<D VAR="u" VALUE="0" />
<D VAR="v" VALUE="0" />

<D VAR="w" VALUE="0" />
<N VAR="p" VALUE="0" USERDEFINEDTYPE="H" />

Do not use much additional information (only for the pressure) and so are valid for both non-linear and linear simulations.

The other type of boundary conditions require an evaluation of U.n on the outflow to decide if the flow is going inwards or outwards. I believe in the linearised case it may be using u’.n in the code which is likely wrong. We need to have a look at the formulation of these outflow condition first the linearised problem and update the code. Probably we should have a warning in this code if the flow is linearised!

If you want to have a look at the formulation I can help getting the code updated.

All the best,

Spencer

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On 7 Oct 2025, at 18:49, Alexander Schukmann <alexander.schukmann@protonmail.com> wrote:

Hello Spencer,

thank you for your quick reply. I obtained good results for the direct stability analysis with the following high-order outflow BC:

<N VAR="u" VALUE="0.0" USERDEFINEDTYPE="HOutflow" />
<N VAR="v" VALUE="0.0" USERDEFINEDTYPE="HOutflow" />

<N VAR="w" VALUE="0.0" USERDEFINEDTYPE="HOutflow" />
<D VAR="p" VALUE="0.0" USERDEFINEDTYPE="HOutflow" />

However, for the adjoint operator, weird artifacts emerge at the outlet, indicating onsetting instability. For this reason I tried the Robin-type BC and failed. As I wrote before, using the following pressure BC, the adjoint results look reasonable:

<D VAR="u" VALUE="0" />
<D VAR="v" VALUE="0" />

<D VAR="w" VALUE="0" />
<N VAR="p" VALUE="0" USERDEFINEDTYPE="H" />

Best,

Alex

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Sherwin, Spencer J <s.sherwin@imperial.ac.uk> schrieb am Dienstag, 7. Oktober 2025 um 8:35 nachm.:

Hi Alex,

I have not had a chance to look closely at this but possibly the issue is that these advanced BCs are only currently set up for non-linear problems. They rely on the total velocity dictating if the flow is inward or outwards but it may be that the stability codes are taking the linearised quantity rather quickly and than the base flow to evaluate this.

Best,

Spencer

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On 6 Oct 2025, at 19:38, Alexander Schukmann <alexander.schukmann@protonmail.com> wrote:

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Hello everyone,

I tried running an Adjoint Stability Analysis including Dongs convective outflow BC enforced through a Robin type specification as detailed in the user guide. I use a 2.5D expansion with version 5.7.0 and obtain satisfactory results with a high-order pressure BC. However, as I switch to a Robin BC, the following error appears:

double free or corruption (!prev)

...indicating segmentation violation.

I've attached some files of a MRE, which is basically just a cylinder at Re = 42. Is there anything wrong with my setup? Any help would be much appreciated.

Best,
Alex

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<session.txt>

<mesh.txt>

<base.txt>
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