I did some testing of the pressure changes on the side of the car caused by my Edgarwit front external air curtains. Each curved curtain is an aluminum extruded aerofoil for low drag. Throttle stop testing has shown a good reduction in overall drag, and tuft testing has shown a slightly better attached airflow pattern on the side of the car.
Measured pressures without Edgarwits:
Measured pressures with Edgarwits:
The converging duct of the Edgarwits obviously causes a pressure build-up - from -110 Pa to +35 Pa in the duct. In testing I found to get the drag reduction, I needed to have the duct converging - I imagine to better squirt air across the face of the wheel. That pressure build-up also occurs ahead of the duct - from -187 up to -71 Pa.
But what is happening on the side panel behind the wheel? The figures show a marked overall increase in pressure:
You can see how consistent the percentage changes are with distance behind the wheel.
Based on the three tests I have done - throttle stop for overall drag, tuft testing for attached flow and now this pressure testing, I assume that the Edgarwits work by giving better flow attachment down the side of the car through countering the disruption caused by the front wheels.