How i can use Vector Expression?

variational formulation
1

I have one expression, but this scalar:

vel_in = Expression("dp/mu * (1/2 * x[2]*x[2] + 16*h*h/(pi*pi))* \
				(1/pow(1,3)*cos(1/4 * pi*x[2]/h)*(1 - cosh(1/4 * pi * x[1]/h)/cosh(1/4 * pi)) - \
				  1/pow(3,3)*cos(3/4 * pi*x[2]/h)*(1 - cosh(3/4 * pi * x[1]/h)/cosh(3/4 * pi)) + \
				  1/pow(5,3)*cos(5/4 * pi*x[2]/h)*(1 - cosh(5/4 * pi * x[1]/h)/cosh(5/4 * pi)) - \
				  1/pow(7,3)*cos(7/4 * pi*x[2]/h)*(1 - cosh(7/4 * pi * x[1]/h)/cosh(7/4 * pi)) + \
				  1/pow(9,3) * cos(9/4 * pi*x[2]/h)*(1 - cosh(9/4 * pi * x[1]/h)/cosh(9/4 * pi)) \
				)", dp=20, h=0.0002, mu=mu, degree = 2)

I want to set this expression on x[1] in boundary condition. I try:

bcp_inflow_top = DirichletBC(V, Constant((0,vel_in,0)), facet_domains, 14)

and try:

bcp_inflow_top = DirichletBC(V, (0,vel_in,0), facet_domains, 14)

But i have error.

2

See for instance: Stokes equations with an iterative solver — DOLFIN documentation

3

Is it possible to solve an equation from a vector for only one scalar component?
I want to solve Poisson’s equation for only one of the components of a vector

4

If you have made a vector space, you would need to collapse the sub space to solve a Poisson equation for only one component. Your questions is very generic, does not contain a minimal example illustrating what you want to do, and does as far as I can tell not relate to the original question.