Vortex street flowmeter has been used in steady flow measurement for about three decades The benefits of this type of flowmeter include high accuracy,good linearty,wide measuring range,and excellent reliability Howeve...Vortex street flowmeter has been used in steady flow measurement for about three decades The benefits of this type of flowmeter include high accuracy,good linearty,wide measuring range,and excellent reliability However,in unsteady flow measurement,the pressure disturbance as well as the noise from the system or surrounding can reduce the signal to noise ratio of the flowmeter seriously Aimed to use vortex street flowmeters in unsteady flow measurement,the characteristics of the vortex shedding induced hydrodynamic vibration around the prism bluff body in a vortex street flowmeter are investigated numerically and by expriments The results show that the hydrodynamic vibrations with 180° phase shift occur at the axisymmetric points of the channel around the bluff body The most intense vibration occurs at the points on the lateral faces close to the base of the prism The results provide therefore a useful reference for developing an anti interference vortex flowmeter using the different ial sensing technique.展开更多
A study to investigate the effects of taper on vortex shedding coherence on High Mast Lighting Towers (HMLT) with models of eight-, twelve-, and sixteen-sided polygonal cross-section was performed in Purdue’s Boeing ...A study to investigate the effects of taper on vortex shedding coherence on High Mast Lighting Towers (HMLT) with models of eight-, twelve-, and sixteen-sided polygonal cross-section was performed in Purdue’s Boeing Low-Speed Wind Tunnel. Partial tower models were mounted on springs to recreate a flutter phenomenon seen on high mast lighting towers and data was taken using a stationary configuration within the wind tunnel. The model was later oscillated at specified frequencies and amplitudes and the resulting wake and surface pressures were recorded and compared to the stationary cases. The researchers aim to study the characteristics of a “lock-in” phenomenon, that is, a region of pole height where there is a vortex cell with a single shedding frequency, instead of different shedding frequencies for different diameters as Strouhal theory dictates. Results show the existence of vortex cell shedding for clamped models. Using a motor and a forcing cam to recreate the elastic movement of the HMLT in ambient conditions has yielded a specific range of diameters to determine the size of the locked in vortex cells. According to standard Department of Transportation manufacturing standards for tapered HMLT, the lock in distance for small excitations (0.254 cm) would be approximately 305 cm in tower height.展开更多
The objective of this study is to apply numerical methods to investigate the effects of the spacing on the vortex shedding of two elastically mounted cylinders in tandem arrangement. 2-D computational simulations are ...The objective of this study is to apply numerical methods to investigate the effects of the spacing on the vortex shedding of two elastically mounted cylinders in tandem arrangement. 2-D computational simulations are carried out at low Reynolds number of 100. The study utilized a commercial software ANSYS FLUENT to carry out the computational simulations. First, a number of test cases, including flows past one and two cylinders with predetermined motions, are simulated to evaluate the solver’s accuracy. The vortex shedding and hydrodynamic forces from the current findings and those from literature show good agreement, which supports the accuracy of the current solver. Multiple simulations were the performed for flow around two elastically mounted cylinders in tandem arrangement. The subsequent relative flow fields demonstrated that for a certain range of spacing, vortex shedding was completely eliminated while it remained completely unaffected or partially reduced for other ranges of spacing. This suggests that the spacing between the two cylinders can be utilized as a passive method of suppressing vortex shedding.展开更多
Owing to the influence of the viscosity of the flow field,the strength of the shedding vortex decreases gradually in the process of backward propagation.Large-scale vortexes constantly break up,forming smaller vortexe...Owing to the influence of the viscosity of the flow field,the strength of the shedding vortex decreases gradually in the process of backward propagation.Large-scale vortexes constantly break up,forming smaller vortexes.In engineering,when numerical simulation of vortex evolution process is carried out,a large grid is needed to be arranged in the area of outflow field far from the boundary layer in order to ensure the calculation efficiency.As a result,small scale vortexes at the far end of the flow field cannot be captured by the sparse grid in this region,resulting in the dissipation or even disappearance of vortexes.In this paper,the effect of grid scale is quantified and compared with the viscous effect through theoretical derivation.The theoretical relationship between the mesh viscosity and the original viscosity of the flow field is established,and the viscosity term in the turbulence model is modified.This method proves to be able to effectively improve the intensity of small-scale shedding vortexes at the far end of the flow field under the condition of sparse grid.The error between the simulation results and the results obtained by using fine mesh is greatly reduced,the calculation time is shortened,and the high-precision and efficient simulation of the flow field is realized.展开更多
As an important lightning protection device in substations,lightning rods are susceptible to vibration and potential structural damage under wind loads.In order to understand their vibration mechanism,it is necessary ...As an important lightning protection device in substations,lightning rods are susceptible to vibration and potential structural damage under wind loads.In order to understand their vibration mechanism,it is necessary to conduct flow analysis.In this study,numerical simulations of the flow field around a 330 kV cylindrical lightning rod with different diameters were performed using the SST k-ωmodel.The flow patterns in different segments of the lightning rod at the same reference wind speed(wind speed at a height of 10 m)and the flow patterns in the same segment at different reference wind speeds were investigated.The variations of lift coefficient,drag coefficient,and vorticity distribution were obtained.The results showed that vortex shedding phenomena occurred in all segments of the lightning rod,and the strength of vortex shedding increased with decreasing diameter.The vorticity magnitude and the root mean square magnitudes of the lift coefficient and drag coefficient also increased accordingly.The time history curves of the lift coefficient and drag coefficient on the surface of the lightning rod exhibited sinusoidal patterns with a single dominant frequency.For the same segment,as the wind speed increased in a certain range,the root mean square values of the lift coefficient and drag coefficient decreased,while their dominant frequencies increased.Moreover,there was a proportional relationship between the dominant frequencies of the lift coefficient and drag coefficient.The findings of this study can provide valuable insights for the refined design of lightning rods with similar structures.展开更多
A numerical study of vortex-induced rotations(VIRs) of an equivalent triangular cylinder, which is free to rotate in the azimuthal direction in a uniform flow, is presented. Based on an immersed boundary method, the n...A numerical study of vortex-induced rotations(VIRs) of an equivalent triangular cylinder, which is free to rotate in the azimuthal direction in a uniform flow, is presented. Based on an immersed boundary method, the numerical model is established, and is verified through the benchmark problem of flow past a freely rotating rectangular body.The computation is performed for a fixed reduced mass of m~*=2.0 and the structural stiffness and damping ratio are set to zero. The effects of Reynolds number(Re=25-180) on the characteristics of VIR are studied. It is found that the dynamic response of the triangular cylinder exhibits four distinct modes with increasing Re: a rest position,periodic rotational oscillation, random rotation and autorotation. For the rotational oscillation mode, the cylinder undergoes a periodic vibration around an equilibrium position with one side facing the incoming flow. Since the rotation effect, the outset of vortex shedding from cylinder shifts to a much lower Reynolds number. Further increase in Re leads to 2 P and P+S vortex shedding modes besides the typical 2 S pattern. Our simulation results also elucidate that the free rotation significantly changes the drag and lift forces. Inspired by these facts, the effect of free rotation on flow-induced vibration of a triangular cylinder in the in-line and transverse directions is investigated. The results show that when the translational vibration is coupled with rotation, the triangular cylinder presents a galloping response instead of vortex-induced vibration(VIV).展开更多
Orientations of laser induced periodic surface structures(LIPSS)are usually considered to be governed by the laser polarization state.In this work,we unveil that fluid dynamics induced by femtosecond(fs)laser ablation...Orientations of laser induced periodic surface structures(LIPSS)are usually considered to be governed by the laser polarization state.In this work,we unveil that fluid dynamics induced by femtosecond(fs)laser ablation in liquid(fs-LAL)can easily break this polarization restriction to produce irregular circular-LIPSS(CLIPPS)and crisscross-LIPSS(CCLIPSS).Fs laser ablation of silicon in water shows formation of diverse LIPSS depending on ablation conditions.At a high power of 700 mW(repetition rate of 100 kHz,pulse duration of 457 fs and wavelength of 1045 nm),single/twin CLIPSS are produced at the bottom of macropores of several microns in diameter due to the formation of strong liquid vortexes and occurrence of the vortex shedding effect.Theoretical simulations validate our speculation about the formation of liquid vortex with an ultrahigh static pressure,which can induce the microstructure trenches and cracks at the sidewalls for fs-LAL of Si and tungsten(W)in water,respectively.At a low power of 50 mW,weak liquid vortexes are produced,which only give birth to curved LIPSS in the valleys of grooves.Consequently,it is deduced that liquid vortex plays a crucial role in the formation of macropores.Mountain-like microstructures induce complex fluid dynamics which can cause the formation of CCLIPSS on them.It is believed that liquid vortexes and fluid dynamics presented in this work open up new possibilities to diversify the morphologies of LIPSS formed by fs-LAL.展开更多
A large eddy simulation of wall-adapting local eddy-viscosity model(LES-WALE)is used to simulate the threedimensional flow around a circular cylinder with a diameter of 0.25 m from sub-critical to super-critical Reyno...A large eddy simulation of wall-adapting local eddy-viscosity model(LES-WALE)is used to simulate the threedimensional flow around a circular cylinder with a diameter of 0.25 m from sub-critical to super-critical Reynolds numbers at 1×10^(5),2.5×10^(5),and 7.2×10^(5),respectively.The present results such as drag crisis,surface pressure distribution,and Strouhal number are in good agreement with the classical experimental data.When entering the critical region,a small plateau was found on the pressure distribution curves,corresponding to the appearance of laminar separation bubbles,and the separation point is delayed and the recirculation bubbles become narrowed and shortened.The tangential velocity of the cylinder surface changes from positive to negative at the separation point.The instantaneous vorticity and timeaveraging separation bubbles embody an unstable feature.Within the separation bubble,the pressure varies dramatically with time,but not with position.The surface pressure fluctuates greatly after the laminar separation bubble appears,and it is gradually stabilized until the basic pressure is reached.The process of laminar separation,transition from laminar flow to turbulent flow and turbulent reattachment is also shown.The three-dimensional Q criterion of vortex structure and the two-dimensional spanwise vorticity reveal the phenomenon that the wake structure narrows with the increase of the Reynolds number.展开更多
The broad implication of the paper is to elucidate the significance of the dynamic heaving motion in the aerodynamic performance of multi-element wings,currently considered as a promising aspect for the improvement of...The broad implication of the paper is to elucidate the significance of the dynamic heaving motion in the aerodynamic performance of multi-element wings,currently considered as a promising aspect for the improvement of the aerodynamic correlation between CFD,wind tunnel and track testing in race car applications.The relationship between the varying aerodynamic forces,the vortex shedding,and the unsteady pressure field of a heaving double-element wing is investigated for a range of mean ride heights,frequencies,and amplitudes,using a two-dimensional(2D)unsteady Reynolds-averaged Navier-Stokes(URANS)approach and an overset mesh method for modelling the moving wing.The analysis of the results shows that at high frequencies,i.e.,k≥5.94 and amplitudes a/c≥0.05 the interaction of the shear vorticity between the two elements results in the generation of cohering leading and trailing edge vortices on the flap,associated to the rapid variation of thrust and downforce enhancement.Both the occurrence and intensity of these vortices are dependent upon the frequency,amplitude,and mean ride height of the heaving wing.The addition of the flap significantly alters the frequency of the shed vortices in the wake and maintains the generation of downforce for longer time in ground proximity.The comparison with the static wing provides evidence that the dynamic motion of a race car wing can be beneficial in terms of performance,or detrimental in terms of aerodynamic correlation.展开更多
The present study stems from the realization that the general problem relating to the analysis of wind-induced vibrations in suspension bridges still requires significant attention.Sidewalk railings,overhaul tracks,an...The present study stems from the realization that the general problem relating to the analysis of wind-induced vibrations in suspension bridges still requires significant attention.Sidewalk railings,overhaul tracks,and deflectors are known to largely affect such dynamics.Here,the influence of a row of water-filled traffic barriers on the response of a sample suspension bridge is investigated numerically.It is shown that the existence of water barriers causes flow separation and non-negligible vortices with respect to the condition with no water barriers.The vortex shedding frequency at the far end is around 41.30 Hz,relatively close to the real vibration frequency.It is also shown how different incoming angles of attack can change the flow field around the bridge cross-section and the vortex detachment frequency.展开更多
Three-stranded rope is widely used in fishing gear and mooring system. Results of numerical simulation are presented for flow around a three-stranded rope in uniform flow. The simulation was carried out to study the h...Three-stranded rope is widely used in fishing gear and mooring system. Results of numerical simulation are presented for flow around a three-stranded rope in uniform flow. The simulation was carried out to study the hydrodynamic characteristics of pressure and velocity fields of steady incompressible laminar and turbulent wakes behind a three-stranded rope. A three-cylinder configuration and single circular cylinder configuration are used to model the three-stranded rope in the two-dimensional simulation. The governing equations, Navier-Stokes equations, are solved by using two-dimensional finite volume method. The turbulence flow is simulated using Standard κ-ε model and Shear-Stress Transport κ-ω(SST) model. The drag of the three-cylinder model and single cylinder model is calculated for different Reynolds numbers by using control volume analysis method. The pressure coefficient is also calculated for the turbulent model and laminar model based on the control surface method. From the comparison of the drag coefficient and the pressure of the single cylinder and three-cylinder models, it is found that the drag coefficients of the three-cylinder model are generally 1.3–1.5 times those of the single circular cylinder for different Reynolds numbers. Comparing the numerical results with water tank test data, the results of the three-cylinder model are closer to the experiment results than the single cylinder model results.展开更多
The characteristics of a uniform-shear flow over a circular cylinder are investigated numerically by using the alternative-direction implicit(ADI) algorithm and a fast Fourier transform(FFT) one in the exponential-pol...The characteristics of a uniform-shear flow over a circular cylinder are investigated numerically by using the alternative-direction implicit(ADI) algorithm and a fast Fourier transform(FFT) one in the exponential-polar coordinates for Re = 150 and0 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 pressure 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
In the present paper,two-and three-dimensional numerical simulations of the flow interference between four cylinders in an in-line square arrangement at Re = 200 are performed.Assisted with the two-dimensional(2-D) nu...In the present paper,two-and three-dimensional numerical simulations of the flow interference between four cylinders in an in-line square arrangement at Re = 200 are performed.Assisted with the two-dimensional(2-D) numerical simulation,the mean and fluctuating forces,Strouhal number(St) and vortex shedding pattern in the wake for each cylinder were analyzed with the spacing ratio(L /D) ranging from 1.5 to 6.0.It was found that,four different vortex modes(viz.,flip-flopping,shielding anti-phase-synchronized,in-phasesynchronized and anti-phase-synchronized) gradually appear with the increase of the L/D ratio.The average drag coefficient of the upstream cylinders is larger than that of the downstream cylinders,while the downstream cylinders usually undergo serious fluctuating forces.When the L/D ratio ranges from 3.0 to 4.0,the dominant frequency of the drag coefficient is equal to the value of St of upstream cylinders.This indicates that a simultaneous resonance in the in-flow and cross-flow directions may occur for some single structures of a multi-body oscillating system.For the 3-D numerical simulation,the L/D and aspect ratios are kept constant as 5.0 and 10,respectively.It was found that some vortices are formed in the wake of the upstream cylinders.Besides,with the same spacing ratio,the calculated drag coefficient and lift coefficient fluctuation are slightly larger than the 2-D results,but with a phase difference.展开更多
The numerical modeling of a 2D flow around a horizontal cylinder near a rigid bed with the gap ratios G/D = 0, 0.2, 0.3 at Reynolds numbers 840, 4500, 9000, and 9500 is investigated by using weakly compressible smooth...The numerical modeling of a 2D flow around a horizontal cylinder near a rigid bed with the gap ratios G/D = 0, 0.2, 0.3 at Reynolds numbers 840, 4500, 9000, and 9500 is investigated by using weakly compressible smoothed particle hydrodynamics. The velocity field and the separation angles from the present simulations are compared with those obtained from the experimental measurements and are in a good agreement. The results show that the maximum value of shear stress on the bed increases as the cylinder closes the bed and suddenly decreases when the cylinder contact the wall.展开更多
Two dimensional numerical simulations of flow around a rotationally oscillating circular cylinder were performed at Re = 1000. A wide range of forcing frequencies, fr, and three values of oscillation amplitudes, A, ar...Two dimensional numerical simulations of flow around a rotationally oscillating circular cylinder were performed at Re = 1000. A wide range of forcing frequencies, fr, and three values of oscillation amplitudes, A, are considered. Different vortex shedding modes are observed for a fixed A at several values of fr, as well as for a fixed fr at different values of A. The 2C mode of vortex shedding was obtained in the present study. It is important to point out that this mode has not been observed by other investigators for rotationally oscillating case. Also, it is verified that this mechanism has great influence on the drag coefficient for high frequency values. Furthermore, the lift and pressure coefficients and the power spectra density are also analyzed.展开更多
Recently,the Liutex-based force field models are proposed and applied to some two-dimensional flows to explore the possibility of direct vortex control strategies.In this paper,we applied the Liutex-based centripetal ...Recently,the Liutex-based force field models are proposed and applied to some two-dimensional flows to explore the possibility of direct vortex control strategies.In this paper,we applied the Liutex-based centripetal force model to three-dimensional flow around a circular cylinder at Re=3900.The turbulent flows around the three-dimensional cylinder are modeled by the delayed detached-eddy simulations based on the two-equation k-ω shear stress transport(SST)model.The cylinder flow without adding the force field model is simulated in the first place.The statistical flow quantities are compared with experimental data to validate the accuracy of the current numerical models.Then the force field model is applied to the momentum equation in a specific control region near the cylinder to study the change of flow field and hydrodynamic performance.The effect of different control region locations with the same force field strength is discussed to find out the most influenced locations on the drag and lift.Based on the screened control region,the influence source term strength is further studied.Variations on the global statistical flow quantities,local instantaneous three-dimensional flow structures are analyzed and discussed.It is concluded that the Liutex-based force field model can effectively change the hydrodynamic behavior of flow past a cylinder and may provide a new direction for performance improvement and optimization.展开更多
This work aims to find the origin and connection of the surface,near-wake,and far-wake structures in the flow encompassing a high-rise building for a high Reynolds number.The origin and interconnection of the stream-w...This work aims to find the origin and connection of the surface,near-wake,and far-wake structures in the flow encompassing a high-rise building for a high Reynolds number.The origin and interconnection of the stream-wise tip vortices,with the other components of the wake,is analysed in this study for the current scenario.The Unsteady Reynolds Averaged Navier-Stokes equations(URANS)together with the realizable k-εturbulence model have been used in this investigation to study the turbulent wake flow following a ground-surface-attached square shape building.A moderately big obstacle aspect ratio of 4,a Reynolds number of 12,000,and a thin evolving boundary layer thickness have been used in the flow modeling.The designed flow addresses the reversed-flows at the outlet during computation to improve the accuracy of the realizable k-e model.The Reynolds stress components are retrieved using the Boussinesq approach.The wake’s principal compositions,including span-wise-side eddies and area of high stream-wise vorticity in the uppermost portion of the wake,are illustrated by both three-dimensional(3D)representations and planner projections of the mean flow distributions.A braided vortex formation,composed of asymmetric hairpin vortexes,is witnessed in the far-wake area.The association of the near-wake vortex structures with the far-wake and near-wall flow,which is associated with the flow strengths,is also discussed.In this investigation,few areas of large stream-wise vorticity magnitude,like tip vortexes,are correlated to the 3D curving of the fluid motion,and tip vortices did not continuously reach to the free end part of the building.The 3D fluid motion interpretation,which combined several measurements of the flow distribution encompassing the cylinder,shows that the time-averaged near-wake structures are formed of two segments of distinct source and section of dominance.Furthermore,addressing reversed-flow during computation shows notable improvement in the results.展开更多
Understanding and control of wake vortices past a circular cylinder is a cardinal problem of interest to ocean engineering.The wake formation and vortex shedding behind a variety of ocean structures such as spars,are ...Understanding and control of wake vortices past a circular cylinder is a cardinal problem of interest to ocean engineering.The wake formation and vortex shedding behind a variety of ocean structures such as spars,are subjected to fatigue failure limiting their life span.The additional influences due to ocean waves and currents further exacerbate these effects.In the present study,flow past an isolated circu-lar cylindrical structure subjected to an oscillatory upstream are numerically investigated.These studies involve high resolution simulations over the low Reynolds number range(100-200).Although the prac-tical range of interest is in high Reynolds number range of 103-105,the flow physics and a number of qualitative and quantitative aspects are similar to the low Reynolds number flows.In the high Reynolds number range,statistical averaging tools in conjunction with suitable closure models would be neces-sary.The control of wake vortices is achieved with the aid of two small rotors located in the aft of the main cylinder.A control algorithm was coupled to determine the quantum of actuation to the rotating elements.Although control of wake vortices was observed for harmonic in-let forcing,residual vortical structures were found to persist at higher amplitudes of oscillation.To study the efficacy of this control,numerical simulations were further extended,when the circular cylinder was flexibly mounted.The con-trol of flow induced vibrations was observed to be reasonably effective in controlling the wake generated behind the main cylinder due to oscillatory upstream.展开更多
A numerical investigation is carried out for the different orientations of a circular cylinder where the upstream cylinder moves with varying anglesα=0˚,30˚,45˚,60˚,90˚with respect to the downstream cylinder in a fix...A numerical investigation is carried out for the different orientations of a circular cylinder where the upstream cylinder moves with varying anglesα=0˚,30˚,45˚,60˚,90˚with respect to the downstream cylinder in a fixed position at a gap ratio(L/D)of 3 where L is the distance from centre to centre between cylinders and D is the diameter of the cylinder.Reynolds number(Re),is based on cylinder diameter,is kept constant at 200 for all the cases with air as the working medium.The vortex shedding formation is analysed when the vortices from the upwind cylinder interact with the downwind cylinder and give impressive flow patterns.It is observed that the drag and lift coefficients increase for both the cylinders with change in angular position.Strouhal number is calculated with the help of Fast Fourier transformation(FFT)of vorticity magnitude and evaluated for each case.It is observed that the effect of upwind cylinder on downwind cylinder is significant forα=30˚,45˚,60˚orientation cases and vortex shedding frequency increases for these cases.Further,the investigation is extending on the laminar forced convection heat transfer performance and variation of Nusselt number for the cylinders.Maximum heat transfer is observed for the downwind cylinder atα=45˚.In contrast,the local Nusselt number does not vary much for the cylinders except for the downwind cylinder atα=0˚.展开更多
基金This project is supported by National Natural Science Foundation of China(No.59835160).
文摘Vortex street flowmeter has been used in steady flow measurement for about three decades The benefits of this type of flowmeter include high accuracy,good linearty,wide measuring range,and excellent reliability However,in unsteady flow measurement,the pressure disturbance as well as the noise from the system or surrounding can reduce the signal to noise ratio of the flowmeter seriously Aimed to use vortex street flowmeters in unsteady flow measurement,the characteristics of the vortex shedding induced hydrodynamic vibration around the prism bluff body in a vortex street flowmeter are investigated numerically and by expriments The results show that the hydrodynamic vibrations with 180° phase shift occur at the axisymmetric points of the channel around the bluff body The most intense vibration occurs at the points on the lateral faces close to the base of the prism The results provide therefore a useful reference for developing an anti interference vortex flowmeter using the different ial sensing technique.
文摘A study to investigate the effects of taper on vortex shedding coherence on High Mast Lighting Towers (HMLT) with models of eight-, twelve-, and sixteen-sided polygonal cross-section was performed in Purdue’s Boeing Low-Speed Wind Tunnel. Partial tower models were mounted on springs to recreate a flutter phenomenon seen on high mast lighting towers and data was taken using a stationary configuration within the wind tunnel. The model was later oscillated at specified frequencies and amplitudes and the resulting wake and surface pressures were recorded and compared to the stationary cases. The researchers aim to study the characteristics of a “lock-in” phenomenon, that is, a region of pole height where there is a vortex cell with a single shedding frequency, instead of different shedding frequencies for different diameters as Strouhal theory dictates. Results show the existence of vortex cell shedding for clamped models. Using a motor and a forcing cam to recreate the elastic movement of the HMLT in ambient conditions has yielded a specific range of diameters to determine the size of the locked in vortex cells. According to standard Department of Transportation manufacturing standards for tapered HMLT, the lock in distance for small excitations (0.254 cm) would be approximately 305 cm in tower height.
文摘The objective of this study is to apply numerical methods to investigate the effects of the spacing on the vortex shedding of two elastically mounted cylinders in tandem arrangement. 2-D computational simulations are carried out at low Reynolds number of 100. The study utilized a commercial software ANSYS FLUENT to carry out the computational simulations. First, a number of test cases, including flows past one and two cylinders with predetermined motions, are simulated to evaluate the solver’s accuracy. The vortex shedding and hydrodynamic forces from the current findings and those from literature show good agreement, which supports the accuracy of the current solver. Multiple simulations were the performed for flow around two elastically mounted cylinders in tandem arrangement. The subsequent relative flow fields demonstrated that for a certain range of spacing, vortex shedding was completely eliminated while it remained completely unaffected or partially reduced for other ranges of spacing. This suggests that the spacing between the two cylinders can be utilized as a passive method of suppressing vortex shedding.
基金Project supported by the National Key Project,China(Grant No.GJXM92579)the National Natural Science Foundation of China(Grant No.12072232)。
文摘Owing to the influence of the viscosity of the flow field,the strength of the shedding vortex decreases gradually in the process of backward propagation.Large-scale vortexes constantly break up,forming smaller vortexes.In engineering,when numerical simulation of vortex evolution process is carried out,a large grid is needed to be arranged in the area of outflow field far from the boundary layer in order to ensure the calculation efficiency.As a result,small scale vortexes at the far end of the flow field cannot be captured by the sparse grid in this region,resulting in the dissipation or even disappearance of vortexes.In this paper,the effect of grid scale is quantified and compared with the viscous effect through theoretical derivation.The theoretical relationship between the mesh viscosity and the original viscosity of the flow field is established,and the viscosity term in the turbulence model is modified.This method proves to be able to effectively improve the intensity of small-scale shedding vortexes at the far end of the flow field under the condition of sparse grid.The error between the simulation results and the results obtained by using fine mesh is greatly reduced,the calculation time is shortened,and the high-precision and efficient simulation of the flow field is realized.
基金supported by State Grid Ningxia Electric Power Co.,Ltd.under Grant 5229CG220006Natural Science Foundation of Ningxia Province under Grant 2022AAC03629.
文摘As an important lightning protection device in substations,lightning rods are susceptible to vibration and potential structural damage under wind loads.In order to understand their vibration mechanism,it is necessary to conduct flow analysis.In this study,numerical simulations of the flow field around a 330 kV cylindrical lightning rod with different diameters were performed using the SST k-ωmodel.The flow patterns in different segments of the lightning rod at the same reference wind speed(wind speed at a height of 10 m)and the flow patterns in the same segment at different reference wind speeds were investigated.The variations of lift coefficient,drag coefficient,and vorticity distribution were obtained.The results showed that vortex shedding phenomena occurred in all segments of the lightning rod,and the strength of vortex shedding increased with decreasing diameter.The vorticity magnitude and the root mean square magnitudes of the lift coefficient and drag coefficient also increased accordingly.The time history curves of the lift coefficient and drag coefficient on the surface of the lightning rod exhibited sinusoidal patterns with a single dominant frequency.For the same segment,as the wind speed increased in a certain range,the root mean square values of the lift coefficient and drag coefficient decreased,while their dominant frequencies increased.Moreover,there was a proportional relationship between the dominant frequencies of the lift coefficient and drag coefficient.The findings of this study can provide valuable insights for the refined design of lightning rods with similar structures.
基金financially supported by the Fundamental Research Funds for the Central Universities(Grant Nos.2018B56414 and2019B12014)the National Natural Science Foundation of China(Grant No.51609077)
文摘A numerical study of vortex-induced rotations(VIRs) of an equivalent triangular cylinder, which is free to rotate in the azimuthal direction in a uniform flow, is presented. Based on an immersed boundary method, the numerical model is established, and is verified through the benchmark problem of flow past a freely rotating rectangular body.The computation is performed for a fixed reduced mass of m~*=2.0 and the structural stiffness and damping ratio are set to zero. The effects of Reynolds number(Re=25-180) on the characteristics of VIR are studied. It is found that the dynamic response of the triangular cylinder exhibits four distinct modes with increasing Re: a rest position,periodic rotational oscillation, random rotation and autorotation. For the rotational oscillation mode, the cylinder undergoes a periodic vibration around an equilibrium position with one side facing the incoming flow. Since the rotation effect, the outset of vortex shedding from cylinder shifts to a much lower Reynolds number. Further increase in Re leads to 2 P and P+S vortex shedding modes besides the typical 2 S pattern. Our simulation results also elucidate that the free rotation significantly changes the drag and lift forces. Inspired by these facts, the effect of free rotation on flow-induced vibration of a triangular cylinder in the in-line and transverse directions is investigated. The results show that when the translational vibration is coupled with rotation, the triangular cylinder presents a galloping response instead of vortex-induced vibration(VIV).
基金This work was supported by the national key R&D program for internation-al collaboration under grant No.2018YFE9103900in part by the Nat-ural Science Foundation of China(NSFC)grant No.11972384 and the Guangdong MEPP Fund,grant No.GDOE[2019]A01.
文摘Orientations of laser induced periodic surface structures(LIPSS)are usually considered to be governed by the laser polarization state.In this work,we unveil that fluid dynamics induced by femtosecond(fs)laser ablation in liquid(fs-LAL)can easily break this polarization restriction to produce irregular circular-LIPSS(CLIPPS)and crisscross-LIPSS(CCLIPSS).Fs laser ablation of silicon in water shows formation of diverse LIPSS depending on ablation conditions.At a high power of 700 mW(repetition rate of 100 kHz,pulse duration of 457 fs and wavelength of 1045 nm),single/twin CLIPSS are produced at the bottom of macropores of several microns in diameter due to the formation of strong liquid vortexes and occurrence of the vortex shedding effect.Theoretical simulations validate our speculation about the formation of liquid vortex with an ultrahigh static pressure,which can induce the microstructure trenches and cracks at the sidewalls for fs-LAL of Si and tungsten(W)in water,respectively.At a low power of 50 mW,weak liquid vortexes are produced,which only give birth to curved LIPSS in the valleys of grooves.Consequently,it is deduced that liquid vortex plays a crucial role in the formation of macropores.Mountain-like microstructures induce complex fluid dynamics which can cause the formation of CCLIPSS on them.It is believed that liquid vortexes and fluid dynamics presented in this work open up new possibilities to diversify the morphologies of LIPSS formed by fs-LAL.
基金The National Natural Science Foundation of China(Grant Nos.12172218 and 52088102)Joint Funds of the National Natural Science Foundation of China(Grant No.U19B2013).
文摘A large eddy simulation of wall-adapting local eddy-viscosity model(LES-WALE)is used to simulate the threedimensional flow around a circular cylinder with a diameter of 0.25 m from sub-critical to super-critical Reynolds numbers at 1×10^(5),2.5×10^(5),and 7.2×10^(5),respectively.The present results such as drag crisis,surface pressure distribution,and Strouhal number are in good agreement with the classical experimental data.When entering the critical region,a small plateau was found on the pressure distribution curves,corresponding to the appearance of laminar separation bubbles,and the separation point is delayed and the recirculation bubbles become narrowed and shortened.The tangential velocity of the cylinder surface changes from positive to negative at the separation point.The instantaneous vorticity and timeaveraging separation bubbles embody an unstable feature.Within the separation bubble,the pressure varies dramatically with time,but not with position.The surface pressure fluctuates greatly after the laminar separation bubble appears,and it is gradually stabilized until the basic pressure is reached.The process of laminar separation,transition from laminar flow to turbulent flow and turbulent reattachment is also shown.The three-dimensional Q criterion of vortex structure and the two-dimensional spanwise vorticity reveal the phenomenon that the wake structure narrows with the increase of the Reynolds number.
文摘The broad implication of the paper is to elucidate the significance of the dynamic heaving motion in the aerodynamic performance of multi-element wings,currently considered as a promising aspect for the improvement of the aerodynamic correlation between CFD,wind tunnel and track testing in race car applications.The relationship between the varying aerodynamic forces,the vortex shedding,and the unsteady pressure field of a heaving double-element wing is investigated for a range of mean ride heights,frequencies,and amplitudes,using a two-dimensional(2D)unsteady Reynolds-averaged Navier-Stokes(URANS)approach and an overset mesh method for modelling the moving wing.The analysis of the results shows that at high frequencies,i.e.,k≥5.94 and amplitudes a/c≥0.05 the interaction of the shear vorticity between the two elements results in the generation of cohering leading and trailing edge vortices on the flap,associated to the rapid variation of thrust and downforce enhancement.Both the occurrence and intensity of these vortices are dependent upon the frequency,amplitude,and mean ride height of the heaving wing.The addition of the flap significantly alters the frequency of the shed vortices in the wake and maintains the generation of downforce for longer time in ground proximity.The comparison with the static wing provides evidence that the dynamic motion of a race car wing can be beneficial in terms of performance,or detrimental in terms of aerodynamic correlation.
文摘The present study stems from the realization that the general problem relating to the analysis of wind-induced vibrations in suspension bridges still requires significant attention.Sidewalk railings,overhaul tracks,and deflectors are known to largely affect such dynamics.Here,the influence of a row of water-filled traffic barriers on the response of a sample suspension bridge is investigated numerically.It is shown that the existence of water barriers causes flow separation and non-negligible vortices with respect to the condition with no water barriers.The vortex shedding frequency at the far end is around 41.30 Hz,relatively close to the real vibration frequency.It is also shown how different incoming angles of attack can change the flow field around the bridge cross-section and the vortex detachment frequency.
基金supported by the National Natural Science Foundation of China (31072246, 30972256)Special Fund for Research on high efficient techniques for Antarctic Krill (20150256)+1 种基金Agro-Scientific Research in the Public Interest (201203018, 201303050-02)the Fundamental Research Funds for Chinese Academy of Fishery Sciences (CAFS) (2012A1301)
文摘Three-stranded rope is widely used in fishing gear and mooring system. Results of numerical simulation are presented for flow around a three-stranded rope in uniform flow. The simulation was carried out to study the hydrodynamic characteristics of pressure and velocity fields of steady incompressible laminar and turbulent wakes behind a three-stranded rope. A three-cylinder configuration and single circular cylinder configuration are used to model the three-stranded rope in the two-dimensional simulation. The governing equations, Navier-Stokes equations, are solved by using two-dimensional finite volume method. The turbulence flow is simulated using Standard κ-ε model and Shear-Stress Transport κ-ω(SST) model. The drag of the three-cylinder model and single cylinder model is calculated for different Reynolds numbers by using control volume analysis method. The pressure coefficient is also calculated for the turbulent model and laminar model based on the control surface method. From the comparison of the drag coefficient and the pressure of the single cylinder and three-cylinder models, it is found that the drag coefficients of the three-cylinder model are generally 1.3–1.5 times those of the single circular cylinder for different Reynolds numbers. Comparing the numerical results with water tank test data, the results of the three-cylinder model are closer to the experiment results than the single cylinder model results.
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 investigated numerically by using the alternative-direction implicit(ADI) algorithm and a fast Fourier transform(FFT) one in the exponential-polar coordinates for Re = 150 and0 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 pressure 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.
基金Sponsored by the National Natural Science Foundation of China(Grant No.50538020)
文摘In the present paper,two-and three-dimensional numerical simulations of the flow interference between four cylinders in an in-line square arrangement at Re = 200 are performed.Assisted with the two-dimensional(2-D) numerical simulation,the mean and fluctuating forces,Strouhal number(St) and vortex shedding pattern in the wake for each cylinder were analyzed with the spacing ratio(L /D) ranging from 1.5 to 6.0.It was found that,four different vortex modes(viz.,flip-flopping,shielding anti-phase-synchronized,in-phasesynchronized and anti-phase-synchronized) gradually appear with the increase of the L/D ratio.The average drag coefficient of the upstream cylinders is larger than that of the downstream cylinders,while the downstream cylinders usually undergo serious fluctuating forces.When the L/D ratio ranges from 3.0 to 4.0,the dominant frequency of the drag coefficient is equal to the value of St of upstream cylinders.This indicates that a simultaneous resonance in the in-flow and cross-flow directions may occur for some single structures of a multi-body oscillating system.For the 3-D numerical simulation,the L/D and aspect ratios are kept constant as 5.0 and 10,respectively.It was found that some vortices are formed in the wake of the upstream cylinders.Besides,with the same spacing ratio,the calculated drag coefficient and lift coefficient fluctuation are slightly larger than the 2-D results,but with a phase difference.
文摘The numerical modeling of a 2D flow around a horizontal cylinder near a rigid bed with the gap ratios G/D = 0, 0.2, 0.3 at Reynolds numbers 840, 4500, 9000, and 9500 is investigated by using weakly compressible smoothed particle hydrodynamics. The velocity field and the separation angles from the present simulations are compared with those obtained from the experimental measurements and are in a good agreement. The results show that the maximum value of shear stress on the bed increases as the cylinder closes the bed and suddenly decreases when the cylinder contact the wall.
文摘Two dimensional numerical simulations of flow around a rotationally oscillating circular cylinder were performed at Re = 1000. A wide range of forcing frequencies, fr, and three values of oscillation amplitudes, A, are considered. Different vortex shedding modes are observed for a fixed A at several values of fr, as well as for a fixed fr at different values of A. The 2C mode of vortex shedding was obtained in the present study. It is important to point out that this mode has not been observed by other investigators for rotationally oscillating case. Also, it is verified that this mechanism has great influence on the drag coefficient for high frequency values. Furthermore, the lift and pressure coefficients and the power spectra density are also analyzed.
基金Projects supported by the National Natural Science Foundation of China(Grant Nos.51909160,51879159)the National Key Research and Development Program of China(Grant Nos.2019YFB1704200,2019YFC0312400).
文摘Recently,the Liutex-based force field models are proposed and applied to some two-dimensional flows to explore the possibility of direct vortex control strategies.In this paper,we applied the Liutex-based centripetal force model to three-dimensional flow around a circular cylinder at Re=3900.The turbulent flows around the three-dimensional cylinder are modeled by the delayed detached-eddy simulations based on the two-equation k-ω shear stress transport(SST)model.The cylinder flow without adding the force field model is simulated in the first place.The statistical flow quantities are compared with experimental data to validate the accuracy of the current numerical models.Then the force field model is applied to the momentum equation in a specific control region near the cylinder to study the change of flow field and hydrodynamic performance.The effect of different control region locations with the same force field strength is discussed to find out the most influenced locations on the drag and lift.Based on the screened control region,the influence source term strength is further studied.Variations on the global statistical flow quantities,local instantaneous three-dimensional flow structures are analyzed and discussed.It is concluded that the Liutex-based force field model can effectively change the hydrodynamic behavior of flow past a cylinder and may provide a new direction for performance improvement and optimization.
基金support for this research gratefully(No.MS20191054)the Faculty Research Grant(CTRG19/SEPS/9 CTRG19/SEPS/15),North South University(NSU),Dhaka,Bangladesh.
文摘This work aims to find the origin and connection of the surface,near-wake,and far-wake structures in the flow encompassing a high-rise building for a high Reynolds number.The origin and interconnection of the stream-wise tip vortices,with the other components of the wake,is analysed in this study for the current scenario.The Unsteady Reynolds Averaged Navier-Stokes equations(URANS)together with the realizable k-εturbulence model have been used in this investigation to study the turbulent wake flow following a ground-surface-attached square shape building.A moderately big obstacle aspect ratio of 4,a Reynolds number of 12,000,and a thin evolving boundary layer thickness have been used in the flow modeling.The designed flow addresses the reversed-flows at the outlet during computation to improve the accuracy of the realizable k-e model.The Reynolds stress components are retrieved using the Boussinesq approach.The wake’s principal compositions,including span-wise-side eddies and area of high stream-wise vorticity in the uppermost portion of the wake,are illustrated by both three-dimensional(3D)representations and planner projections of the mean flow distributions.A braided vortex formation,composed of asymmetric hairpin vortexes,is witnessed in the far-wake area.The association of the near-wake vortex structures with the far-wake and near-wall flow,which is associated with the flow strengths,is also discussed.In this investigation,few areas of large stream-wise vorticity magnitude,like tip vortexes,are correlated to the 3D curving of the fluid motion,and tip vortices did not continuously reach to the free end part of the building.The 3D fluid motion interpretation,which combined several measurements of the flow distribution encompassing the cylinder,shows that the time-averaged near-wake structures are formed of two segments of distinct source and section of dominance.Furthermore,addressing reversed-flow during computation shows notable improvement in the results.
文摘Understanding and control of wake vortices past a circular cylinder is a cardinal problem of interest to ocean engineering.The wake formation and vortex shedding behind a variety of ocean structures such as spars,are subjected to fatigue failure limiting their life span.The additional influences due to ocean waves and currents further exacerbate these effects.In the present study,flow past an isolated circu-lar cylindrical structure subjected to an oscillatory upstream are numerically investigated.These studies involve high resolution simulations over the low Reynolds number range(100-200).Although the prac-tical range of interest is in high Reynolds number range of 103-105,the flow physics and a number of qualitative and quantitative aspects are similar to the low Reynolds number flows.In the high Reynolds number range,statistical averaging tools in conjunction with suitable closure models would be neces-sary.The control of wake vortices is achieved with the aid of two small rotors located in the aft of the main cylinder.A control algorithm was coupled to determine the quantum of actuation to the rotating elements.Although control of wake vortices was observed for harmonic in-let forcing,residual vortical structures were found to persist at higher amplitudes of oscillation.To study the efficacy of this control,numerical simulations were further extended,when the circular cylinder was flexibly mounted.The con-trol of flow induced vibrations was observed to be reasonably effective in controlling the wake generated behind the main cylinder due to oscillatory upstream.
文摘A numerical investigation is carried out for the different orientations of a circular cylinder where the upstream cylinder moves with varying anglesα=0˚,30˚,45˚,60˚,90˚with respect to the downstream cylinder in a fixed position at a gap ratio(L/D)of 3 where L is the distance from centre to centre between cylinders and D is the diameter of the cylinder.Reynolds number(Re),is based on cylinder diameter,is kept constant at 200 for all the cases with air as the working medium.The vortex shedding formation is analysed when the vortices from the upwind cylinder interact with the downwind cylinder and give impressive flow patterns.It is observed that the drag and lift coefficients increase for both the cylinders with change in angular position.Strouhal number is calculated with the help of Fast Fourier transformation(FFT)of vorticity magnitude and evaluated for each case.It is observed that the effect of upwind cylinder on downwind cylinder is significant forα=30˚,45˚,60˚orientation cases and vortex shedding frequency increases for these cases.Further,the investigation is extending on the laminar forced convection heat transfer performance and variation of Nusselt number for the cylinders.Maximum heat transfer is observed for the downwind cylinder atα=45˚.In contrast,the local Nusselt number does not vary much for the cylinders except for the downwind cylinder atα=0˚.
