Fetching normals and tangents of a Tri6

[rebpoli]

Hi,

It seems the get_tangents method only works on faces (sides of a Tet, for example).
I need tangents and normals for a Tri6.
Do libmesh interfaces provide that?

Thanks,
Renato

[jwpeterson]

You can call:

const std::vector<RealGradient>& dxyzdxi = fe->get_dxyzdxi();
const std::vector<RealGradient>& dxyzdeta = fe->get_dxyzdeta();

Then the "face" normal to the Tri6 at quadrature point qp should be given by:

dxyzdxi[qp].cross(dxyzdeta[qp]).unit()

[rebpoli]

Thanks, If I got it properly, I would have: Point tan1 = dxyzdxi[qp].unit(); Point tan2 = dxyzdxi[qp].unit(); Point normal = dxyzdxi[qp].cross(dxyzdeta[qp]).unit(); Is that right? Em qua., 3 de nov. de 2021 às 16:01, John W. Peterson < ***@***.***> escreveu:

…

You can call: const std::vector<RealGradient>& dxyzdxi = fe->get_dxyzdxi(); const std::vector<RealGradient>& dxyzdeta = fe->get_dxyzdeta(); Then the "face" normal to the Tri6 at quadrature point qp should be given by: dxyzdxi[qp].cross(dxyzdeta[qp]).unit() — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub <#3073 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AMOVERZCZHKD3XFRKOZ7B33UKGIJXANCNFSM5HJVIWQA> . Triage notifications on the go with GitHub Mobile for iOS <https://apps.apple.com/app/apple-store/id1477376905?ct=notification-email&mt=8&pt=524675> or Android <https://play.google.com/store/apps/details?id=com.github.android&referrer=utm_campaign%3Dnotification-email%26utm_medium%3Demail%26utm_source%3Dgithub>.

[jwpeterson]

Yes, I think that should work, but it would be good if you could check that the results you get seem reasonable.

[jwpeterson]

Actually your "tan2" should use dxyzdeta.

[rebpoli]

Sorry for the typo. Well .. Below is what I got (on the left would be the expected, on the right the resulting). I can see that *dxyzdxi* is not orthogonal to *dxyzdeta*. Is that expected? Point tan_s = dxyzdxi[qp].unit(); Point tan_t = dxyzdeta[qp].unit(); Point normal_f = dxyzdxi[qp].cross(dxyzdeta[qp]).unit(); tan_s: (x,y,z)=( -1, 0, 0) tan_t: (x,y,z)=(-0.672813, 0, 0.739813) normal_f:(x,y,z)=( 0, 1, 0) Em qua., 3 de nov. de 2021 às 16:14, John W. Peterson < ***@***.***> escreveu:

…

Actually your "tan2" should use dxyzdeta. — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub <#3073 (reply in thread)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AMOVER43KZILJITYSUHYVC3UKGJ27ANCNFSM5HJVIWQA> . Triage notifications on the go with GitHub Mobile for iOS <https://apps.apple.com/app/apple-store/id1477376905?ct=notification-email&mt=8&pt=524675> or Android <https://play.google.com/store/apps/details?id=com.github.android&referrer=utm_campaign%3Dnotification-email%26utm_medium%3Demail%26utm_source%3Dgithub>.

[jwpeterson]

OK, I have actually only used this formula with Quad4 and Tri3 and it seemed to work OK for this, but maybe it doesn't generalize? I would suggest you find the orthogonal projection of tan_t onto tan_s in the plane spanned by tan_s and tan_t and use that for the second tangent vector instead if it's important for the two tangent directions to be orthogonal...

[rebpoli]

Thanks! Em qua., 3 de nov. de 2021 às 16:36, John W. Peterson < ***@***.***> escreveu:

…

OK, I have actually only used this formula with Quad4 and Tri3 and it seemed to work OK for this, but maybe it doesn't generalize? I would suggest you find the orthogonal projection of tan_t onto tan_s in the plane spanned by tan_s and tan_t and use that for the second tangent vector instead if it's important for the two tangent directions to be orthogonal... — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub <#3073 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AMOVERZ56Y53W7AJBGVL6IDUKGMOJANCNFSM5HJVIWQA> . Triage notifications on the go with GitHub Mobile for iOS <https://apps.apple.com/app/apple-store/id1477376905?ct=notification-email&mt=8&pt=524675> or Android <https://play.google.com/store/apps/details?id=com.github.android&referrer=utm_campaign%3Dnotification-email%26utm_medium%3Demail%26utm_source%3Dgithub>.