Macro-element footprint on ReynoldsStress
******************* This email originates from outside Imperial. Do not click on links and attachments unless you recognise the sender. If you trust the sender, add them to your safe senders list https://spam.ic.ac.uk/SpamConsole/Senders.aspx to disable email stamping for this address. ******************* Dear All I am doing a quasi 3-d simulation with a periodic direction in Z. When I use the "ReynoldsStresses" filter, I get a smooth time-averaged velocity, but I see the footprints of macro-elements on the Reynolds stress contours. I wonder why this can happen, as the Reynolds stresses are derived directly from the velocity and as far as I can tell, the representation of velocity contours is fine. Is there any reason that can explain this? Kind regards syavash
Hi Ehsan, I'm pretty sure this is linked the continuity of the solution spatial derivatives across elements boundaries, I've always had similar "issues" with derived quantities, like vorticity and Reynolds stresses. Please keep in mind that the Reynolds stresses are usually evaluated by taking the spatial derivatives of the continuous velocity field, once you calculate the derivative it's highly likely that you end up with a "less smooth" function and possibly discontinuous across large elements boundaries. I would rather try to refine a little bit in order to preserve gradients smoothness across the elements boundaries. That is, scaling down the elements size relatively below the sharp gradients length scale. Or try some post-processing interpolation to smooth out the gradients. Hope it helps Regards Saad On Thu, Sep 28, 2023, 15:17 Ehsan Asgari <eh.asgari@gmail.com> wrote:
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Dear All
I am doing a quasi 3-d simulation with a periodic direction in Z. When I use the "ReynoldsStresses" filter, I get a smooth time-averaged velocity, but I see the footprints of macro-elements on the Reynolds stress contours. I wonder why this can happen, as the Reynolds stresses are derived directly from the velocity and as far as I can tell, the representation of velocity contours is fine.
Is there any reason that can explain this?
Kind regards syavash
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Hi Saad Thank you for your reply. I understand the C1 continuity problem with for example vorticity. But to my understanding, Reynolds stresses are local quantities and generally don't require spatial derivation. Or I might be missing something here with the finite-element formulation? Please correct me if I'm wrong. Kind regards syavash On Thu, Sep 28, 2023 at 11:52 AM Debbahi Saad <saad.debbahi@gmail.com> wrote:
Hi Ehsan,
I'm pretty sure this is linked the continuity of the solution spatial derivatives across elements boundaries, I've always had similar "issues" with derived quantities, like vorticity and Reynolds stresses. Please keep in mind that the Reynolds stresses are usually evaluated by taking the spatial derivatives of the continuous velocity field, once you calculate the derivative it's highly likely that you end up with a "less smooth" function and possibly discontinuous across large elements boundaries.
I would rather try to refine a little bit in order to preserve gradients smoothness across the elements boundaries. That is, scaling down the elements size relatively below the sharp gradients length scale.
Or try some post-processing interpolation to smooth out the gradients.
Hope it helps
Regards
Saad
On Thu, Sep 28, 2023, 15:17 Ehsan Asgari <eh.asgari@gmail.com> wrote:
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Dear All
I am doing a quasi 3-d simulation with a periodic direction in Z. When I use the "ReynoldsStresses" filter, I get a smooth time-averaged velocity, but I see the footprints of macro-elements on the Reynolds stress contours. I wonder why this can happen, as the Reynolds stresses are derived directly from the velocity and as far as I can tell, the representation of velocity contours is fine.
Is there any reason that can explain this?
Kind regards syavash
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participants (2)
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Debbahi Saad
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Ehsan Asgari