Simulating and investigating the impact of bedform geometric features on flow structure in three-dimensional dunes

Riverbed forms are formed by changing the power of water flow in rivers and changing the carrying capacity of sediment flows. The riverbed forms are noteworthy investigated from the hydraulic and environmental point of view. For many years, river engineers have investigated the flow structure in the presence of sandy riverbed landforms under laboratory and field conditions. Also, many laboratory studies have been conducted on two-dimensional dunes, and very few studies have been conducted on three-dimensional dunes, which have been conducted in field conditions and with very limited capabilities. It can be safely stated that this is a major gap in river engineering science. Due to the limitations in laboratory and field studies, including the difficulties of Hydraulic data collection in field conditions and the inability to create a variety of hydraulic and geometric conditions in controlled laboratory conditions, numerical methods have been considered and can accurately examine the flow structure on river bed forms. Hence, to fill the gap in previous research, the main target of this research is the investigation of the various geometric conditions of three-dimensional dunes and their effects on the structure of turbulent flow passing through these three-dimensional bedforms. In this research, simulations on three-dimensional dunes (Lobe and Saddle) were performed using computational fluid mechanics (CFD). The experimental geometry included a laboratory channel with a length of 15.75 m, a width of 90 cm, and a height of 60 cm, as well as three-dimensional dunes built in the channel bed. Hydraulic conditions and boundary conditions were created in OpenFoam software, and meshing was also created using the block-Mesh file in the same software. Simulations were performed in OpenFoam software. First, the validation was carried out with experiments conducted in the laboratory channel of Isfahan University of Technology. At this stage, the optimal mesh was selected. Coarse meshing led to faster simulation convergence, but due to the coarseness of the cells, the simulation results were not reliable. On the other hand, finer meshing gave more accurate results but increased the simulation time. By changing the meshing and validating the simulation results with laboratory results, the optimal mesh was selected. It should be noted that the simulations were performed using the supercomputer system of Isfahan University of Technology. With the aim of examining the intended objectives, the effect of changes in three parameters, including bed form angle, bed form wavelength, and the curvature of the three-dimensional dune crest line, was investigated. It should be noted that as the angle changes, the wavelength remains constant, which inevitably increases the height of the 3D dune. The results showed that for the lobe bed form, with the decrease in the exit angle of the bedform, the velocity and Reynolds shear stress increased. Meanwhile, for the saddle bedform, the velocity increased and the Reynolds shear stress decreased with the decrease of the exit angle. For both the Lobe and Saddle bed forms, negative velocities were observed near the bed and four selected profiles, indicating the occurrence of flow separation near the bed. The results showed that by increasing the exit angle of the 3D bed form in both the Lobe and Saddle 3D bed forms, the thickness of the flow separation zone increased. On the other hand, the decrease in the wavelength of the three-dimensional lobe and saddle dunes led to a decrease in the velocity and an increase in the Reynolds shear stress. In this section, the results showed that the thickness of the flow separation zone increased with decreasing wavelength. Also, with the increase in the crest line curvature in the 3D lobe bed form, the velocity increased in the first half of the bed form wavelength. Although in the second half of the bed form wavelength, the increased velocity with increasing crest line curvature in the outer layer of the flow was clear, in the inner layer, the velocity difference was not significant, and the velocity profiles overlapped over a large part of the depth. The results for the lobe bed form showed that with increasing crest line curvature, the Reynolds shear stress decreased throughout the bed wavelength. Meanwhile, for another 3D dune bed form, the saddle, increasing crest line curvature led to a decrease in velocity. Also, a comparison of Reynolds shear stress values for the 3D saddle dune bed form showed that with increasing crest line curvature, Reynolds shear stress increased in most cases. In many previous studies, the turbulent flow structure for the two-dimensional dunes has been investigated in the laboratory and in the field, and three-dimensional dunes have been studied to a limited extent in field conditions. Given this strong need to identify the flow structure on three-dimensional dunes, the effect of changing the geometric parameters of three-dimensional lobe and saddle dunes on the flow structure was investigated in this study. The results showed that for the lobe bed form, with a decrease in the exit angle of the bed, the velocity and Reynolds shear stress increased, and the thickness of the flow separation zone decreased. Meanwhile, for the saddle bed form, with a decrease in the exit angle, the velocity increased, and the Reynolds shear stress decreased. Therefore, despite the increase in velocity, an increase in the exit angle can reduce the flow turbulence zone and have a positive effect on the aquatic habitat in the river. Also, a decrease in the wavelength of the three-dimensional lobe and saddle dunes led to a decrease in velocity and an increase in the thickness of the flow separation zone. An increase in the curvature of the crest line in the lobe bed form resulted in an increase in velocity and a decrease in shear stress. Meanwhile, for the saddle bed shape, increasing the crest line curvature has led to a decrease in velocity and, in most cases, an increase in Reynolds shear stress. Therefore, in general, it can be concluded that increasing the exit angle and wavelength can have positive effects on the river environment.