31 research outputs found
Cross-helicity effects and turbulent transport in magnetohydrodynamic flow
In the presence of large-scale vortical motions and/or magnetic-field
strains, the turbulent cross helicity (velocity--magnetic-field correlation in
fluctuations) may contribute to the turbulent electromotive force and the
Reynolds stress. These effects of cross helicity are considered to balance the
primary effects of turbulence such as the turbulent magnetic diffusivity in
magnetic-field evolution and the eddy viscosity in the momentum transport. The
cross-helicity effects may suppress the enhanced transports due to turbulence.
Physical interpretation of the effects is presented with special emphasis on
the difference between the cross-helicity effect and the usual or
helicity effect in the dynamo action. The relative importance of the
cross-helicity effect in dynamo action is validated with the aid of a direct
numerical simulation (DNS) of the Kolmogorov flow with an imposed magnetic
field. Several mechanisms that provide turbulence with the cross helicity are
also discussed.Comment: 10 pages, 6 figures, Journal of Physics Conference Series: 13th
European Turbulence Conference (ETC13
Non-equilibrium turbulent transport in convective plumes obtained from closure theory
Non-equilibrium property of turbulence modifies characteristics of turbulent
transport. With the aid of response-function formalism, such non-equilibrium
effects in turbulent transport can be represented by the temporal variation of
the turbulent energy () and its dissipation rate () along the
mean stream through the advective derivatives of and .
Applications of this effect to the turbulent convection with plumes are
considered for the first time in this work. The non-equilibrium transport
effects associated with plumes are addressed in two aspects. Firstly, the
effect associated with a single plume is evaluated using data measured in the
recent plume/jet experiments. The second argument is developed for the
collective turbulent transport associated with multiple plumes mimicking the
stellar convection zone. In this second case, for the purpose of capturing the
plume motions into the advective derivatives, use has to be made of the
time--space double averaging procedure, where the turbulent fluctuations are
divided into the coherent or dispersion component (which represents plume
motions) and incoherent or random component. With the aid of the transport
equations of the coherent velocity stress and the incoherent counterpart, the
interaction between the dispersion and random fluctuations are also discussed
in the context of convective turbulent flows with plumes. It is shown from
these analyses that the non-equilibrium effect associated with plume motions is
of a great deal of relevance in the convective turbulence modelling.Comment: 22 pages, 6 figures, 2 tables, submitted to Atmosphere. Turbulence
from Earth to Planets, Stars and Galaxies - Commemorative Issue Dedicated to
the Memory of Jackson Rae Herrin
Analysis of fast turbulent reconnection with self-consistent determination of turbulence timescale
We present results of Reynolds-averaged turbulence model simulation on the
problem of magnetic reconnection. In the model, in addition to the mean
density, momentum, magnetic field, and energy equations, the evolution
equations of the turbulent cross-helicity , turbulent energy and its
dissipation rate are simultaneously solved to calculate the rate
of magnetic reconnection for a Harris-type current sheet. In contrast to
previous works based on algebraic modeling, the turbulence timescale is
self-determined by the nonlinear evolutions of and , their
ratio being a timescale. We compare the reconnection rate produced by our
mean-field model to the resistive non-turbulent MHD rate. To test whether
different regimes of reconnection are produced, we vary the initial strength of
turbulent energy and study the effect on the amount of magnetic flux
reconnected in time.Comment: 10 pages, 7 figure