Bottom Vortex Research Articles

Taylor vortex flow in presence of internal baffles

We present a study on Taylor vortex flow in between a rotating inner cylinder and a stationary outer cylinder with vertical or horizontal baffles. A mineral oil is used as the working fluid in the annular channel of 60 mm in height and 11.6 mm in gap width. The flow pattern in the annulus is investigated by particle image velocimetry (PIV) and computational fluid dynamics (CFD). Both the experimental and simulation results show that the presence of vertical baffles influences the vertical positions of vortices—it is primarily significant for the bottom vortex. Increasing number of vertical baffles gradually decreases the azimuthal variation in the vertical position. The baffles also lead to the occurrence of a circulation flow immediately upstream and downstream each baffle plate in the radial–azimuthal plane. On the other hand, an annular horizontal baffle with sufficient width alters the vortex structure in the gap, for instance, fewer vortices are observed. With two horizontal baffles in the annulus, the flow pattern is largely affected by the baffle width and the distance between the two baffles. This study provides a better understanding of Taylor vortex in presence of internal baffles, which will be important for practical applications of Taylor vortex devices.

The Role of Bottom Vortex Stretching on the Path of the North Atlantic Western Boundary Current and on the Northern Recirculation Gyre

Abstract The mechanisms affecting the path of the depth-integrated North Atlantic western boundary current and the formation of the northern recirculation gyre are investigated using a hierarchy of models, namely, a robust diagnostic model, a prognostic model using a global 1° ocean general circulation model coupled to a two-dimensional atmospheric energy balance model with a hydrological cycle, a simple numerical barotropic model, and an analytic model. The results herein suggest that the path of this boundary current and the formation of the northern recirculation gyre are sensitive to both the magnitude of lateral viscosity and the strength of the deep western boundary current (DWBC). In particular, it is shown that bottom vortex stretching induced by a downslope DWBC near the south of the Grand Banks leads to the formation of a cyclonic northern recirculation gyre and keeps the path of the depth-integrated western boundary current downstream of Cape Hatteras separated from the North American coast. Both south of the Grand Banks and at the crossover region of the DWBC and Gulf Stream, the downslope DWBC induces strong bottom downwelling over the steep continental slope, and the magnitude of the bottom downwelling is locally stronger than surface Ekman pumping velocity, providing strong positive vorticity through bottom vortex-stretching effects. The bottom vortex-stretching effect is also present in an extensive area in the North Atlantic, and the contribution to the North Atlantic subpolar and subtropical gyres is on the same order as the local surface wind stress curl. Analytic solutions show that the bottom vortex stretching is important near the western boundary only when the continental slope is wider than the Munk frictional layer scale.

Transition phenomena in the wake of a square cylinder

The transition phenomena in the wake of a square cylinder were investigated. The existence of mode A and mode B instabilities in the wake of a square cylinder was demonstrated. The critical Reynolds numbers for the inception of these instability modes were identified through the determination of discontinuities in the St–Re curves, and were found to have mean values of 160 and 204 for the onset of mode A and B instabilities, respectively. The spectra and time traces of the wake streamwise velocity component were found to display three distinct patterns in laminar, mode A and mode B flow regimes. Streamwise vortices with different wavelength at various Reynolds numbers were observed through different measures. The symmetries and evolution of the secondary vortices were observed using laser-induced-fluorescent dye. It was found that, just like the case of a circular cylinder, the secondary vortices from the top and bottom rows were out-of-phase with each other in the mode A regime, but in-phase with each other in the mode B regime. From the flow visualization, it was qualitatively proven that there is stronger interaction between braid regions in the mode B regime. At the same time, analysis of PIV measurements quantitatively demonstrated the presence of the stronger cross flow in mode B regime when compared to the mode A regime. It suggests that the in-phase symmetry of the mode B instability is the result of strong interaction between the top and bottom vortex rows. It was also observed that although the vorticity of the secondary vortices in the mode A regime was smaller, its circulation was more than twice that of mode B instability. Compared to primary vortices, the circulations of both mode A and mode B vortices were much smaller, which indicates that the secondary vortices most likely originate from the primary vortices. The wavelengths of the streamwise vortices in the mode A and B regimes were measured using the auto-correlation method, and were found to be 5.1 (±0.1) D , 1.3 (±0.1) D , and 1.1 (±0.1) D at Re=183 (mode A), 228 and 377 (both mode B), respectively. From the present investigation, mode A instability was likely to be due to the joint-effects of the deformation of primary vortex cores and the stretching of vortex sheets in the braid region. On the other hand, mode B instability was thought to originate from the “imprinting” process.

Thermocapillary convection in an annular cylindrical container

The liquid–gas interface of a liquid in an annular container is subjected to a temperature gradient. Since shear stress on the free liquid surface depends on the temperature it shall create a variable shear stress on the surface which in turn yields by viscous traction a thermocapillary convection in the bulk of the liquid. For constant temperature T 2 > T 1 on the outer cylindrical wall and T 1 on the inner wall a steady convection shall emerge. The morphology of the ensuing flow is investigated as a function of the diameter ratio and the liquid height ratio and exhibits the growth of bottom rim vortex rings, which unite to single vortex rings of opposite flow direction to the original single strong vortex ring above it. The results also show how such a system depends on the magnitude of the diameter ratio and the liquid height ratio. The evaluation of the results exhibits the evolution of vortex rings.

Reynolds stress modelling of rectangular open-channel flow

A Reynolds stress model for the numerical simulation of uniform 3D turbulent open-channel flows is described. The finite volume method is used for the numerical solution of the flow equations and transport equations of the Reynolds stress components. The overall solution strategy is the SIMPLER algorithm, and the power-law scheme is used to discretize the convection and diffusion terms in the governing equations. The developed model is applied to a flow at a Reynolds number of 77000 in a rectangular channel with a width to depth ratio of 2. The simulated mean flow and turbulence structures are compared with measured and computed data from the literature. The computed flow vectors in the plane normal to the streamwise direction show a small vortex, called inner secondary currents, located at the juncture of the sidewall and the free surface as well as the free surface and bottom vortices. This small vortex causes a significant increase in the wall shear stress in the vicinity of the free surface. A budget analysis of the streamwise vorticity is carried out. It is found that both production terms by anisotropy of Reynolds normal stress and by Reynolds shear stress contribute to the generation of secondary currents. Copyright © 2006 John Wiley & Sons, Ltd.

Regional cooling in the South Pacific sector of the Southern Ocean due to global warming

[1] This study investigates processes leading to regional cooling in global warming experiments conducted with the NCAR fully coupled Community Climate System Model (CCSM3). While several previous studies have investigated regional cooling in the South Pacific sector of the Southern Ocean, the mechanism leading to this response remains unclear. We find that changes in ocean circulation offer the key to understanding the regional cooling. The regional cooling in the South Pacific sector of the Southern Ocean, especially the area north of the Ross Sea, results mainly through an advective mechanism. The ocean topography contributes strongly to the change of ocean circulation through changes in bottom vortex stretching associated with a decrease in the Antarctic Circumpolar Current transport.

INVESTIGATION ON THREE-DIMENSIONAL TURBULENT STRUCTURE IN UNIFORM OPEN-CHANNEL AND CLOSED DUCT FLOWS

The present study has carried out highly accurate measurements of secondary currents in fully-developed straight open channel flows by making use of a powerful two-color Laser Doppler Anemometer system with a direct digital signal processing. The primary and secondary velocity distributions, the maximum-velocity-dip phenomena, the bed and side-wall shear stress distributions and also the turbulence characteristics were made clear in channels of the aspect ratio of 2. The effects of free surface on three-dimensional structure were then examined by comparison with the data of closed duct flow. Most noticeable feature is that a strong free-surface vortex is produced due to high an-isotropy of turbulence which is caused by the existence of free surface. This vortex produces a velocity dip and also restrains the development of the bottom vortex.

The “insulating” effect of a steep continental slope

Geochemical evidence shows that the midcontinental slope off the Mid‐Atlantic Bight constitutes an important sink for substances attached to fine particles. It is reasonable to infer that near‐bottom currents in this region are generally much weaker than those over the adjacent continental shelf, allowing the settling out of fine particles and their incorporation into permanent sediment. A partial dynamical explanation of this phenomenon may be sought in the “insulating” effect of a steep slope. At levels below the main thermocline the bottom pressure perturbation associated with steady flow is governed by a parabolic equation which expresses vorticity tendency balance between bottom stress curl and vortex stretching. A heat conduction analogy is appropriate, in which the friction coefficient corresponds to conductivity and the bottom slope corresponds to heat capacity. Over the continental slope the latter is large, suppressing pressure gradients originating from the shelf. Thus steady, near‐bottom currents over the middle slope are generally weak. Exceptions to this rule are locations subject to intense winter cooling and deep convection, which may generate strong thermohaline near‐bottom flow. Oscillatory motions of importance (which may be taken to be coastally trapped waves and topographic waves propagating shoreward from deep water) are weak partly because an efficient local generation mechanism is lacking. Disturbances originating from the Gulf Stream decay as the depth reduces below 3 km, by a mechanism the details of which are not clear at present. The findings in the Mid‐Atlantic Bight are likely to apply to other continental slopes distant from western boundary currents and not subject to intense winter cooling.