PIV measurements of anisotropy and inhomogeneity in decaying fractal generated turbulence

An experimental investigation of the turbulence generated by low-blockage-ratio space-filling square fractal grids is performed by means of particle image velocimetry (PIV) in an open-circuit wind tunnel. Careful corrections are performed to account for and eliminate sources of noise and error that become significant in very-low-turbulence-intensity flows such as highly decayed grid turbulence. The signal-to-noise ratio varies between 20 and 70, but despite noise and some spatial filtering, it is possible to accurately measure the turbulent viscous dissipation within +/- 10%. The results confirm that the proportionality relation L-u/lambda alpha Re-lambda (where L-u is the integral length scale, lambda is the Taylor microscale and Re-lambda is the Reynolds number based on the Taylor microscale), stemming from the scaling of the turbulent dissipation epsilon = C(epsilon)k(3/2)/L-u (where k is the turbulent kinetic energy), does not apply to the turbulence generated by square fractal grids. More importantly, the semi-empirical constant C-epsilon is not at all constant for this flow, varying by more than a factor of 4 over the observed decay range.