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
As reported in a previous work by Lin et al.(Acta Meeh Sin, 2018. https://doi.org/10.1007/s 10409-018-0758-z), an interesting phenomenon was discovered based on the analysis of wavy vortex and vorticity distribution i...As reported in a previous work by Lin et al.(Acta Meeh Sin, 2018. https://doi.org/10.1007/s 10409-018-0758-z), an interesting phenomenon was discovered based on the analysis of wavy vortex and vorticity distribution in the shear layers and near wake of a peak-perforated conic shroud, and two sign laws were summarized. In the present paper, the theory of a vortex-induced vortex is introduced to explore mechanisms in a wavy vortex and applicable sign laws for uniform and incompressible flow past a fixed bluff body. Based on the analysis of the nearest-wall How, two vortex-induced models for streamwise and vertical vortex pairs, respectively, are proposed under two boundary cases, denoting the induced vorticity introduced or distributed on and near the walls. As a result, the first sign law, for only streamwise and vertical components of vorticity, and the second sign law, for three components of vorticity, are obtained under their own particular conditions. The first sign law reveals the intrinsic physical relationship between streamwise and vertical vorticities, independent of the distribution of spanwise vortices in the whole flow field. It is also confirmed that the spanwise vortices, as well as the shear layers and wake width, distributed wavily across the span, are attributed to the introduced streamwise or vertical vortices. The two sign laws for vorticity are independent of the disturbed spanwise wavelength and the Reynolds number. Through the analysis of flow past the conic shroud, the two sign laws are successfully used to summarize typical spacial distributions of vorticity in three flow regions: on and near the front cylinder surfaces, the separated shear layers and the near wake.展开更多
In the present paper,the spatio-temporal evolution of vorticity in the first wake instability,i.e.,(pure)mode A,is investigated in order to understand the wake vortex dynamics and sign relationships among vorticity co...In the present paper,the spatio-temporal evolution of vorticity in the first wake instability,i.e.,(pure)mode A,is investigated in order to understand the wake vortex dynamics and sign relationships among vorticity components.Direct numerical simulation(DNS)for the flow past a circular cylinder is performed,typically at a Reynolds number of 200,in the three-dimensional(3-D)wake transition.According to characteristics of time histories of fluid forces,three different stages are identified as the computational transition,the initial stage and fully developed wake.In the second initial stage,the original two-dimensional spanwise vortices become obviously three-dimensional associated with the streamwise or vertical vorticity intensified up to about 0.1.As a matter of fact,these additional vorticities,caused by the intrinsic 3-D instability,are already generated firstly on cylinder surfaces early in the computational transition,indicating that the three-dimensionality appeared early near the cylinder.The evolution of additional components of vorticity with features the same as mode A shows that(pure)mode A can be already formed in the late computational transition.Through careful analysis of the vorticity field on the front surface,in the shear layers and near wake at typical times,two sign laws are obtained.They illustrate intrinsic relationships among three vorticity components,irrelevant to the wavelength or Fourier mode and Reynolds number in(pure)mode A.Most importantly,the origin of streamwise vortices is found and explained by a new physical mechanism based on the theory of vortex-induced vortex.As a result,the whole process of formation and shedding vortices with these vorticities is firstly and completely illustrated.Other characteristics are presented in detail.展开更多
In order to find the intrinsic physical mechanism of the original Kármán vortex wavily distorted across the span due to the introduction of three-dimensional (3-D) geometric disturbances,a flow past a peak-p...In order to find the intrinsic physical mechanism of the original Kármán vortex wavily distorted across the span due to the introduction of three-dimensional (3-D) geometric disturbances,a flow past a peak-perforated conic shroud is numerically simulated at a Reynolds number of 100.Based on previous work by Meiburg and Lasheras (1988),the streamwise and vertical interactions with spanwise vortices are introduced and analyzed.Then vortex-shedding patterns in the near wake for different flow regimes are reinspected and illustrated from the view of these two interactions.Generally,in regime Ⅰ,spanwise vortices are a little distorted due to the weak interaction.Then in regime Ⅱ,spanwise vortices,even though curved obviously,are still shed synchronously with moderate streamwise and vertical interactions.But in regime Ⅲ,violently wavy spanwise vortices in some vortex-shedding patterns,typically an Ω-type vortex,are mainly attributed to the strong vertical interactions,while other cases,such as multiple vortex-shedding patterns in sub-regime Ⅲ-D,are resulted from complex streamwise and vertical interactions.A special phenomenon,spacial distribution of streamwise and vertical components of vorticity with specific signs in the near wake,is analyzed based on two models of streamwise and vertical vortices in explaining physical reasons of top and bottom shear layers wavily varied across the span.Then these two models and above two interactions are unified.Finally two sign laws are summarized:the first sign law for streamwise and vertical components of vorticity is positive in the upper shear layer,but negative in the lower shear layer,while the second sign law for three vorticity components is always negative in the wake.展开更多
The intrinsic physical relationship of vorticity between modes A and B in the three-dimensional wake transition is investigated.Direct numerical simulations for the flow past a square-section cylinder are carried out ...The intrinsic physical relationship of vorticity between modes A and B in the three-dimensional wake transition is investigated.Direct numerical simulations for the flow past a square-section cylinder are carried out at Reynolds numbers of 180 and 250,associated with modes A and B,respectively.Based on the analysis of spacial distributions of vorticity in the near wake,characteristics of the vertical vorticity in modes A and B are identified.Moreover,the relationship of three vorticity components with specific signs is summarized into two sign laws,as intrinsic physical relationships between two instability modes.By the theory of vortex-induced vortex,such two sign laws confirm that there are two and only two kinds of vortex-shedding patterns in the near wake,just corresponding to modes A and B.In brief,along the free stream direction,mode A can be described by the parallel shedding vertical vortices with the same sign,while mode B is described by the parallel shedding streamwise vortices with the same sign.Finally,it is found out that the|-type vortex is a basic kind of vortex structure in both modes A and B.展开更多
The specific sign of Reynolds stress in the boundary layer on a flat plate at zero incidence is newly interpreted in present paper based on the theory of vortex-induced vortex. It avoids some problems appeared in a tr...The specific sign of Reynolds stress in the boundary layer on a flat plate at zero incidence is newly interpreted in present paper based on the theory of vortex-induced vortex. It avoids some problems appeared in a traditional explanation, on the basis of relationship between mean and fluctuating flows due to the transport of momentum. Through the analysis of local flow field in the immediate neighborhood of wall, the characteristics of Reynolds stress are identified through introducing turbulence-induced small-scale streamwise eddies above the flat plate. The positive Reynolds stress is theoretically verified. And such new interpretation illustrates that the generation of Reynolds stress, as well as fluctuating velocity, is intrinsically independent of the mean flow. But its specific sign would be determined by the mean flow due to the inertial forces. Other features,such as the intensity relationship among three components of fluctuating velocity, are also presented.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.
文摘As reported in a previous work by Lin et al.(Acta Meeh Sin, 2018. https://doi.org/10.1007/s 10409-018-0758-z), an interesting phenomenon was discovered based on the analysis of wavy vortex and vorticity distribution in the shear layers and near wake of a peak-perforated conic shroud, and two sign laws were summarized. In the present paper, the theory of a vortex-induced vortex is introduced to explore mechanisms in a wavy vortex and applicable sign laws for uniform and incompressible flow past a fixed bluff body. Based on the analysis of the nearest-wall How, two vortex-induced models for streamwise and vertical vortex pairs, respectively, are proposed under two boundary cases, denoting the induced vorticity introduced or distributed on and near the walls. As a result, the first sign law, for only streamwise and vertical components of vorticity, and the second sign law, for three components of vorticity, are obtained under their own particular conditions. The first sign law reveals the intrinsic physical relationship between streamwise and vertical vorticities, independent of the distribution of spanwise vortices in the whole flow field. It is also confirmed that the spanwise vortices, as well as the shear layers and wake width, distributed wavily across the span, are attributed to the introduced streamwise or vertical vortices. The two sign laws for vorticity are independent of the disturbed spanwise wavelength and the Reynolds number. Through the analysis of flow past the conic shroud, the two sign laws are successfully used to summarize typical spacial distributions of vorticity in three flow regions: on and near the front cylinder surfaces, the separated shear layers and the near wake.
文摘In the present paper,the spatio-temporal evolution of vorticity in the first wake instability,i.e.,(pure)mode A,is investigated in order to understand the wake vortex dynamics and sign relationships among vorticity components.Direct numerical simulation(DNS)for the flow past a circular cylinder is performed,typically at a Reynolds number of 200,in the three-dimensional(3-D)wake transition.According to characteristics of time histories of fluid forces,three different stages are identified as the computational transition,the initial stage and fully developed wake.In the second initial stage,the original two-dimensional spanwise vortices become obviously three-dimensional associated with the streamwise or vertical vorticity intensified up to about 0.1.As a matter of fact,these additional vorticities,caused by the intrinsic 3-D instability,are already generated firstly on cylinder surfaces early in the computational transition,indicating that the three-dimensionality appeared early near the cylinder.The evolution of additional components of vorticity with features the same as mode A shows that(pure)mode A can be already formed in the late computational transition.Through careful analysis of the vorticity field on the front surface,in the shear layers and near wake at typical times,two sign laws are obtained.They illustrate intrinsic relationships among three vorticity components,irrelevant to the wavelength or Fourier mode and Reynolds number in(pure)mode A.Most importantly,the origin of streamwise vortices is found and explained by a new physical mechanism based on the theory of vortex-induced vortex.As a result,the whole process of formation and shedding vortices with these vorticities is firstly and completely illustrated.Other characteristics are presented in detail.
文摘In order to find the intrinsic physical mechanism of the original Kármán vortex wavily distorted across the span due to the introduction of three-dimensional (3-D) geometric disturbances,a flow past a peak-perforated conic shroud is numerically simulated at a Reynolds number of 100.Based on previous work by Meiburg and Lasheras (1988),the streamwise and vertical interactions with spanwise vortices are introduced and analyzed.Then vortex-shedding patterns in the near wake for different flow regimes are reinspected and illustrated from the view of these two interactions.Generally,in regime Ⅰ,spanwise vortices are a little distorted due to the weak interaction.Then in regime Ⅱ,spanwise vortices,even though curved obviously,are still shed synchronously with moderate streamwise and vertical interactions.But in regime Ⅲ,violently wavy spanwise vortices in some vortex-shedding patterns,typically an Ω-type vortex,are mainly attributed to the strong vertical interactions,while other cases,such as multiple vortex-shedding patterns in sub-regime Ⅲ-D,are resulted from complex streamwise and vertical interactions.A special phenomenon,spacial distribution of streamwise and vertical components of vorticity with specific signs in the near wake,is analyzed based on two models of streamwise and vertical vortices in explaining physical reasons of top and bottom shear layers wavily varied across the span.Then these two models and above two interactions are unified.Finally two sign laws are summarized:the first sign law for streamwise and vertical components of vorticity is positive in the upper shear layer,but negative in the lower shear layer,while the second sign law for three vorticity components is always negative in the wake.
基金financially supported by the Strategic Priority Research Program of the Chinese Academy of Science (Grant XDB22030101)
文摘The intrinsic physical relationship of vorticity between modes A and B in the three-dimensional wake transition is investigated.Direct numerical simulations for the flow past a square-section cylinder are carried out at Reynolds numbers of 180 and 250,associated with modes A and B,respectively.Based on the analysis of spacial distributions of vorticity in the near wake,characteristics of the vertical vorticity in modes A and B are identified.Moreover,the relationship of three vorticity components with specific signs is summarized into two sign laws,as intrinsic physical relationships between two instability modes.By the theory of vortex-induced vortex,such two sign laws confirm that there are two and only two kinds of vortex-shedding patterns in the near wake,just corresponding to modes A and B.In brief,along the free stream direction,mode A can be described by the parallel shedding vertical vortices with the same sign,while mode B is described by the parallel shedding streamwise vortices with the same sign.Finally,it is found out that the|-type vortex is a basic kind of vortex structure in both modes A and B.
基金financially supported by the Strategic Priority Research Program of the Chinese Academy of Science (Grant XDB22030101)
文摘The specific sign of Reynolds stress in the boundary layer on a flat plate at zero incidence is newly interpreted in present paper based on the theory of vortex-induced vortex. It avoids some problems appeared in a traditional explanation, on the basis of relationship between mean and fluctuating flows due to the transport of momentum. Through the analysis of local flow field in the immediate neighborhood of wall, the characteristics of Reynolds stress are identified through introducing turbulence-induced small-scale streamwise eddies above the flat plate. The positive Reynolds stress is theoretically verified. And such new interpretation illustrates that the generation of Reynolds stress, as well as fluctuating velocity, is intrinsically independent of the mean flow. But its specific sign would be determined by the mean flow due to the inertial forces. Other features,such as the intensity relationship among three components of fluctuating velocity, are also presented.
