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
