Low Vortex Research Articles

Seasonal transition features of large-scale moisture transport in the Asian-Australian monsoon region

Using NCEP/NCAR reanalysis data for the period of 1957–2001, the climatological seasonal transition features of large-scale vertically integrated moisture transport (VIMT) in the Asian-Australian monsoon region are investigated in this paper. The basic features of the seasonal transition of VIMT from winter to summer are the establishment of the summertime “great moisture river” pattern (named the GMR pattern) and its eastward expansion, associated with a series of climatological events which occurred in some “key periods”, which include the occurrence of the notable southerly VIMT over the Indochina Peninsula in mid March, the activity of the low VIMT vortex around Sri Lanka in late April, and the onset of the South China Sea summer monsoon in mid May, among others. However, during the transition from summer to winter, the characteristics are mainly exhibited by the establishment of the easterly VIMT belt located in the tropical area, accompanied by some events occurring in “key periods”. Further analyses disclose a great difference between the Indian and East Asian monsoon regions when viewed from the meridional migration of the westerly VIMT during the seasonal change process, according to which the Asian monsoon region can be easily divided into two parts along the western side of the Indochina Peninsula and it may also denote different formation mechanisms between the two regions.

302 光合成微生物の低アスペクト比Taylor渦による培養実験(流体工学(1))

302 光合成微生物の低アスペクト比Taylor渦による培養実験(流体工学(1))

Velocity and turbulence measurements in a separating boundary layer with and without passive flow control

The present paper reports the results of a detailed experimental study on low profile vortex generators (VGs) used to control boundary layer separation on a large-scale flat plate with prescribed adverse pressure gradients. This activity is part of a joint European research program on Aggressive Intermediate Duct Aerodynamics. The inlet turbulent boundary layer and the pressure gradient over the flat plate are representative of aggressive turbine intermediate ducts. By regulating the inclination of the wall opposite to the flat plate, different pressure gradients, typical of turbine intermediate ducts, can be obtained. To avoid separation on the movable wall, boundary layer suction is applied. Previous measurements showed the effectiveness of VGs in delaying separation and revealed their optimum configuration for the different prescribed pressure gradients. In the present work, laser Doppler velocimetry (LDV) is applied to the most significant pressure gradient case, in order to obtain a more thorough knowledge of the near-wall flow field. Velocity and turbulence profiles are determined up to the near-wall region in order to provide an in-depth analysis of turbulent boundary layer at separation conditions, with and without application of control devices. LDV allowed high spatial resolution and accurate statistical analysis of the boundary layer velocities. The results show velocity and turbulence profiles typical of separated turbulent boundary layers for the baseline case, and non-conventional unseparated boundary layer profiles when VGs are installed on the flat plate.

Dynamic Coupling-Decoupling Crossover in the Current-Driven Vortex State inTl2Ba2CaCu2O8Probed by the Josephson Plasma Resonance

We have used terahertz spectroscopy to measure the Josephson plasma resonance in the superconductor Tl2Ba2CaCu2O8+delta. This allows us to probe the longitudinal ordering of pancake vortices as a function of applied ab-plane current in a 2.5 kG c-axis magnetic field. With increasing current in the low temperature vortex solid phase, we observe a decrease in the interlayer phase coherence consistent with a progressive misalignment of the pancake vortices in neighboring layers. In the high temperature vortex liquid phase, an increase in the longitudinal ordering occurs above a certain threshold current. Our results show evidence of a current-driven coupling-decoupling crossover in the pinned liquid phase.

Finite number of vortices and bending of finite vortex lines in a confined rotating Bose-Einstein condensate

The minimal energy configurations of finite Nv-body vortices in a rotating trapped Bose-Einstein condensate is studied analytically by extending the previous work [Y. Castin, R. Dum, Eur. Phys. J. D 7, 399 (1999)], and taking into account the finite size effects on z-direction and the bending of finite vortex lines. The calculation of the energy of the vortices as a function of the rotation frequency of the trap gives number, curvature, configuration of vortices and width of vortex cores self-consistently. The numerical results show that (1) the simplest regular polynomial of the several vortex configurations is energetically favored; while the hexagonal vortex lattice is more stable than square lattice; (2) bending is more stable then straight vortex line along the z-axis for λ<1; (3) the boundary effect is obvious: compared with the estimation made under infinite boundary, the finite size effect leads to a lower vortex density, while the adding vortex bending results in a less higher density because of the expansion. The results are in well agreement with the other authors' ones.

Simple Model for Turbulent Tip Vortices

V ORTICES are interwoven into the fabric of fluid mechanics transporting mass momentum and energy in the majority of natural and industrial flows. In technology these are either produced deliberately to accomplish the task, improve the function of devices, or emerge as a parasitic by-product of fluid motion. Aeronautical applications are very susceptible to these flow manifestations. Consequences of lift, tip vortices cause considerable drag, noise, and/or hazard in fixedand/or rotating-wing aircraft. Although complicated in nature simple mathematical models are routinely used to elaborate on some of their fundamental properties [1,2]. It is a well-known fact that, irrespective of the host flowfield, intense vortices are analogous. Theoretically, intense (or strong) vortices are those where the tangential velocity component is orders of magnitude larger than the radial and axial. In practice, however, the property is also maintained by vortices where the swirl velocity component is dominant, but not necessarily of a magnitude enormously greater than the other two. Under these conditions the vortex appears to develop in amanner that the swirling action ignores the secondarylike flow in the azimuthal plane. Consequently, simple Rankine-like formulations have been used widely to model a variety of geophysical, wingtip, cyclone chamber, ship propeller, intake, and other types of vortices. The physics of laminar vortices is relatively well known. Simple exact solutions of the Navier–Stokes equations are due to Rankine [3], Oseen [4], Lamb [5], Burgers [6], and others. Every one of them represents a possible solution, applicable to low vortex Reynolds numbers (Re ) defined as the total circulation divided by the kinematic viscosity. In antithesis, our grasp reduces exponentially for turbulent vortices. Certainly this is not accidental. Turbulent flows are considerably more complex both analytically and experimentally. Even today the turbulent vortex is among the not very well-explored territories in aerodynamics. Not long ago, Ramasamy and Leishman [7] examined turbulent helicopter tip vortices using state of the art instruments and experimental techniques. Their high-resolution visualizations revealed that these types of whirls display the already familiar path to turbulence (probably slow, through spectral development). Inside the core, they have confirmed that helicopter tip vortices do enjoy laminarlike conditions. Approaching the core from the origin, past the laminar core and when a critical local Reynolds number is reached, the flow enters a transition region. This condition persists until a second critical Reynolds number where the flowfield changes into the “turbulent state” at larger radii. Most important of all, the accompanied high-fidelity velocity data (at Re 48; 000) exposed the following particular behavior for the azimuthal velocity component. In the region where the vortex is turbulent, the velocity decreases at a rate noticeably smaller than that of a laminar vortex. As a consequence of the new experimental evidence it became amply evident that high Re helicopter tip vortices cannot be modeled by the laminar formulations of the past. Instead one has to seek analytical representations of the phenomenon where at least the most fundamental effects of turbulence are included. Previous, theoretically more involved developments on the subject are those of Newman [8], Inversen [9], Tang [10], and Ramasamy and Leishman [7]. Here we present a new simple mathematically convenient formulation that accounts for the flattening effect in the tangential velocity profile.

A generic, mass conservative local grid refinement technique for lattice‐Boltzmann schemes

AbstractA generic, mass conservative local grid refinement technique for the lattice‐Boltzmann method (LBM) is proposed. As a volumetric description of the lattice‐Boltzmann equation is applied, mass conservation can be imposed by allowing the lattice‐Boltzmann particles to move from coarse grid cells to fine grid cells and vice versa in the propagation step. In contrast to most existing techniques, no spatial and temporal interpolation of particle densities is applied. Moreover, since the communication between the coarse and the fine grids is independent on the collision step, the method can be used for any LBM scheme.It was found that the method is second‐order accurate in space for 2‐D Poiseuille flow and different grid setups. The method was also applied to the case of 2‐D lid driven cavity flow at Re=1000, where half of the cavity was locally refined. It was found that the locations of the two lower vortices could be captured accurately. Finally, a direct numerical simulation (DNS) of turbulent channel flow at Reτ=360 was performed where the grid was locally refined near the walls of the channel. Good first‐ and second‐order turbulence statistics were obtained, showing the applicability of the local grid refinement technique for complex flows. Copyright © 2005 John Wiley &amp; Sons, Ltd.

Quasi-geostrophic vortices in compressible atmospheres

This paper considers the effect of an exponential variation in the background density field (as exists in compressible atmospheres) on the structure and dynamics of the quasi-geostrophic system, and compares the results with the corresponding Boussinesq limit in which background density variations are assumed small. The behaviour of the compressible system is understood via a closed-form analytic expression for the Green's function of the inversion operator relating potential vorticity and streamfunction. This expression makes explicit the anisotropy of the Green's function, inherited from the density profile, which has a slow, algebraic decay directly above the source and an exponential decay in all other directions. An immediate consequence for finite-volume vortices is a differential rotation of upper and lower levels that results in counterintuitive behaviour during the nonlinear evolution of ellipsoidal vortices, in which vortex destruction is confined to the lower vortex and wave activity is seen to propagate downwards. This is in contrast to the Boussinesq limit, which exhibits symmetric destruction of the upper and lower vortex, and in contrast to naive expectations based on a consideration of the mass distribution alone, which would lead to greater destruction of the upper vortex. Finally, the presence of a horizontal lower boundary introduces a strong barotropic component that is absent in the unbounded case (the presence of an upper boundary has almost no effect). The lower boundary also alters the differential rotation in the lower vortex with important consequences for the nonlinear evolution: for very small separation between the lower boundary and the vortex, the differential rotation is reversed leading to strong deformations of the middle vortex; for a critical separation, the vortex is stabilized by the reduction of the differential rotation, and remains coherent over remarkably long times.

Field dependence of the vortex core size

We argue that in clean high-$\kappa$ type II superconductors, the low temperature vortex core size (defined as the coherence length $\xi$) in high fields should decrease with increasing applied field in qualitative agreement with experimental data. Calculations are done for the Fermi sphere and cylinder (with the field parallel to the cylinder axis). The results for clean materials at T=0 can be represented as $\xi(H)/\xi(H_{c2}) = U (H/H_{c2})$ with $U$ being an universal function.

Jupiter's cyclones and anticyclones vorticity from Voyager and Galileo images

We have measured the internal velocity field in jovian synoptic-scale cyclones and anticyclones by tracking cloud elements in very high spatial resolution images obtained by the Voyager 1 and 2 (in 1979) and Galileo (1996–2000) spacecrafts. In total we have studied 24 different closed vortices (6 cyclones, 18 anticyclones) spanning a latitude range from ∼60° N to 60° S and with East–West sizes larger than ∼ 2000 km . The tangential component of the velocity as a function of the distance to the vortex center and position angle is used to retrieve the vorticity field. We find that the velocity increases in all the vortices from a nearly quiescent center to a maximum at the vortex periphery, with a record of about 180 m s −1 for the GRS. The vorticity of cyclones and anticyclones increases in general toward their periphery with absolute values in the range from ∼ 2 – 14 × 10 −5 s −1 . There is a marked tendency to increase the vortices vorticity with their latitude location. However the vorticity does not depend on the vortex size, circulation sense, or ambient background meridional wind shear. The vortex Rossby number ranges from ∼0.2 to 0.5. A study of the interaction between the Great Red Spot with other vortices show that the GRS does not change its vorticity upon their absorption. The two White Ovals mergers showed contradictory results, with greater vorticity in the case of BE, but lower vorticity in the case of BA, although data are poorer for this last case. We present the case of a short lived but large coherent cyclone at −59° that was embedded in a weakly anticyclone wind shear domain. We show that jovian vortices do not follow the simple Kida vortex relationship between vorticity and aspect ratio as it has been previously suggested.

COMBUSTION OF NORMAL AND LOW CALORIFIC FUELS IN HIGH TEMPERATURE AND OXYGEN DEFICIENT ENVIRONMENT

Combustion characteristics of two different gaseous fuels (a low calorific value fuel and methane fuel) have been examined using high temperature and low oxygen concentration combustion air. The momentum flux ratio between the fuel jet and the combustion airflow was kept constant to provide similarity in mixing between the different experimental cases to understand the role of fuel jet property on combustion. Direct flame photography, 2-D Particle image velocimetry (PIV), Light Emission Spectroscopy and chemiluminescent NOX analyzer was used as the diagnostics. These diagnostics allowed information on global flame features, mean and rms components of axial and radial velocity, axial strain rates and vorticity, the spatial distribution of combustion intermediate species, such as, OH and CH, and overall NOX emission levels. The results indicate a slower mixing during high temperature air combustion with low calorific value fuel as compared to methane fuel. The results showed higher turbulence levels and higher axial strain rates for low calorific fuel jets as compared to methane fuel jet during the high temperature air combustion condition. This results in less intense (or mild) combustion conditions with the result of increased flame length and volume and lower NOX emissions. Even for the normal methane fuel high temperature and oxygen deficient combustion conditions provided lower NOX emission. Furthermore, the high temperatures obtained for methane combustion provided lower vorticity and axial strain rates than the low calorific value fuel due to the suppression of vortical structure formation from the stronger heat release. In the case of low calorific value fuel, higher fuel jet velocity into low-density high temperature air leads to longer jet length. This jet causes a local stagnation to the upstream cross-flow to create local higher value of turbulence levels immediately upstream of the jet. The spatial distribution of the flame generated radicals (OH and CH) revealed significant ignition delay of the LCV fuel jet and a far more uniform distribution of the intermediate species. The methane fuel jet showed a prolonged reaction zone and faster ignition at high temperature and oxygen deficient conditions when compared to normal temperature air combustion of methane.

Laboratory Studies of Low Vortices in a Low Froude Number FlowPasting over an Isolated Obstacle

The present paper mainly focuses on the generation and developmentof lee vortices for the low Froude number density stratified flowpassing over an isolated obstacle. A series of laboratory experimentswere carried out by towing-tank equipments to simulate the airflowpassing over a large terrain. Based on the similarity principle, theFroude number of the stratified flow, the stratified condition, and therotation parameter are chosen in the specified scope. Our studies showthat the lee vortices can appear in the non-rotational and rotationalcases. For the non-rotational fluid, the lee vortices generate onlyunder the condition of strong stratification and the small Froudenumber (0.1<Fr<0.3). However, the lee vortices can be formed underthe condition of relatively strong stratification and the properFroude number (0.1<Fr<0.5) for the rotational fluid. Theresults of the divided experiments including non-rotational androtational cases show that the formation mechanism of lee vortices isdifferent for non-rotational flow and rotational flow. The leevortices are created by the tilting term of perturbation velocity forthe non-rotational flow, but for the rotational flow, the β-effectassociated with the horizontal convergence and divergence of thehorizontal perturbation velocity is more important.

Stable higher-order vortices and quasivortices in the discrete nonlinear Schrödinger equation

Vortex solitons with the topological charge S=3 , and "quasivortex" (multipole) solitons, which exist instead of the vortices with S=2 and 4, are constructed on a square lattice in the discrete nonlinear Schrödinger equation (true vortices with S=2 were known before, but they are unstable). For each type of solitary wave, its stability interval is found, in terms of the intersite coupling constant. The interval shrinks with increase of S . At couplings above a critical value, oscillatory instabilities set in, resulting in breakup of the vortex or quasivortex into lattice solitons with a lower vorticity. Such localized states may be observed in optical guiding structures, and in Bose-Einstein condensates loaded into optical lattices.

Vortex sound in the presence of a wedge with inhomogeneous surface flow impedance

The low Mach number vortex sound scattered by a sharp wedge formed by a rigid and an acoustically softer material is analyzed theoretically in the present investigation. A general expression for the time-varying far field potential due to the incompressible vortex motion around the edge of the wedge is derived to the leading order of magnitude. Results show that the introduction of a softer material increases the far field sound pulse magnitude. Also, it is found that the lower the impedance of the material, the louder the far field sound will be. In addition, it is shown that the effective fluid density inside the porous material is more important that the flow resistance in affecting the far field sound pulse magnitude when a weak vortex is concerned.

Vortex solid-liquid transition inBi2Sr2CaCu2O8+δwith a high density of strong pins

The introduction of a large density of columnar defects in %underdoped Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ crystals does not, at sufficiently low vortex densities, increase the irreversibility line beyond the first order transition (FOT) field of pristine crystals. At such low fields, the flux line wandering length $r_{w}$ behaves as in pristine %Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ crystals. Next, vortex positional correlations along the $c$--axis in the vortex Bose glass at fields above the FOT are smaller than in the low--field vortex solid. Third, the Bose-glass-to-vortex liquid transition is signaled by a rapid decrease in c-axis phase correlations. These observations are understood in terms of the ``discrete superconductor'' model.

Low field vortex dynamics of BSCCO-2223 superconducting thin films

Highly oriented BSCCO thin films were produced by r.f. magnetron sputtering on MgO(1 0 0) substrates. The magnetic properties of these single high T c-phase with different granularity were analyzed in the low magnetic field region 0< H<0.08 T. The T irr( H) data for all the samples fitted well to a de Almeida–Thoules (AT)-like power law. On the other hand the irreversibility line (IL) was shifted to lower temperatures and both critical current density and activation energy decreased by reducing the grain sizes.

A study of the effect of topography on the merging of vortices

Eight sets of numerical experiments are performed in 48 hours of integtation by using a barotropic primitive equation model with a topographic term so as to investigate the effect of topography on the merging of vortices. It is pointed out that the introduction of topography may change the track of vortices, and it causes the low vortices and vorticity lumps to be detained on the southeast side of the topography, thus creating a favorable condition for the merging of the low vortex and vorticity lumps. It is also shown that the effect of topography may cause double mergers of vortices in a horizontally shearing basic flow, and it can strengthen the low vortex remarkably.

Land surface influences on the West African summer monsoon: Implications for synoptic disturbances

The study examines regional atmospheric model (RM) simulations of the mean June–September (JJAS) climate and the implications of the mean state for the model representation of African wave disturbances (AWD). Two simulations are made with a version of the RM that computes soil moisture with multivariate functions that statistically relate it to rainfall, surface temperature, albedo, vegetation and terrain slope. These simulations differ according to the assignment of ground temperatures (Tg). While the control experiment is based on the fully interactive computation of soil moisture, the second experiment tests the response to fixed Tg whose seasonal means are more realistic than in the control within a swath along the Gulf of Guinea coast. A third simulation is made with the RM coupled to a sophisticated land surface process model (RM2). Results show a rather acute sensitivity of the mean circulation to land surface processes. The more realistic meridional temperature gradient created by fixing Tg in turn increased the vertical wind shear over West Africa and eliminated unrealistic westerly circulation at 700 mb. AWD composites were transformed from intense closed cyclonic circulations with copious rainfall to more realistic open waves that organized more moderate precipitation maxima. Lower vorticity variances in the specified change experiment imply that the open waves were characterized by more moderate vorticity extremes. Corresponding spectral amplitudes for 3–6 day periodicities of the 700 mb meridional wind were 40–80% of control values within the swath of maximum AWD activity. From among the three simulations, RM2 achieved seasonal mean precipitation, temperature, energy flux and circulation distributions that, despite some unrealistic features, were closest to observational evidence. RM2 AWD were much less intense and favored slightly longer periods. Results demonstrate that discrepancies in modeled ground temperatures caused by underestimating the cloudiness that intercepts short wave solar flux along a narrow swath of the West African coast have far-reaching consequences for the simulation of both the mean summer climate and individual synoptic disturbances. The study implies that this cooling along the Gulf of Guinea coast prevents AWD from developing into more intense storms with heavier precipitation.

A Spectral Model for Process Studies of Rotating, Density-Stratified Flows

A numerical model designed for three-dimensional process studies of rotating, stratified flows is described. The model is freely available, parallel, and portable across a range of computer architectures. The underlying numerics are high quality, based on spectral expansions, and third-order time stepping. Optional submodels include accurate calculation of Lagrangian trajectories. Special consideration has been taken to ensure ease of use by geophysical, as distinguished from computational, scientists. The mathematical and computational methods underlying the model are presented here as are several illustrative applications highlighting the model capabilities and the types of flows amenable to simulation using the model. Sample applications include forced inertial gravity waves, parametric subharmonic instability, shear-driven mixing layers, instability of a low Froude number vortex street, and geostrophic adjustment of intermittent, isolated mixing patches.

The dynamics of vortices on S2 near the Bradlow limit

The explicit solutions of the Bogomolny equations for N vortices on a sphere of radius R2&amp;gt;N are not known. In particular, this has prevented the use of the geodesic approximation to describe the low energy vortex dynamics. In this article we introduce an approximate general solution of the equations, valid for R2≳N, which has many properties of the true solutions, including the same moduli space CPN. Within the framework of the geodesic approximation, the metric on the moduli space is then computed to be proportional to the Fubini–Study metric, which leads to a complete description of the particle dynamics.