PIV measurements of anisotropy and inhomogeneity in decaying fractal generated turbulence

Abstract

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 Lu /λ ∝ Reλ (where Lu is the integral length scale, λ is the Taylor microscale and Reλ is the Reynolds number based on the Taylor microscale), stemming from the scaling of the turbulent dissipation ε = Cε k3/2 /Lu (where k is the turbulent kinetic energy), does not apply to the turbulence generated by square fractal grids. More importantly, the semi-empirical constant Cε is not at all constant for this flow, varying by more than a factor of 4 over the observed decay range.