SWIRLING TURBULENT-FLOW THROUGH A CURVED PIPE .2. RECOVERY FROM SWIRL AND BEND CURVATURE

An experimental study of swirling turbulent flow through a curved bend and its downstream tangent has been carried out. This study reports on the recovery from swirl and bend curvature and relies on measurements obtained in the downstream tangent and data reported in Part 1 to assess the recovery. Unlike the non-swirling flow case, the present measurements show that the cross-stream secondary flow is dominated by the decay of the solid-body rotation and the total wall shear stress measured at the inner and outer bend (furthest away from the bend center of curvature) is approximately equal. The shear distribution is fairly uniform, even at 1 D downstream of the bend exit. At 49 D downstream of the bend exit, the mean axial velocity has recovered to its measured profile at 18 D upstream of the bend entrance. Furthermore, the mean tangential velocity is close to zero everywhere and the turbulent shear and normal stresses take another 15 D to approximately approach their stationary straight pipe values. Therefore, complete flow recovery from swirl and bend curvature takes a total length of about 85D from the bend entrance. This compares with a recovery length of about 78 D for bend curvature alone. The recovery length is substantially shorter than that measured previously in swirling flow through straight pipes and is a consequence of the angular momentum decreasing by approximately 74% across the curved bend. Consequently, the effect of bend curvature is to accelerate swirl decay in a pipe flow.