# calculating partial derivatives

I am trying to solve a vorticity problem. I have got the exact expression for vorticity and I am trying to match the numerical vorticity. My exact velocity is represented as vel and my exact vorticity as vor. The expression for vor and vel are given below:

class exact(Expression):

def eval(self, value, x):

value[0] = x[1]**2

def value_shape(self):

return (2,)

vel = exact()

class vortex(Expression):

def eval(self, value, x):

value[0] = 2*(x[0]-x[1])

vor = vortex()

The vorticity is obtained from the velocity by setting vor = v_x-u_y where vel=(u,v) and v_x is the partial derivative of v

wrt x and u_y is the partial derivative of u wrt y.

Just to check I put this in my code:

exvor = Function(Z)

exvor = interpolate(vor,Z)

uvor = Function(Z)

uvor = project(

curlerr = errornorm(exvor, uvor,'L2')

where Z = FunctionSpace(

and mesh = UnitSquare(10,10)

In principle I am just using the partial derivative operator on the exact velocity to get the vorticity. So I expect the obtained vorticity to match with the Expression for vorticity given above.

But the curlerr = 0.0197852169412

I changed the number of points in the mesh from 10 to 100 but the curlerr is 0.000620929306036.

I expect it to reach machine precision (10^-15) as it has been achieved for the other quantities in the same code. I am

basically trying to do a patch test. What could be the problem?

## Question information

- Language:
- English Edit question

- Status:
- Answered

- For:
- DOLFIN Edit question

- Assignee:
- No assignee Edit question

- Last query:
- 2013-04-19

- Last reply:
- 2013-05-09

Jan Blechta (blechta) said : | #1 |

On Fri, 19 Apr 2013 19:06:13 -0000

Souvik Roy <email address hidden> wrote:

> New question #227070 on DOLFIN:

> https:/

>

> I am trying to solve a vorticity problem. I have got the exact

> expression for vorticity and I am trying to match the numerical

> vorticity. My exact velocity is represented as vel and my exact

> vorticity as vor. The expression for vor and vel are given below:

>

> class exact(Expression):

> def eval(self, value, x):

> value[0] = x[1]**2

> value[1] = x[0]**2

> def value_shape(self):

> return (2,)

>

> vel = exact()

>

> class vortex(Expression):

> def eval(self, value, x):

> value[0] = 2*(x[0]-x[1])

>

> vor = vortex()

>

> The vorticity is obtained from the velocity by setting vor = v_x-u_y

> where vel=(u,v) and v_x is the partial derivative of v wrt x and u_y

> is the partial derivative of u wrt y.

>

> Just to check I put this in my code:

>

> exvor = Function(Z)

> exvor = interpolate(vor,Z)

>

> uvor = Function(Z)

> uvor = project(

>

> curlerr = errornorm(exvor, uvor,'L2')

>

> where Z = FunctionSpace(

>

> and mesh = UnitSquare(10,10)

>

> In principle I am just using the partial derivative operator on the

> exact velocity to get the vorticity. So I expect the obtained

> vorticity to match with the Expression for vorticity given above.

>

> But the curlerr = 0.0197852169412

>

> I changed the number of points in the mesh from 10 to 100 but the

> curlerr is 0.000620929306036. I expect it to reach machine precision

> (10^-15) as it has been achieved for the other quantities in the same

> code. I am basically trying to do a patch test. What could be the

> problem?

Matching FE spaces. For example for CG2 velocity its derivatives are

from DG1.

Anders Logg (logg) said : | #2 |

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Anders Logg (logg) said : | #3 |

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## Can you help with this problem?

Provide an answer of your own, or ask Souvik Roy for more information if necessary.