Avoid Mass Increase with Pressure Outlet

This is a support page for the following forum thread.

In a channel flow, mass is observed to gradually increase in the channel if the same velocity profile is applied on inlet and outlet. If on the other hand the inlet uses a Poiseuille velocity profile, and the outlet imposes a constant pressure, the total mass in the channel remains bounded. On the following figure, this is illustrated for the flow around a cylinder. Blue shows the time evolution of the average density in the channel for a pressure outlet, and red for a velocity outlet.

This picture was created from a slightly modified version of the example code “cylinder2d” in OpenLB r0.5. The code can be downloaded here (you need to install OpenLB). Call it by typing ”./cylinder2d pressure” or ”./cylinder2d velocity”.