  A new type of grid turbulence generator, the 3D sparse grid (3DS), is a
co-planar arrangement of co-frames each containing a different length scale of
grid elements [Malik, N. A. US Patent No. US 9,599,269 B2 (2017)] and
possessing a much bigger parameter space than the flat 2D fractal square grid
(2DF). Using DNS we compare the characteristics of the turbulence (mean flow,
turbulence intensity, energy spectrum) generated by different types of 3DS
grids. The peak intensities generated by 3DS can exceed the peaks generated by
the 2DF by 80\%; we observe that a 3DS with blockage ratio 24\% produces
turbulence similar to the 2DF with blockage ratio 32\% implying lower energy
input for the same turbulence.
