Spectral triad interaction ($\mathbf{k}+\mathbf{p}+\mathbf{q}=0$)

The quadratic nonlinear term of the Navier-Stokes equation couples three Fourier modes whose wave vectors close to a triangle. These spectral triad interactions transfer kinetic energy between wave numbers. A turbulence closure can model their triple correlations while retaining more scale information than a prescribed local power law.