# Probes¶

Probes can be used to ease postprocessing and to extract numbers directly from the simulation instead of having to write XDMF files which are large and may be somewhat interpolated from the “real” value inside the simulation. Often XDMF and probes are used at the same time, probes write their values every time step and XDMF may be written every 100 time steps for disk saving purposes.

The probes list contains dictionaries describing each probe. An example can be seen below under PointProbe. Some common configuration options for all the probes are:

name

Name of the probe, often used in the name of the output file

type

The probe type. One of

• IsoSurface

• LineProbe

• PlaneProbe

• PointProbe

enabled

Default on, you can turn of a probe if you want

file_name

Where to save the data. Normally a default value based on the name is used

write_interval

How often to write the probe results, default 1, every time step

custom_hook

When to run the probe. Normally run at the end oe each time step, but you can give the name of a hook here instead (typically MultiPhaseModelUpdated). Not all probes support this.

## IsoSurface¶

Only implemented in 2D for now. Outputs the position of an ISO surface. Can be used to save the free surface. Another way (for 3D) is to use XDMF output and create a Contour in Paraview as a post-processing step.

field

Name of the field to study, e.g., c

value

The value of the field at the ISO surface, e.g. 0.5 for the free surface in a VOF simulation

## LineProbe¶

field

Name of the field to study, e.g., u0

startpos

A list of numbers, the coordinates of a point in the domain

endpos

A list of numbers, the coordinates of a point in the domain

Npoints

Number of probing points along the line segment from startpos to endpos

## PlaneProbe¶

Saves an XDMF plot file of the specified field intersected by a plane. Sometimes it can be useful to have 2D slices of 3D simulations since the 2D slices are smaller in size and can be written more often without too much IO.

field

Name of the field to study, e.g., u0. You can also give a list, [u0, u1, u2], but the functions in the list must share the same function space (most likely DG2 in this case)

plane_point

A list of numbers, the coordinates of a point on the plane

plane_normal

A list of numbers, the normal direction of the plane

xlim, ylim, zlim

Lists of two numbers specifying limits to the extents of the plane. By default the plane is as large as the intersection with the 3D mesh allows.

## PointProbe¶

Probe one or more fields in given points

probe_points

A list of function names and the coordinates of the points to probe. The name of each probe must also be given so that you can figure out which value belongs to which point. See example below for the syntax

probes:
-   name: pressure_probes
enabled: yes
type: PointProbe
probe_points:
-   ['p', 'probe1', -1, 0.5, 0.3]
-   py$['p', 'probe2', L - 2, 1e-3, 1e-3] - py$ ['c', 'cprobe', L - 2, 1e-3, 1e-3]