In continuum mechanics, an energy cascade involves the transfer of energy from large scales of motion to the small scales (called a direct energy cascade) or a transfer of energy from the small scales to the large scales (called an inverse energy cascade). This transfer of energy between different scales requires that the dynamics of the system is nonlinear. Strictly speaking, a cascade requires the energy transfer to be local in scale (only between fluctuations of nearly the same size), evoking a cascading waterfall from pool to pool without long-range transfers across the scale domain.
Big whirls have little whirls
that feed on their velocity,
And little whirls have lesser whirls
and so on to viscosity
—Lewis F. Richardson, 1922[1]
This concept plays an important role in the study of well-developed turbulence. It was memorably expressed in this poem by Lewis F. Richardson in the 1920s. Energy cascades are also important for wind waves in the theory of wave turbulence.
Consider for instance turbulence generated by the air flow around a tall building: the energy-containing eddies generated by flow separation have sizes of the order of tens of meters. Somewhere downstream, dissipation by viscosity takes place, for the most part, in eddies at the Kolmogorov microscales: of the order of a millimetre for the present case. At these intermediate scales, there is neither a direct forcing of the flow nor a significant amount of viscous dissipation, but there is a net nonlinear transfer of energy from the large scales to the small scales.
This intermediate range of scales, if present, is called the inertial subrange. The dynamics at these scales is described by use of self-similarity, or by assumptions – for turbulence closure – on the statistical properties of the flow in the inertial subrange. A pioneering work was the deduction by Andrey Kolmogorov in the 1940s of the expected wavenumber spectrum in the turbulence inertial subrange.
- ^ Richardson, Lewis Fry (1922). Weather Prediction by Numerical Processes. Boston: Cambridge University Press. p. 66. ISBN 9780511618291. Retrieved 2019-02-23.