Drag of Open Windows

[71GSSDemon]

I believe I watched a wind tunnel test that showed an open window with a window net to have near identical aero as a closed window.  Does anyone here have back to back tests of this?  or even CFD analysis?

[slowalex]

There seem to be a number of studies that have reached the conclusion that open windows increase drag.

Study on the effect of window opening on the drag characteristics of a car
https://www.researchgate.net/publication/287701249_Study_on_the_effect_of_window_opening_on_the_drag_characteristics_of_a_car
https://trid.trb.org/View/1336456

The results obtained from both simulation and experimental data showed that the car with all windows open produced higher drag than a car with all windows closed.

The Aerodynamics of Open Windows
https://occamsracers.com/2023/08/12/the-aerodynamics-of-open-windows/

The focus of this study was on circuit racing, but the conclusion is that there is a measurable increase in drag and lap times, with open windows on a moderately fast circuit. He mentions Lime Rock as one example, and also lists some mitigation strategies, including window nets with more fabric than holes, if you must keep the windows open.

[aircap]

Years ago I was headed down I-40 around 80MPH, and the kids opened both back windows at the same time. The car immediately slowed down to 75.