Generation Of Large-scale Flows Research Articles

Large-scale flow generation by inhomogeneous helicity.

The effect of kinetic helicity (velocity-vorticity correlation) on turbulent momentum transport is investigated. The turbulent kinetic helicity (pseudoscalar) enters the Reynolds stress (mirror-symmetric tensor) expression in the form of a helicity gradient as the coupling coefficient for the mean vorticity and/or the angular velocity (axial vector), which suggests the possibility of mean-flow generation in the presence of inhomogeneous helicity. This inhomogeneous helicity effect, which was previously confirmed at the level of a turbulence- or closure-model simulation, is examined with the aid of direct numerical simulations of rotating turbulence with nonuniform helicity sustained by an external forcing. The numerical simulations show that the spatial distribution of the Reynolds stress is in agreement with the helicity-related term coupled with the angular velocity, and that a large-scale flow is generated in the direction of angular velocity. Such a large-scale flow is not induced in the case of homogeneous turbulent helicity. This result confirms the validity of the inhomogeneous helicity effect in large-scale flow generation and suggests that a vortex dynamo is possible even in incompressible turbulence where there is no baroclinicity effect.

Aspects of flow generation and saturation indrift-wave turbulence

The generation of large-scale flows by turbulence in plasmas andfluids has attracted a great deal of attention in the lastyears. The interplay between turbulence and sheared zonal flowshas been identified as a possible candidate to explain thetransition from L to H mode as well as the setup of transportbarriers in general, while radial flows are made responsible forincreased levels of transport. Using simplified equations forlow-frequency plasma turbulence of the drift-wave type weconsider some general aspects of flow generation and address theproblem of saturation of drift-wave turbulence.

Astrophysical significance of the anisotropic kinetic alpha effect

The generation of large scale flows by the anisotropic kinetic alpha (AKA) effect is investigated in simulations with a suitable time-dependent space- and time-periodic anisotropic forcing lacking parity invariance. The forcing pattern moves relative to the fluid, which leads to a breaking of the Galilean invariance as required for the AKA effect to exist. The AKA effect is found to produce a clear large scale flow pattern when the Reynolds number, R, is small as only a few modes are excited in linear theory. In this case the non-vanishing components of the AKA tensor are dynamically independent of the Reynolds number. For larger values of R, many more modes are excited and the components of the AKA tensor are found to decrease rapidly with increasing value of R. However, once there is a magnetic field (imposed and of sufficient strength, or dynamo-generated and saturated) the field begins to suppress the AKA effect, regardless of the value of R. It is argued that the AKA effect is unlikely to be astrophysically significant unless the magnetic field is weak and R is small.

Symmetry breaking, anomalous scaling, and large-scale flow generation in a convection cell.

We consider a convection process in thin loops of different geometries. At Ra=Ra(')(cr) a first transition leading to the generation of corner vortices is observed. At higher Ra (Ra>Ra(cr)) a coherent large-scale flow, which persists for a very long time, sets up. The mean velocity nu mass flux m, and the Nusselt number Nu in this flow scale with Ra as nu proportional to m proportional to Ra0.45 and Nu proportional to Ra0.9, respectively, in a wide range of r=(Ra-Ra(cr))/Ra(cr) variation. The "normal" scaling nu proportional to sqrt[Ra] is detected as r-->0 and its range shrinks with decrease of the aspect ratio. The time evolution of the coherent flow is well described by the Landau amplitude equation with the appropriate selection of the Ra-dependent Landau constants. Analysis of the aspect ratio influence on the range of validity of anomalous scaling, observed in this paper, indicates the important role played by both thermal boundary conditions and geometry of the system.

Role of ion diamagnetic effects in the generation of large scale flows in toroidal ion temperature gradient mode turbulence

The secondary instability of large scale flows, such as zonal flows and streamers, in toroidal ion temperature gradient turbulence is investigated. It is shown that diamagnetic effects (finite pressure fluctuations), such as those for toroidal ion temperature gradient modes, significantly increase the total Reynolds force. In general, pressure fluctuations have a finite phase shift relative to the electrostatic potential. It is shown that both parts (in-phase and out-of-phase) contribute to the diamagnetic Reynolds stress tensor. Also, the out-of-phase component of the pressure fluctuations is responsible for anomalous energy flux. It is shown here, that a specific contribution of the out-of-phase component to the Reynolds stress tensor allows us to decouple zonal flow dynamics from slow evolution of the ion pressure profile. General equations for the zonal flow and streamer instabilities in toroidal ion temperature gradient turbulence are derived and the growth rates are determined.

Large-scale flow generation in turbulent convection

In a horizontal layer of fluid heated from below and cooled from above, cellular convection with horizontal length scale comparable to the layer depth occurs for small enough values of the Rayleigh number. As the Rayleigh number is increased, cellular flow disappears and is replaced by a random array of transient plumes. Upon further increase, these plumes drift in one direction near the bottom and in the opposite direction near the top of the layer with the axes of plumes tilted in such a way that horizontal momentum is transported upward via the Reynolds stress. With the onset of this large-scale flow, the largest scale of motion has increased from that comparable to the layer depth to a scale comparable to the layer width. The conditions for occurrence and determination of the direction of this large-scale circulation are described.