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Flow Characteristics and Mixing Transportation of Contaminant in Shallow Wakes

Author: ZuoZhiFeng
Tutor: LiLing
School: Tsinghua University
Course: Hydraulic Engineering
Keywords: shallow wakes flow across cylinder flow patterns contaminant characteristics of mixing transportation
CLC: X143
Type: Master's thesis
Year: 2010
Downloads: 70
Quote: 0
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Abstract


When the transverse scale of obstacle is much larger than vertical scale of flow, shallow wake forms behind the obstacle. Shallow wakes are observed widely in nature, such as wakes behind mountains, peninsulas, islands, and piers. Investigations with shallow wakes are not only able to further understand flow characteristics of fluid, but also optimize the arrangement of sewage pipes and coastal engineering, and accelerate the removal of contaminants in wakes behind islands, peninsulas and mountains.The paper mainly numerically investigates flow characteristics of wakes and mixing transportation of contaminants behind circular cylinders in shallow water. First, wake structures and mixing transportation behind single circular cylinder in shallow water are simulated with various shallow wake parameter S. Two-dimensional (2D) shallow water equations and three-dimensional (3D) Reynolds-averaged Nervier-Stokes equations models are adopted along with renormalization group k-εand standard k-εturbulence models. Then wake patterns and contamination characteristics behind two and three circular cylinders arranged side-by-side in shallow water are simulated with different S and center-to-enter gap space G between cylinders. At last, in order to investigate the effect of piles group on flow structures, the geometry of nine cylinders arranged in square is modeled with two different G values.The results show that, in general, 2D shallow water equations model predicts flow structures of wakes behind cylinder well, and RNG k-εturbulence model displays large-scale vortex structures better than standard k-εturbulence model. Wake patterns of single cylinder are determined uniquely by S. With the increase of S, wakes become more and more stable due to the increasing bottom friction. There are vortex street, unsteady bubble and steady bubble wake behind single cylinder in sequence. In contrast, wake patterns of multiply cylinders are influenced by not only S, but also G. With the increase of G, there are single bluff-body flow, biased flow, symmetry mixed flow and symmetry separated flow behind two cylinders, and single bluff-body flow, biased flow, symmetry flow and asymmetry flow behind three cylinders. The influence of gap in the single bluff-body flow can be neglected. In the biased flow, the wake of cylinder which is squeezed by biased flow has smaller scale of wake scope than the other. The wake of symmetry flow is symmetrical with the center line. Besides, the numerical experiments show that the arrangement of multiply cylinders promote the instability of wakes compared with single cylinder. When S equals to 0.6, wakes of single cylinder have been stable, yet there are still oscillating vortex structures behind two and three cylinders.The prediction of mixing transportation of contaminant uses convection and diffusion equations model. Continuous point source is located 5D (D is the diameter of cylinder) downstream of inlet boundary. Results show that contaminant mainly transports along with flow. The distribution of concentration has close relation with flow structures in wake region.

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CLC: > Environmental science, safety science > The basic theory for the Environment and Science > Environmental Earth Science > Environmental Hydrology
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