Flow structures in a tripped turbulent boundary layer flow An investigation using time-resolved tomographic PIV

Abstract

The spatial and temporal development of turbulent boundary layer flows is governed by the self-organization of coherent structures like hairpin-like or arch vortices and spanwise alternating wall bounded low- and highspeed streaks. For a detailed analysis of the topologies of the wall normal fluid exchange namely of the four quadrants of the “instantaneous” Reynolds stresses Q1,…,Q4 a time-resolved 3D-3C measurement technique is desired. In this feasibility study the tomographic PIV technique (Elsinga et al. 2005, PIV ‘05) 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 4 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 with 250 μsec time steps.