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Techn. Universiẗat, Diss., 2006--Darmstadt.

Thesis (Ph. D.)--Lehigh University, 2002. / Includes vita. Includes bibliographical references (leaves 162-169).

This study on the hydrodynamics of fishing gear focuses on fish nets. A multi-scale concept has been introduced. By decomposing the fish net structure into 4 distinct scales of flow structure interaction (twine, node, mesh and substructure scales) the complexity is reduced with the potential to integrate new knowledge at each scale to form an overall picture of flow-gear interactions. Within the scope and time-frame of the project, experiments were carried out at the twine and mesh scales. Two sets of experiments were designed at twine scale. The first featured synchronous velocity and drag force measurements on various rigid cylinders consisting of circular cylinders and cylinders inspired by twisted twine. The second aimed to study the cylinder near-wakes in greater detail than previously using PIV. At mesh scale, experiments using rigid bi-plane grids were conducted with individual focus on investigating grid turbulence and flow-grid interactions respectively. The twisted cylinders did not affect mean drag, likely due to free-stream turbulence acting on the boundary layers and free shear layers possibly negating the effects of the twisted cylinder geometry. In the near-wake, the twisted cylinders deflected the free stream into the wake and secondary vortices were introduced, shed along the cylinder span, de-correlating the flow field and in one instance, destroying regular vortex shedding. At mesh scale, for low solidities, a limiting value for mesh length was found where the total drag was dominated by individual contribution of bars. The key to altering the turbulence properties of the flow was found to be alteration of the spectral energy in the largest turbulence scales. For turbulence generation, the spectral energy should be increased and for turbulence suppression, spectral energy should be decreased in the largest turbulence scales.

No description available.

Recently, granules under vertical vibrations receive many attentions due to their importance in theoretical research and engineering application. In this paper, a two-dimensional Particle Image Velocimetry (PIV) system was used to examine the f/2 stripe pattern forming in a vertically vibrated granular layer. Since the PIV sampling frequency does not match with the vibrating frequency, a special identification-coupling method was adopted to combine the images taken in different cycles to offer the information in one complete cycle. The measured velocity vectors showed exactly the particle motions at various stages of a motion cycle, illustrating the alternating peaks and valleys on the layer top. Furthermore, quantitative results on the temporal evolution of velocity profiles were obtained and some other interesting phenomena were observed, such as the appearance of local structures (e.g. dual-phase layer structure) and the moving feature of the 'standing point'. The mechanism accounting for the occurrence of stripes on the surface is also discussed. This work will be of interest to a better understanding on pattern formation in the vibrating granular bed. / Singapore-MIT Alliance (SMA)

The overarching objective of this research project was to gain improved basic understanding of the fluid mechanisms governing the development of secondary flow structures in complex, three-dimensional inlet ducts. To accomplish this objective, particle image velocimetry measurements were employed to document the mean and turbulent flow properties within the complex flow regions. Complimentary, surface oil flow visualizations and static pressures were obtained to aid in the interpretation of the PIV data. Using these diagnostic techniques, the formation of a pair of counter-rotating vortices was revealed. Two-dimensional PIV measurements were conducted along 20 planes near the two bends of the duct model. All data was collected with an incoming freestream of 40 m/s. Over 2000 image pairs were collected for each measurement location, which were then processed and averaged to generate mean velocity, variance intensity, and velocity gradient statistics. The data was analyzed and it was determined that the experimental PIV data corresponded well with the qualitative flow visualization. This research will contribute to the particle image velocimetry database and subsequent analyses, which will provide additional insight into the flow structure and provide a new database for numerical model validation.

The overarching objective of this research project was to gain improved basic understanding of the fluid mechanisms governing the development of secondary flow structures in complex, three-dimensional inlet ducts. To accomplish this objective, particle image velocimetry measurements were employed to document the mean and turbulent flow properties within the complex flow regions. Complimentary, surface oil flow visualizations and static pressures were obtained to aid in the interpretation of the PIV data. Using these diagnostic techniques, the formation of a pair of counter-rotating vortices was revealed. Two-dimensional PIV measurements were conducted along 20 planes near the two bends of the duct model. All data was collected with an incoming freestream of 40 m/s. Over 2000 image pairs were collected for each measurement location, which were then processed and averaged to generate mean velocity, variance intensity, and velocity gradient statistics. The data was analyzed and it was determined that the experimental PIV data corresponded well with the qualitative flow visualization. This research will contribute to the particle image velocimetry database and subsequent analyses, which will provide additional insight into the flow structure and provide a new database for numerical model validation.