Numerical and experimental study of three dimensional flow structures and water surface fluctuations at a right-angled confluence.

Hidete Yoshimura, Ichiro Fujita, Ryohei Moriguchi

Friday 3 july 2015

8:45 - 9:00h at Amazon (level 1)

Themes: (T) Water engineering, (ST) River and coastal engineering

Parallel session: 14E. Engineering - River

The flow structure at a river confluence is very complicated due to the effect of the interaction between main and branch streams. In particular, local bed scour, sediment transport and water surface variations take place caused by large-scale coherent vortices and a recirculating region is sometimes generated during flood making the flow more complicated. Therefore river confluence can be an important location from a river engineering point of view. However, the flow structure with three dimensionality and the mechanisms of water surface variations and bed variations have not been made clear sufficiently in previous researchers. This study focused on a fixed channel confluence with a right angle as one of the river confluences. Numerical analyses by a large eddy simulation (LES) with a volume of fluid (VOF) method capable of treating free surface boundary were performed to investigate three dimensional structures of turbulence flow and water surface variations at the confluence. In the LES high-resolution and massive grids were used in order to fully resolve eddies without any wall models. Moreover, detailed measurements of surface flow and inner flow were conducted by an experimental method using PIV analysis. The velocity ratios between the main and branch streams were changed in both the LES and PIV to investigate how they affect structures of separation zone, flow unsteadiness and deformation of water surface. As a result, it was found that the shapes of the separation zone and water surface were varied by changing the velocity ratio. Moreover, complex structures of turbulence flow were captured by the result of the LES analysis.