Study on flow-induced-vibration evaluation of large-diameter pipings in a sodium-cooled fast reactor. (2) A large-eddy simulation of turbulent flow in a short-elbow pipe

A turbulent flow in a short-elbow pipe was computed with a FEM-based large-eddy simulation code to examine the fundamental features of the flow, especially, pressure fluctuation to cause unsteady fluid force on the short-elbow pipe. The results at Re=3.2x105 and 1.2x106 show that two separation regions exist on the interior side of the elbow around 45-degree (middle of elbow) and 90-degree (end of elbow) positions. The statistical quantities of pressure fluctuation such as deviation, skewness and flatness were computed and analyzed. It is found that high pressure-fluctuation is located in the downstream of the second separation region, and near the twin vortex (or Dean vortices). This indicates that the pressure fluctuation is probably caused by both the shear layer instability at the separation and the twin vortex oscillation. Fluid force exerted on the pipe wall was also computed via surface integration of pressure, showing dominant fluctuations in vertical and out-of-plane (normal to the plane of pipe center line) directions at Strouhal number of about 0.6. (author)