On Wed, Apr 16, 2014 at 5:38 PM, Sang pham van <[email protected]> wrote:
> Yes, It does converge toward 0-flow when I refine the mesh ! > > In the code, coordinates are set by > ierr = > DMDASetUniformCoordinates(da_prop,0.0+0.5*dx,1.0-0.5*dx,0.0+0.5*dy,1.0-0.5*dy,0.,0);CHKERRQ(ierr); > Does it set coordinate for elements center here? > When I want to try non-uniform grid, should I just modify coordinates > attached with the DM, or I need more implementation? > Yes, I think that should be enough. Matt > Many thanks. > Sang > > > > On Wed, Apr 16, 2014 at 6:23 PM, Matthew Knepley <[email protected]>wrote: > >> On Wed, Apr 16, 2014 at 4:48 PM, Sang pham van <[email protected]>wrote: >> >>> Hi Jed, >>> >>> I modified the ex43 code to enforce no-slip BCs on all boundaries. >>> I run the code with volume force (0,-1) and isoviscosity. The expected >>> result is Vx = Vy = 0 everywhere, and linearly decreasing pressure (from to >>> to bottom). >>> In the attached is plot of velocity field and pressure, so there is >>> still a (light) flow in middle of the domain. Do you know why the solution >>> is that, and what should I do to get the expected result? >>> >> >> It sounds like it is due to discretization error. Your incompressibility >> constraint is not verified element-wise >> (I think ex43 is penalized Q1-Q1), so you can have some flow here. Refine >> it and see if it converges toward >> 0 flow. >> >> Matt >> >> >>> Thank you. >>> >>> Sang >>> >>> >>> >>> >>> On Wed, Mar 26, 2014 at 2:38 PM, Jed Brown <[email protected]> wrote: >>> >>>> Sang pham van <[email protected]> writes: >>>> >>>> > Hi Dave, >>>> > I guess you are the one contributed the ex42 in KSP's examples. I >>>> want to >>>> > modify the example to solve for stokes flow driven by volume force in >>>> 3D >>>> > duct. Please help me to understand the code by answering the following >>>> > questions: >>>> > >>>> > 1. Firstly, just for confirmation, the equations you're solving are: >>>> > \nu * \nabla \cdot \nabla U - \nabla P = 0 and >>>> >>>> For variable viscosity, it must be formulated as in the example: >>>> >>>> \nabla\cdot (\nu D U) - \nabla P = 0 >>>> >>>> where D U = (\nabla U + (\nabla U)^T)/2 >>>> >>>> > \nabla \cdot U = 0 >>>> > >>>> > where U = (Ux,Uy,Uz), \nu is variable viscosity? >>>> > >>>> > 2. Are U and P defined at all nodes? (I googled the Q1Q1 element, it >>>> looks >>>> > like a box element with U and P defined at 8 corners). >>>> >>>> Yes. >>>> >>>> > 3. Are nodes' coordinate defined though the DA coordinates? >>>> >>>> Yes, though they are set to be uniform. >>>> >>>> > 4. How can I enforce noslip BC, and where should I plug in volume >>>> force? >>>> >>>> Enforce the Dirichlet condition for the entire node. >>>> >>> >>> >> >> >> -- >> What most experimenters take for granted before they begin their >> experiments is infinitely more interesting than any results to which their >> experiments lead. >> -- Norbert Wiener >> > > -- What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead. -- Norbert Wiener
