Tomographic Particle-Image Velocimetry in a Fully Developed Turbulent Channel Flow to Measure Dissipation Elements

Abstract A new method to describe small scale statistical information from passive scalar fields has been proposed by Wang and Peters (2006). They used direct numerical simulations (DNS) of homogeneous shear flow to introduce the innovative concept. This novel method determines the local minimum and maximum points of a fluctuating scalar field via gradient trajectories starting from every grid point in the direction of the steepest ascending and descending scalar gradients. Relying on gradient trajectories, a dissipation ele-ment is defined as the region of all the grid points the trajectories of which share the same pair of maximum and minimum points. The procedure has also been successfully applied to various DNS fields of homogene-ous shear turbulence using the three velocity components and the kinetic energy as scalar fields. To validate statistical properties of these elements derived from DNS (Wang and Peters 2006, 2008), dissipation ele-ments are for the first time determined based on experimental data of a fully developed turbulent channel flow. The dissipation elements are deduced from the gradients of the instantaneous fluctuation of the three velocity components u′, v′, and w ′ and the instantaneous kinetic energy k′, respectively. The required 3D ve-locity data is obtained investigating a 17.82 × 17.82 × 2.7 mm 3 (0.356 δ × 0.356 δ × 0.054 δ) test volume using tomographic particle-image velocimetry (Tomo-PIV). The measurements are conducted at a Reynolds number of 1.7 × 10 4 based on the channel half-height δ and the bulk velocity U. Detection and analysis of dissipation elements from the experimental velocity data are presented. The statistical results are compare