# Fast interpolation between two meshes

I have a mesh and I solve a problem with FEniCS using that mesh.

Every time step I need to evaluate the solution obtained with FEniCS in a specific set of points (large). I can use the function interpolate but I was thinking that since my points are always the same if I could use something similar to assemble. This assemble would correspond to generating a matrix with the evaluation of each basis functions at each point where I wish to interpolate. Then, interpolation would be obtained just by multiplying my solution vector by this matrix.

Is this possible?

Thank you.

-artur palha

## Question information

- Language:
- English Edit question

- Status:
- Solved

- For:
- DOLFIN Edit question

- Assignee:
- No assignee Edit question

- Solved by:
- Mikael Mortensen

- Solved:
- 2013-03-06

- Last query:
- 2013-03-06

- Last reply:
- 2013-03-05

## This question was reopened

Hi

Den Mar 4, 2013 kl. 2:41 PM skrev Artur Palha:

> New question #223355 on DOLFIN:

> https:/

>

> I have a mesh and I solve a problem with FEniCS using that mesh.

>

> Every time step I need to evaluate the solution obtained with FEniCS in a specific set of points (large). I can use the function interpolate but I was thinking that since my points are always the same if I could use something similar to assemble. This assemble would correspond to generating a matrix with the evaluation of each basis functions at each point where I wish to interpolate. Then, interpolation would be obtained just by multiplying my solution vector by this matrix.

>

> Is this possible?

If I understand you correctly it sort of is, but it's not part of the basic FEniCS package. I do turbulence simulations and typically need to probe one or many locations many times during a simulation. I use the pdesys (https:/

If you install CBC.PDESys the following demo will interpolate a solution onto a structured 2D mesh each timestep (you could alternatively use another FEniCS mesh to achieve the same)

from cbc.cfd.tools.Probe import *

# Create a solution Function that we want to probe many times. Any mesh will do

mesh = UnitSquareMesh(20, 20)

V = VectorFunctionS

f0 = Expression(

# Create a structured mesh on a rectangle [0.25, 0.75] x [0.25, 0.75]

N = 20

xx = linspace(0.25, 0.75, N)

xx = xx.repeat(

x = zeros((N*N, 3))

for i in range(N):

for j in range(N):

x[i*N + j, 0 ] = xx[j, i] # x-value

x[i*N + j, 1 ] = xx[i, j] # y-value

# Note that we could also use another FEniCS mesh here instead, but that only works in serial

# Create probes for all points in x

num_evals = 10

#probes = CppProbes(x, V) # Fast C++ version

probes = Probes(x, V, num_evals) # Slower Python version

# Evaluate the solution Function f inside a loop and probe it

for j in range(num_evals):

# Modify the solution Function

f0.t = j*0.5

f = interpolate(f0, V)

# Evaluate all probes

probes(f)

# Check the first probe on each processor

probe_0 = probes[0]

print "Global index and position of probe = ", probe_0[0], probe_0[

print "Values f[0] = ", probe_0[

print "Values f[1] = ", probe_0[

if MPI.num_processes() == 1:

# plot the solution if running with just one proc

from scitools.std import surfc

z = zeros((N, N, num_evals))

for i in range(N):

for j in range(N):

val = probes[i*N + j][1].get_probe(0)

z[i, j, :] = val[:]

plot(f[0])

surfc(

Best regards

Mikael

>

> Thank you.

>

> -artur palha

>

> --

> You received this question notification because you are a member of

> DOLFIN Team, which is an answer contact for DOLFIN.

Artur Palha (artur-palha) said : | #2 |

Thank you Mikael, I will give a try on that.

-artur palha

Artur Palha (artur-palha) said : | #3 |

Thanks Mikael Mortensen, that solved my question.