how to extract vector field components

Question

Dear all,
i've solved Navier-stokes stationary problem in 2D. I would like to obtain velocity field components, e.g. ux and uy. How can i do it ?

Answer 1

You can simply form the dot-product with a unit vector.

n = Expression(('0.0','1.0'))
uy = dot(field,n)

I did not find a direct way to do it.

Comments

Thank you very much !!!! Could I ask you how to evaluate the vorticity values on different points of domain (2D square cavity) ?

Best reguards
Francesco

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Just use

x = np.array([0.0,0.0])
print test_field(x)

this will also split it up in x and y direction

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Last question ( i hope )
I found the solution of stationary Navier-Stokes (for square cavity) , I would estimate the vorticity in the different points but I always find some errors:
u #velocity field
w = curl (u) # vorticity field
np.array center = ([0.5,0.5])
print w (center) # -> error #

Best reguards

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Could be anything, could you please post the error meassage?

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sure,

#

Traceback (most recent call last):
File "nspf.py", line 202, in
print vor(center)
File "/home/rbnics/install/fenics/2016-03-09-93f759d/petsc-3.6.3/trilinos-/lib/python2.7/site-packages/ufl/exproperators.py", line 285, in _call
return _eval(self, arg, mapping, component)
File "/home/rbnics/install/fenics/2016-03-09-93f759d/petsc-3.6.3/trilinos-/lib/python2.7/site-packages/ufl/exproperators.py", line 277, in _eval
return f.evaluate(coord, mapping, component, index_values)
File "/home/rbnics/install/fenics/2016-03-09-93f759d/petsc-3.6.3/trilinos-/lib/python2.7/site-packages/ufl/algebra.py", line 101, in evaluate
return sum(o.evaluate(x, mapping, component, index_values) for o in self.ufl_operands)
File "/home/rbnics/install/fenics/2016-03-09-93f759d/petsc-3.6.3/trilinos-/lib/python2.7/site-packages/ufl/algebra.py", line 101, in
return sum(o.evaluate(x, mapping, component, index_values) for o in self.ufl_operands)
File "/home/rbnics/install/fenics/2016-03-09-93f759d/petsc-3.6.3/trilinos-/lib/python2.7/site-packages/ufl/indexed.py", line 90, in evaluate
return A.evaluate(x, mapping, component, index_values)
File "/home/rbnics/install/fenics/2016-03-09-93f759d/petsc-3.6.3/trilinos-/lib/python2.7/site-packages/ufl/differentiation.py", line 154, in evaluate
derivatives=derivatives)
File "/home/rbnics/install/fenics/2016-03-09-93f759d/petsc-3.6.3/trilinos-/lib/python2.7/site-packages/ufl/core/terminal.py", line 67, in evaluate
return self.ufl_evaluate(x, component, derivatives)
File "/home/rbnics/install/fenics/2016-03-09-93f759d/petsc-3.6.3/trilinos-/lib/python2.7/dist-packages/dolfin/functions/function.py", line 463, in ufl_evaluate

#

assert derivatives == () # TODO: Handle derivatives
AssertionError

#

Thank you !!!

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I can confirm this behaviour...

Code:

UV.vector()[:] = ev_r # ev_r is a result from SLEPc

xx = np.array([1., 2.]) # point within the 2d domain

print UV(xx) # Works fine

H_phi = grad_rz(UV)/(mu02.np.pi*f)

print H_phi(xx) # crashes (see below)

Funny enough, I can plot H_phi (using plot(H_phi))

Traceback (most recent call last):
  File "test18.py", line 159, in 
    print H_phi(xx)
  File "/usr/lib/python2.7/dist-packages/ufl/exproperators.py", line 269, in _call
    return _eval(self, arg, mapping, component)
  File "/usr/lib/python2.7/dist-packages/ufl/exproperators.py", line 261, in _eval
    return f.evaluate(coord, mapping, component, index_values)
  File "/usr/lib/python2.7/dist-packages/ufl/algebra.py", line 259, in evaluate
    a = a.evaluate(x, mapping, component, index_values)
  File "/usr/lib/python2.7/dist-packages/ufl/algebra.py", line 101, in evaluate
    return sum(o.evaluate(x, mapping, component, index_values) for o in self.ufl_operands)
  File "/usr/lib/python2.7/dist-packages/ufl/algebra.py", line 101, in 
    return sum(o.evaluate(x, mapping, component, index_values) for o in self.ufl_operands)
  File "/usr/lib/python2.7/dist-packages/ufl/indexed.py", line 93, in evaluate
    return A.evaluate(x, mapping, component, index_values)
  File "/usr/lib/python2.7/dist-packages/ufl/differentiation.py", line 150, in evaluate
    derivatives=derivatives)
  File "/usr/lib/python2.7/dist-packages/ufl/core/terminal.py", line 80, in evaluate
    return self.ufl_evaluate(x, component, derivatives)
  File "/usr/lib/python2.7/dist-packages/dolfin/functions/function.py", line 465, in ufl_evaluate
    assert derivatives == () # TODO: Handle derivatives
AssertionError

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Seems like I found the problem: The result remains an ufl algebra object until it is evaluated. That's probably why it works with plot. Projecting it seems to help:

H_phi = dolfin.project(grad_rz(UV)/(mu0*2.*np.pi*f))
print H_phi(xx)

Works...