Decay of secondary motion downstream bends in turbulent pipe flows

Decay of secondary motions downstream 180{degree sign} pipe bends are investigated using Large Eddy Simulations (LES) for bend radii of 1{less than or equal to} r_B/d_h {less than or equal to} 3.375 at a Reynolds number of Re_h = 10,000. he velocity and turbulence characteristics are validated against experimental data for a straight pipe section as well as against experimental- and DNS data for a 90{degree sign} pipe bend. As the bend radius decreases, a larger magnitude of turbulence intensity is induced immediately downstream of the bend and for the largest magnitude, the highest gradient of the decay turbulence intensity is observed. As a result, the recovery length needed to re-establish the velocity profile downstream of the pipe bend decreases. Turbulence is transported at a higher rate, indicating that the recovery of the velocity profile is driven by turbulence transport. Secondary motions are induced by the curvature of the pipe bend and as the bend radius decreases, the magnitude of the secondary motion increases. The results show how the secondary motion decay in magnitude as the flow moves downstream the pipe bend. At the outlet of the bend, secondary motions are dominating at the walls and in the bulk flow. As the fluid moves further downstream, the secondary flows dominate close to the walls and at a length of x/d_h=5, a negligible difference in secondary motion is observed for the different bend radii.