# Are there known turbulent nonlinear equations where the cascade is a thermal gradient?

In a recent answer (here: The equipartition theorem in momentum space ), I suggested that if you have an appropriate first order equation (in the answer I used a second order equation, but it is more likely to be true for a first order equation) of the form:

$$i\partial_t \phi = (\nabla^2)^N \phi + \lambda \phi^3 + f + D$$

(EDIT: I first had $|\phi|^2\phi$, which is the nonlinear Schrodinger equation, but this has the same frequency shifting property as the relativistic version--- the frequencies are not completely perturbatively decoupled even with a resonance condition. The only reason I went to first order was to fix this annoying problem, and there is no condition of being physical. Although this will not conserve the total $|\phi|^2$, it has a conserved energy.)

Where f is a driving force with only long-wavlength components, and D is a damping force which only affects short wavelength components. The field $\phi$ is a complex scalar. Then if N is large and $\lambda$ is sufficiently small (by scaling, $\phi$'s magnitude can substitute for $\lambda$, so the limit of small $\lambda$ is really the limit of small $|\phi|$), the cascade due to the nonlinear term is a thermal gradient draining energy from the hot low frequency modes to the cold high-frequency modes.

I don't know if this model is in the literature, of if the thermal analysis is accurate (I am pretty sure it is correct, because the resonance matching does lead to local flow of energy in |k|, but I am not sure if there isn't a block to thermalization due to some problem with mixing in each |k| modes separately).

In the original answer, I used a second order equation, which has frequencies of both signs. This allows some extra mixing between distant modes, which shifts the frequency of low modes based on the occupation of high modes. While I don't think this changes the cascade significantly, I wanted a clean example, so I used a first order equation here. (EDIT: The clean example does not use the absolute value, so that the frequencies have to strictly additively combine.)

Is this model analyzed in the literature? Does it thermalize into a thermal gradient cascade?

• This looks like some kind of a QFT version of the Lorenz energy cycle ... What is a "thermal gradient cascade"? In the Lorenz energy cycle, some long-wave forces establish a meridional temperature gradient which is baroclinically unstable and the available potential energy is transfered to baroclinic waves which are finally dissipated. In the steady state, the maintanance of the temperature gradient by large scale external forcings and dissipation at smaller scales leads to a downward energy cascade. Discribing such processes or turbulence in general by QFT would be awesome :-D. – Dilaton Feb 17 '12 at 11:37
• A thermal gradient cascade is a model for turbulence where each k mode has a Boltzmann distribution at a temperature T(k) where T is smoothly varying with k, and is hot at small k and cold at high k, and establishes a thermal gradient according to the k-thermal conductivity. It is a personal model for turbulence which I like, but it is certainly false in Navier Stokes. I made up the equation to find a case where it is true. The "thermal gradient" is not thermal at all, not in the traditional sense, so it is not related to fluid flow thermal instabilities like what I think you are describing. – Ron Maimon Feb 17 '12 at 15:25
• Ah ok, I see that You mean something different (which sounds nevertheless intresting to me and I +1ed) ... Thanks for the clarification Ron. – Dilaton Feb 17 '12 at 15:57