ABSTRACT
For the modelling of river dynamics secondary flow, the helical motion which occurs perpendicular to the primary flow direction, is essential. Recently, modelling on unstructured grids is rapidly becoming more accessible for river engineers. Unstructured modelling allows greater flexibility in setting up a model, where complex boundaries can be more accurately captured than when using a structured curvilinear grid. In previous numerical studies the parameterization of the secondary flow is generally computed on structured grids. In the presence of non-uniformity of unstructured grids, the calculation of the secondary flow undergoes into spatial inaccuracies because of difficulties in the reconstruction of the velocity gradients with sufficient accuracy. In this study, a new method based on least-square reconstruction technique is used to compute the velocity gradients on uniform and non-uniform triangular grids. This method leads to sufficiently high spatial accuracy which allows the computation of the secondary flow intensity with sufficient accuracy. This technique is validated with theoretical and experimental studies conducted by Kalkwijk & Booij (1986). Good agreement is found between the computed secondary flow intensity and the past theoretical and experimental studies. Moreover, in order to study the effect of the spiral flow on the bed morphology, we simulated morphology in a curved bend and found that phenomenologically the pattern from previous Delft3D implementation for structured grids is well reproduced. Further validation with experiments is currently being performed.
