Numerical Investigation of the Relative Submergence Effects on the Logarithmic Velocity Profiles in a Straight Channel with Gravel Bed

The study of turbulent flow field passing over the rough beds is still one of the important and controversial issues after a few years. In the past, most researchers have focused on flow with a high and medium relative submergence (the ratio of water depth to aggregate size), but only a few studies have been done on the low relative submergence flows. In this study, the gravel bed open-channel flow in a straight channel has been numerically modeled and studied using the Flow-3D package. The numerical model has been calibrated and verified using available data in the literature. During simulation, six different relative submergences were analyzed and the coefficients of Von Karman, integral constant and zero displacement planes determined. Results showed that in flat-bed conditions, the Von Karman coefficient is 0.41, but in the case of rough bed and relatively low submergence conditions, this value changes with relative submergence. These changes ranged from 0.29 to 0.43. It was also observed that with increasing depth, the Von Karman coefficient initially decreased and increased consecutively. Also, the zero displacement plane has no coherent relationship with relative submergence at all and it cannot be commented definitively. At the end, a new experimental formulation for the Von-Karman coefficient has been presented as a two-function equation with relative submergence boundary.