Ph.D Thesis


Ph.D StudentAlexander Svizher
SubjectExperimental Study of the Evolution of the Coherent
Structures in Wall Bounded Shear Flows Using HPIV
DepartmentDepartment of Aerospace Engineering
Supervisor Full Professors Cohen Jacob


Abstract

Coherent structures (counter-rotating vortex pairs and hairpin vortices) artificially generated in a plane Poiseuille air flow have been studied using a 3D Holographic Particle Image Velocimetry (HPIV) method as well as a 2D visualization technique. During the 2D experiments the main characteristics of the generated structures have been mapped as a function of the base flow Reynolds number and the nondimensional injection velocity.

The HPIV system is a modified version of the hybrid setup, previously employed in turbulent water flows. Accordingly, separate reconstruction of holograms, successively recorded on the same photoplate, is provided by using two reference beams. The positioning of the photoplate within the image of the sample volume accompanied by special alignment procedures, minimizes the apparent displacement caused by the misalignment of the reconstruction waves. A novel method is employed for detecting in-focus particles. The use of the phase hologram takes advantage of the high diffraction efficiency. Special attention is given to the problem of multiple images generated during the reconstruction of the phase hologram.

The 3D structure of the hairpin vortex and a set of its associated 2D velocity and vertical vorticity fields are simultaneously measured by the HPIV method. Good agreement between this data and corresponding results obtained previously by Direct Numerical Simulations (DNS) is demonstrated. The experimental results support the view, that the generation of hairpins is governed by a basic mechanism, the important elements of which are the shear of the base flow and an initial disturbance having a sufficient large amplitude.