On the Physics of Turbulence Entrainment: A Comparative Study*

 

            This study elucidates the Physics of Turbulence Entrainment or spreading. We present a comparative study of spreading in forced 2D hydrodynamics, forced 2D MHD and a forced extended Hasegawa–Mima Model (i.e. with zonal flows), all implemented with spatially localized forcing. The 2D Hydro and 2D MHD studies recover ballistic expansion of the turbulent region, intimately connected to “free flying” dipolar vortices. For drift waves, the spreading flux is mapped in the space of forcing strength and zonal flow drag. Both waves and turbulent mixing mediate spreading, with Kubo Number indicating which is dominant. A Dimits-like regime occurs at weak drag and forcing. Zonal flow quenching emerges as intimately linked to spreading. Reynolds power density bursts track the breakdown of the Dimits regime, and indicate the increase in spreading due to turbulent mixing. A basic experiment to study turbulence spreading is discussed.