Inflation layers over inlets/outlets

ergk-10 · June 29, 2021, 08:56

I am setting up a transient simulation with an inlet and outlet that will only be activated after a certain amount of time, until which they will be modelled as walls. As walls it is of course prudent to have an inflation layer at these surfaces to capture the large velocity gradient, however to keep the mesh simple I am considering leaving the mesh unchanged once the inlet/outlet BCs are activated.

I am now wondering if there is a downside to having an inflation layer over an inlet/outlet surface beyond maybe having more elements than are really necessary?

In general, what are the implications of using an inflation layer at an inlet/outlet? Are there accuracy/stability reasons to avoid this? Or is it simply overkill in most situations?

Thanks for any thoughts!

colinda1 · July 22, 2021, 04:24

On a converged simulation it should not have an adverse impact I believe. There may be even a (small) positive impact because of the lower numerical dissipation in case of streamwise flow field gradients in the inlet region.

The convergence may be slower and this not only because of the higher cell count but especially because of the local time stepping. The local time step is linked directly to the local cell size through the CFL number. So you would have a very low time step and it may take time to really converge. Because of the very small time step it may even look like your simulation is converged as residuals and quantities of interest vary so little that they look constant.

June 29, 2021, 08:56	Inflation layers over inlets/outlets	#1
ergk-10 New Member ergk-10 Join Date: Mar 2021 Posts: 3 Rep Power: 5	I am setting up a transient simulation with an inlet and outlet that will only be activated after a certain amount of time, until which they will be modelled as walls. As walls it is of course prudent to have an inflation layer at these surfaces to capture the large velocity gradient, however to keep the mesh simple I am considering leaving the mesh unchanged once the inlet/outlet BCs are activated. I am now wondering if there is a downside to having an inflation layer over an inlet/outlet surface beyond maybe having more elements than are really necessary? In general, what are the implications of using an inflation layer at an inlet/outlet? Are there accuracy/stability reasons to avoid this? Or is it simply overkill in most situations? Thanks for any thoughts!

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July 22, 2021, 04:24		#2
colinda1 Senior Member Colinda Join Date: May 2012 Location: Brussels Posts: 153 Rep Power: 14	On a converged simulation it should not have an adverse impact I believe. There may be even a (small) positive impact because of the lower numerical dissipation in case of streamwise flow field gradients in the inlet region. The convergence may be slower and this not only because of the higher cell count but especially because of the local time stepping. The local time step is linked directly to the local cell size through the CFL number. So you would have a very low time step and it may take time to really converge. Because of the very small time step it may even look like your simulation is converged as residuals and quantities of interest vary so little that they look constant.