Circulation in turbulent flow through a contraction

We study experimentally the statistical properties and evolution of circulation in a turbulent flow passing through a smooth 2-D contraction. The turbulence is generated with active grids to reach R(lambda similar or equal to)220 at the inlet to the 2.5:1 contraction. We employ time-resolved 3-D Lagrangian Particle Tracking technique with the Shake-The-Box algorithm to obtain volumetric velocity fields which we use to calculate the simultaneous circulation in three perpendicular planes. Forming a circulation vector and studying the PDFs of the relative strength of its components, we can quantify how the mean strain enhances and orients coherent vortical structures with the streamwise direction. This is further studied with streamwise space and time correlations of the circulations over a range of loop sizes. The streamwise component of the circulation, over same-size square loops, shows increased integral length, while the other two components are less affected. The circulation around the compressive direction weakens and reaches prominent negative correlation values, suggesting buckling or sharp reorientation of transverse vortices. The PDFs of circulation transit from non-Gaussian to Gaussian behaviour as the loop size is increased from dissipative to large scales.