Lagrangian 3D particle tracking for turbulent flow in rapid contraction*

The present work discusses the experimental investigation of a turbulent flow through rapid contraction using the Lagrangian particle tracking approach “Shake-The-Box” method. The experimental setup was created based on the work of (Mugundhan et al., 2020), it comprises an active bi-planar grid with flaps that can be rotated in either synchronous or random mode with a max rotational speed of 210 rpm and the maximum Reynolds number based on the Taylor microscale Re_λ ≈220. The characteristics of the turbulent flow inside the 2D and 3D contractions with contraction ratios of 4:1 and 16:1 respectively were investigated. To resolve the flow field, 50,000 time-resolved volumetric velocity and vorticity fields were used with 136,000 tracked particles. A total of 45 volumes we obtained at different streamwise locations within the contraction to understand the evolution of vortical structures along with the contraction. The progression of the RMS fluctuations was compared against existing literature to understand the influence of the contractions in the alignment and amplification of coherent vertical structures. Additionally, the contraction and stretching of vertical structures in the YZ plane, due to the extensional strain in the streamwise direction.