Evolution of turbulent circulation through a smooth contraction

We study the statistical properties and evolution of circulation in a turbulent flow advected through a smooth 2.5:1, 2-D contraction. The turbulence is generated by active grids in a constant-head vertical water tunnel. The grid is operated in synchronous and random modes, yielding a Taylor-Reynolds number, Re_λ ~ 220 at the inlet to the contraction. We employ time-resolved tomographic particle image velocimetry with the shake-the-box algorithm to obtain volumetric velocity fields used to characterize the circulation in three perpendicular planes, computed over square loops of different sizes which mostly lie in the inertial range. Circulation computed over closed loops can characterize the vortical coherent structures in a more representative way than the local vorticity. The mean stretching strengthens the streamwise circulation, whereas the transverse compression weakens the transverse circulation. Integral scales based on spatial correlation of streamwise circulation, were the largest compared to the other two directions. The probability density function of circulation transitioned from non-Gaussian to Gaussian behavior as the loop size increased from the dissipative scales to large scales and this transition persisted even under the influence of straining. The moments of these distribution exhibited a sub-Kolmogorov scaling exponents.