[MEM]

Hello Everyone,
I have simulated an incompressible flow inside a pipe using a steady-state RANS model. Inlet boundary conditions are inlet velocity and zero gradient pressure, and at the outlet, I have zero-gradient boundary conditions.

Cross-sectional streamlines exactly at the inlet are outward from the centre toward the wall, and slightly after inlet, they are changing direction, and then became inward from the wall toward the centre. This phenomenon appears multiple times along the length of the pipe. (Picture is attached)

Does anyone know why this happens?