Abstract
The paper introduces a three-dimensional numerical model that solves the Reynolds Averaged Navier-Stokes (RANS) equations on a curvilinear grid system using a novel nested grid approach. The main benefit of the model is the possibility to model locally complex hydraulic features in large rivers like the flow field at hydraulic structures. The entire study domain in such a case can be discretized with a coarser resolution, whereas a much finer resolution can be applied to a defined zone of the obstructions, where a detailed description of the flow field is needed. The model is tested on a laboratory experiment carried out at the Georgia Institute of Technology, where the flow field around a single and two double circular cylinders in a flatbed flume was studied. Simulated flow velocity, turbulent kinetic energy and bed shear stress distributions are in good agreement with measurements. However, deviations downstream of the piers indicate the limitation of the steady state description of the flow in the unstable wake zone. Nevertheless, the nested grid approach presented herein is a promising step towards the modeling of the local scouring phenomenon due to the relatively low computational demand.
Highlights
Local scouring around hydraulic structures can lead to their failure, knowledge of the mechanism of scouring and the prediction of the characteristic scour geometry are of great importance during the design of such structures
Two different data sets are available from the laboratory experiments
More detailed flow measurements were done for the multiple pier cases, so that three longitudinal 2D planes of flow velocities and turbulent kinetic energy (TKE) along the centerline of pier 1, the centerline of the flume and the centerline of pier 2 are available for comparison
Summary
Local scouring around hydraulic structures can lead to their failure, knowledge of the mechanism of scouring and the prediction of the characteristic scour geometry are of great importance during the design of such structures. Characteristic components of the flow are (Graf and Istiarto, 2002): strong downflow at the face of the pier, the formation of the horseshoe vortex around the pier, which is caused by the influence of adverse pressure gradients on the approaching boundary layer, and upwelling vortex shedding downstream of the pier The reproduction of this complex flow field with computational modeling is a challenging task for researchers and practitioners. Generalized formulas are based on idealized flow conditions, like steady flow in a rectangular channel with homogeneous bed material Those formulas are not suitable in all river engineering cases with a more complex geometry. Due to increasing computer performance, CFD models have become suitable tools to carry out flow analysis for different river engineering problems Their applicability has to be proven using laboratory or field measurements
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