Decay of homogenious incompressible isotropic turbulence

This case is based on the set-up of Comte-Bellot and S. Corrsin [1]

The mesh comprises a block-structured cube divided into 64x64x64 cells, with
a box length of 9*pi cm.

Turbulence is initialised using the target energy spectrum supplied in [1] (see
table 3).

For more information, see:

    https://www.openfoam.com/documentation/cpp-guide/html/verification-validation-turbulent-decay-homogeneous-isotropic-turbulence.html

[1] G. Comte-Bellot and S. Corrsin, "The use of a contraction to improve the 
    isotropy of grid-generated turbulence," J. Fluid Mech. 25, 657 (1966).
