  We investigate wind tunnel turbulence generated by both conventional and
multi-scale grids. Measurements were made in a tunnel which has a large
test-section, so that possible side wall effects are very small and the length
assures that the turbulence has time to settle down to a homogeneous shear-free
state. The conventional and multi-scale grids were all designed to produce
turbulence with the same integral scale, so that a direct comparison could be
made between the different flows. Our primary finding is that the behavior of
the turbulence behind our multi-scale grids is virtually identical to that
behind the equivalent conventional grid. In particular, all flows exhibit a
power-law decay of energy, $u^2 \sim t^{-n}$, where $n$ is very close to the
classical Saffman exponent of $n = 6/5$. Moreover, all spectra exhibit
classical Kolmogorov scaling, with the spectra collapsing on the integral
scales at small $k$, and on the Kolmogorov micro-scales at large $k$. Our
results are at odds with some other experiments performed on similar
multi-scale grids, where significantly higher energy decay exponents and
turbulence levels have been reported.
