When running FLACS simulations, you often want to obtain the resulting flow field data at specific locations in the simulation domain. You can achieve this by defining monitor points at these locations and specifying which variables you want to have output there.
Viewing and defining monitor points in CASD.

Adding monitor points

To add a new monitor point, press Add in the Monitor points section of the CASD scenario menu. Double-click the monitor point (or press Edit) to enter the point’s position. If you intend to simulate a fire scenario, you also have to assign a direction to the monitor point. This is the direction along which the incident radiative flux is calculated, and it is specified as a vector. If you do not specify a direction then the maximum value of all directions will be output.


Where to place monitor points

Avoid putting monitor points exactly on a grid line or within fractions of a grid cell size from a wall. Usually it is best to enter monitor points according to the grid, not according to the geometry. For example, if you place two monitor points close to both sides of a solid wall they may not even be in two different control volumes. Further, during the porosity calculation, Flacs may adjust the wall to the nearest control volume face (grid line) and therefore the wall may move to the wrong side of a monitor point! Also, ensure that none of the monitor points is inside a fully blocked control volume.

How to position monitor points.
How to avoid positioning monitor points, and how to do it properly.

Verifying monitor point locations

Check monitor point locations visually in Flowvis 2D and 3D plots: to show the monitor points, tick off the Monitor points checkbox in the Options section of the properties sidebar. Monitor points that are placed outside the simulation domain will be red. Monitor points that are too close to or inside obstructing geometry will also be red. These monitor points will not give results in the r1-file.

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Enable displaying monitor points in Flowvis to verify their positions. Note the red monitor points in the closed geometry in the 2D plot on the right.

Selecting output variables

Select the output variables to be written for a monitor point by editing the monitor point in CASD. If you want to edit several monitor points at the same time, mark them and click Edit. Typically, you would like the same set of variables to be output at all monitoring locations; you can achieve this by selecting all monitor points, right-clicking and selecting Edit in the context menu. When editing monitor points, you can choose one or several variables by left-clicking or dragging.

Which variables to output

The most commonly used monitor point variables for explosion simulations are pressure (P), dynamic pressure (DRAG), panel pressure (PP), pressure impulse (P_IMP) and sometimes flow velocity (UVW). For dispersion calculations, you would typically require the volume gas concentration (FMOLE) and flow velocity (UVW).

Output frequency and file

Flacs writes monitor point data to the scalar-time output file (r1-file, e.g. r1000000.dat3) once every MODD time steps (default: MODD=1, i.e. every time step). You can set MODD in the Simulation and output control section of the CASD scenario menu, or in the scenario file (cs-file). Normally you want the monitor point output very often, e.g. in every time step for an explosion simulation, while 3D field data is sufficient to have much less often.

Related articles

How to convert monitor point data using r1file (binary to ASCII)

Advanced topics covered in the user manual