Visualisation postprocessor for spherical velocities

ASPECT
1

Hi all,
Standard velocity visualisation output for paraview give results in full vectors (arrows) and separated in x,y,z-directions. However, regarding a spherical set-up (eg. spherical shell, chunk or sphere) I would like to visualise the velocity field also in separate radial and tangential velocities.

I was wondering if someone already has written a postprocessor that creates vtu-files containing these spherical velocities and is willing to share it?

Or if anyone could give me a heads-up regarding an existing postprocessor which I could try to modify to make it myself and, of course, any tips in how to do so.

Thanks in advance,
Erik

2

Hi Erik,
I do not have such a postprocessor, but you are right it would be very useful to have one. If nobody has one available and if you want to create one on your own I would start with a copy of one of the existing postprocessors that creates a vector or tensor quantity, e.g. source/postprocess/visualization/gravity.cc or source/postprocess/visualization/shear_stress.cc. The function evaluate_vector_field in these postprocessors is called once per cell whenever output is written, and the input parameter input_data contains the positions and velocities at every location we want to write output for in this cell. E.g. you can do input_data.solution_values[q][this->introspection().component_indices.velocities[d]] to access the velocity (as Tensor<1,dim>) at the q’th output vertex of that cell. Correspondingly input_data.evaluation_points[q] accesses the related positions (as Point<dim> in cartesian coordinates). You would then need to convert the velocities using the position into radius/longitude/latitude components and write those into the computed_quantities parameter. I think this would make a fine addition to the main code so please let us know if you do this or need any help.

Best,
Rene

3

Actually, if you’ve already extracted the velocity as in

  const Tensor<1,dim> velocity = ...

and assuming that you have also gotten the gravity vector somehow (there are examples for both of these things)

  const Tensor<1,dim> gravity = ...

then you can get the vertical (scalar) component of the velocity via

  const double vertical_velocity = (velocity * gravity) / gravity.norm();

and then the horizontal part of the velocity vector is simply

  const Tensor<1,dim> horizontal_velocity
    = velocity - vertical_velocity * gravity/gravity.norm();

This has the advantage that you don’t have to convert between different coordinate systems. It also works in 2d/3d, and in spherical/Cartesian coordinate systems. The only assumption is that the gravity vector indicates a good vertical direction – which is generally true.

Best
W.

4

The file source/postprocess/visualization/vertical_heat_flux.cc shows how to work around the issue if the gravity vector is zero.