Transient inlet velocity - Reversed Flow
 

Hello, I am trying to run an unsteady simulation with pulsatile velocity, approximately a sine function, contained in a boundary profile file. Even if I try a simple geometry like a cylinder there is always reversed flow expressed by negative vectors in the descending portion of the pulsatile function near the walls. Is this physically valid? How can I avoid this? Thanks in advance.

Eduarda

Re: Transient inlet velocity - Reversed Flow
 

Hey

Here is what "I think" is happening

In a real internal flow with an alternating positive velocity, the minimum velocity near the walls is zero (no-slip condition). (Unless you have a small nozzle (inlet) ejecting fluid in a large tube. In this case two vortices will appear on the sides (2D model).

In the CFD modeling of a pulsating inlet velocity, as the velocity increases, most of the mass gets accelerated in the core of the flow. In order to satisfy conservation of mass across a cross section at a specific moment in time, some mass gets reversed near the walls (again, mass is consereved at this cross section at this moment in time, i.e, what goes in must come out). {Don't quote me on that!}

Please do the following and teach me:

1)See if the mass flow rate is constant in a single moment of time at different cross sections (i.e. at a point where you have a reveresed flow and at a point downstream where you don't have any).

2)See if changing the walls to Symmetry boundary condition eleminate the negative sign (Of course this is not what you want, but see its effect).

3)Try having an extremely fine grid near the walls. This might supress the problem.

Regards

AAA