Shallow dead zone dynamics: Experimental description and analysis of the three-dimensional flow structure of single dead zones and dead zone sequences, with special focus on their turbulent characteristics, coherent structures and water surface oscillations.
Final Report Abstract
Dead zones can be defined as large scale recirculating volumes of water that are stagnant compare with a faster flow occurring in an adjacent main stream. These zones can be generated by the normal evolution of the fluvial morphology or by the presence of hydraulics structures, such as harbours and groyne fields. A good understanding and description of the physics of the dead zones in rivers is crucial for the improvement and development of tools to predict the concentration of substances in shallow flows. Dead zones can be classified in single dead zones and dead zone sequences, based on their geometrical layout, and in emerged and submerged dead ones, based on their flow conditions. The main goal of this work was the understanding of the detailed fluid dynamics of shallow dead zones for emerged and submerged flow conditions. To achieve the goal new experimental techniques, based on particle velocimetry and flow visualization were developed and used to describe the flow within the dead zones. Due to the complex flow structure found during the experimental phase, numerical Large Eddy Simulations were also performed. It was shown in this work that the momentum deficit observed in the transversal velocity profile was due to the momentum transfer necessary to sustain the generation of a secondary current located at the main channel and parallel to the dead zone sequence. The characteristics of the mixing layer impingement and it connection with the velocity profile were explained. In addition it was found out that the relation between the horizontal dimensions of the dead zone and its water depth affect importantly the mass exchange rate between main channel and dead zone. During the submerged condition the flow within the dead zones changes importantly. It was found a large-scale oscillation of the mixing layer formed between dead zone and main channel, and it was explain that its generation was connected to a vortex sequence of vertical axis located at the immediately adjacent main channel. The dead zone formed after a single topographical obstruction was also investigated. It was found that the mixing layer formed in this case does not growth as observed for analogous deeper cases. This behaviour was connected with a scale jump located approximately at the interface between the primary and secondary gyre of the dead zone. A hypothesis to explain this observation is the impingement of large scale vortices at the reattachment zone that are able to create an intermittent change of the velocity at the position of the scale jump generation.
Publications
- (2008). On flow dynamics and coherent structures in submerged groyne fields. RiverFlow Conference, Izmir, Turkey. International Association of Hydraulic Engineering and Research (IAHR)
Brevis, W., Weitbrecht, V., Niño, Y. and Jirka, G.
- (2008). Three-dimensionality of the flow structure of emerged shallow dead zone sequences. 2nd International Symposium of Shallow Flows, Hong Kong
Brevis, W., Jirka, G. H., Niño, Y. and V. Weitbrecht
- (2009). Hydrodynamics of submerged groyne flow fields for two-aspect ratios. River, Coastal and Estuarine Morphodynamics. Santa Fe, Argentina
Brevis, W., Jirka , G.H. and Y. Niño
- (2009). Numerical study of flow in emerged and submerged groyne fields. International Workshop on Environmental Hydraulics. Valencia, Spain
Garcia-Villalba, M. and Brevis, W.
- (2010). Flow structure in a single groyne field. Differences and similarities with groyne fields in sequence. Latin-American congress of hydraulics engineering, IAHR, The International Association of Hydraulics and Research. Punta del Este. Uruguay
Alfaro, P., Brevis, W. and G. H. Jirka
- (2010). Mixing layer structure and impingement characteristics in emerged groyne fields. Latin-American congress of hydraulics engineering, IAHR, The International Association of Hydraulics and Research. Punta del Este. Uruguay
Brevis, W., Garcia-Villalba, M., and G. H. Jirka
- (2011). On the integration of cross-correlation and relaxation algorithms for Particle Tracking Velocimetry: The ICCRM method. Experiments in Fluids, Volume 50, Issue 1, Page 135
Brevis, W., Niño, Y. and Jirka, G.H.
