ufl team mailing list archive

[Martin Sandve Alnæs <martinal@xxxxxxxxx>]

On Fri, Apr 24, 2009 at 1:22 PM, Garth N. Wells <gnw20@xxxxxxxxx> wrote:
>
>
> Martin Sandve Alnæs wrote:
>>
>> On Fri, Apr 24, 2009 at 1:00 PM, Garth N. Wells <gnw20@xxxxxxxxx> wrote:
>>>
>>> To get upwinding in a DG formulation of the advection-diffusion eqn with
>>> the FFC .form language, we used
>>>
>>> vector = VectorElement("Lagrange", "triangle", 2)
>>> scalar = FiniteElement("Discontinuous Lagrange", "triangle", 1)
>>> constant = FiniteElement("Discontinuous Lagrange", "triangle", 0)
>>>
>>> u  = TrialFunction(scalar)
>>> b  = Function(vector)
>>> of = Function(constant)
>>>
>>> def upwind(u, b):
>>>  return [b[i]('+')*(of('+')*u('+') + of('-')*u('-')) for i in range(2)]
>>>
>>> to return the product b*u, with u taken from the 'upwind' side. How
>>> should the function 'upwind' look using UFL?
>>
>> Lets see...
>>
>> def upwind(u, b):
>>    s = of('+')*u('+') + of('-')*u('-')
>>    return [b[i]('+')*s for i in range(2)]
>>
>> This returns a list, so it's a vector-valued expression?
>> s is a scalar expression, and b is a vector-valued expression,
>> so I would simply do
>>
>> def upwind(u, b):
>>    s = of('+')*u('+') + of('-')*u('-')
>>    return b('+')*s
>>
>> Does that look right?
>>
>
> Yes. Even simpler: return b('+')*(of('+')*u('+') + of('-')*u('-')).
>
> I keep forgetting that simple things are simple with UFL ;).
>
> Garth

:)

In general, if you have a list you can make an ufl expression
of it using as_vector(mylist), or as_matrix or as_tensor.

Martin