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 ]
