Large-scale volumetric characterization of a turbulent boundary layer flow

Abstract

We report on an experimental undertaking with the goal of detecting so-called turbulent superstructures within a turbulent boundary layer and describing their temporal development while propagating from a region of zero pressure gradient (ZPG) into a region of adverse pressure gradient (APG). The flow was characterized by performing Lagrangian Particle Tracking (LPT) of Helium-filled soap bubbles (HFSBs) using the Shake-The-Box (STB, Schanz et al. 2016) algorithm. This work will present first results, but will concentrate on the description of the experimental efforts and the evaluation approaches that were necessary to successfully conduct this investigation. The tracking of a large number of tracer particles (several 100,000) within an extensive volume (nearly three meters in length) was made possible by the use of a system consisting of 12 cameras, forming three successive volumetric systems. Calibration and evaluation methods for fusing the different systems into one large camera system are described. Challenges in calibrating camera systems with a large field-of-view due to small-scale distortion effects introduced by the observation windows are discussed and correction methods are introduced. Methods for achieving a perfect transition between overlapping camera systems are introduced. For higher flow velocities, a double illumination approach was created in order to limit the pixel displacement between frames. To the knowledge of the authors, this is the first time such an approach was realized for time-resolved image sequences.