טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentAlex Liberzon
SubjectCharacterization of Coherent Structures in Turbulent
Flows
DepartmentDepartment of Mechanical Engineering
Supervisor Professor Emeritus Hetsroni Gad (Deceased)


Abstract

This thesis is an experimental investigation on a turbulent boundary layer in a flume. The ultimate target of the research is the three dimensional characterization of the spatial features of the coherent structures.

The large scale structures in the turbulent boundary layer in a flume, are investigated experimentally by using the non-intrusive optical technique - Stereoscopic Particle Image Velocimetry (SPIV) and combination of the SPIV technique with another non-intrusive method of Hot-Foil Infrared Imaging. The uniqueness of this combined analysis is its capability to relate the coherent structures to the low- and high-speed streaks at the near-wall region. The need for the three dimensional measurements led to the development of a novel, multi-plane stereoscopic velocimetry technique, namely XPIV, based on the defocus, stereoscopic and multi-plane illumination concepts. The technique provides three dimensional and statistically significant velocity data.

In the present work, the unbiased statistical analysis method, Proper Orthogonal Decomposition (POD) has been applied to thee experimental (SPIV) and numerical (DNS) data. The analysis is aimed at the spatial characteristics of the linear combination of the first POD modes of the instantaneous vorticity components, which extracts the most dominant parts of the vorticity fluctuations. The analysis show the quasi-streamwise vortical structure of several hundred wall units length, inclined upward at 80 and with the spanwise spacing between the structures of one hundred wall units. The three dimensional view proposes the existence of the large scale, quasi-streamwise structure, elongated in the streamwise direction.

The experimental research, in three dimensions, revealing the kinematic and dynamic characteristics of the coherent motions in turbulent boundary layer flow, improves our understanding of the turbulent phenomenon and turbulent flow transport mechanisms.