The vortex structures of the mean turbulent flow field in a 90-degree bend pipe.

We analyse the vortex structures of the mean turbulent flow field in a 90-degree bend pipe at bulk Reynolds number 5300 and radius of curvature 2.5 times the pipe radius. The Reynolds averaged Navier–Stokes equations are solved for the mean turbulent flow field in the finite volume framework of ANSYS Fluent. The Reynolds stress model is adopted for closure of turbulence quantities. Notwithstanding any scale resolving simulation, we show that a detailed exploration of the flow, turbulence, and vortex structures in the mean turbulent flow field inside a pipe bend is possible, through the paradigm based on complex conjugate eigenvalues of the velocity gradient tensor. The convex contour of the inner side of the bend is found to be the key vortex-generating surface. We find that the vortices start to gain swirling strength just after entering the bend. The strength further decays, rather than getting enhanced, within the remaining part of the bend. The outer side of the bend, on the other hand, appears to maintain a straining flow regime. Fluid particles move between the vortex region and the straining region through the strain-dominated vortex regions, the structure of which is also unveiled through the eigenvalue-based vortex defining paradigm. Both the straining and the vortex zone inject turbulence. However, the vortex-induced turbulence suffers significant dissipation within the bend region. The vortices transported downstream of the bend help in generating turbulence to recover the loss of turbulence in the bend region. [ABSTRACT FROM AUTHOR]