Simultaneous two-scale 3D-PTV measurements in turbulence under the influence of system rotation

We present a technique capable of measuring in a Lagrangian manner the three components of the flow velocity simultaneously at two different spatial resolutions. We use two synchronized 3D Particle Tracking Velocimetry (3D-PTV) systems equipped with filters to separate the two types of seeding particles and measure in volumes of 55  ×  55  ×  50 mm³ and 15  ×  15  ×  15 mm³, respectively. In this way, we obtain (a) the velocity field in a large observation volume with focus on the large-scale flow features and (b) velocity derivatives in a small observation volume providing small-scale quantities associated with vorticity and strain. The small observation volume is located in the center of the large one, which allows the small-scale quantities to be related to the large-scale flow patterns. To illustrate the capability of the simultaneous two-scale 3D Particle Tracking Velocimetry (STS 3D-PTV) method, the behavior of vorticity and strain is analyzed in shear-free turbulence under the influence of system rotation. We find that in the quasi-2D region, vortex stretching is strongly suppressed only inside columnar vortex tubes.