Physics of Zonal Flows

Zonal flows, by which we mean azimuthally symmetric band-like shear flows, are ubiquitous phenomena in nature and the laboratory. It is now widely recognized that zonal flows are a key constituent in virtually all cases and regimes of drift wave turbulence, indeed, so much so that this classic problem is now frequently referred to as 'drift wave-zonal flow turbulence'. In this lecture note, we present new viewpoints and unifying concepts which facilitate understanding of zonal flow physics, via theory, computation and their confrontation with the results of laboratory experiment. Special emphasis is placed on identifying avenues for further progress. We briefly survey issues such as (i) mechanism of zonal flows excitation, (ii) back interaction on turbulence, (iii) saturation mechanism of zonal flows, (iv) energy partition between fluctuations and flows, (v) turbulent transport coefficient dressed by zonal flows, and (vi) experimental efforts to verify these fundamental processes