From: David Knezevic <dknez@MIT.EDU>  20090913 00:32:55

Quoting John Peterson <peterson@...>: > On Sat, Sep 12, 2009 at 4:08 PM, Ted Kord <teddy.kord@...> wrote: >> 2009/9/12 David Knezevic <dknez@...> >> >>> Roy Stogner wrote: >>> >>>> >>>> On Sat, 12 Sep 2009, David Knezevic wrote: >>>> >>>> Ted Kord wrote: >>>>> >>>> >>>> How do I apply a Neumann B.C at an interelement 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 interelement >>>>> 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 * diracdelta(x16) >> >> 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 pointload 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 interelement flux... 