A flow past a circular-section cylinder with a perforated conic shroud, in which the perforation is located at the peak of the conic disturbance as the shroud installed on the cylinder and uniformly distributed with s...A flow past a circular-section cylinder with a perforated conic shroud, in which the perforation is located at the peak of the conic disturbance as the shroud installed on the cylinder and uniformly distributed with several circular holes, is numerically simulated at a Reynolds number of 100. Two factors in the perforation are taken into account, i.e. the attack angle relative to the direction of incoming flow and diameter of holes. The effect of such perforation on the drag, lift and vortex-shedding frequency is mainly investigated. Results have shown that variation of the attack angle has a little effect, especially on the drag and vortex-shedding frequency, except in certain cases due to the varied vortex-shedding patterns in the near wake. The increasing hole diameter still exhibits a little effect on the drag and frequency of vortex shedding, but really reduces the lift, in particular at larger wavelength, such as the lift reduction reaching almost 66% 68% after introducing the perforation.展开更多
Leeward of natural elevations, like mountains and hills, the air flow becomes turbulent and often times damaging and hazardous to aviation and downwind populations. There is currently a trend for massive construction ...Leeward of natural elevations, like mountains and hills, the air flow becomes turbulent and often times damaging and hazardous to aviation and downwind populations. There is currently a trend for massive construction projects, the result of which are megastructures that behave similarly to these natural elevations and create analogous turbulence conditions. Examples five mega projects were analyzed, and it was estimated that the Reynolds number variation in these buildings, is from 6.10g and 7.109, for winds between 10 m/s and 50 m/s. In this work, the authors present a first numerical approach to this phenomenon by calculating the Strouhal numbers induced by winds blowing against large-volume bodies, in the range of high Reynolds numbers. For this study, satellite images depicting von K^irm^n cloud streets leeward of isolated islands were used. The methodology employed was based on a satellite image where streets watch von K^rnfin vortex, from NOAA-ARL （National Oceanic and Atmospheric Administration-Air Resource Laboratory） prognosis was obtained for a grid point near the island, then determined the inversion layer and meteorological data （wind, temperature and pressure）, was measured from the satellite image the distances separating the vortices to calculate the period, the Reynolds number and Strouhal. The studied results of the cases are displayed graphically, where it is possible to observe a data dispersion as well as a rising trend of the Strouhal number as the Reynolds number increases.展开更多
The supercritical Strouhal and critical Reynolds numbers associated with the drag crisis of circular cylinders in cross-flow are known to be dependent on surface roughness and turbulence levels.To better understand th...The supercritical Strouhal and critical Reynolds numbers associated with the drag crisis of circular cylinders in cross-flow are known to be dependent on surface roughness and turbulence levels.To better understand the role of free-stream turbulence,we consider the traditional model of oscillating lift forcing together with a randomly fluctuating disturbance associated with fluid turbulence.To the extent that these two components are additive,statistically independent,and Gaussian,we are able to derive a closed-form expression describing the relationship between the effective shedding rate and free-stream turbulence.A semi-empirical model of the critical Reynolds number accounting for fluid turbulence is also presented.展开更多
The lattice Boltzmann method(LBM)is employed to simulate flow around two staggered cylinders within a confined channel.The numerical model is validated against existing experimental data by comparing drag coefficients...The lattice Boltzmann method(LBM)is employed to simulate flow around two staggered cylinders within a confined channel.The numerical model is validated against existing experimental data by comparing drag coefficients and Strouhal numbers in the single-cylinder configuration.The study systematically investigates the influence of vertical(h)and horizontal(l)spacing between the cylinders,as well as the Reynolds number(Re=0.1–160),on the hydrodynamic forces,streamline patterns,and vortex dynamics.Results indicate that reducing the horizontal spacing l suppresses flow separation behind the upstream cylinder,while either excessively small or large vertical spacing h diminishes separation in the downstream cylinder.The onset of periodic vortex shedding is delayed due to inter-cylinder interactions,with the critical Reynolds number increasing to Rec=71–112,significantly higher than that of a single-cylinder case(Re_(c)≈69).During the vortex shedding regime,the downstream cylinder exhibits a greater lift force fluctuation compared to the upstream cylinder.At Re=160,the root-mean-square lift coefficient(C′_(L))ranges from approximately 0.17 to 0.56 for the downstream cylinder,and from 0.018 to 0.4 for the upstream one.The shedding frequency,characterized by the Strouhal number(St),increases with Reynolds number,reaching St=0.12–0.18 at Re=160.Variations in h and l significantly influence St,with a decrease in l or an increase in h lowering the shedding frequency—this effect is more pronounced in the horizontal direction.展开更多
Cylindrical cross sections are critical components in offshore structures, including jacket platform legs, pipelines, mooring lines, and risers. Thesecylindrical structures are subjected to vortex-induced vibrations (...Cylindrical cross sections are critical components in offshore structures, including jacket platform legs, pipelines, mooring lines, and risers. Thesecylindrical structures are subjected to vortex-induced vibrations (VIV) due to strong ocean currents, where vortices generated during fluid flowresult in significant vibrations in crossflow and in-flow directions. Such vibrations can lead to severe damage to platforms, cables, and risersystems. Consequently, mitigating VIV caused by vortex-induced forces is important. This study investigates the hydrodynamic performance offive strake models relative to a bare cylinder at moderate Reynolds numbers. The models encompass one conventional continuous helical strake(HS) and four helical discrete strake (HDS) with varying segment spacing between the fins. The hydrodynamic performance, specifically liftand drag force coefficients, was computed using a Reynolds averaged Navier –Stokes-based CFD solver and validated with experimentalmeasurements. The conventional HS suppresses 95% of the lift force but increases the drag force by up to a maximum of 48% in measurements.The HDS suppress the lift force by 70%–88% and increase the drag force by 15%–30%, which is less than the increase observed with the HS.Flow visualization showed that HS and HDS cylinders mitigate vortex-induced forces by altering the vortex-shedding pattern along the length ofthe cylinder. The HDS achieves a reduction in drag compared with the conventional continuous HS. The segment spacing is found to significantlyimpact the reduction in vortex-induced forces.展开更多
基金supported by the National Key Scientific Instrument and Equipment Development Program of China(Grant No.2011YQ120048)
文摘A flow past a circular-section cylinder with a perforated conic shroud, in which the perforation is located at the peak of the conic disturbance as the shroud installed on the cylinder and uniformly distributed with several circular holes, is numerically simulated at a Reynolds number of 100. Two factors in the perforation are taken into account, i.e. the attack angle relative to the direction of incoming flow and diameter of holes. The effect of such perforation on the drag, lift and vortex-shedding frequency is mainly investigated. Results have shown that variation of the attack angle has a little effect, especially on the drag and vortex-shedding frequency, except in certain cases due to the varied vortex-shedding patterns in the near wake. The increasing hole diameter still exhibits a little effect on the drag and frequency of vortex shedding, but really reduces the lift, in particular at larger wavelength, such as the lift reduction reaching almost 66% 68% after introducing the perforation.
文摘Leeward of natural elevations, like mountains and hills, the air flow becomes turbulent and often times damaging and hazardous to aviation and downwind populations. There is currently a trend for massive construction projects, the result of which are megastructures that behave similarly to these natural elevations and create analogous turbulence conditions. Examples five mega projects were analyzed, and it was estimated that the Reynolds number variation in these buildings, is from 6.10g and 7.109, for winds between 10 m/s and 50 m/s. In this work, the authors present a first numerical approach to this phenomenon by calculating the Strouhal numbers induced by winds blowing against large-volume bodies, in the range of high Reynolds numbers. For this study, satellite images depicting von K^irm^n cloud streets leeward of isolated islands were used. The methodology employed was based on a satellite image where streets watch von K^rnfin vortex, from NOAA-ARL （National Oceanic and Atmospheric Administration-Air Resource Laboratory） prognosis was obtained for a grid point near the island, then determined the inversion layer and meteorological data （wind, temperature and pressure）, was measured from the satellite image the distances separating the vortices to calculate the period, the Reynolds number and Strouhal. The studied results of the cases are displayed graphically, where it is possible to observe a data dispersion as well as a rising trend of the Strouhal number as the Reynolds number increases.
文摘The supercritical Strouhal and critical Reynolds numbers associated with the drag crisis of circular cylinders in cross-flow are known to be dependent on surface roughness and turbulence levels.To better understand the role of free-stream turbulence,we consider the traditional model of oscillating lift forcing together with a randomly fluctuating disturbance associated with fluid turbulence.To the extent that these two components are additive,statistically independent,and Gaussian,we are able to derive a closed-form expression describing the relationship between the effective shedding rate and free-stream turbulence.A semi-empirical model of the critical Reynolds number accounting for fluid turbulence is also presented.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.12372251 and 12132015).
文摘The lattice Boltzmann method(LBM)is employed to simulate flow around two staggered cylinders within a confined channel.The numerical model is validated against existing experimental data by comparing drag coefficients and Strouhal numbers in the single-cylinder configuration.The study systematically investigates the influence of vertical(h)and horizontal(l)spacing between the cylinders,as well as the Reynolds number(Re=0.1–160),on the hydrodynamic forces,streamline patterns,and vortex dynamics.Results indicate that reducing the horizontal spacing l suppresses flow separation behind the upstream cylinder,while either excessively small or large vertical spacing h diminishes separation in the downstream cylinder.The onset of periodic vortex shedding is delayed due to inter-cylinder interactions,with the critical Reynolds number increasing to Rec=71–112,significantly higher than that of a single-cylinder case(Re_(c)≈69).During the vortex shedding regime,the downstream cylinder exhibits a greater lift force fluctuation compared to the upstream cylinder.At Re=160,the root-mean-square lift coefficient(C′_(L))ranges from approximately 0.17 to 0.56 for the downstream cylinder,and from 0.018 to 0.4 for the upstream one.The shedding frequency,characterized by the Strouhal number(St),increases with Reynolds number,reaching St=0.12–0.18 at Re=160.Variations in h and l significantly influence St,with a decrease in l or an increase in h lowering the shedding frequency—this effect is more pronounced in the horizontal direction.
文摘Cylindrical cross sections are critical components in offshore structures, including jacket platform legs, pipelines, mooring lines, and risers. Thesecylindrical structures are subjected to vortex-induced vibrations (VIV) due to strong ocean currents, where vortices generated during fluid flowresult in significant vibrations in crossflow and in-flow directions. Such vibrations can lead to severe damage to platforms, cables, and risersystems. Consequently, mitigating VIV caused by vortex-induced forces is important. This study investigates the hydrodynamic performance offive strake models relative to a bare cylinder at moderate Reynolds numbers. The models encompass one conventional continuous helical strake(HS) and four helical discrete strake (HDS) with varying segment spacing between the fins. The hydrodynamic performance, specifically liftand drag force coefficients, was computed using a Reynolds averaged Navier –Stokes-based CFD solver and validated with experimentalmeasurements. The conventional HS suppresses 95% of the lift force but increases the drag force by up to a maximum of 48% in measurements.The HDS suppress the lift force by 70%–88% and increase the drag force by 15%–30%, which is less than the increase observed with the HS.Flow visualization showed that HS and HDS cylinders mitigate vortex-induced forces by altering the vortex-shedding pattern along the length ofthe cylinder. The HDS achieves a reduction in drag compared with the conventional continuous HS. The segment spacing is found to significantlyimpact the reduction in vortex-induced forces.
