[CIG-SHORT] Velocity BCs in PyLith

[Brad Aagaard]

Tabrez-

Dirichlet boundary conditions (displacement and velocity boundary conditions) 
are explained in section 6.2 of the PyLith manual. The equation in section 
6.2, u(t) = u0 + (t - tref) v0, defines the displacements as a function of 
time. The default values for tref and v0 are 0 seconds and 0 m/s, 
respectively.

The default database for Dirichlet BCs is FixedDOFDB. This hardwires the 
boundary condition values to zero (zero displacements for the Dirichlet db or 
zero velocities for the Dirichlet rate_db). If you want nonzero values, you 
must change the spatial database being used. Unfortunately, in the current 
implementation there is no error message if you try setting the values for 
the FixedDOFDB; it just ignores them. If you want uniform values, use the 
spatialdata.spatialdb.UniformDB spatial database. You can also specify a 
spatialdata.spatialdb.SimpleDB spatial database.

For example,

[pylithapp.timedependent.bc.x_pos]
rate_db = spatialdata.spatialdb.UniformDB
rate_db.values = [dof-0,dof-1,dof-2]
rate_db.data = [1.0,2.0,3.0]

BUGFIX: There was indeed a bug in setting the reference time. The reference 
time from the input was not transferred to the boundary condition. Since 
there is an obvious workaround for this bug (simply adjust the displacements 
and rates to use a reference time of 0.0), we will not make a special bugfix 
release to resolve this issue. This bug has been fixed in the development 
version so the fix will be included in the next release.

Brad

On Tuesday 13 May 2008, Tabrez Ali wrote:
> PyLith Developers/Users
>
> What is the proper way to apply velocity BCs in PyLith 1.1.x?
>
> Lets suppose I have a displacement BC specified in "bc1.spatialdb"
> (attached) and if I want to have the same as my velocity BC (i.e., same
> displacements applied at some delta t) then what do I have to add to the
> following:
>
> [pylithapp.timedependent.bc.bc1]
> fixed_dof = [0, 1, 2]
> label = 1
> db = spatialdata.spatialdb.SimpleDB
> db.label = Dirichlet BC 1
> db.iohandler.filename = bc1.spatialdb
>
> Specifying  the "rate_db.values" and "rate_db.data" doesnt seem to work.
> Also what exactly is "reference_t"? Do I have to specify that too?
>
> T