Numerical simulation of flow structures over dunes to develop an empirical relationship for separation zone dimensions

Rivers have non-uniform beds, which are called bedforms. Depending on the hydraulic conditions, these shapes have different types that cause resistance to the flow. Despite a great number of research works and experiments on bedforms, no accurate and comprehensive equation has been proposed to estimate separation zone dimensions formed on the lee side of the dunes. The present study is mainly focused on simulating the flow motion numerically on dunes in open channels to evaluate the effect of dune geometry on the flow separation zone. Twenty-nine simulations were conducted to study the effect of the geometry of five types of dunes with different angles and heights under different hydraulic conditions and bed roughnesses. RANS and DES turbulence models were used to simulate small and large-scale dunes, respectively. The results of the numerical model were compared with the experimental results of previous researchers to validate the work, indicating the appropriate accuracy of the numerical model. Bursting events analysis were carried out to understand dominant events phenomena, it was observed that all the events have high fluctuations near the bed and none of them is dominant. However, this could decrease and diverge by getting closer to the water surface, so that all the events would tend to converge in the vicinity of the water surface. Empirical equations were derived to estimate flow separation dimensions in terms of bed characteristics and flow conditions. The equations showed that flow separations zone dimensions are highly related to lee side angle of the dunes.