# What is the correct direction of turbulence energy cascade?

[1] Pope, Stephen B., Turbulent flows. Cambridge university press, 2000.

[2] Bejan, Adrian. Entropy generation minimization: the method of thermodynamic optimization of finite-size systems and finite-time processes. CRC press, 2013.

[4] Chen, Shiyi, Robert E. Ecke, Gregory L. Eyink, Michael Rivera, Minping Wan, and Zuoli Xiao. "Physical mechanism of the two-dimensional inverse energy cascade." Physical review letters 96, no. 8 (2006): 084502.

The answer depends on what kind of fluid theory you are considering.

• In the 3D viscous incompressible flow the kinetic energy is transferred from large scale eddies to small scale eddies, in particular if you inject energy at a wavenumber $$k_F$$ kinetic energy will be transferred to the wavenumbers k s.t. $$k>k_F$$. This is well know fact both numerically and theoretically, see the Kolmogorov41 model (which doesn't rely on NS) theory which prescribes an energy spectrum: $$E(k)= C \epsilon^{2/3}k^{-5/3}$$, where $$\eta$$ is the dissipation rate.

• In 2D visc. incompr. flow the kinetic energy is transferred from small to large eddies, in particular if you inject energy at $$k_F$$ your energy will be transferred both to the larger and smaller wavenumbers, this fact can be flagged as an inverse canscade. See in this respect Kraichnan-Leith-Batchelor phenomenology according which $$E(k) \approx \epsilon^{2/3} k^{-5/3}\theta(kk_F)$$ where $$\eta$$ is the Kolmogorov scale. In other words, you have two different power law behaviors. This double cascade is due to the presence of two inviscid quadratic invariants: energy and enstrophy.

Let's go back to Gibson's point. In an older paper he states:

Everything that wiggles is not turbulence

From this point we should be aware that his defintion of turbulence is different from the standard one. Moreover he states that:

Eddies form at the Kolmogorov scale (Fig. 1), pair with neighboring eddies, and these pairs pair with neighboring pairs, etc. ...

I believe he is referring to the large structures formation rather than the energy carried from them. The point of the energy cascade is to define how energy is transferred across the scales not how energy transfer determines the vortex merging or structure formation. In addition the presence of backscatter can be emergent in other more complicated theories, in fact Gibson presented an exotic topic like dark matter planets.

• So, the idea of Gibbs is that turbulence does not exist in 1D because you can only "wiggle"? How about energy cascade in 1D (is it direct or inverse)? Oct 20, 2022 at 10:30
• @Quillo see the following paper for more information: journals.aps.org/pre/abstract/10.1103/PhysRevE.93.053101 Oct 21, 2022 at 14:36
• I found this paper yesterday, but it's very unclear to me. They have a 1-D model that, however, is not just Navier-Stokes reduced to 1-D but some other effective model that has been derived in another paper (onlinelibrary.wiley.com/doi/10.1002/cpa.3160380605 ). Do you know this stuff? in case, may you give me a couple of coordinates to navigate? Oct 21, 2022 at 15:14
• @Quillo Thank you for the consideration. As you see they start from Euler eq, so no viscosity involved. They get the velocity from the vorticity field using Biot-Savart. They also play with the smoothness of the fields involved. Oct 22, 2022 at 17:56

Depends on the flow! In homogeneous isotropic turbulence, energy may go in both directions, though there is a clear preference for it going to the smallest eddies (called forward cascade), in line with the theories of Kolmogorov and many others. When energy goes the other way, it is called backscatter.

In this paper (arXiv link), they found that forward cascade happens twice as often as backscatter.