Flachwasserwirbel: Strömungs- und Transportdynamik von Einzelwirbeln bzw. Wirbelpaaren mit vorwiegend zwei-dimensionalem Verhalten induziert durch Flachwassergeometrie bzw. Dichteschichtung. Experimentelle Untersuchungen mit PIV-LIF Techniken, analytische Modelle und numerische Simulationen.
Final Report Abstract
Shallow-water flows are very common in many hydraulic, environmental, and geophysical flows, where the horizontal length scale of the flow is much larger than the flow depth. For flow scale much below the Rossby radius (free from the Coriolis acceleration), the geometrical confinement is the only limiting condition toward two-dimensionality. Due to the vertical confinement of the flow, the flow instabilities grow in the horizontal dimension and lead to the generation of large-scale predominantly two-dimensional coherent structures (2DCS) through the inverse energy cascade. The 2DCSs frequently appear in form of a single vortex (monopole), dipole, tripole, or further multiple vortices based on the distribution of vortex patches. After their generation, these structures continuously interact with ambient flows and neighboring vortices until dissipated by the viscosity. Studies on the dynamics of individual 2DCS can provide physical insight into turbulent flows, which can be regarded as a superposition of large numbers of single vortices with various scales. The main object of the current research project is to investigate the correlation between the flow instabilities and the flow conditions with a single large-scale vortex or its combination as a precursor to more complex multiple vortex system. The major findings from the studies are as follows: Firstly, the evolution and dynamics of a shallow flow vortex system with high initial Reynolds numbers are investigated experimentally without background rotation. A single vortex is generated by rotating a water mass at the center of a experimental tank using a bottomless cylinder with internal sectors. The surface velocity field is observed via Particle Image Velocimetry (PIV). The experimentally observed vorticity fields indicate that strong shallowness (the ratio of the cylinder diameter to the water depth) and high Reynolds number contribute to the formation of large-scale coherent structures in form of a tripolar vortex system. The shallow water vortices with high initial Reynolds numbers experience the transition from turbulent to laminar regimes in their decay process. Secondly, we experimentally investigate the interaction of a dipolar vortex with a submerged island. The flow fields are measured with surface PIV. The trajectories of the vortex dipoles approaching a submerged island are deflected by the island and rejoined to form a dipole system again after passing the island. This implies that a vortex approaching a no-slip bottom topography rebounds due to the creation of vorticity at the bottom in a viscous boundary layer. The scatter plots of vorticity and stream function are compared with 2D inviscid vortex dipole theory. It is found that island topography increase the non-linearirty due to the weak linkage between two monopoles. In addition to the advancement of our knowledge on the shallow-water vortices, the project also contribute to promote awareness on engineering and scientific importance on the shallow flows to the students and other fellow researchers through the lectures and seminars. The developed lecture courses for undergraduate students provided the opportunity to get familiar with shallow-water vortices and their applications in environmental fluid mechanics. By giving talks and seminars at international conferences and other universities, we disseminate the major results of our research and receive feedbacks for further research issues. For the future works, further experimental studies are necessary to investigate the detailed vertical structure of shallow water vortices. It is shown that the effects of three-dimensional instabilities can influence on the dynamics of large-scale predominantly two-dimensional flow. To apply the knowledge on the single shallow-water vortices from this research to real field, a study on the interaction of large-scale vortices with background flow should be planned. Interactions between small- and large-scale turbulence also need to be done in the near future to understand turbulence properties in terms of the energy exchange between different vortex scales. Numerical simulations for a shallow vortex with high Reynolds numbers can be conducted to resolve flow instabilities around free shear flow boundary. The results from this research can be practically applied to further understand the mixing and transport process in the coastal ocean and improve the current numerical models. Shallow-water vortices have tremendous impacts on transporting and mixing mass and momentum in and out from the near shore to the open ocean in the coastal ocean. The improved knowledge on the shallow-water vortices will be helpful to develop the management and prediction programs for the coastal water quality and mass transport. The detailed knowledge on the instability properties of shallow-water vortices can be incorporated to the existing numerical models for better accuracy and predictability. The results will help numerical modelers to improve their models to simulate the two-dimensional coherent flow structures and their impact on mixing and transport processes.
Publications
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“Experimental studies on vortex dynamics in shallow flows”. Fifth International Symposium on Environmental Hydraulics, Tempe, Arizona, USA, Dec. 4-7, 2007
Rasheduzzaman, M., V. Weitbrecht, and G.H. Jirka
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“Laboratory experiments on the evolution and dynamics of shallow water vortices”. Second International Symposium on Shallow Flows, Hong Kong, China, Dec. 10-12, 2008
Rasheduzzaman, M., D.-G. Seol , T. Bleninger , and G. H. Jirka
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“Shallow flow vortex dynamics: experimental simulations”. EUROMECH Colloquium No. 501 “Mixing of Coastal, Estuarine and Riverine Shallow Flows”, Ancona, Italy, Jun. 8-12, 2008
Rasheduzzaman, M., T. Bleninger , and G.H. Jirka
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“An experimental study on the vortex interaction with a submerged island”. Eighth International Conference on Civil & Environmental Engineering, ICCEE 2009, Busan, Korea, Oct. 28-30, 2009
Seol , D.-G. and G.H. Jirka
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“Shallow vortex dynamics”. Sixth Symposium on River, Coastal and Estuarine Morhodynamics, RCEM 2009, Santa Fe, Argentina, Sept. 21-25, 2009
Jirka, G.H. and D.-G. Seol
