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Originally Posted by Phase
From studies I’ve read, the wake size will be smaller than just the measured “ edges” of the cavity fins if I taper them in because then that wake extends inwards with a different vortex layer. It forms an “ invisible boat tail” that the outside air flow skims across. Not to mentioned the rear base pressure increases too from the cavity affect. I’ll be tuft testing them and then heat molding the taper inwards while maintaining attached airflow.
Was thinking of doing the same thing under the car with a diffuser, pretty much just acting as a horizontal box cavity fin. It should also help with snow and rain build up on the back of the car and keep the backup camera cleaner too I believe
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Yeah, if you can find the sweet-spot, where the inward curve is just right, you'll pick up a smidgeon of pressure recovery.
You'll have the 'smaller' wake, AND it will be of higher base pressure.
Is there a reason for 'saw-toothing' the fins?
The pressure at the trailing edge of the 'point' is going to be greater than at the shorter inclined locations, a setup for shear-induced vorticity, something you want to avoid like the plague.
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The SAAB 9.3 was tested with and without a rear diffuser, and it had no effect at all on drag, as the car had no complete belly pan ahead of it. If you haven't already paneled in the underbody, you might run into the same situation. Don't know. It's a $4,000 answer.
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Also, you have a 'cavity', but a 'box-cavity' is an entirely different animal.
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The tearing edges will help reduce vorticity, and the wake will kind of 'look' like an imaginary boat-tail, but it has no facility for pressure recovery, as with an actual boat-tail, tapered cavity, or box-cavity.
And measuring from the IONIQ blueprint, there's no chance of any 'phantom' tail affect until you've got 140-inches of boat-tail behind the car.