Baselines and deltas¶

Most CFD development is comparison: is this change better or worse than what we had? Bramble builds that in. A baseline is the case you compare against, and a delta is the difference from it. Bramble has more than one kind of baseline, and it helps to keep them apart.

	Run baseline	Table-view delta baseline	Image delta baseline
Set on	The Run, in Create Run or Edit Run	The Results page, with Set Delta Baseline	The postpro viewer, with Generate Image Deltas
Affects	The Run's setup, pre-checks, delta postpro and default delta values	Only the delta values in that table view	Only the images you generate
Changes the Run?	Yes	No	No

The Run baseline¶

A Run's baseline is the Run it's a modification of.

You set it when you create or edit a Run, or by creating a child Run from an existing one. A Project can also have a Project baseline that new Runs start with. See set a baseline.

Attitudes are matched by name. Each Attitude is paired with the baseline Attitude whose Map entry has the same name, so a sweep can be compared point by point with an earlier sweep, even if the Maps differ slightly. An Attitude with no partner shows "Attitude not in baseline" and gets no deltas.

The baseline shapes the Run from the start. Because it's set up front, Bramble uses it well before any results exist:

Setup. New Attitudes copy the baseline Attitude's CFD setup, checks template and postpro selections. A modification starts as a like-for-like copy, so the only differences are the ones you make.
Pre-checks. The checks can compare the case with the baseline, for example to flag a changed mesh recipe or plot the geometry that's been added and removed. Without a baseline, those comparisons are skipped.
Delta postpro. After solving, the Create Delta PostPro stage generates delta plots and movies of the flow variables against the baseline Attitude.
Delta values. The Results table shows each Attitude's forces with a delta, in brackets, to its own baseline, unless the table view sets a different baseline.

Extra deltas. An Attitude can also generate delta postpro against other Attitudes, from any Run you can open. Add them in the Extra PostPro Deltas row of the PostPro Setup tab, or, for delta geometry plots, in Rerun Prechecks. See configure postpro.

Delta postpro¶

Delta plots and movies show where in the flow a change made a difference.

Delta postpro is generated at the Create Delta PostPro stage, from the Attitude's Delta PostPro Selections. Delta variables are named after the baseline, in the form "Δ<variable>: <baseline Attitude>".

Because delta postpro needs the baseline's solution, the timing matters:

The baseline hasn't solved yet. If a child Attitude gets to Create Delta PostPro first, the stage shows Waiting and the rest of the lifecycle carries on. When the baseline finishes Mesh & Solve, Bramble generates the deltas automatically.
The baseline changes later. Delta postpro that has already run isn't regenerated. Use Re-run in the Create Delta PostPro row. See re-run post-processing.
The baseline is archived. Archived Attitudes are skipped as baselines and extra deltas. See archive Attitudes.

Acoustic (zCFD) methodologies don't generate delta plots.

Table-view delta baselines¶

A table view can compare against a different baseline without changing any Run.

The Results table's delta values default to each Attitude's own Run baseline. Sometimes you want a different view: everything against one reference Attitude, or one sweep against another. Set Delta Baseline on the Results page does that for the current table view. It's saved with the view, and it doesn't change any Run.

There are three modes:

Attitude: every row is compared with one chosen Attitude.
Run: each Attitude is compared with the same-named Attitude in a chosen Run. Useful for runs that use the same Map.
Map Relative: within each Run, every Attitude is compared with that Run's Attitude of the chosen name. Useful for seeing how Attitudes develop within a Run.

Each column shows its delta as a percentage or an absolute number, as the table view sets. The Delta Units buttons override that for every column. See compare against a baseline.

Image deltas¶

An image delta compares two existing plots, pixel by pixel.

In the postpro viewer, Generate Image Deltas makes a delta image between any two Attitudes' plots, with its own Delta Baseline. It works from the images already generated, so neither simulation is reprocessed, and the two Attitudes don't need a baseline relationship. See generate image deltas.