The quality of the inflow across the propeller is closely related with the hydrodynamic performance and the noise characteristics of the propeller. For a submarine, with a horseshoe vortex generated at the junction of...The quality of the inflow across the propeller is closely related with the hydrodynamic performance and the noise characteristics of the propeller. For a submarine, with a horseshoe vortex generated at the junction of the main body and the appendages, the submarine wake is dominated by a kind of highly non-uniform flow field, which has an adverse effect on the performance of the submarine propeller. In order to control the horseshoe vortex and improve the quality of the submarine wake, the flow field around a submarine model is simulated by the detached eddies simulation （DES） method, and the vortex configuration is displayed using the second invariant of the velocity derivative tensor. The state and the transition process of the horseshoe vortex are analyzed, then a modified method to break the vortex core by a vortex baffle is proposed. The flow numerical simulation is carried out to study the effect of this method. Numerical simulations show that, with the breakdown of the vortex core, many unstable vortices are shed and the energy of the horseshoe vortex is dissipated quickly, and the uniformity of the submarine wake is improved. The submarine wake test in a wind tunnel has verified the effect of the method to control the horseshoe vortex. The vortex baffle can improve the wake uniformity in cases of high Reynolds numbers as well, and it does not have adverse effects on the maneuverability and the speed ability of the submarine.展开更多
A time domain model is presented to study the vibrations of long slender cylinders placed in shear flow. Long slender cylinders such as risers and tension legs are widely used in the field of ocean engineering. They a...A time domain model is presented to study the vibrations of long slender cylinders placed in shear flow. Long slender cylinders such as risers and tension legs are widely used in the field of ocean engineering. They are subjected to vortex-induced vibrations（VIV） when placed within a transverse incident flow. A three dimensional model coupled with wake oscillators is formulated to describe the response of the slender cylinder in cross-flow and in-line directions. The wake oscillators are distributed along the cylinder and the vortex-shedding frequency is derived from the local current velocity. A non-linear fiuid force model is accounted for the coupled effect between cross-flow and in-line vibrations. The comparisons with the published experimental data show that the dynamic features of VIV of long slender cylinder placed in shear flow can be obtained by the proposed model,such as the spanwise average displacement,vibration frequency,dominant mode and the combination of standing and traveling waves. The simulation in a uniform flow is also conducted and the result is compared with the case of nonuniform flow. It is concluded that the flow shear characteristic has significantly changed the cylinder vibration behavior.展开更多
A numerical study based on a wake oscillator model was conducted to determine the response performance of vortex-induced vibration(VIV) on a long flexible cylinder with pinned-pinned boundary conditions subjected to l...A numerical study based on a wake oscillator model was conducted to determine the response performance of vortex-induced vibration(VIV) on a long flexible cylinder with pinned-pinned boundary conditions subjected to linear and exponential shear flows. The coupling equations of a structural vibration model and wake oscillator model were solved using a standard central finite difference method of the second order. The VIV response characteristics including the structural displacement, structural frequency, structural wavenumber, standing wave behavior,travelling wave behavior, structural velocity, lift force coefficient and transferred energy from the fluid to the structure with different flow profiles were compared. The numerical results show that the VIV displacement is a combination of standing waves and travelling waves. For linear shear flow, standing waves and travelling waves dominate the VIV response within the low-velocity and high-velocity zones, respectively. The negative values of the transferred energy only occur within the low-velocity zone. However, for exponential shear flow, travelling waves dominate the VIV response and the negative energy occurs along the entire length of the cylinder.展开更多
This paper addresses the necessity to make a physical interpretation of a highly complex three-dimensional tip clearance flow field study for high-speed mixed-flow compressor having stage exit static pressure to inlet...This paper addresses the necessity to make a physical interpretation of a highly complex three-dimensional tip clearance flow field study for high-speed mixed-flow compressor having stage exit static pressure to inlet total pressure ratio of 3.8 with 39,836 rpm rotor speed.The four different tip configurations namely the constant(l Z 0.016 and 0.019)and variable(l Z 0.011(inlet)-0.019(exit)and 0.019(inlet)-0.022(exit))tip clearances were numerically analysed using available experimental data-set.The numerical investigation reveals that in contrast to the classic jet-wake pattern,two anomalous velocity profiles formed at the impeller exit which results in pressure losses in the vaneless diffuser.Near the impeller inlet,the tip leakage flow rolls up to discrete tip leakage vortex structure for each tip clearance configuration.This results in the formation of a region of momentum deficit,recirculation zone,which gets weakened as it moves downstream.The tip clearance configuration is observed to profoundly influence the extent and vorticity of the tip leakage vortex.In the splitter blade passage,the tip leakage flow and Coriolis flow interact with passage flow,resulting in the formation of two secondary passage vortices that move downstream along the pressure and suction surface of the splitter blade.The tip clearance configuration directly influences the impeller exit jetwake pattern by modulating the secondary passage vortices trajectory and vorticity.Moreover,off-design analysis for tip clearances l Z 0.016 and l Z 0.019,depict distinctive tip leakage vortex characteristics.When operating near the stall conditions(80%of design mass flow rate),l Z 0.019 exhibits bubble shape tip leakage vortex breakdown occurring near the impeller inlet.This result in a substantial change in the tip leakage vortex nature;expansion of the recirculation zone and early weakening of the vorticity in the tip leakage vortex.It is observed that vortex breakdown plays a vital role in characteristics of the passage flow field structure and 展开更多
The near wake of a circular cylinder in linearly stratified flows of finite depth was experimentally investigated by means of flow visualization and measurements of vortex shedding frequencies, at Reynolds numbers 3.5...The near wake of a circular cylinder in linearly stratified flows of finite depth was experimentally investigated by means of flow visualization and measurements of vortex shedding frequencies, at Reynolds numbers 3.5 × 103-1.2 × 104 and stratification parameters kd 0-2.0. The non-dimensional parameter kd is defined as kd = Nd/U, where N is the Brunt-Vaisala frequency, d, the diameter of the cylinder, and U, the approaching flow velocity. The study demonstrates that as kd increases from zero, the vortex shedding from a circular cylinder progressively strengthens, while the Strouhal number gradually becomes lower than that for homogeneous flow. This phenomenon can be explained by the effect of the increasingly stable stratification which enhances the two-dimensionality of the near-wake flow of the circular cylinder;the enhanced two-dimensionality of the flow strengthens the roll-up of the separated shear layer. Above a certain value of kd, however, vortex formation and shedding are strongly suppressed and the Strouhal number rises sharply. This observation is attributable to the development of stationary lee waves downstream of the circular cylinder because the lee waves strongly suppress vertical fluid motions.展开更多
The Lorentz force generated by electromagnetic field on the surface of the cylinder in the electrolyte solution may modify the structure of the flow boundary layer effectively. The transient control process of Lorentz...The Lorentz force generated by electromagnetic field on the surface of the cylinder in the electrolyte solution may modify the structure of the flow boundary layer effectively. The transient control process of Lorentz force is investigated experimentally for lift amplification and vibration suppression. The experiments are conducted in a rotating annular tank filled with a low-conducting electrolyte. A cylinder with an electro-magnetic actuator is placed into the electrolyte. The lift force of cylinder is measured using the strain gages attached to a fixed beam, and the flow fields are visualized by the dye markers. The results show that the upper vortex on the cylinder is suppressed, and the wake becomes a line and leans to the lower side under the action of upside Lorentz force while the lower vortex on the cylinder is suppressed and limited in a small region. Therefore, the value of lift increases with the variation of flow field. However, the vortexes on the cylinder are suppressed fully under the action of symmetrical Lorentz force which leads to the suppression of lift oscillation and then the vibration of cylinder are suppressed fully.展开更多
Numerical study on near wake flows of a flat plate in three kinds of oncoming flows is made by using the discrete vortex model and improved vorticity creation method. For steady oncoming flow, both gross and detailed ...Numerical study on near wake flows of a flat plate in three kinds of oncoming flows is made by using the discrete vortex model and improved vorticity creation method. For steady oncoming flow, both gross and detailed features of the wake flow are calculated and discussed. Then, in harmonic oscillatory oncoming flow two different wake flow patterns with K_c=2,4 and 10 are obtained respectively. Our results present a new wake flow pattern for low K_c numbers (K_c<5) describing vortex shedding, pairing and moving in a period of the oscillatory flow starting from rest. The calculated drag and inertia force coefficients are closer to experimental data from the U-tube than the previous results of vortex simulation. For in-line combined oncoming flow the vortex lock-in and dynamic characteristics are simulated. The results are shown to be in good agreement with experiments.展开更多
This study demonstrates an active flow control for deflecting a direction of wake vortex structures behind a NACA0012 airfoil using an active morphing flap. Two-dimensional direct numerical simulations are performed f...This study demonstrates an active flow control for deflecting a direction of wake vortex structures behind a NACA0012 airfoil using an active morphing flap. Two-dimensional direct numerical simulations are performed for flows at the chord Reynolds number of 10,000, and the vortex pattern in the controlled and noncontrolled wakes as well as the effect of an actuation frequency on the control ability are rigorously investigated. It is found that there is an optimum actuation-frequency regime at around <em>F <sup>+</sup></em> = 2.00 which is normalized by the chord length and freestream velocity. The wake vortex pattern of the well-controlled case is classified as the 2P wake pattern according to the Williamson’s categorization [<a href="#ref1">1</a>] [<a href="#ref2">2</a>], where the forced oscillation frequency corresponds to the natural vortex shedding frequency without control. The present classification of wake vortex patterns and finding of the optimum frequency regime in the wake deflection control can lead to a more robust design suitable for vortex-induced-vibration (VIV) related engineering systems.展开更多
For complex aerodynamic and hydrodynamic problems,the analysis of vortex is very important.The Liutex method is an eigenvalue-based method which is local,accurate,and unique,which can give an accurate definition of vo...For complex aerodynamic and hydrodynamic problems,the analysis of vortex is very important.The Liutex method is an eigenvalue-based method which is local,accurate,and unique,which can give an accurate definition of vortex,so the control of vortex can be implemented and effectively guaranteed.Based on Liutex method,two methodologies of centripetal force model and counter-rotation force model were proposed to illustrate the vortex dynamics and possibly strengthen or weaken the vortices.In this paper,the Liutex-based centripetal force model is applied by adding a source term to the Navier-Stokes equations.In order to investigate the influence of the constructed Liutex force model on the 3-dimensional flow around a slow-fat ship,the calm-water drag calculation result of JBC ship is regarded as the initial flow field,and the new resistance and wake performances of the ship are obtained after applying the centripetal force model to the flow field with different strengths.Several views of the comparisons of the new steady flow fields are shown,and the parametric study results indicate that the Liutex-based centripetal force model can effectively change the resistance and wake performances of the JBC ship,which provides a new idea and theoretical basis for the comprehensive hydrodynamic performance optimization of the ship hull.展开更多
The flow around two plates having asymmetric beveled trailing edge placed side-by-side in a uniform stream were investigated both numerically and experimentally. Two plates with spacing ratio of 1.0 and beveled traili...The flow around two plates having asymmetric beveled trailing edge placed side-by-side in a uniform stream were investigated both numerically and experimentally. Two plates with spacing ratio of 1.0 and beveled trailing edge of angle 20° were tested at Reynolds numbers equal to 3.97×10^4. The numerical simulation results displayed that the flow separates on one plate beveled trailing edge surface. The vortex formation and shedding from this plate led to deflecting the flow toward the other plate and consequently the wake behind the plates showed an asymmetric development. The photographs obtained from the flow visualization confirmed the numerical results concerning the flow separation and vortex formation. It was also found that the computed pressure distribution on the two plate surfaces was different, particularly on the beveled trailing edge surfaces and on the fiat sides.展开更多
The characteristics of a uniform-shear flow over a circular cylinder are in- vestigated numerically by using the alternative-direction implicit （ADI） algorithm and a fast Fourier transform （FFT） one in the exponen...The characteristics of a uniform-shear flow over a circular cylinder are in- vestigated numerically by using the alternative-direction implicit （ADI） algorithm and a fast Fourier transform （FFT） one in the exponential-polar coordinates for Re = 150 and 0 ≤ K ≤ 0.46. The diagram of lift-drag phase, implying the detail information about the fluctuations of drag and lift as well as the flow patterns in the wake and fluctuating pres- sure on the cylinder surface, is used to describe the effects of the shear rate on the flow. Results show that the upper （or lower） closed curve of a phase diagram corresponds to the first （or second） half shedding cycle. The lift-drag phase diagram will move down-left with the increase of shear rate K such that the lift is exerted from the upper side to the lower side, and the drag on the first half shedding cycle is smaller than that on the second half.展开更多
基金supported by the National Natural Science Foun-dation of China(Grant No.51209213)supported by the Science Study Foundation of Naval University of Engineering (Grant No. HGDQNJJ12004)
文摘The quality of the inflow across the propeller is closely related with the hydrodynamic performance and the noise characteristics of the propeller. For a submarine, with a horseshoe vortex generated at the junction of the main body and the appendages, the submarine wake is dominated by a kind of highly non-uniform flow field, which has an adverse effect on the performance of the submarine propeller. In order to control the horseshoe vortex and improve the quality of the submarine wake, the flow field around a submarine model is simulated by the detached eddies simulation （DES） method, and the vortex configuration is displayed using the second invariant of the velocity derivative tensor. The state and the transition process of the horseshoe vortex are analyzed, then a modified method to break the vortex core by a vortex baffle is proposed. The flow numerical simulation is carried out to study the effect of this method. Numerical simulations show that, with the breakdown of the vortex core, many unstable vortices are shed and the energy of the horseshoe vortex is dissipated quickly, and the uniformity of the submarine wake is improved. The submarine wake test in a wind tunnel has verified the effect of the method to control the horseshoe vortex. The vortex baffle can improve the wake uniformity in cases of high Reynolds numbers as well, and it does not have adverse effects on the maneuverability and the speed ability of the submarine.
基金supported by the National Natural Science Foundation of China (10532070)the Knowledge Innovation Program of Chinese Academy of Sciences (KJCX2-YW-L07)the LNM Initial Funding for Young Investigators
文摘A time domain model is presented to study the vibrations of long slender cylinders placed in shear flow. Long slender cylinders such as risers and tension legs are widely used in the field of ocean engineering. They are subjected to vortex-induced vibrations（VIV） when placed within a transverse incident flow. A three dimensional model coupled with wake oscillators is formulated to describe the response of the slender cylinder in cross-flow and in-line directions. The wake oscillators are distributed along the cylinder and the vortex-shedding frequency is derived from the local current velocity. A non-linear fiuid force model is accounted for the coupled effect between cross-flow and in-line vibrations. The comparisons with the published experimental data show that the dynamic features of VIV of long slender cylinder placed in shear flow can be obtained by the proposed model,such as the spanwise average displacement,vibration frequency,dominant mode and the combination of standing and traveling waves. The simulation in a uniform flow is also conducted and the result is compared with the case of nonuniform flow. It is concluded that the flow shear characteristic has significantly changed the cylinder vibration behavior.
基金financially supported by the National Natural Science Foundation of China(Grant Nos.51609206,51522902 and 51579040)
文摘A numerical study based on a wake oscillator model was conducted to determine the response performance of vortex-induced vibration(VIV) on a long flexible cylinder with pinned-pinned boundary conditions subjected to linear and exponential shear flows. The coupling equations of a structural vibration model and wake oscillator model were solved using a standard central finite difference method of the second order. The VIV response characteristics including the structural displacement, structural frequency, structural wavenumber, standing wave behavior,travelling wave behavior, structural velocity, lift force coefficient and transferred energy from the fluid to the structure with different flow profiles were compared. The numerical results show that the VIV displacement is a combination of standing waves and travelling waves. For linear shear flow, standing waves and travelling waves dominate the VIV response within the low-velocity and high-velocity zones, respectively. The negative values of the transferred energy only occur within the low-velocity zone. However, for exponential shear flow, travelling waves dominate the VIV response and the negative energy occurs along the entire length of the cylinder.
文摘This paper addresses the necessity to make a physical interpretation of a highly complex three-dimensional tip clearance flow field study for high-speed mixed-flow compressor having stage exit static pressure to inlet total pressure ratio of 3.8 with 39,836 rpm rotor speed.The four different tip configurations namely the constant(l Z 0.016 and 0.019)and variable(l Z 0.011(inlet)-0.019(exit)and 0.019(inlet)-0.022(exit))tip clearances were numerically analysed using available experimental data-set.The numerical investigation reveals that in contrast to the classic jet-wake pattern,two anomalous velocity profiles formed at the impeller exit which results in pressure losses in the vaneless diffuser.Near the impeller inlet,the tip leakage flow rolls up to discrete tip leakage vortex structure for each tip clearance configuration.This results in the formation of a region of momentum deficit,recirculation zone,which gets weakened as it moves downstream.The tip clearance configuration is observed to profoundly influence the extent and vorticity of the tip leakage vortex.In the splitter blade passage,the tip leakage flow and Coriolis flow interact with passage flow,resulting in the formation of two secondary passage vortices that move downstream along the pressure and suction surface of the splitter blade.The tip clearance configuration directly influences the impeller exit jetwake pattern by modulating the secondary passage vortices trajectory and vorticity.Moreover,off-design analysis for tip clearances l Z 0.016 and l Z 0.019,depict distinctive tip leakage vortex characteristics.When operating near the stall conditions(80%of design mass flow rate),l Z 0.019 exhibits bubble shape tip leakage vortex breakdown occurring near the impeller inlet.This result in a substantial change in the tip leakage vortex nature;expansion of the recirculation zone and early weakening of the vorticity in the tip leakage vortex.It is observed that vortex breakdown plays a vital role in characteristics of the passage flow field structure and
文摘The near wake of a circular cylinder in linearly stratified flows of finite depth was experimentally investigated by means of flow visualization and measurements of vortex shedding frequencies, at Reynolds numbers 3.5 × 103-1.2 × 104 and stratification parameters kd 0-2.0. The non-dimensional parameter kd is defined as kd = Nd/U, where N is the Brunt-Vaisala frequency, d, the diameter of the cylinder, and U, the approaching flow velocity. The study demonstrates that as kd increases from zero, the vortex shedding from a circular cylinder progressively strengthens, while the Strouhal number gradually becomes lower than that for homogeneous flow. This phenomenon can be explained by the effect of the increasingly stable stratification which enhances the two-dimensionality of the near-wake flow of the circular cylinder;the enhanced two-dimensionality of the flow strengthens the roll-up of the separated shear layer. Above a certain value of kd, however, vortex formation and shedding are strongly suppressed and the Strouhal number rises sharply. This observation is attributable to the development of stationary lee waves downstream of the circular cylinder because the lee waves strongly suppress vertical fluid motions.
基金Sponsored by the National Nature Science Foundation of China ( 11202102)pecialized Research Fund for Doctoral Program of High Educatio n ( 20123219120050)
文摘The Lorentz force generated by electromagnetic field on the surface of the cylinder in the electrolyte solution may modify the structure of the flow boundary layer effectively. The transient control process of Lorentz force is investigated experimentally for lift amplification and vibration suppression. The experiments are conducted in a rotating annular tank filled with a low-conducting electrolyte. A cylinder with an electro-magnetic actuator is placed into the electrolyte. The lift force of cylinder is measured using the strain gages attached to a fixed beam, and the flow fields are visualized by the dye markers. The results show that the upper vortex on the cylinder is suppressed, and the wake becomes a line and leans to the lower side under the action of upside Lorentz force while the lower vortex on the cylinder is suppressed and limited in a small region. Therefore, the value of lift increases with the variation of flow field. However, the vortexes on the cylinder are suppressed fully under the action of symmetrical Lorentz force which leads to the suppression of lift oscillation and then the vibration of cylinder are suppressed fully.
基金The project supported by National Natural Science Fundation of China and LNM of Institute of Mechanics. CAS .
文摘Numerical study on near wake flows of a flat plate in three kinds of oncoming flows is made by using the discrete vortex model and improved vorticity creation method. For steady oncoming flow, both gross and detailed features of the wake flow are calculated and discussed. Then, in harmonic oscillatory oncoming flow two different wake flow patterns with K_c=2,4 and 10 are obtained respectively. Our results present a new wake flow pattern for low K_c numbers (K_c<5) describing vortex shedding, pairing and moving in a period of the oscillatory flow starting from rest. The calculated drag and inertia force coefficients are closer to experimental data from the U-tube than the previous results of vortex simulation. For in-line combined oncoming flow the vortex lock-in and dynamic characteristics are simulated. The results are shown to be in good agreement with experiments.
文摘This study demonstrates an active flow control for deflecting a direction of wake vortex structures behind a NACA0012 airfoil using an active morphing flap. Two-dimensional direct numerical simulations are performed for flows at the chord Reynolds number of 10,000, and the vortex pattern in the controlled and noncontrolled wakes as well as the effect of an actuation frequency on the control ability are rigorously investigated. It is found that there is an optimum actuation-frequency regime at around <em>F <sup>+</sup></em> = 2.00 which is normalized by the chord length and freestream velocity. The wake vortex pattern of the well-controlled case is classified as the 2P wake pattern according to the Williamson’s categorization [<a href="#ref1">1</a>] [<a href="#ref2">2</a>], where the forced oscillation frequency corresponds to the natural vortex shedding frequency without control. The present classification of wake vortex patterns and finding of the optimum frequency regime in the wake deflection control can lead to a more robust design suitable for vortex-induced-vibration (VIV) related engineering systems.
基金Projects supported by the National Key Research and Development Program of China(Grant Nos.2019YFB1704200,2019YFC0312400)the National Natural Science Foundation of China(Grant Nos.51879159,51909160).
文摘For complex aerodynamic and hydrodynamic problems,the analysis of vortex is very important.The Liutex method is an eigenvalue-based method which is local,accurate,and unique,which can give an accurate definition of vortex,so the control of vortex can be implemented and effectively guaranteed.Based on Liutex method,two methodologies of centripetal force model and counter-rotation force model were proposed to illustrate the vortex dynamics and possibly strengthen or weaken the vortices.In this paper,the Liutex-based centripetal force model is applied by adding a source term to the Navier-Stokes equations.In order to investigate the influence of the constructed Liutex force model on the 3-dimensional flow around a slow-fat ship,the calm-water drag calculation result of JBC ship is regarded as the initial flow field,and the new resistance and wake performances of the ship are obtained after applying the centripetal force model to the flow field with different strengths.Several views of the comparisons of the new steady flow fields are shown,and the parametric study results indicate that the Liutex-based centripetal force model can effectively change the resistance and wake performances of the JBC ship,which provides a new idea and theoretical basis for the comprehensive hydrodynamic performance optimization of the ship hull.
文摘The flow around two plates having asymmetric beveled trailing edge placed side-by-side in a uniform stream were investigated both numerically and experimentally. Two plates with spacing ratio of 1.0 and beveled trailing edge of angle 20° were tested at Reynolds numbers equal to 3.97×10^4. The numerical simulation results displayed that the flow separates on one plate beveled trailing edge surface. The vortex formation and shedding from this plate led to deflecting the flow toward the other plate and consequently the wake behind the plates showed an asymmetric development. The photographs obtained from the flow visualization confirmed the numerical results concerning the flow separation and vortex formation. It was also found that the computed pressure distribution on the two plate surfaces was different, particularly on the beveled trailing edge surfaces and on the fiat sides.
基金supported by the National Natural Science Foundation of China(Nos.11202102 and11172140)the Specialized Research Fund for Doctoral Program of High Education(No.20123219120050)
文摘The characteristics of a uniform-shear flow over a circular cylinder are in- vestigated numerically by using the alternative-direction implicit （ADI） algorithm and a fast Fourier transform （FFT） one in the exponential-polar coordinates for Re = 150 and 0 ≤ K ≤ 0.46. The diagram of lift-drag phase, implying the detail information about the fluctuations of drag and lift as well as the flow patterns in the wake and fluctuating pres- sure on the cylinder surface, is used to describe the effects of the shear rate on the flow. Results show that the upper （or lower） closed curve of a phase diagram corresponds to the first （or second） half shedding cycle. The lift-drag phase diagram will move down-left with the increase of shear rate K such that the lift is exerted from the upper side to the lower side, and the drag on the first half shedding cycle is smaller than that on the second half.
