Quoting John Peterson <[email protected]>: > On Sat, Sep 12, 2009 at 4:08 PM, Ted Kord <[email protected]> wrote: >> 2009/9/12 David Knezevic <[email protected]> >> >>> Roy Stogner wrote: >>> >>>> >>>> On Sat, 12 Sep 2009, David Knezevic wrote: >>>> >>>> Ted Kord wrote: >>>>> >>>> >>>> How do I apply a Neumann B.C at an inter-element boundary? >>>>>> >>>>> >>>>> The same way as a usual Neumann BC... the only trick is that you have to >>>>> find which internal element to apply it to. One way to do this would be >>>>> to set the subdomain_id of elements on one side of the inter-element >>>>> boundary to 1 and on the other side to 2, and then search for elements >>>>> with subdomain_id = 1 that have a neighbor with subdomain_id = 2, and >>>>> apply the Neumann BC to the appropriate side of those elements. >>>>> >>>> >>>> The trouble with this is that you'll still have the entries in your >>>> matrix from the shape functions which stretch between the element on >>>> one side of the boundary and on the other. If you have a slit in your >>>> domain on which you want to weakly impose boundary conditions, you >>>> need to make it an actual topologically broken slit, and then it's >>>> just another set of exterior boundaries. >>>> >>> >>> I was thinking of imposing an internal flux between internal elements (e.g. >>> as a type of forcing, but inside the domain rather than on the >>> boundary). In >>> that situation an "internal" Neumann condition does the job --- the >>> variational formulation takes care of everything for you... >>> >>> - Dave >>> >> >> The problem I actually have is that there's a concentrated load at a single >> point, say x = 16 (domain: 0 < x < 20) which is represented mathematically >> as : >> >> -0.5 - 30 * dirac-delta(x-16) >> >> As far as I know, this, i.e., -30 will have to be applied as a Neumann B.C >> at that point. > > I wouldn't think of a point-load as a boundary condition ... it's not > a boundary condition. > > Assuming the delta function falls on a node in the mesh, you can just > modify the load vector entry for the basis function associated to that > row. > > -- > John >
Yeah, this is equivalent to what I described above... multiplying the delta function by a test function and integrating is equivalent to sampling the test function, so it ends up looking like an "internal Neumann condition" which imposes an inter-element flux... ------------------------------------------------------------------------------ Let Crystal Reports handle the reporting - Free Crystal Reports 2008 30-Day trial. Simplify your report design, integration and deployment - and focus on what you do best, core application coding. Discover what's new with Crystal Reports now. http://p.sf.net/sfu/bobj-july _______________________________________________ Libmesh-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/libmesh-users
