Investigation of a turbulent spot using time-resolved tomographic PIV

Kurzfassung

The formation and development of a turbulent spot in a laminar boundary layer flow is governed by the self-organization of generic substructures like hairpin-like vortices and spanwise alternating wall bounded low- and high-speed streaks which effectively produce new turbulent flow structures out of the oncoming laminar flow reaching the spot at the rear edge. Although the main substructures of a spot are identified and the exchange topologies in wall normal directions inside the spot are depicted roughly by means of dual plane stereo PIV (Schröder and Kompenhans 2004) the principle growth mechanism of the turbulent spot itself is still not fully understood. The analysis of this growth process would also help to understand the function of very similar structures for the turbulent exchange in fully developed turbulent boundary layer flows. In this feasibility study the tomographic PIV technique (Elsinga et al. 2005) has been applied to time resolved PIV recordings. Four high speed CMOS cameras are imaging tracer particles which were illuminated in a volume inside a boundary layer flow at 5 kHz by using two high repetitive Nd:YAG pulse lasers. The instantaneously acquired single particle images of these cameras have been used for a three dimensional tomographic reconstruction of the light intensity distribution of the particle images in a volume of voxels (volume elements) virtually representing the measurement volume. Each of two subsequently acquired and reconstructed particle image distributions are cross-correlated in small interrogation volumes using iterative multi-grid schemes with volume-deformation in order to determine a time series of instantaneous 3D-3C velocity vector fields. The measurement volume with a size of ~ 34 x 19 x 30 mm³ was located near the wall in a flat plate boundary layer flow with zero pressure gradients and downstream of a local disturbance source. At a free stream velocity of U = 7 m/s a turbulent spot grows downstream inside a laminar flat plate boundary layer flow after introducing a short initial flow injection and convects through the measurement volume. The time resolved tomographic PIV method enables capturing the spatio-temporal development of the complete flow structures and in particular the rapid formation process of hairpin-like vortices at the trailing-edge of the spot. In comparison an experiment with a wire-tripped flat plate turbulent boundary layer flow has been performed in order to reveal the differences of both flow types. The measurement method offers the possibility to determine the complete time dependent three-dimensional velocity gradient- tensor within the measurement volume. The 3D vorticity operator has been applied to the dataset in order to identify the three dimensional development of the vortical structures inside the turbulent spot, which enables the analysis of the role of coherent structures for flow exchange mechanisms in such wall bounded turbulent flows.