Turbulent shear flow investigations using 4D-PTV “Shake-The-Box” (STB)

Kurzfassung

For all PIV based approaches the spatial resolution of the local instantaneous velocity measurement is limited by the correlation window size (and the light-sheet thickness), which acts as a low-pass filter biasing the measurement in presence of strong velocity gradients. Further basic limitations of PIV are the low dynamic velocity range (DVR) which is typically in the order of 1:100 and the dynamic spatial range (DSR), which is basically limited by the resolution of the used camera sensor (and the correlation window size). Several methods have been developed in order to overcome these limitations (image deformation and variational approaches); but only particle-tracking (PTV) have been found capable of delivering reliable results for the mean flow statistics in strong-shear flow or near interfaces and walls. While the full Reynolds stress tensor in such flows is only achievable by long-term statistics of3D-PTV, 4D-PTV „Shake-The-Box“ and FlowFit enables experimental investigations of flow velocities and topologies in time and space as well as global and local statistics by spatially high-resolved volumes in an Eulerian- and a Lagrangian- reference frame at the same time. The principle of „Shake-The-Box“(STB) and the difference between Eulerian and Lagrangian frames of reference are explained briefly. Results of STB measurements in the Periodic Hill (ERCOFTAC Test Case 81) flow and of a Large-Scale Thermal Plume using Helium-Filled-Soap- Bubbles (HFSB) are presented. Both experimental investigations demonstrate the advantages of STB for gaining high velocity estimation accuracies at high seeding densities without spatial filtering and bias errors.