Thanks Daniel. I have to admit, I'm a bit surprised that these types of problems are a challenge in FiPy, but I appreciate the information.
On Thu, Aug 29, 2019 at 3:06 PM Daniel Wheeler <daniel.wheel...@gmail.com> wrote: > Hi Daniel, > > Solving the flow equations isn't easy and we certainly don't provide > much guidance on how to solve them with FiPy. We do have the Stokes > cavity example [1] and reactive wetting example [2], which might be a > good place for you to start. The Stokes cavity example demonstrates > the simple algorithm [3] which basically turns the continuity equation > into an equation that solves for pressure. However, this becomes more > complicated if the convection terms are included in the momentum > equation. > > I have solved compressible flow problems with FiPy for my research. > FWIW, I have code for this, see [4, 5]. However, I don't think that > the code is much use to you other than as a reference for what is > possible with FiPy given enough persistence. > > You might also want to try Dolfyn [6]. It might have an example that > you can use right out of the box. > > Cheers, > > Daniel > > [1]: > https://www.ctcms.nist.gov/fipy/examples/flow/generated/examples.flow.stokesCavity.html > [2]: > https://www.ctcms.nist.gov/fipy/examples/reactiveWetting/generated/examples.reactiveWetting.liquidVapor1D.html#module-examples.reactiveWetting.liquidVapor1D > [3]: https://en.wikipedia.org/wiki/SIMPLE_algorithm > [4]: https://gist.github.com/wd15/c28ab796cb3d9781482b01fb67a7ec2d > [5]: https://journals.aps.org/pre/abstract/10.1103/PhysRevE.82.051601 > [6]: https://www.dolfyn.net/index_en.html > > On Thu, Aug 29, 2019 at 10:33 AM Daniel DeSantis <desan...@gmail.com> > wrote: > > > > Hello to the FiPy Team: > > > > I was recently reviewing some old work and thinking of converting it > over to FiPy and realized that, while I had done some successful work in > FiPy (with your assistance several times), there are a few gaps in my > knowledge of how to apply FiPy to some CFD type problems. I apologize for > asking questions you may feel are clear in your guide, but I am trying to > learn how to use your awesome program a bit better so I don't have to keep > asking questions. > > > > I'm essentially looking at coupling the continuity equation for a fluid > with the equations for motion or the equations of change. So, we start with > this: > > > > \nabla(\rho*v) = -\frac{\partial \rho}{\partial t} > > > > But frequently I have a steady state situation in which: > > \nabla(\rho*v) = 0 > > > > In either case, I'm not sure how the continuity equation up in FiPy. Do > you have any suggestions? Should I set up multiple convection terms like > this: > > > > PowerLawConvectionTerm(coeff=rho, var = v_x) + > PowerLawConvectionTerm(coeff=rho, var = v_y) + > PowerLawConvectionTerm(coeff=rho, var = v_z) = 0 > > > > Similarly, we have momentum equations as: > > > > \nabla(\rho v_x) = -\frac{\partial{P}}{\partial x} +\rho g_x + \mu > \nabla^2v_x > > > > \nabla(\rho v_y) = -\frac{\partial{P}}{\partial y} +\rho g_y + \mu > \nabla^2v_y > > > > \nabla(\rho v_z) = -\frac{\partial{P}}{\partial z} +\rho g_z + \mu > \nabla^2v_z > > > > I frequently am able to ignore the \rho g_i term but if I didn't, I > believe I could treat this as a source term in Fipy, and simply add it to > my equation. I understand how to do the diffusion and convection terms, > from your guides. > > > > However, I don't know what to do with the pressure term. I don't usually > have a consistent pressure in all dimensions so I would think that the > pressure term should be a cell variable, but I don't know how to add it. I > tried treating it as P.faceGrad, but I get errors. I've tried it as a > PowerLawConvectionTerm, and I don't seem to get a result. Perhaps the > problem is with the continuity equation? > > > > As a sort of classic example, I've uploaded a code that describes two > plates with a fluid in between. In it, fluid is flowing between two plates. > The bottom plate is stationary, the top one is moving. There's a pressure > difference over the length of the plates. It's at steady state, but I'll > probably end up playing with it as a non-steady state system eventually. > I've tried it before, and perhaps I'm graphing it incorrectly, but it seems > like it just presents a steady state solution. > > > > If I've got this completely wrong, could you suggest how to set up a > continuity equation coupled with a momentum equation and, perhaps how to > handle a first order differential term like the pressure gradient? > > > > Thank you, > > > > -- > > Daniel DeSantis > > > > > > _______________________________________________ > > fipy mailing list > > fipy@nist.gov > > http://www.ctcms.nist.gov/fipy > > [ NIST internal ONLY: https://email.nist.gov/mailman/listinfo/fipy ] > > > > -- > Daniel Wheeler > > _______________________________________________ > fipy mailing list > fipy@nist.gov > http://www.ctcms.nist.gov/fipy > [ NIST internal ONLY: https://email.nist.gov/mailman/listinfo/fipy ] > -- Daniel DeSantis
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