The Humble Delta Cp Plot

When a simulation completes, we want to quickly determine what the effect of our test was.

How have the forces and flow metrics changed, and perhaps more importantly, where and why have they changed?

bramble produces a detailed set of forces, flow metrics and post-processing when a simulation completes.  This post looks at one set of these images, the ‘delta Cp’, as is it is key for helping us identify where the flow structures have changed.

But first the ‘how’…

Forces and Flow Metrics

The first thing we’ll look at are the averaged forces and flow metrics.  This might include drag, downforce (or lift), centre of pressure (balance), mass flows and other flow metrics.

Moreover, we’ll look at the change in these performance metrics versus the baseline… did they get better or worse.

bramble’s results view is customisable and can be configured to display any number of different data sets.  Perhaps you want to see a drag breakdown, or maybe radiator mass-flows.  Whatever it is, you can load your previously saved tables and display the data you want.

To make analysis even easier, you can get bramble to insert force deltas into the table, conveniently coloured green or red depending if the change is good or bad.

Delta Cp Plots

Next we will want to know where these changes occurred.  That is what the ‘delta Cp’ can be used for.  It is an image that shows our model coloured by the change (the delta) in static pressure coefficient (Cp) versus the run’s baseline.

In this palette, greens/blues show reductions in pressure and yellows/reds show increases.  The areas with the largest changes will point us to where the flow fields are likely have changed the most.

With experience, aerodynamicists will come to know what the pressure changes indicate before looking at more detailed post-processing.  For example, in image above the red streaks at the leading edge of the floor are a sure sign of a weakening of the bargeboard/floor leading edge vortices.

Flow Visualisations

Now that we know where key flow structures have changed, we can jump to the post-processing (images and movies) that best show the flow in this region.

But that’s a story for another day…

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