Hello,

I want to calculate laminar incompressible flows around an circular cylinder.

I have problem in understanding the outlet boundary condition.

I have been recommended to set the normal component of the velocity to zero at the outlet.

But can I use the mean value of pressure as the outlet boundary condition ? As the outlet surface is far from the circular cylinder (20 D), may I set the pressure over the complete boundary surface equal to the ambient value.

Which choice is better ?

All your insights and advise is appreciated, Ning.

(1) you can use a far-field boundary condition and set the flow field there to constant values. Make sure that the overall continuity is satisfied on the field boundary. (2). you can use a parallel downstream condition where the gradient of flow variables is zero in the downstream coordinate direction. (3) you can specify the downstream pressure, as long as it is consistent with your formulation. (4). you can specify any velocity profile at the outlet as long as the overall continuity is satisfied. The flow will re-adjust itself to these boundary conditions.(5). you can examine these results to see which one fit your needs.

Based on the physics of wakes in subsonic flows (notwithstanding potential difficulties of implementing it into the code), it is usually easier to assume constant static pressure at the exit boundary. The velocity profile created by the wake behind a cylinder usually decays very slowly.