# [aspect-devel] Boundary condition in Velocity

FELIPE ORELLANA ROVIROSA f_orellana at berkeley.edu
Tue Mar 15 00:16:07 PDT 2016

``` Hi all,

I am trying to implement a Dirichlet boundary condition in Velocity on a
3D convecting system, perhaps a time-dependent BC, somehow following the
cookbook's 2D example.

I did this:
set Prescribed velocity boundary indicators = top: function
set Tangential velocity boundary indicators = left, right, front, back,
bottom

and then outside the subsection Model settings, I created the corresponding

subsection Boundary velocity model
subsection Function
set Variable names      = x,z,t
set Function expression = if(x> 1 + 0.5*t, 1, -1); 0
# set Function expression = 1                                 ##( this
is a simpler trial)
end
end

but it does not work, it produces Parsing errors:

ERROR: FunctionParser failed to parse
ERROR: FunctionParser failed to parse
ERROR: FunctionParser failed to parse
'Boundary velocity model.Function'
'Boundary velocity model.Function'
'Boundary velocity model.Function'
with expression
with expression
with expression
'    '    'ERROR: FunctionParser failed to parse
if(x> 1 + 0.5*t, 1, -1); 0    'Boundary velocity model.Function'
'if(x> 1 + 0.5*t, 1, -1); 0if(x> 1 + 0.5*t, 1, -1); 0with expression
''    'if(x> 1 + 0.5*t, 1, -1); 0'ERROR: FunctionParser failed to parse
'Boundary velocity model.Function'
with expression
'if(x> 1 + 0.5*t, 1, -1); 0'ERROR: FunctionParser failed to parse
'Boundary velocity model.Function'
...
..

I have tried changing upper case to lower case in the function declaration,
I have tried giving the function a specific name (a nickname that
distinguishes from the rest of the code), none of these worked.

I am pretty sure someone has done this, I guess there is an extra thing to
set.