Identification of vortex structures in flow fields using tomographic PIV method

Abstract

We carried out the implementation of visualization techniques that aim at the interpretation of the complex turbulent structures in fluid flow. Turbulent structures help in understanding various phenomena of the flow field such as energy dissipation and drag reduction. However, few studies were found regarding vortex sheet structure identification for experimental data. In the current study, these sheet structures were visualized. Experimental data for the three-dimensional instantaneous velocity vector field was measured using the Tomographic PIV method. Two different kinds of flow settings, pipe flow and whirlpool flow, were measured and analyzed. For comparison of results, pipe measurements were recorded at varying Reynolds number and whirlpool flow were recorded with constant stirring and decaying turbulence. Q criterion analysis, which evaluates regions of higher vorticity than strain, was employed for visualization of vortex tubes in a time series flow field measurement. Vortex sheets were visualized using the Aij matrix analysis that utilizes second-order velocity gradient tensor and its eigenvalues for identifying regions with a high correlation and magnitudes of vorticity and strain. The relative position of the tube and sheet structures visualized using the two methods were reviewed for the experimental data of pipe flow and whirlpool flow. Comparison of the experimentally resolved structures with structures of DNS data was studied for pipe flow and a close resemblance was observed.

Keywords

• tomographic piv
• turbulent structures
• q criterion
• vortex tube
• aij criterion
• vortex sheet