A comprehensive review of zonal flow phenomena in plasmas is presented. While the emphasis is on zonal flows in laboratory plasmas, planetary zonal flows are discussed as well. The review presents the status of theory, numerical simulation and experiments relevant to zonal flows. The emphasis is on developing an integrated understanding of the dynamics of drift wave–zonal flow turbulence by combining detailed studies of the generation of zonal flows by drift waves, the back-interaction of zonal flows on the drift waves, and the various feedback loops by which the system regulates and organizes itself. The implications of zonal flow phenomena for confinement in, and the phenomena of fusion devices are discussed. Special attention is given to the comparison of experiment with theory and to identifying directions for progress in future research.

/pdf/zonal-flows-in-plasma-a-review-3byloag115.pdf

Zonal flows in plasma—a review

One of the scientific success stories of fusion research over the past decade is the development of the ExB shear stabilization model to explain the formation of transport barriers in magnetic confinement devices. This model was originally developed to explain the transport barrier formed at the plasma edge in tokamaks after the L (low) to H (high) transition. This concept has the universality needed to explain the edge transport barriers seen in limiter and divertor tokamaks, stellarators, and mirror machines. More recently, this model has been applied to explain the further confinement improvement from H (high)-mode to VH (very high)-mode seen in some tokamaks, where the edge transport barrier becomes wider. Most recently, this paradigm has been applied to the core transport barriers formed in plasmas with negative or low magnetic shear in the plasma core. These examples of confinement improvement are of considerable physical interest; it is not often that a system self-organizes to a higher energy state with reduced turbulence and transport when an additional source of free energy is applied to it. The transport decrease that is associated with ExB velocity shear effects also has significant practical consequences for fusion research. The fundamental physics involved in transport reduction is the effect of ExB shear on the growth, radial extent and phase correlation of turbulent eddies in the plasma. The same fundamental transport reduction process can be operational in various portions of the plasma because there are a number ways to change the radial electric field Er. An important theme in this area is the synergistic effect of ExB velocity shear and magnetic shear. Although the ExB velocity shear appears to have an effect on broader classes of microturbulence, magnetic shear can mitigate some potentially harmful effects of ExB velocity shear and facilitate turbulence stabilization.

/pdf/effects-of-e-b-velocity-shear-and-magnetic-shear-on-5f3wdz73f7.pdf

Effects of E×B velocity shear and magnetic shear on turbulence and transport in magnetic confinement devices

The dynamics and stability of thin liquid films have fascinated scientists over many decades: the observations of regular wave patterns in film flows down a windowpane or along guttering, the patterning of dewetting droplets, and the fingering of viscous flows down a slope are all examples that are familiar in daily life. Thin film flows occur over a wide range of length scales and are central to numerous areas of engineering, geophysics, and biophysics; these include nanofluidics and microfluidics, coating flows, intensive processing, lava flows, dynamics of continental ice sheets, tear-film rupture, and surfactant replacement therapy. These flows have attracted considerable attention in the literature, which have resulted in many significant developments in experimental, analytical, and numerical research in this area. These include advances in understanding dewetting, thermocapillary- and surfactant-driven films, falling films and films flowing over structured, compliant, and rapidly rotating substrates, and evaporating films as well as those manipulated via use of electric fields to produce nanoscale patterns. These developments are reviewed in this paper and open problems and exciting research avenues in this thriving area of fluid mechanics are also highlighted.

Dynamics and stability of thin liquid films

A short resume is given about the nature of exceptional points (EPs) followed by discussions about their ubiquitous occurrence in a great variety of physical problems. EPs feature in classical as well as in quantum mechanical problems. They are associated with symmetry breaking for -symmetric Hamiltonians, where a great number of experiments has been performed, in particular in optics, and to an increasing extent in atomic and molecular physics. EPs are involved in quantum phase transition and quantum chaos; they produce dramatic effects in multichannel scattering, specific time dependence and more. In nuclear physics, they are associated with instabilities and continuum problems. Being spectral singularities they also affect approximation schemes.This article is part of a special issue of Journal of Physics A: Mathematical and Theoretical devoted to 'Quantum physics with non-Hermitian operators'.

https://iopscience.iop.org/article/10.1088/1751-8113/45/44/444016/pdf

The physics of exceptional points

The present understanding of convection in rotating spherical fluid shells is reviewed and some recent results on coherent structures and magnetic field generation are considered in detail. The constraints exerted by the Coriolis force leads to unusual properties not found in nonrotating systems, such as vacillations, localized convection, and chaotic relaxation oscillations. The central role played by the differential rotation generated by the Reynolds stress of convection in the case of Prandtl numbers of the order unity or less is emphasized. Magnetic fields generated through the dynamo process offer new degrees of freedom. Through the braking of the differential rotation the Lorentz force contributes to an enhanced heat transport.

Convective flows in rapidly rotating spheres and their dynamo action

The nonlinear equations describing convection in the form of thermal Rossby waves in a rotating annulus are solved both by an analytical perturbation theory and by a numerical method. It is shown that even in the absence of curvature of the surfaces bounding the fluid annulus in the axial direction a mean flow is generated by Reynolds stresses. The good agreement between analytical expressions and numerical results indicates that the former are valid over a larger domain of the parameter space than may be expected on the basis of the analysis of convection rolls in a non-rotating layer. This is caused in part by the reduced release of potential energy accompanying the reduced convective heat transport owing to the drift of the convection columns. The effect of curvature causes the replacement of the basic mode of convection by a different mode characterized by a double roll structure. The associated zonal mean flow is typically stronger than in the case without curvature.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0022112086000095

Convection in a rotating cylindrical annulus: thermal Rossby waves

The instabilities of convection columns (also called thermal Rossby waves) in a cylindrical annulus rotating about its axis and heated from the outside are investigated as a function of the Prandtl number P and the Coriolis parameter η* When this latter parameter is sufficiently large, it is found that the primary solution observed at the onset of convection becomes unstable when the Rayleigh number exceeds its critical value by a relatively small amount Transitions occur to columnar convection which is non-symmetric with respect to the mid-plane of the small-gap annular layer Further transitions introduce convection flows that vacillate in time or tend to split the row of columns into an inner and an outer row of separately propagating waves Of special interest is the regime of non-symmetric convection, which exhibits decreasing Nusselt number with increasing Rayleigh number, and the indication of a period doubling sequence associated with vacillating convection

Convection in a rotating cylindrical annulus. Part 2. Transitions to asymmetric and vacillating flow

A new class of steady solutions is derived describing convection rolls which do not reflect the symmetry of the physical conditions of the convection layer. As does the class of mixed solutions considered by Segel (1962) and by Knobloch and Guckenheimer (1983) the new class arises from a wavelength doubling bifurcation. The new class is distinguished by a tilt of the convection rolls which gives rise to a finite mean horizontal component of vorticity. An analytic theory is derived for small amplitudes of motion in the case of stress-free boundaries. The theory is extended to higher amplitudes by numerical computations. The new solution shares with the solution of Segel, Knobloch and Guckenheimer the property that it is unstable for large Prandtl numbersP with respect to disturbances which tend to establish the wellknown symmetric solutions, but becomes stable with respect to these disturbances for Prandtl numbers\(P \lessapprox 0.296\).

Subharmonic and asymmetric convection rolls

We study the effect of proportional feedback control on the onset and development of finite-wavelength Rayleigh-Benard-Marangoni (RBM) convection using weakly nonlinear theory as applied to Nield's model, which includes both thermocapillarity and buoyancy but ignores deformation of the free surface. A two-layer model configuration is used, which has a purely conducting gas layer on top of the liquid. In the feedback control analysis, a control action in the form of temperature or heat flux is considered. Both measurement and control action are assumed to be continuous in space and time. Besides demonstrating that stabilization of the basic state can be achieved on a linear basis, the results also indicate that a wide range of weakly nonlinear flow properties can also be altered by the linear and nonlinear control processes used here. These include changing the nature of hexagonal convection and the amount of subcritical hysteresis associated with subcritical bifurcation

Feedback control of weakly nonlinear Rayleigh-Bénard-Marangoni convection

The thermocapillary and shear-induced instabilities of a thin heated layer of liquid bounded from the top by a deformable free surface and at the bottom by a horizontally oscillating plate are studied for both Earth-bound and microgravity conditions. Finite-wavelength thermocapillary convection can be stabilized very significantly by the oscillatory shear, whereas shear-induced instabilities are greatly stabilized if the Marangoni number is negative. For long-wavelength thermocapillary convection, oscillatory shear can stabilize or destabilize the basic state, depending primarily on the imposed forcing frequency. With microgravity, significant stabilization of the dominant long-wavelength convection can be achieved by carefully selecting the imposed frequency.

A. C. Or

Papers

Convection in a rotating cylindrical annulus: thermal Rossby waves

Convection in a rotating cylindrical annulus. Part 2. Transitions to asymmetric and vacillating flow

Subharmonic and asymmetric convection rolls

Feedback control of weakly nonlinear Rayleigh-Bénard-Marangoni convection

Thermocapillary and oscillatory-shear instabilities in a layer of liquid with a deformable surface