The flow past a circular-section cylinder with a conic shroud perforated with four holes at the peak was simulated numerically at Re = 100, considering two factors, viz.the angle of attack and the diameter of the hole...The flow past a circular-section cylinder with a conic shroud perforated with four holes at the peak was simulated numerically at Re = 100, considering two factors, viz.the angle of attack and the diameter of the holes. The effects of the perforated conic shroud on the vortex shedding pattern in the near wake was mainly investigated, as well as the time history of the drag and lift forces. In the investigated parameter space, three flow regimes were generally identified, corresponding to weak, moderate, and strong disturbance effects.In regime I, the wake can mainly be described by alternately shedding Kármán or Kármán-like vortices. In regime II, the spanwise vortices are obviously disturbed along the span due to the appearance of additional vorticity components and their interactions with the spanwise vortices, but still shed in synchronization along the spanwise direction. In regime III,the typical Kármán vortices partially or totally disappear,and some new vortex shedding patterns appear, such as-type, obliquely shedding, and crossed spanwise vortices with opposite sign. Corresponding to these complex vortex shedding patterns in the near wake, the fluid forces no longer oscillate regularly at a single vortex shedding frequency, but rather with a lower modulation frequency and multiple amplitudes. An overview of these flow regimes is presented.展开更多
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 Ho...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
An attempt has been made to explore whether the power relation can be obtained from theoretical considerations. The classical laminar and turbulent boundary layer concepts have been employed to determine appropriate v...An attempt has been made to explore whether the power relation can be obtained from theoretical considerations. The classical laminar and turbulent boundary layer concepts have been employed to determine appropriate values of the scaling lengths associated with vortex shedding and shear layer frequencies to predict the power law relationship with Reynolds number. The predicted results are in good agreement with experimental results. The findings will provide a greater insight into the overall phenomenon involved.展开更多
A comprehensive hot wire investigation of the flow around a circular cylinder is carried out in an 18" × 18" wind tunnel to look into the dominant frequencies at the stagnation, separation and separated shear l...A comprehensive hot wire investigation of the flow around a circular cylinder is carried out in an 18" × 18" wind tunnel to look into the dominant frequencies at the stagnation, separation and separated shear layers in the transition Reynolds number range. The majority of the experiments are carried out at Reynolds number of 4.5×104, with additional transition frequency tests at Reynolds numbers of 2.9×104, 3.3×104 and 9.7×104 respectively. The results are analysed in terms of power spectral density. While the frequency associated with stagnation is found to be essentially due to vortex shedding, frequency doubling of vortex shedding is also evident in the separated shear layers. Two peaks associated with transition frequencies are detected and their possible implications are presented.展开更多
The present study aims at the investigation of the effects of turbulence-chemistry interaction on combus- tion instabilities using a probability density function (PDF) method. The instantaneous quantities in the flo...The present study aims at the investigation of the effects of turbulence-chemistry interaction on combus- tion instabilities using a probability density function (PDF) method. The instantaneous quantities in the flow field were decomposed into the Favre-averaged variables and the stochastic fluctuations, which were calculated by unsteady Reynolds averaged Navier-Stokes (U-RANS) equations and the PDF model, respectively. A joint fluctuating velocity- frequency-composition PDF was used. The governing equa- tions are solved by a consistent hybrid finite volume/Monte- Carlo algorithm on triangular unstructured meshes. A non- reacting flow behind a triangular-shaped bluff body flame stabilizer in a rectilinear combustor was simulated by the present method. The results demonstrate the capability of the present method to capture the large-scale coherent struc- tures. The triple decomposition was performed, by divid- ing the coherent Favre-averaged velocity into time-averaged value and periodical coherent part, to analyze the coherent and incoherent contributions to Reynolds stresses. A sim- ple modification to the coefficients in the turbulent frequency model will help to improve the simulation results. Unsteady flow fields were depicted by streamlines and vorticity con- tours. Moreover, the association between turbulence produc- tion and vorticity saddle points is illustrated.展开更多
The suppression method of vortex shedding from a circular cylinder has been studied experimentally in the Reynolds number range from 300 to 1600. The test is performed in a water channel. The model cylinder is 1 cm in...The suppression method of vortex shedding from a circular cylinder has been studied experimentally in the Reynolds number range from 300 to 1600. The test is performed in a water channel. The model cylinder is 1 cm in diameter and 38 cm in length. A row of small rods of 0.18 cm in diameter and 1.5 cm in length are perpendicularly connected to the surface of the model cylinder and distributed along the meridian, The distance between the neighboring rods and the angle of attack of the rods can be changed so that the suppression effect on vortex shedding can be adjusted. The results show that vortex shedding can be suppressed effectively if the distance between the neighboring rods is smaller than 3 times and the cylinder diameter and the angle of attack is in the range of 30degreesless than or equal tobeta<90&DEG;.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
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
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
In this study, the vortex-induced vibrations of a cylinder near a rigid plane boundary in a steady flow are studied experimentally. The phenomenon of vortex-induced vibrations of the cylinder near the rigid plane boun...In this study, the vortex-induced vibrations of a cylinder near a rigid plane boundary in a steady flow are studied experimentally. The phenomenon of vortex-induced vibrations of the cylinder near the rigid plane boundary is reproduced in the flume. The vortex shedding frequency and mode are also measured by the methods of hot film velocimeter and hydrogen bubbles. A parametric study is carded out to investigate the influences of reduced velocity, gap-to-diameter ratio, stability parameter and mass ratio on the amplitude and frequency responses of the cylinder. Experimental results indicate: (1) the Strouhal number (St) is around 0.2 for the stationary cylinder near a plane boundary in the sub-criti- cal flow regime; (2) with increasing gap-to-diameter ratio (eo/D), the amplitude ratio (A/D) gets larger but frequency ratio (f/fn) has a slight variation for the case of larger values of eo/D(eo/D 〉 0.66 in this study); (3) there is a clear difference of amplitude and frequency responses of the cylin- derbetween the larger gap-to-diameter ratios (e0/D 〉 0.66) and the smaller ones (e0/D 〈 0.3); (4) the vibration of the cylinder is easier to occur and the range of vibration in terms of Vr number becomes more extensive with decrease of the stability parameter, but the frequency response is affected slightly by the stability parameter; (5) with decreasing mass ratio, the width of the lock-in ranges in terms of Vr and the frequency ratio (f/fn) become larger.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
Flow visualization was used to investigate experimentally the evolution process from symmetrical shedding to staggered shedding of the starting vortex and the phenomenon of secondary separation on an elliptic cylinder...Flow visualization was used to investigate experimentally the evolution process from symmetrical shedding to staggered shedding of the starting vortex and the phenomenon of secondary separation on an elliptic cylinder at moderate Reynolds numbers.The vortex structure of the flow separation was studied.The temporal variation of separation angle and length of wake vortex were given. The photographs and experimental results provided basis for further investigation of the complicated feature of the starting process of unsteady separated flows around an elliptic cylinder.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 propulsive performance and vortex shedding of oscillating foil, whichmimics biological locomotion, were numerically investigated. The objectives of this study were todeal with unsteady force, in particular thrust ...The propulsive performance and vortex shedding of oscillating foil, whichmimics biological locomotion, were numerically investigated. The objectives of this study were todeal with unsteady force, in particular thrust force, exerted on the foil in pitching and plungingmotions, and to explore the relation of the propulsive performance with vortex structures near thefoil and vortex shedding in the near wake. The two-dimensional incompressible Navier-Stokesequations in the vorticity and stream-function formulation were solved by fourth-order essentiallycompact finite difference schemes for the space derivatives and a fourth-order Runge-Kutta schemefor the time advancement. To reveal the mechanism of the propulsive performance, the unsteady forceand the shedding of the trailing- and leading-edge vortices of the foil were analyzed. The effectsof some typical factors, such as the frequency and amplitude of the oscillation, the phasedifference between the pitching and plunging motions, and the thickness ratio of the foil, on thevortex shedding and unsteady force were discussed.展开更多
In this work, a three-dimensional (3D) Computational Fluid Dynamics (CFD) model was built to simulate the tail fin motion of a fish robot actuated by a piezoceramic composite actuator, and to determine the maximum...In this work, a three-dimensional (3D) Computational Fluid Dynamics (CFD) model was built to simulate the tail fin motion of a fish robot actuated by a piezoceramic composite actuator, and to determine the maximum thrust tail-beat frequency. A simulation of the tail fin at a tail-beat frequency was performed to confirm measured thrust data from a previous study. The computed and measured thrusts were in good agreement. A series of thrust simulations were conducted for various tail-beat frequencies to confirm the maximum thrust frequency that was obtained from thrust measurements in the previous study. The largest thrust was calculated at a tail-beat frequency of 3.7 Hz and vortices around the tail were fully separated. The calculated maximum thrust tail-beat frequency was in good agreement with the measured frequency. Flow characteristics during tail fin motion were examined to explain why the largest thrust occurred at this particular tail-beat frequency.展开更多
The Xu & Yan scale-adaptive simulation (XYSAS) model is employed to simulate the flows past wavy cylinders at Reynolds number 8 × 10 3.This approach yields results in good agreement with experimental measureme...The Xu & Yan scale-adaptive simulation (XYSAS) model is employed to simulate the flows past wavy cylinders at Reynolds number 8 × 10 3.This approach yields results in good agreement with experimental measurements.The mean flow field and near wake vortex structure are replicated and compared with that of a corresponding circular cylinder.The effects of wavelength ratios λ/D m from 3 to 7,together with the amplitude ratios a /D m of 0.091 and 0.25,are fully investigated.Owing to the wavy configuration,a maximum reduction of Strouhal number and root-meansquare (r.m.s) fluctuating lift coefficients are up to 50% and 92%,respectively,which means the vortex induced vibration (VIV) could be effectively alleviated at certain larger values of λ/D m and a /D m.Also,the drag coefficients can be reduced by 30%.It is found that the flow field presents contrary patterns with the increase of λ/D m.The free shear layer becomes much more stable and rolls up into mature vortex only further downstream when λ/D m falls in the range of 5-7.The amplitude ratio a /D m greatly changes the separation line,and subsequently influences the wake structures.展开更多
The objective of this study is to deal with unsteady force acting on a pitching foil in shear flow and to study the relation of the force characteristics with vortex shedding near the foil. The two dimensional incomp...The objective of this study is to deal with unsteady force acting on a pitching foil in shear flow and to study the relation of the force characteristics with vortex shedding near the foil. The two dimensional incompressible Navier Stokes equations in the vorticity and stream function formulation were solved with the fourth order essentially compact finite difference schemes for the space derivatives and a fourth order Runge Kutta scheme for the time advancement. The force characteristics and vortex shedding of the pitching foil in shear flows were investigated. The effects of some typical factors, including the incoming flow shear, the oscillating frequency and amplitude, on the vortex shedding and force behavior were analyzed and discussed.展开更多
Large eddy simulation is performed to study three-dimensional wave-current interaction with a square cylinder at different Reynolds numbers, ranging from 1,000 to 600,000. The Keulegan-Carpenter number is relevantly a...Large eddy simulation is performed to study three-dimensional wave-current interaction with a square cylinder at different Reynolds numbers, ranging from 1,000 to 600,000. The Keulegan-Carpenter number is relevantly a constant of 0.6 for all cases. The Strouhal number, the mean and the RMS values of the effective drag coefficient in the streamwise and transverse directions are computed for various Reynolds numbers, and the velocity of a rep- resentative point in the turbulent zone is simulated to find the turbulent feature. It is found that the wave-current interaction should be considered as three-dimensional flow when the Reynolds number is high; under wave-current effect, there exists a critical Reynolds number, and when the Reynolds number is smaller than the critical one, current effect on wave can be nearly neglected; conversely, with the Reynolds number increasing, wave-currentstructure interaction is sensitive to the Reynolds number.展开更多
基金supported by the National Key Scientific Instrument and Equipment Development Program of China (Grant 2011YQ120048)
文摘The flow past a circular-section cylinder with a conic shroud perforated with four holes at the peak was simulated numerically at Re = 100, considering two factors, viz.the angle of attack and the diameter of the holes. The effects of the perforated conic shroud on the vortex shedding pattern in the near wake was mainly investigated, as well as the time history of the drag and lift forces. In the investigated parameter space, three flow regimes were generally identified, corresponding to weak, moderate, and strong disturbance effects.In regime I, the wake can mainly be described by alternately shedding Kármán or Kármán-like vortices. In regime II, the spanwise vortices are obviously disturbed along the span due to the appearance of additional vorticity components and their interactions with the spanwise vortices, but still shed in synchronization along the spanwise direction. In regime III,the typical Kármán vortices partially or totally disappear,and some new vortex shedding patterns appear, such as-type, obliquely shedding, and crossed spanwise vortices with opposite sign. Corresponding to these complex vortex shedding patterns in the near wake, the fluid forces no longer oscillate regularly at a single vortex shedding frequency, but rather with a lower modulation frequency and multiple amplitudes. An overview of these flow regimes is presented.
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An attempt has been made to explore whether the power relation can be obtained from theoretical considerations. The classical laminar and turbulent boundary layer concepts have been employed to determine appropriate values of the scaling lengths associated with vortex shedding and shear layer frequencies to predict the power law relationship with Reynolds number. The predicted results are in good agreement with experimental results. The findings will provide a greater insight into the overall phenomenon involved.
文摘A comprehensive hot wire investigation of the flow around a circular cylinder is carried out in an 18" × 18" wind tunnel to look into the dominant frequencies at the stagnation, separation and separated shear layers in the transition Reynolds number range. The majority of the experiments are carried out at Reynolds number of 4.5×104, with additional transition frequency tests at Reynolds numbers of 2.9×104, 3.3×104 and 9.7×104 respectively. The results are analysed in terms of power spectral density. While the frequency associated with stagnation is found to be essentially due to vortex shedding, frequency doubling of vortex shedding is also evident in the separated shear layers. Two peaks associated with transition frequencies are detected and their possible implications are presented.
基金supported by the National Natural Science Foundation of China (50936005)the Fundamental Research Fundsfor the Central Universities (WK2090130008,WK2090130011)
文摘The present study aims at the investigation of the effects of turbulence-chemistry interaction on combus- tion instabilities using a probability density function (PDF) method. The instantaneous quantities in the flow field were decomposed into the Favre-averaged variables and the stochastic fluctuations, which were calculated by unsteady Reynolds averaged Navier-Stokes (U-RANS) equations and the PDF model, respectively. A joint fluctuating velocity- frequency-composition PDF was used. The governing equa- tions are solved by a consistent hybrid finite volume/Monte- Carlo algorithm on triangular unstructured meshes. A non- reacting flow behind a triangular-shaped bluff body flame stabilizer in a rectilinear combustor was simulated by the present method. The results demonstrate the capability of the present method to capture the large-scale coherent struc- tures. The triple decomposition was performed, by divid- ing the coherent Favre-averaged velocity into time-averaged value and periodical coherent part, to analyze the coherent and incoherent contributions to Reynolds stresses. A sim- ple modification to the coefficients in the turbulent frequency model will help to improve the simulation results. Unsteady flow fields were depicted by streamlines and vorticity con- tours. Moreover, the association between turbulence produc- tion and vorticity saddle points is illustrated.
基金This study is financially supported by the National Natural Science Foundaion of China(Grant No.10172087)
文摘The suppression method of vortex shedding from a circular cylinder has been studied experimentally in the Reynolds number range from 300 to 1600. The test is performed in a water channel. The model cylinder is 1 cm in diameter and 38 cm in length. A row of small rods of 0.18 cm in diameter and 1.5 cm in length are perpendicularly connected to the surface of the model cylinder and distributed along the meridian, The distance between the neighboring rods and the angle of attack of the rods can be changed so that the suppression effect on vortex shedding can be adjusted. The results show that vortex shedding can be suppressed effectively if the distance between the neighboring rods is smaller than 3 times and the cylinder diameter and the angle of attack is in the range of 30degreesless than or equal tobeta<90&DEG;.
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.
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the National Natural Science Foundation of China (50509022, 10532070)Knowledge Innovation Program of the Chinese Academy of Sciences (KJCX2-YW-L02)
文摘In this study, the vortex-induced vibrations of a cylinder near a rigid plane boundary in a steady flow are studied experimentally. The phenomenon of vortex-induced vibrations of the cylinder near the rigid plane boundary is reproduced in the flume. The vortex shedding frequency and mode are also measured by the methods of hot film velocimeter and hydrogen bubbles. A parametric study is carded out to investigate the influences of reduced velocity, gap-to-diameter ratio, stability parameter and mass ratio on the amplitude and frequency responses of the cylinder. Experimental results indicate: (1) the Strouhal number (St) is around 0.2 for the stationary cylinder near a plane boundary in the sub-criti- cal flow regime; (2) with increasing gap-to-diameter ratio (eo/D), the amplitude ratio (A/D) gets larger but frequency ratio (f/fn) has a slight variation for the case of larger values of eo/D(eo/D 〉 0.66 in this study); (3) there is a clear difference of amplitude and frequency responses of the cylin- derbetween the larger gap-to-diameter ratios (e0/D 〉 0.66) and the smaller ones (e0/D 〈 0.3); (4) the vibration of the cylinder is easier to occur and the range of vibration in terms of Vr number becomes more extensive with decrease of the stability parameter, but the frequency response is affected slightly by the stability parameter; (5) with decreasing mass ratio, the width of the lock-in ranges in terms of Vr and the frequency ratio (f/fn) become larger.
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 project supported by the National Natural Science Foundation of China.
文摘Flow visualization was used to investigate experimentally the evolution process from symmetrical shedding to staggered shedding of the starting vortex and the phenomenon of secondary separation on an elliptic cylinder at moderate Reynolds numbers.The vortex structure of the flow separation was studied.The temporal variation of separation angle and length of wake vortex were given. The photographs and experimental results provided basis for further investigation of the complicated feature of the starting process of unsteady separated flows around an elliptic cylinder.
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 propulsive performance and vortex shedding of oscillating foil, whichmimics biological locomotion, were numerically investigated. The objectives of this study were todeal with unsteady force, in particular thrust force, exerted on the foil in pitching and plungingmotions, and to explore the relation of the propulsive performance with vortex structures near thefoil and vortex shedding in the near wake. The two-dimensional incompressible Navier-Stokesequations in the vorticity and stream-function formulation were solved by fourth-order essentiallycompact finite difference schemes for the space derivatives and a fourth-order Runge-Kutta schemefor the time advancement. To reveal the mechanism of the propulsive performance, the unsteady forceand the shedding of the trailing- and leading-edge vortices of the foil were analyzed. The effectsof some typical factors, such as the frequency and amplitude of the oscillation, the phasedifference between the pitching and plunging motions, and the thickness ratio of the foil, on thevortex shedding and unsteady force were discussed.
文摘In this work, a three-dimensional (3D) Computational Fluid Dynamics (CFD) model was built to simulate the tail fin motion of a fish robot actuated by a piezoceramic composite actuator, and to determine the maximum thrust tail-beat frequency. A simulation of the tail fin at a tail-beat frequency was performed to confirm measured thrust data from a previous study. The computed and measured thrusts were in good agreement. A series of thrust simulations were conducted for various tail-beat frequencies to confirm the maximum thrust frequency that was obtained from thrust measurements in the previous study. The largest thrust was calculated at a tail-beat frequency of 3.7 Hz and vortices around the tail were fully separated. The calculated maximum thrust tail-beat frequency was in good agreement with the measured frequency. Flow characteristics during tail fin motion were examined to explain why the largest thrust occurred at this particular tail-beat frequency.
基金supported by the National Basic Research Program of China (2009CB724104)the National Natural Science Foundation of China (90716010)
文摘The Xu & Yan scale-adaptive simulation (XYSAS) model is employed to simulate the flows past wavy cylinders at Reynolds number 8 × 10 3.This approach yields results in good agreement with experimental measurements.The mean flow field and near wake vortex structure are replicated and compared with that of a corresponding circular cylinder.The effects of wavelength ratios λ/D m from 3 to 7,together with the amplitude ratios a /D m of 0.091 and 0.25,are fully investigated.Owing to the wavy configuration,a maximum reduction of Strouhal number and root-meansquare (r.m.s) fluctuating lift coefficients are up to 50% and 92%,respectively,which means the vortex induced vibration (VIV) could be effectively alleviated at certain larger values of λ/D m and a /D m.Also,the drag coefficients can be reduced by 30%.It is found that the flow field presents contrary patterns with the increase of λ/D m.The free shear layer becomes much more stable and rolls up into mature vortex only further downstream when λ/D m falls in the range of 5-7.The amplitude ratio a /D m greatly changes the separation line,and subsequently influences the wake structures.
基金This work was supported by the National Natural Science Foundation of China (Grant Nos: 10332040 10125210+3 种基金10302028) the Innovation Project of the Chinese Academy of Sciences (Grant Nos: KJCX SW L04 KJCX2 SW L2) and the Programme of Hundred Talents of the Chinese Academy of Sciences.
文摘The objective of this study is to deal with unsteady force acting on a pitching foil in shear flow and to study the relation of the force characteristics with vortex shedding near the foil. The two dimensional incompressible Navier Stokes equations in the vorticity and stream function formulation were solved with the fourth order essentially compact finite difference schemes for the space derivatives and a fourth order Runge Kutta scheme for the time advancement. The force characteristics and vortex shedding of the pitching foil in shear flows were investigated. The effects of some typical factors, including the incoming flow shear, the oscillating frequency and amplitude, on the vortex shedding and force behavior were analyzed and discussed.
基金supported by the National Natural Science Foundation of China (No. 51178397)Technological Research and Development Programs of the Ministry of Railways (No. 2010G004-L)
文摘Large eddy simulation is performed to study three-dimensional wave-current interaction with a square cylinder at different Reynolds numbers, ranging from 1,000 to 600,000. The Keulegan-Carpenter number is relevantly a constant of 0.6 for all cases. The Strouhal number, the mean and the RMS values of the effective drag coefficient in the streamwise and transverse directions are computed for various Reynolds numbers, and the velocity of a rep- resentative point in the turbulent zone is simulated to find the turbulent feature. It is found that the wave-current interaction should be considered as three-dimensional flow when the Reynolds number is high; under wave-current effect, there exists a critical Reynolds number, and when the Reynolds number is smaller than the critical one, current effect on wave can be nearly neglected; conversely, with the Reynolds number increasing, wave-currentstructure interaction is sensitive to the Reynolds number.