## [Libmesh-users] About FEBase::dphi , FEBase::dphideta

 [Libmesh-users] About FEBase::dphi , FEBase::dphideta From: Shengli Xu - 2006-01-14 08:59:41 Attachments: Message as HTML ```Hello, Buddy Is it correctly that dphi=3D[d(phi)/d(x), d(phi)/d(y)]? dphideta is shape function derivatives in the eta direction. Is eta ont of the axis of local element coordination? If it is so, Which direction is eta= ? If the local element coordiantion is (psi, eta, gamma), Why do not define dphidpsi, dphidgamma? Shenglixu ```

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 [Libmesh-users] About FEBase::dphi , FEBase::dphideta From: Shengli Xu - 2006-01-14 08:59:41 Attachments: Message as HTML ```Hello, Buddy Is it correctly that dphi=3D[d(phi)/d(x), d(phi)/d(y)]? dphideta is shape function derivatives in the eta direction. Is eta ont of the axis of local element coordination? If it is so, Which direction is eta= ? If the local element coordiantion is (psi, eta, gamma), Why do not define dphidpsi, dphidgamma? Shenglixu ```
 Re: [Libmesh-users] About FEBase::dphi , FEBase::dphideta From: Roy Stogner - 2006-01-14 16:09:48 ```On Sat, 14 Jan 2006, Shengli Xu wrote: > Is it correctly that dphi=[d(phi)/d(x), d(phi)/d(y)]? Assuming you're either talking about the private member FEBase::dphi or about a reference returned by FEBase::get_dphi(), then the answer is yes in 2D. In 3D d(phi)/d(z) is there also. > dphideta is shape function derivatives in the eta direction. Is eta ont of > the axis of local element coordination? If it is so, Which direction is eta? On the diagrams of 2D master elements in include/geom/face_*.h, eta is up. On the diagrams of 3D master elements in include/geom/cell_*.h, I think eta points into the page. > If the local element coordiantion is (psi, eta, gamma), Why do not define > dphidpsi, dphidgamma? The local element coordinates are < xi, eta, zeta >. There are dphidxi, dphideta, dphidzeta defined as protected variables in FEBase, but user code shouldn't need them, it should be using the x y z derivatives instead. What does a PDE care about the orientation of your master element? --- Roy ```