carried out based on a nanosecond resolution photogrammetry system,and the propagation velocities of shock waves in different directions are directly measured using a double exposure technique on a single frame.In the experiment,the velocity of the shock wave at the beginning of the breakdown was measured up to nearly 4000 m/s.The early shock wave front was ellipsoidal,and the propagation velocity in the laser incident direction was generally slower than that in the perpendicular direction,decaying to the speed of sound in water within 1000 ns after the breakdown,and the wave front gradually approached to a circle.The variability of the shock wave front pressure ratio in the laser propagation direction and the vertical direction implies that the linear superposition theory applicable to the far field is not applicable to the near field.There may be more complex mechanism for the near-field shock wave propagation process.
基金Project supported by the National Naturel Science Foundation of China(Grant Nos.51836010,51609244)the National Science-Technology Support Plan of China(Grant No.2015BAD20B01)
文摘In the present study, an analytical concept model is built, using a two-phase model for the sediment transport in a pure acceleration-skewed oscillatory sheet flow. The analytical model is based on the asymmetric wave theory, the irregular boundary layer theory and the exponential concentration distribution theory, to be used for analyzing the phase lag and the boundary layer development related to the acceleration skewness. The two-phase model is applied for the calculations of the instantaneous erosion depth, the sediment flux, the boundary layer thickness and the sediment transport rate, as well as the differences between the positive acceleration stage and the negative acceleration stage caused by the acceleration skewness, as very important in the net current and sediment transport. The effects of the sediment diameter and the phase lag are explained by a comparison with the instantaneous type empirical formula, as is closely related to the acceleration skewness. With the analytical concept model and the two-phase model, the generation of the net sediment transport in the pure acceleration-skewed flows is clearly explained. The phase lag effect is important for the instantaneous sediment transport in the pure acceleration-skewed flow, whereas the boundary layer development difference between the positive acceleration stage and the negative acceleration stage plays a major role in the determination of the net sediment transport.
基金the National Natural Science Foundation of China(Grant Nos.51836010,51779258).
文摘The internal flow in an axial flow rotating machinery is affected by the rotating characteristics, often accompanied by a strong rotating separation under small flow conditions. At present, the very large eddy simulation (VLES) model commonly used for the separation flow simulation still has certain limitations in simulating such rotating separation flow: (1) The Reynolds stress level is overestimated in the near-wall region. (2) The influence of the rotating effect cannot be effectively considered. The above two limitations affect the simulation accuracy of the VLES model for the rotating separation flow under small flow conditions in the axial flow rotating machinery. The objective of this paper is to provide a new hybrid unsteady Reynolds average Navier-Stokes/large eddy simulation (URANS/LES) model suitable for the simulation of the rotating separation flow in an axial flow rotating machinery. Compared with the original VLES method, the modifications are as follows: (1) A Reynolds stress damping function in the near-wall region is introduced to reduce the overestimation of the Reynolds stress caused by the near-wall Reynolds average Navier-Stokes (RANS) behavior of the VLES model. (2) A control function driven by the vortex is introduced to reflect the influence of the rotating effect. Three typical cases are used to verify the calculation accuracy of the modified model. It is shown that the modified model can capture more turbulent vortices based on the URANS grids, and the prediction accuracy of the rotating separation flow is effectively improved. Compared with the original VLES model, the modified model can accurately predict the head change in the hump region of the axial flow pump.
基金Supported by the National Natural Science simulating the unsteady eddying motions⑴.Foundation of China(Grant Nos.51836010,51779258 and 51839001)the National Key Research and Development Program of China(Grant No.2018YFB0606103)the Nature Science Foundation of Beijing(Gmat No.3182018).
文摘A novel Omega(Ω)-driven dynamic partially-averaged Navier-Stokes(PANS)model is proposed in this paper.The ratio of the modeled-to-total turbulent kinetic energies fk is dynamically adjusted by the rigid vorticity ratio(the ratio of the rigid vorticity to the total vorticity),the key parameter of the Ω vortex identification method.Three classical flow cases with rotation and curvature are used to test the model.The results show that the turbulent viscosity is effectively adjusted by the new dynamic fk and the LES-like mode is activated,which can help the revelation of more turbulence information and improve the prediction accuracy.The new PANS model does not contain any explicit dependency on the grid size and enjoys good adaptability to the flow fields,and can be used for efficient engineering computations of the turbulent flows in the hydraulic machinery.
基金supported by the National Natural Science Foundation of China(Grant Nos.51479196,51779257)the National Natural Science Foundation for the Youth of China(Grant No.51909132).
文摘An experimental study to investigate various features of subsurface vortices and their evolutions inside a closed pump intake under different pressure conditions at free surface,and to analyze the influence of a pipe bell with different mean axial velocities at the same water level on flow patterns is presented.For all cases,three different flow regimes are closely observed:various side-wall attached,back-wall attached and submerged vortices structures,and their evolutions are subject to the free-surface pressure for a critical flow rate,the number and strength of all these vortex rise as the absolute value of the pressure increases for a higher flow rate,for a lower flow rate,whether the side-wall vortices occur or not is related closely to the pressure magnitude,and there are no vortices for lower absolute pressure values,the number and the strength of the side-wall vortices increase as the absolute pressure values increase.Besides,based on statistical analyses,the side-wall vortex probability on the right side along the stream-wise direction is much higher than that on the opposite side for all operation cases,particularly single vortex probability,and the back-wall vortex probabilities are quite higher with values of more than 90%,meanwhile,the multi-vortices probability is much higher than that of the single-vortex.The captured vortices have been qualitatively classified into different“vortex zones”:no vortex zones and vortex zones including transition,single-and multi-vortex.The Reynolds number and the absolute value pressure are the most important parameters which influence the existence and the number of these vortices,and the increase of the two parameters reduces the no vortex transition zone and causes bigger difference in the back-wall and side-wall vortex zones.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.U22A20238,52209117)supported by the 2115 Talent Development Program of China Agricultural University.
文摘The cavitation bubble precipitation refers to the formation process of the spherical cavities,known as cavitation bubbles,as the ambient pressure of water decreases.In the fields of hydraulic machinery,the saturated vapor pressure of clean water is often used as the pressure threshold for cavitation occurrence.However,the engineering practice has demonstrated that,the incipient cavitation pressure may be significantly higher than the saturated vapor pressure,especially in sand-laden water conditions.Therefore,to determine a reasonable cavitation pressure threshold and ensure the accurate cavitation flow simulations and effective assessment of cavitation risks for hydraulic machinery operating in sand-laden water conditions,an experimental investigation is conducted.First,a high-precision experimental setup based on the vacuum pump,high-frequency pressure sensor and high-speed camera is constructed.This setup allows for the continuous pressure reduction in water,acquisition of high-precision pressure data and tracking of the entire cavitation bubble precipitation process.Second,based on the experiments in clean water conditions,the relationship between the cavitation bubble precipitation degree and pressure is established,and two key states of incipient cavitation and boiling cavitation are defined.Third,based on the experiments in sand-laden water conditions,it is observed that the numerous cavitation nuclei on sand surfaces make both the incipient and boiling cavitation pressure in sand-laden water higher than those in clean water.The quantitative relationship between the sand concentration and diameter,and the cavitation pressure is established,providing a more reasonable cavitation pressure threshold.This investigation enhances the understanding of cavitation bubble precipitation in sand-laden water and supports the development of more accurate cavitation models for hydraulic machinery operating in sand-laden water conditions.
基金the Beijing Municipal Science and Technology Project (Grant No.Z181100005518013)the National Natural Science Foundation of China (Grant Nos.51779258,51621061).
文摘An elastic model that combines the dead-end and the orifice at the downstream end is proposed in order to study the hydraulic transients during the rapid filling in small-scale pipelines with submerged outlet.In this model,the unsteady water flow is solved by the method of characteristics,the water-air interface is traced by a shock fitting method,and the air phase is controlled by the polytropic law.The model is validated by a filling test in a pipeline with orifice.Then the proposed model is applied to the filling transient analysis in a small-scale pipeline with submerged outlet.Finally,the effects of the submerged depth of the downstream reservoir on the filling transients are analyzed.This model provides a reasonable approximation of the submerged outlet boundary condition in small-scale pipelines.
