Dissipation Rate Estimation in a Highly Turbulent Isotropic Flow Using 2D-PIV

In experimental turbulent flows, the estimation of the dissipation rate of turbulent kinetic energy, epsilon, is a challenge. The dimensional analysis approach is the simplest of the many available strategies, where epsilon = C(epsilon)k(3/2)/L. Although the proportionality constant, C-epsilon, is commonly stated to be on the order of unity, there is little experimental evidence to verify this claim for zero-mean stirred-chamber configurations in general, nor is there detailed information on how C-epsilon might systematically vary with flow conditions. Given the importance of zero-mean chambers for both practical and fundamental studies on turbulent flows, reliable data on the magnitude of C-epsilon would be an asset. The goal of the present investigation is to rigorously determine epsilon in turbulent helium gas using medium-resolution particle image velocimetry (PIV) combined with the corrected spatial gradient method-these results lead directly to C-epsilon. Helium maintains relatively large Kolmogorov length scales, eta, due to its high kinematic viscosity, making it possible to resolve spatial velocity gradients in strongly turbulent fields (k <= 17.6 m(2) s(-2)) with only modest magnification while avoiding many of the difficulties associated with micro-PIV. The results confirm that the vector spacing, Delta x, must be less than eta to properly calculate the spatial velocity gradients-a recommendation that has not been universally agreed upon. We provide comprehensive C-epsilon results up to Re-lambda = 220 by varying the fan speed, fan count, and chamber pressure. C-epsilon eventually falls to a value of similar to 0.5, although the true asymptotic value of C-epsilon-if it exists-remains elusive.

Automated summary

Estimating the dissipation rate of turbulent kinetic energy in experimental turbulent flows is a challenge. This study rigorously determines epsilon in turbulent helium gas using particle image velocimetry combined with the spatial gradient method, providing comprehensive results of C-epsilon. The results show that C-epsilon eventually falls to around 0.5, but the true asymptotic value remains elusive.