Reynolds stress: Difference between revisions

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In the context of fusion plasmas, the Reynolds stress is a mechanism for generation of sheared flow from turbulence.
It was dark when I woke. This is a ray of sunishne.
<ref>[http://dx.doi.org/10.1088/0741-3335/43/10/308 S.B. Korsholm et al, ''Reynolds stress and shear flow generation'', Plasma Phys. Control. Fusion '''43''' (2001) 1377]</ref>
 
Starting from the incompressible momentum balance equation, neglecting the dissipative pressure tensor, in slab coordinates (think of ''x'' as radial, ''y'' as poloidal, and ''z'' as toroidal):
<ref>R. Balescu, ''Aspects of Anomalous Transport in Plasmas'', Institute of Physics Pub., Bristol and Philadelphia, 2005, ISBN 9780750310307</ref>
 
:<math>\frac{\partial u_y}{\partial t} + \nabla_x \left ( u_x u_y \right ) = -\nabla_y P + \frac{1}{\rho} \left ( \vec{j} \times \vec{B} \right )_y</math>
 
Averaging over a [[Flux surface|magnetic surface]] (i.e., over ''y''), the right-hand side cancels:
 
:<math>\frac{\partial u_y}{\partial t} + \nabla_x \left ( u_x u_y \right ) = 0</math>
 
It may seem as if one has lost all information concerning the background field.  
However, this is not true, as the choice of the ''x,y,z'' coordinate system depends, precisely, on the background magnetic field (and, in particular, on the cited flux surfaces).
The corresponding anisotropy is in fact essential to the effectiveness of the Reynolds Stress mechanism.
 
Now, writing the flow as the sum of a mean and a fluctuating part
 
:<math>u = \bar{u} + \tilde{u}</math>
 
one obtains
 
:<math>\frac{\partial \bar{u}_y}{\partial t} + \nabla_x \left \langle \tilde{u}_x \tilde{u}_y \right \rangle = 0</math>
 
Here, the Reynolds stress tensor appears:
 
:<math>R_{xy} = \left \langle \tilde{u}_x \tilde{u}_y \right \rangle</math>
 
Thus, a non-zero value of the gradient of the Reynolds stress (of fluctuating flow components) can drive a laminar flow. Obviously, <math>\tilde{u}_x</math> and <math>\tilde{u}_y</math> must be ''correlated'' for this to work.
This correlation occurs naturally in the presence of a background (mean) gradient driving turbulent transport.
 
Once the laminar flow shear develops, it may suppress small-scale turbulence, leading to a reduction of transport.
<ref>[http://link.aps.org/doi/10.1103/RevModPhys.72.109 P. W. Terry, ''Suppression of turbulence and transport by sheared flow'', Rev. Mod. Phys. '''72''' (2000) 109–165]</ref>
 
== See also ==
 
* [[H-mode]]
* [[Internal Transport Barrier]]
 
== References ==
<references />

Revision as of 09:28, 16 July 2011

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