Development and applications of film-based and digital holographic particle image velocimetry for both large and small scale flow measurements.

摘要

In this dissertation, new advances on Holographic Particle Image Velocimetry for fluid flow measurements and their applications are presented. At first, a film-based holographic particle image velocimetry system (Zhang et al. Exp. in Fluids, 1996) is improved upon by the hybrid HPIV system. By introducing a mirror into the recording setup, two orthogonal views of the same particle field are recorded onto a same recording media. By carefully matching each individual particle from two views, we obtain 3D particle locations with 7mum uncertainty in all three directions. A proof-of-principle system is constructed and used to measure the flow behind a rising bubble. With advances in digital recording media, digital holography becomes a very promising 3D flow measurement technique. The basic principles of digital holographic particle image velocimetry are presented in this dissertation first, and a system incorporating single-beam two-view concept is used to measure the flow around a free swimming copepod. To address the need in understanding small scale near wall flow, a high-resolution Digital Holographic Microscope (DHM) is developed. Digital Holographic Microscopy (DHM) enables measurements of 3D locations and displacements of microscopic objects in space. It has the potential of revolutionizing microscopy, especially while studying small-scale dynamic phenomena. The dissertation introduces this technique, and then demonstrates its implementations in tracking microorganisms, and in performing 3D velocity measurement of turbulent shear flows. The primary focus is placed upon the near-wall region of a turbulent boundary layer over a smooth wall, covering the viscous sublayer, buffer layer and lower portion of logarithmic layer (0 y+150.) The Reynolds number based on ut=tw/r is 1,400. The measurements are performed at a resolution of one wall unit in all directions. The resolution is sufficient for resolving buffer layer structures and for measuring instantaneous wall shear stress distributions from velocity gradients in the sublayer. The data provides detailed statistics on the spatial distribution of wall shear stress along with the characteristic flow structures. Included are streamwise counter-rotating vortex pairs, multiple streamwise vortices and other structures. Conditional sampling based on local shear stress magnitudes identifies characteristic length scales of Deltaz +=70 and Deltay+=∼10, and its associated flow pattern. In the region of high stress, the conditionally averaged flow consists of a sweeping motion induced by a counter rotating pair of streamwise vortices. These vortices seem to be the major contributors to the local high shear stress but not the only ones. Statistics on the local strain and geometric alignment between strain and vorticity shows that the high shear generating vortices are inclined to the free stream direction at 45°. The dissertation will conclude with the studies using digital holographic microscope to investigate the swimming behaviors of dinoflagellates and how their behavior is modified by interacting with themselves and external stimuli. Our studies, for the first time, quantitatively show the trends on behavioral modifications of dinoflagellates in the presence of prey and kinematical quantities that distinguish the differences between species.