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- - **Problem with very high gradients**
(*http://www.cfd-online.com/Forums/openfoam-solving/109823-problem-very-high-gradients.html*)

Problem with very high gradients2 Attachment(s)
Hi Foamers,
i'm trying to implent a new boundary condition for which i need the second derivative of U in normal direction. (slip velocity) In order to get a second order derivative (which isn't laplacian) i'll do: Code:
` volScalarField Ux(U.component(0));` | Ux,xx Uy,xx Uz,xx | d2U=| Ux,yy Uy,yy Uz,yy | | Ux,xx Uy,xx Uz,zz | This works great but only to a certain dimension of the mesh. Attached to this post there are 2 cases and a modified rhoPimpleFoam solver. the first case "microKanal2"(Kanal:channel in german) has a height of 1.68e-3 meters. The velocity field result in a parabola with an increasing maximum for higher x values. Therefore the first derivative dU shows in y direction a linear function. The tensor compoent(3) Ux,yy should be a a constant value for a cross section of the channel, but changing with the position of x (see picture). the second case "microKanal" has a height of 1.68e-4 meters. For component 3 of the tensor there are following values (see attached picture) The field isn't smooth at all it suddenly jumps to another value. A possible reason for that could be that double isn't accurate enough. Does anyone know what the problem is and how it can be solved? Best regards Henning http://dl.dropbox.com/u/9273087/microKanal2.tar.gz http://dl.dropbox.com/u/9273087/microKanal.tar.gz http://dl.dropbox.com/u/9273087/rhoPimpleFoam.tar.gz |

hello
Where did you write these codes? |

the code is in the rhoPimpleFoam.C after the pimple loop
or what do you mean exactly? |

Hello Henning
I am facing the same problem. I need to calculate second derivative e.g for field F i need F_xx, F_yy. I did it the same way as you. gradF = fvc::grad(F) // vector of first derivatives gradgradF = fvc::grad(gradF) // tensor of second derivatives as far as gradF is fine/smooth, gradgradF is not anymore ... Were you able to solve this problem ? Best ZMM |

sadly not.
for what do you need it? |

Dear Henning,
please see my post located at: http://www.cfd-online.com/Forums/ope...erivative.html where I am writing about other way of dealing with second order mixed derivative. What do you think about that ? Best ZMM |

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