Direct Measurement of the Turbulent Kinetic Energy Viscous Dissipation Rate Behind a Grid and a Circular Cylinder

A method is presented to evaluate the turbulent kinetic energy viscous dissipation rate, ε, from the direct measurement of the mean squared turbulent velocity spatial gradients 2ji)x/u ( ∂ ∂. The latter have been estimated using Laser Doppler Anemometer to measure simultaneously the velocities in two different points in the flow. To minimise the effect of virtual particle bias and of geometry bias the control volume dimensions have been reduced collecting the light in side scatter and using short focal length lenses. The methodology has been tested in the isotropic and homogeneous region of a turbulent flow downstream of a grid where ε is proportional to the kinetic energy decay. The measurements were carried out in a water rig for a Reynolds number ReM of 5500, based on a mesh size M (5.5 mm) of the grid, along the centreline in a range between 23-35 M. Isotropic and homogenous conditions were satisfactorily achieved. In agreement with theory, the turbulent kinetic energy decayed exponentially along the centreline with a decay coefficient of 1.3. In Figure 1 the viscous dissipation rate derived from the kinetic energy decay (blue squares) is compared with the direct measurement ε (red circles) for different distances from the grid. The average difference between the two values along the centreline is about 7%. Further measurements were taken in the wake of a cylinder for a Reynolds number Red of 7200. The aim was to separate the contributions to the total kinetic energy viscous dissipation rate due to the periodic and the turbulent motions