Turbulence around a scoured bridge abutment

Abstract

In this paper the turbulent field developing around a scoured 45° wing–wall bridge abutment is numerically investigated. Three different geometrical conditions are considered: the beginning of the process, the logarithmic phase and the equilibrium stage of the scour. The bathymetric data are taken from physical experiments with equivalent geometry. The flow field is computed using a wall-resolving large eddy simulation. The dynamics of the coherent structures around the obstacle and inside the scour-hole is investigated and its influence on the turbulence characteristics as well as on the modeling of the problem is discussed. The results of the present study are found to be in satisfactory agreement with data obtained in high Reynolds number laboratory experiments, highlighting the fact that the dynamics of turbulent structures in such type of flows is nearly independent of Reynolds number, at least in the range of values of laboratory-scale experiments. The dynamics of the large-scale structures is found to change from the flat bed configuration to the scoured ones. In the flat bed case, trains of structures originate upstream the obstacle and are advected downstream around the obstacle. In the scoured cases, flow separation occurs at the upstream edge of the scour and larger structures are generated compared to the flat bed case. In particular, the maximum vortex strength was found in a scoured intermediate configuration. In all the cases large-scale fluctuations are generated in the upstream corner region, where a large-scale vertical corner vortex is present. This phenomenon does not have a counterpart in the case of the flow over piers. The reliability of the eddy viscosity RANS-like closure models was also evaluated by taking advantage of the LES data. It was found that such simple eddy-viscosity models are not able to accurately capture complex three-dimensional field developing in such a geometry. On the other hand, the analysis of turbulence anisotropy in the scoured configurations suggests that return-to-isotropy nonlinear models may be more suited to study this class of problems. The results of the present study show that turbulence in the scour configurations exhibits higher space and time complexity compared to the flat bed case. As a consequence, local erosion formulations, able to predict the first stage of the scouring may not be suited to simulate the process during its own advanced stages.

Keywords:

• bridge abutment
• large-eddy simulations
• scour-hole
• coherent structures
• RANS modeling

Aknowledgements

This study is supported by MIUR under the Program PRIN07, research project: MOMICS (Models and measurements of flow-sediment interactions at spatial and temporal scales of physical interest). The numerical simulations of the present research were carried out at the supercomputing facilities of the CILEA, CASPUR and CINECA.