Numerical Modelling of Turbulent Flow over Rough Surfaces (Numerieke modellering van turbulente stroming over ruwe oppervlaktes)
Numerical Modelling of Turbulent Flow over Rough Surfaces
Heredia Gomez, Marcelo; S0110906
The numerical modeling of turbulent flows over rough walls is one of the most important issues in computational fluid dynamics (CFD). The presen ce of roughness adds more difficulties and uncertainties. Roughness is p resent in most of the cases in nature like rivers, channels and coastal waters; therefore, it has to be characterized to be accounted for in mod elling works, avoiding the use of more expensive techniques with respect to the computational point of view. Thus, the main scope of the present doctoral research is the characterization of the roughness in open-chan nel flows and the proposal of new approaches to simulate flow over rough walls avoiding the use of computationally expensive methods to be appli ed under clear and sediment-laden flow. Therefore, the characterization of the roughness should be complemented by the use of proper turbulence models which take into account the flow processes near to bottom surfaces e.g. low-Reynolds turbulence mod els. This is done considering that turbulence is present in most of the flow processes, especially over irregular surfaces. Thus, the most prope r turbulence models found in literature are two-layer turbulence models, which are a combination of two approaches, the standard transport equat ions for the turbulent quantities far from the wall, and the use of alge braic or empirical approaches close to the wall, in order to reduce the demand of finer numerical grids close to irregular surfaces. Following, two-layer turbulence models are assessed and applied to flow over rough bottoms in order to determine their advantages and limitation s of their performance. The main limitation of current two-layer turbule nce models proposed in literature is the proper simulation of turbulent quantities close to the bottom with the presence of protrusions or rough ness. Therefore, a new two-layer turbulence model for flow over rough bo ttoms is developed and proposed. This model will overcome the limitation s of current two-layer models and can be applied in clear water and sedi ment-laden flows. For this purpose LES (Large Eddy Simulation) data gene rated from simulations of flow over wavy walls developed at the Vrije Un iversiteit Brussel are applied for the development of the new model. The n, the proposed model is calibrated with the LES (Large Eddy Simulation) data reproducing the asymptotic behavior of turbulent quantities near t he wall and its variation due to roughness. The proposed model is tested and applied to DNS (Direct Numerical Simulation) and experimental cases of open-channel flow over rough bottoms under clear water conditions. Finally, the presence of sediments in suspension will add more complexit ies that should need to be considered in modeling works. Therefore, the proposed model is applied to an experimental case of sediment-laden flow to simulate the observed data. In this type of flow cases, it is necess ary to take into account that the presence of sediment particles at the bottom surface will (in the case of sand) increase the roughness and the sediments in suspension will increase the resistance to flow. In conclusion, the contributions proposed in the present research lead t o suggestions for further research work on the effects of sediments and rough walls on turbulent open-channel flow, even considering the presenc e of mobile beds, and more complex conditions.