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Numerical flow simulations are used to study the effect of coherent structures on transport in the context of a two-field, two-dimensional model of dissipative drift-wave turbulence. The presence and nature of structures are found to depend on the adiabaticity parameter α=k∥2 VT2/2νeiωs which controls the degree to which the electrons respond to parallel electric fields. Transport estimates based on quasilinear and mixing-length models are compared with the simulations. In the regime with long-lived coherent structures, the turbulent particle transport predicted by a standard quasilinear or mean-field estimate is found to exceed that actually observed in the presence of coherent structures.

The statistical dynamics of a two-field model of dissipative drift wave turbulence is investigated using the EDQNM (eddy damped quasinormal Markovian) closure method [J. Fluid Mech. 41, 363 (1970)]. The analyses include studies of statistical closure equations, derivation of an H theorem, and its application to formulation of selective decay hypotheses for turbulent relaxation process. The results show that the dynamics of the two-field model is fundamentally different from that of the familiar, one-field Hasegawa–Mima model [Phys. Fluids 21, 87 (1978)]. In particular, density fluctuations nonlinearly couple to small scales, as does enstrophy. This transfer process is nonlinearly regulated by the dynamics of the density–vorticity cross correlation. Since density perturbations are not simply related to potential perturbations, as is vorticity, their transfer rate is greater. As a result, turbulent relaxation processes exhibit both dynamic alignment of density and vorticity and coherent vortex formation.

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The development of trapped structures in decaying and saturated drift wave turbulence is studied via computer simulation. A two-dimensional electrostatic fluid model is used. The turbulence that evolves in the pure decay runs (i.e., no nonadiabatic electrons) is characterized by tightly bound monopole vortices and a very narrow frequency spectrum. For the studies of saturated turbulence, a new nonadiabatic electron model is introduced, which gives the qualitatively correct response to a coherent trapped structure. This model takes into account the effects of broadening and shifting of the frequency spectrum. These effects are found to be quite important. Trapped structures are observed in many of the saturated simulations, even in the presence of moderately broad frequency spectra. The extent of the trapping varies dramatically, becoming a much stronger effect as the average electric field increases.