Showing papers on "Vortex published in 1995"
TL;DR: In this article, the authors propose a definition of vortex in an incompressible flow in terms of the eigenvalues of the symmetric tensor, which captures the pressure minimum in a plane perpendicular to the vortex axis at high Reynolds numbers, and also accurately defines vortex cores at low Reynolds numbers.
Abstract: Considerable confusion surrounds the longstanding question of what constitutes a vortex, especially in a turbulent flow. This question, frequently misunderstood as academic, has recently acquired particular significance since coherent structures (CS) in turbulent flows are now commonly regarded as vortices. An objective definition of a vortex should permit the use of vortex dynamics concepts to educe CS, to explain formation and evolutionary dynamics of CS, to explore the role of CS in turbulence phenomena, and to develop viable turbulence models and control strategies for turbulence phenomena. We propose a definition of a vortex in an incompressible flow in terms of the eigenvalues of the symmetric tensor ${\bm {\cal S}}^2 + {\bm \Omega}^2$ are respectively the symmetric and antisymmetric parts of the velocity gradient tensor ${\bm \Delta}{\bm u}$. This definition captures the pressure minimum in a plane perpendicular to the vortex axis at high Reynolds numbers, and also accurately defines vortex cores at low Reynolds numbers, unlike a pressure-minimum criterion. We compare our definition with prior schemes/definitions using exact and numerical solutions of the Euler and Navier–Stokes equations for a variety of laminar and turbulent flows. In contrast to definitions based on the positive second invariant of ${\bm \Delta}{\bm u}$ or the complex eigenvalues of ${\bm \Delta}{\bm u}$, our definition accurately identifies the vortex core in flows where the vortex geometry is intuitively clear.
5,837 citations
TL;DR: In this paper, a quasi-cyclic and spatially organized process of regeneration of near-wall structures is observed, composed of three distinct phases: formation of streaks by streamwise vortices, breakdown of the streaks, and regeneration of the streamwise Vortices.
Abstract: Direct numerical simulations of a highly constrained plane Couette flow are employed to study the dynamics of the structures found in the near-wall region of turbulent flows. Starting from a fully developed turbulent flow, the dimensions of the computational domain are reduced to near the minimum values which will sustain turbulence. A remarkably well-defined, quasi-cyclic and spatially organized process of regeneration of near-wall structures is observed. This process is composed of three distinct phases: formation of streaks by streamwise vortices, breakdown of the streaks, and regeneration of the streamwise vortices. Each phase sets the stage for the next, and these processes are analysed in detail. The most novel results concern vortex regeneration, which is found to be a direct result of the breakdown of streaks that were originally formed by the vortices, and particular emphasis is placed on this process. The spanwise width of the computational domain corresponds closely to the typically observed spanwise spacing of near-wall streaks. When the width of the domain is further reduced, turbulence is no longer sustained. It is suggested that the observed spacing arises because the time scales of streak formation, breakdown and vortex regeneration become mismatched when the streak spacing is too small, and the regeneration cycle at that scale is broken.
978 citations
TL;DR: In this article, the authors describe the properties of the triangular flux-line lattice (FLL), which is more or less perturbed by material inhomogeneities that pin the flux lines, and also by thermal fluctuations.
Abstract: Magnetic flux can penetrate a type-II superconductor in the form of Abrikosov vortices (also called flux lines, flux tubes, or fluxons) each carrying a quantum of magnetic flux phi 0=h/2e. These tiny vortices of supercurrent tend to arrange themselves in a triangular flux-line lattice (FLL), which is more or less perturbed by material inhomogeneities that pin the flux lines, and in high-Tc superconductors (HTSCs) also by thermal fluctuations. Many properties of the FLL are well described by the phenomenological Ginzburg-Landau theory or by the electromagnetic London theory, which treats the vortex core as a singularity. In Nb alloys and HTSCs the FLL is very soft mainly because of the large magnetic penetration depth lambda . The shear modulus of the FLL is c66~1/ lambda 2, and the tilt modulus c44(k)~(1+k2 lambda 2)-1 is dispersive and becomes very small for short distortion wavelengths 2 pi /k<< lambda . This softness is enhanced further by the pronounced anisotropy and layered structure of HTSCs, which strongly increases the penetration depth for currents along the c axis of these (nearly uniaxial) crystals and may even cause a decoupling of two-dimensional vortex lattices in the Cu-O layers. Thermal fluctuations and softening may `melt` the FLL and cause thermally activated depinning of the flux lines or ofthe two-dimensional `pancake vortices` in the layers. Various phase transitions are predicted for the FLL in layered HTSCs. Although large pinning forces and high critical currents have been achieved, the small depinning energy so far prevents the application of HTSCs as conductors at high temperatures except in cases when the applied current and the surrounding magnetic field are small.
866 citations
TL;DR: In this article, the authors describe the properties of the triangular flux-line lattice (FLL), which is more or less perturbed by material inhomogeneities that pin the flux lines and also by thermal fluctuations.
Abstract: Magnetic flux can penetrate a type-II superconductor in form of Abrikosov vortices. These tend to arrange in a triangular flux-line lattice (FLL) which is more or less perturbed by material inhomogeneities that pin the flux lines, and in high-$T_c$ supercon- ductors (HTSC's) also by thermal fluctuations. Many properties of the FLL are well described by the phenomenological Ginzburg-Landau theory or by the electromagnetic London theory, which treats the vortex core as a singularity. In Nb alloys and HTSC's the FLL is very soft mainly because of the large magnetic penetration depth: The shear modulus of the FLL is thus small and the tilt modulus is dispersive and becomes very small for short distortion wavelength. This softness of the FLL is enhanced further by the pronounced anisotropy and layered structure of HTSC's, which strongly increases the penetration depth for currents along the c-axis of these uniaxial crystals and may even cause a decoupling of two-dimensional vortex lattices in the Cu-O layers. Thermal fluctuations and softening may melt the FLL and cause thermally activated depinning of the flux lines or of the 2D pancake vortices in the layers. Various phase transitions are predicted for the FLL in layered HTSC's. The linear and nonlinear magnetic response of HTSC's gives rise to interesting effects which strongly depend on the geometry of the experiment.
812 citations
TL;DR: In this article, an ensemble-averaged statistics at constant phase of the turbulent near-wake flow (Reynolds number ≈ 21400 around a square cylinder) were obtained from two-component laser-Doppler measurements.
Abstract: Ensemble-averaged statistics at constant phase of the turbulent near-wake flow (Reynolds number ≈ 21400 around a square cylinder have been obtained from two-component laser-Doppler measurements. Phase was defined with reference to a signal taken from a pressure sensor located at the midpoint of a cylinder sidewall. The distinction is drawn between the near wake where the shed vortices are ‘mature’ and distinct and a base region where the vortices grow to maturity and are then shed. Differences in length and velocity scales and vortex celerities between the flow around a square cylinder and the more frequently studied flow around a circular cylinder are discussed. Scaling arguments based on the circulation discharged into the near wake are proposed to explain the differences. The relationship between flow topology and turbulence is also considered with vorticity saddles and streamline saddles being distinguished. While general agreement with previous studies of flow around a circular cylinder is found with regard to essential flow features in the near wake, some previously overlooked details are highlighted, e.g. the possibility of high Reynolds shear stresses in regions of peak vorticity, or asymmetries near the streamline saddle. The base region is examined in more detail than in previous studies, and vorticity saddles, zero-vorticity points, and streamline saddles are observed to differ in importance at different stages of the shedding process.
649 citations
TL;DR: In this article, the near field behavior of a wingtip vortex flow has been studied computationally and experimentally in an interactive fashion, using the method of artificial compressibility to solve the three-dimensional, incompressible, Navier-Stokes equations with experimentally determined boundary conditions and a modified Baldwin-Barth turbulence model.
Abstract: The near-field behavior of a wingtip vortex flow has been studied computationally and experimentally in an interactive fashion. The computational approach involved using the method of artificial compressibility to solve the three-dimensional, incompressible, Navier-Stokes equations with experimentally determined boundary conditions and a modified Baldwin-Barth turbulence model. Inaccuracies caused by the finite difference technique, grid resolution, and turbulence modeling have been explored. The complete geometry case was computed using 1.5 million grid points and compared with mean velocity measurements on the suction side of the wing and in the near wake. Good agreement between the computed and measured flowfields has been obtained. The velocity distribution in the vortex core compares to within 3% of the experiment.
511 citations
TL;DR: The dynamics of quasi-geostrophic flow with uniform potential vorticity reduces to the evolution of buoyancy, or potential temperature, on horizontal boundaries as discussed by the authors, but a different relationship between the flow and the advected scalar creates several distinctive features, such as an elliptical vortex, the start-up vortex shed by flow over a mountain, the instability of temperature filaments, the edge wave critical layer, and mixing in an overturning edge wave.
Abstract: The dynamics of quasi-geostrophic flow with uniform potential vorticity reduces to the evolution of buoyancy, or potential temperature, on horizontal boundaries. There is a formal resemblance to two-dimensional flow, with surface temperature playing the role of vorticity, but a different relationship between the flow and the advected scalar creates several distinctive features. A series of examples are described which highlight some of these features: the evolution of an elliptical vortex; the start-up vortex shed by flow over a mountain; the instability of temperature filaments; the ‘edge wave’ critical layer; and mixing in an overturning edge wave. Characteristics of the direct cascade of the tracer variance to small scales in homogeneous turbulence, as well as the inverse energy cascade, are also described. In addition to its geophysical relevance, the ubiquitous generation of secondary instabilities and the possibility of finite-time collapse make this system a potentially important, numerically tractable, testbed for turbulence theories.
510 citations
TL;DR: In this article, the lattice of magnetic flux lines that can permeate a type ii superconductor, such as the high-transition-temperature copper oxide materials, was found to be first-order.
Abstract: The lattice of magnetic flux lines that can permeate a type ii superconductor, such as the high-transition-temperature copper oxide materials, melts from a solid-like stale to a liquid-like state at a temperature below the superconducting transition temperature. Contrary to the predictions of mean-field theory, this phase transition in Bi2Sr2CaCu2O8 is found to be first-order. The vortex liquid discontinuously expands on freezing.
483 citations
TL;DR: In this article, the initial response of the vortex to vertical shear is to tilt in the plane of the shear, and as soon as a tilt is established, the upper and lower-level centres begin to rotate cyclonically about the mid-level center.
Abstract: The behaviour of initially-barotropic vortices in vertically-sheared environmental flows is investigated. the strength and structure of the vortices used are representative of tropical cyclones. the calculations are performed using a primitive-equation numerical model on an f-plane. It is found that the initial response of the vortex to the vertical shear is to tilt in the plane of the shear. As soon as a tilt is established, the upper- and lower-level centres begin to rotate cyclonically about the mid-level centre. This rotation can be understood in terms of upper- and lower-level potential-vorticity anomalies which are displaced in the horizontal relative to one another. the flow associated with the vertical projection of each anomaly advects the other anomaly, leading to the observed cyclonic rotation. the rotation rate decreases with time, so that the direction of tilt becomes constant, but the magnitude of the tilt continues to increase. We argue that the observed rotation acts to oppose the destructive action of the vertical shear on the vortex, even in the absence of diabatic processes.
The role of the vertical circulation is considered in detail. It is shown that the vertical circulation develops in a manner which is consistent with the model flow remaining balanced. It is found that the mesoscale nature of the vertical circulation leads to a distortion of the axisymmetric vortex. This results in the inner core having a smaller vertical tilt than the outer region. the vertical circulation does not act on a large enough scale to explain why the vortex is not destroyed by the vertical shear.
The behaviour of the vortex is found to depend on various parameters. Results are presented where the vertical shear, the strength and size of the vortex, the Coriolis parameter, and the static stability are varied. With the exception of the vertical shear, altering any of these parameters alters the vertical penetration of a potential-vorticity anomaly. the results show that increasing the penetration depth leads to an increase in the rotation rate of the upper- and lower-level vortex centres about the mid-level centre, and to a reduction in the magnitude of the vertical tilt.
403 citations
TL;DR: In this article, the transition of the cylinder wake is investigated experimentally in a water channel and is computed numerically using a finite-difference scheme, and four physically different instabilities are observed: a local vortex-adhesion mode, three near-wake instabilities associated with three different spanwise wavelengths of approximately 1, 2, and 4 diam.
Abstract: The transition of the cylinder wake is investigated experimentally in a water channel and is computed numerically using a finite‐difference scheme. Four physically different instabilities are observed: a local ‘‘vortex‐adhesion mode,’’ and three near‐wake instabilities, which are associated with three different spanwise wavelengths of approximately 1, 2, and 4 diam. All four instability processes can originate in a narrow Reynolds‐number interval between 160 and 230, and may give rise to different transition scenarios. Thus, Williamson’s [Phys. Fluids 31, 3165 (1988)] experimental observation of a hard transition is for the first time numerically reproduced, and is found to be induced by the vortex‐adhesion mode. Without vortex adhesion, a soft onset of three‐dimensionality is numerically and experimentally obtained. A control‐wire technique is proposed, which suppresses transition up to a Reynolds number of 230.
400 citations
TL;DR: In this paper, the development of a viscous incompressible flow generated from a circular cylinder impulsively started into rectilinear motion is studied computationally, and an adaptative numerical scheme based on vortex methods is used to integrate the vorticity/velocity formulation of the Navier-Stokes equations for a wide range of Reynolds numbers (Re = 40 to 9500).
Abstract: The development of a two-dimensional viscous incompressible flow generated from a circular cylinder impulsively started into rectilinear motion is studied computationally. An adaptative numerical scheme, based on vortex methods, is used to integrate the vorticity/velocity formulation of the Navier–Stokes equations for a wide range of Reynolds numbers (Re = 40 to 9500). A novel technique is implemented to resolve diffusion effects and enforce the no-slip boundary condition. The Biot–Savart law is employed to compute the velocities, thus eliminating the need for imposing the far-field boundary conditions. An efficient fast summation algorithm was implemented that allows a large number of computational elements, thus producing unprecedented high-resolution simulations. Results are compared to those from other theoretical, experimental and computational works and the relation between the unsteady vorticity field and the forces experienced by the body is discussed.
TL;DR: The zero-field spectra are reproducible over large areas of the sample and show a multiple peak structure and the vortices appear arranged in an oblique lattice in which the primitive vectors are nearly equal and form an angle of approximately 77°.
Abstract: We report the observation of the flux line lattice in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ by scanning tunneling microscopy. The measurements were carried out at 4.2 K and in a magnetic field of 6 T applied along the $c$ axis. The vortices appear arranged in an oblique lattice in which the primitive vectors are nearly equal and form an angle of approximately 77\ifmmode^\circ\else\textdegree\fi{}. We also report local tunneling spectroscopy into a vortex core which reveals two peaks separated by about 11 meV. The zero-field spectra are reproducible over large areas of the sample and show a multiple peak structure.
TL;DR: In this paper, the authors used a 3D large-eddy simulation model of the ocean surface mixed layer that includes the Craik-Leibovich vortex force to parameterize the interaction of surface waves with mean currents.
Abstract: Numerical experiments were performed using a three-dimensional large-eddy simulation model of the ocean surface mixed layer that includes the Craik-Leibovich vortex force to parameterize the interaction of surface waves with mean currents. Results from the experiments show that the vortex force generates Langmuir circulations that can dominate vertical mixing. The simulated vertical velocity fields show linear, small-scale, coherent structures near the surface that extend downwind across the model domain. In the interior of the mixed layer, scales of motion increase to eddy sizes that are roughly equivalent to the mixed-layer depth. Cases with the vortex force have stronger circulations near the surface in contrast to cases with only heat flux and wind stress, particularly when the heat flux is positive. Calculations of the velocity variance and turbulence dissipation rates for cases with and without the vortex force, surface cooling, and wind stress indicate that wave-current interactions are a dominant mixing process in the upper mixed layer. Heat flux calculations show that the entrainment rate at the mixed-layer base can be up to two times greater when the vortex force is included. In a case with reduced wind stress, turbulence dissipation rates remained high near the surface because of the vortexmore » force interaction with preexisting inertial currents. In deep mixed layers ({approximately}250 m) the simulations show that Langmuir circulations can vertically transport water 145 m during conditions of surface heating. Observations of turbulence dissipation rates and the vertical temperature structure support the model results. 42 refs., 20 figs., 21 tabs.« less
TL;DR: In this paper, multiple smoke wires are used to investigate the secondary flow near the endwall of a plane cascade with blade shapes used in high-performance turbine stages, and traces of the smoke generated by the wires are visualized with a laser light sheet illuminating various cross sections around the cascade.
Abstract: Multiple smoke wires are used to investigate the secondary flow near the endwall of a plane cascade with blade shapes used in high-performance turbine stages. The wires are positioned parallel to the endwall and ahead of the cascade, within and outside the endwall boundary layer. The traces of the smoke generated by the wires are visualized with a laser light sheet illuminating various cross sections around the cascade. During the experiment, a periodically fluctuating horseshoe vortex system of varying number of vortices is observed near the leading edge of the cascade. A series of photographs and video tapes was taken in the cascade to trace these vortices. The development and evolution of the horseshoe vortex and the passage vortex are clearly resolved in the photographs. The interation between the suction side leg of the horseshoe vortex and the passage vortex is also observed in the experiment. A vortex induced by the passage vortex, starting about one-fourth of the curvilinear distance along the blade on the suction surface, is also found. This vortex stays close to the suction surface and above the passage vortex in the laminar flow region on the blade. From this flow visualization, a model describing the secondary flows in a cascade is proposed and compared with previous published models. Some naphthalene mass transfer results from a blade near an endwall are cited and compared with the current model. The flows inferred from the two techniques are in good agreement.
TL;DR: In this article, the motion of solid particles near the wall in a turbulent boundary layer was investigated experimentally in a water flume by flow visualization techniques and by LDA, and the results showed that particle motion, as well as entrainment and deposition processes, are controlled by the action of coherent wall structures, which appear to be funnel vortices.
Abstract: The motion of solid particles near the wall in a turbulent boundary layer was investigated experimentally in a water flume by flow visualization techniques and by LDA. The particles were of polystyrene (specific density ∼1.05) with diameters ranging from 100 to 900 μm. Results show that particle motion, as well as entrainment and deposition processes, are controlled by the action of coherent wall structures, which appear to be funnel vortices. The behavior of the particles is consistent with the motion and effects of such vortices. The vortices appear to cause the formation of particle streaks near the wall, to create suitable conditions for particle entrainment, and to assist in particle deposition by conveying them from the outer flow to the wall region.
TL;DR: In this paper, the authors reported the observation of vortex formation upon the absorption of a thermal neutron in a rotating container of superfluid $3$He-B. The nuclear reaction n + $^3$H = p + $ 3$H + 0.76MeV heats a cigar shaped region of the superfluid into the normal phase.
Abstract: We report the observation of vortex formation upon the absorption of a thermal neutron in a rotating container of superfluid $^3$He-B. The nuclear reaction n + $^3$He = p + $^3$H + 0.76MeV heats a cigar shaped region of the superfluid into the normal phase. The subsequent cooling of this region back through the superfluid transition results in the nucleation of quantized vortices. Depending on the superflow velocity, sufficiently large vortex rings grow under the influence of the Magnus force and escape into the container volume where they are detected individually with nuclear magnetic resonance. The larger the superflow velocity the smaller the rings which can expand. Thus it is possible to obtain information about the morphology of the initial defect network. We suggest that the nucleation of vortices during the rapid cool-down into the superfluid phase is similar to the formation of defects during cosmological phase transitions in the early universe.
TL;DR: In this article, it was shown that there is a critical value of the Chern-Simons coupling parameter so that, below the value, there exists self-dual doubly periodic vortex solutions, and above the value the vortices are absent.
Abstract: It is shown that there is a critical value of the Chern-Simons coupling parameter so that, below the value, there exists self-dual doubly periodic vortex solutions, and, above the value, the vortices are absent. Solutions of such a nature indicate the existence of dyon condensates carrying quantized electric and magnetic charges.
TL;DR: The quasiparticle spectrum around a single vortex line in a superconductor with {ital d}{sub {ital x}}{sup 2}{minus}{ital y}{sup 2} symmetry is studied.
Abstract: We study the quasiparticle spectrum around a single vortex line in a superconductor with ${\mathit{d}}_{\mathit{x}}^{2}$-${\mathit{y}}^{2}$ symmetry. For E(T), the density of states around a vortex exhibits a fourfold symmetry, in sharp constrast to a superconductor with s-wave symmetry. This should be readily seen in scanning tunneling microscopy experiments.
TL;DR: In this article, a class of wing-type vortex generators, which can easily be incorporated into heat transfer surfaces, is considered in fully developed and developing channel flows with respect to heat transfer enhancement and pressure loss penalty.
TL;DR: In this article, the local structure of flamelets in premixed turbulent combustion is investigated using two and three-dimensional direct numerical simulations with simple chemistry models and effects of complex chemistry are considered through recent two-dimensional DNS performed with realistic chemical schemes.
TL;DR: In this paper, the authors measured the instantaneous velocity fields in the x −y plane of a zero pressure gradient turbulent boundary layer using particle image velocimetry and found that there exist random, time-varying zones in the u −ν fields in which the streamwise momentum is remarkably uniform.
Abstract: Instantaneous velocity fields in the x‐y plane of a zero pressure gradient turbulent boundary layer are measured using particle image velocimetry. It is found that there exist random, time‐varying zones in the u‐ν fields in which the streamwise momentum is remarkably uniform. The largest dimension of a typical zone is proportional to the boundary layer thickness. The zone closest to the wall contains viscous‐inertial inclined structures similar to those found in low Reynolds number wall turbulence. A second zone is located above the wall zone in a region that coincides roughly with the logarithmic layer. The wake region of the boundary layer contains a complicated, time‐varying pattern of several nearly‐constant‐momentum zones. The zones are separated from each other and from the free stream by thin viscous shear layers that contain concentrations of spanwise vorticity.
TL;DR: The observed distribution of vortex avalanche sizes shows a power-law behavior over two decades, proving that the vortex dynamics in the Bean state is characterized by avalanches of many length scales.
Abstract: We monitor the dynamics of superconducting vortices in the Bean state, as the system is driven to the threshold of instability by the slow ramping of an external field. Individual avalanches, containing as few as 50 vortices, are detected in real time. Thus our experiment is the superconducting analog of monitoring the granular avalanches produced by slowly dropping sand on a sandpile. The observed distribution of vortex avalanche sizes shows a power-law behavior over two decades, proving that the vortex dynamics in the Bean state is characterized by avalanches of many length scales.
TL;DR: In this paper, the well-known investigations of vortex breakdown are supplemented with an exact analytic representation of this phenomenon on the basis of the complete Navier-Stokes equations for the case of a potential swirl of the input flow about the axis of symmetry.
Abstract: The well-known investigations of vortex breakdown are supplemented with an exact analytic representation of this phenomenon on the basis of the complete Navier-Stokes equations for the case of a potential swirl of the input flow about the axis of symmetry.
TL;DR: In this article, smoke-wire and surface oil-flow techniques are employed to visualize the flow patterns and evolution of vortex shedding and shear-layer instability of a NACA 0012 cantilever wing.
Abstract: Flow patterns and characteristics of vortex shedding and shear-layer instability of a NACA 0012 cantilever wing are experimentally studied. Smoke-wire and surface oil-flow techniques are employed to visualize the flow patterns and evolution of vortex shedding. Hot-wire anemometers are used to characterize the frequency domain of the unsteady flow structures. Several characteristic flow modes are classified in the domain of chord Reynolds number and root angle of attack. Effects of the juncture and wing tip are discussed. Vortex shedding can be classified into four characteristic modes. Vortex shedding at low and high angles of attack are found to have different dominant mechanisms. Effects of the juncture and wing tip on the vortex shedding are discussed. Shear-layer instabilities are found to be closely related to the behaviors of the vortex shedding. Behaviors of the shear-layer instabilities can be traced back to the characteristics of the boundary layer on the suction surface of the airfoil.
TL;DR: In this article, a quasi-steady vortex flow field around circular piers in a quasidequilibrium scour hole under a clear water regime has been investigated experimentally, and the main characteristic features of the flow, to be modeled, are a relatively large secondary vortex flow within the scour holes and skewed velocity distributions along the circumference of the pier.
Abstract: The three-dimensional quasi-steady vortex flow field around circular piers in a quasi-equilibrium scour hole under a clear water regime has been investigated experimentally. The main characteristic features of the flow, to be modeled, are a relatively large secondary vortex flow within the scour hole and skewed velocity distributions along the circumference of the pier. The flow field has been divided into a number of parts according to the flow characteristics. The components of quasi-steady velocity for different parts have been theoretically expressed in the form of equations satisfying the continuity equation and the requirements of fitting an individual profile of the measured velocity components. Using the suitable functions, the equations of the velocity components derived for different parts have been combined and matched at the junctions of these parts to get a single equation for each velocity component. The measured data have been utilized to determine the coefficients and exponents used in these equations through curve fittings. The proposed flow model, which corresponds closely with the observations, can be utilized to simulate the flow field in prototype.
TL;DR: In this article, the Strouhal numbers of the observed oscillating and coalescing systems agree reasonably well with those appearing in the literature for wall-mounted circular cylinders, and the relationship between the unsteady horseshoe vortex motions in the wake is studied for a velocity ratio of 4.
Abstract: The horseshoe vortex system resulting from the interaction between a laminar boundary layer and a round transverse jet was studied over a range of Reynolds numbers and velocity ratios using hydrogen bubble wire visualization in a water channel. The study shows that the horseshoe vortex system can be steady, oscillating, or coalescing, depending on the flow conditions. Topological concepts are used to interpret the observed flow patterns and compare these patterns with those observed and computed upstream of wall‐mounted circular cylinders. The Strouhal numbers of the observed oscillating and coalescing systems agree reasonably well with those appearing in the literature for wall‐mounted circular cylinders. The relationship between the unsteady horseshoe vortex motions and the unsteady vortex motions in the wake is studied for a velocity ratio of 4. Here it is shown that the oscillating regime occurs at the same frequency as the wake and the coalescing regime occurs at approximately double the frequency of...
TL;DR: Experiments on magnetized electron columns show that this relaxation can be arrested by spontaneous ``cooling`` of the chaotic vortex motions, leading to regular lattices of vortices within a uniform background of weaker vorticity.
Abstract: Two-dimensional turbulence normally relaxes through vortex merger and filamentation, with energy flowing to large scales and enstrophy dissipated on fine scales. Experiments on magnetized electron columns show that this relaxation can be arrested by spontaneous ``cooling`` of the chaotic vortex motions, leading to regular lattices of vortices within a uniform background of weaker vorticity. {copyright} {ital 1995} {ital The} {ital American} {ital Physical} {ital Society}.
Abstract: An experimental study was made of the effect of a periodic velocity perturbation on the separation bubble downstream of the sharp-edged blunt face of a circular cylinder aligned coaxially with the free stream. Velocity fluctuations were produced with an acoustic driver located within the cylinder and a small circumferential gap located immediately downstream of the fixed separation line to allow communication with the external flow. The flow could be considerably modified when forced at frequencies lower than the initial Kelvin-Helmholtz frequencies of the free shear layer, and with associated vortex wavelengths comparable to the bubble height. Reattachment length, bubble height, pressure at separation, and average pressure on the face were all reduced. The effects on the large-scale structures were studied on flow photographs obtained by the smoke-wire technique. The forcing increased the entrainment near the leading edge. It was concluded that the final vortex of the shear layer before reattachment is an important element of the flow structure. There are two different instabilities involved, the Kelvin-Helmholtz instability of the free shear layer and the “shedding” type instability of the entire bubble. A method of frequency scaling is proposed which correlates data for a variety of bubbles and supports an analogy with Karman vortex shedding.
TL;DR: An interactive system on a graphics workstation is implemented to permit a viewer to examine, in 3D, the evolution of the vortical structures in a complex, unsteady flow.
Abstract: Presents a method for visualizing unsteady flow by displaying its vortices. The vortices are identified by using a vorticity-predictor pressure-corrector scheme that follows vortex cores. The cross-sections of a vortex at each point along the core can be represented by a Fourier series. A vortex can be faithfully reconstructed from the series as a simple quadrilateral mesh, or its reconstruction can be enhanced to indicate helical motion. The mesh can reduce the representation of the flow features by a factor of 1000 or more compared with the volumetric dataset. With this amount of reduction, it is possible to implement an interactive system on a graphics workstation to permit a viewer to examine, in 3D, the evolution of the vortical structures in a complex, unsteady flow. >
TL;DR: In this article, a very high resolution atmospheric general circulation model, T106-L19, has been used for the simulation of hurricanes in a multi-year numerical experiment, and it was found that the number of storms in particular areas appear to depend on the intensity of the Hadley-Walker cell.
Abstract: A very high resolution atmospheric general circulation model, T106-L19, has been used for the simulation of hurricanes in a multi-year numerical experiment. Individual storms as well as their geographical and seasonal distribution agree remarkably well with observations. In spite of the fact that only the thermal and dynamical structure of the storms have been used as criteria of their identification, practically all of them occur in areas where the sea surface temperature is higher or equal to 26 °C. There are considerable variations from year to year in the number of storms in spite of the fact that there are no interannual variations in the SST pattern. It is found that the number of storms in particular areas appear to depend on the intensity of the Hadley-Walker cell. The result is clearly resolution-dependant. At lower horizonal resolution, T42, for example, the intensity of the storms is significantly reduced and their overall structure is less realistic, including their vertical form and extent. DOI: 10.1034/j.1600-0870.1995.t01-1-00003.x