Open Window Drag

All of the aero analysis I've seen shows cars in an optimal (closed window) state, yet production based cars are generally required to run with at least the front side windows open. So it would only make sense for GRM to have a discussion about what would be the best way to minimize drag on a car with open front side windows?

1. Connect cockpit to a high pressure area (base of windshield, front of car)
2. Connect cockpit to a low pressure area (base of rear window, rear of car)
3. Seal cockpit off from both high and low pressure areas.

While I understand that the ultimate results may vary from car to car, I have to imagine there are some general guidelines that would be more widely applicable.

Purely as an interesting data point. Our one racecar(truck) has no side windows and no rear window(well, we cut the back roof off a sedan). Wind hits the driver in the back of the head. There is a lot of swirling going on inside the cabin. We need interior yarn testing.

In reply to buzzboy :

But for a truck with a much smaller slider opening in the rear window, it seems to draw air neatly through the cabin while also reducing interior turbulence. I've also seen some race cars put holes near the base of the polycarbonate rear window, presumably to similar effect. What that actually means for overall drag though, I have found little discussion and no data.

Conversely, redirecting high pressure air to low pressure areas has been used to reduce drag elsewhere too. I'm namely thinking the 'air skirts' being used on many (most?) new cars, to redirect air from the front of the car through a slit behind the leading edge of the front fenders. Could similar benefits be achieved for the cockpit, drawing from there or the base of the windshield?

I think having high pressure in the cockpit will be more beneficial.  Between these options:

1. high pressure in the cabin (I think this is the most likely to be able to maintain and the most stable, so the least draggy)
2. low pressure in the cabin (I think this will be difficult to maintain, with so much open area available for air to flow in)
3. neutral pressure (I think this will be least draggy in laminar flow, but unstable, turbulent, and more draggy in the real world)

But I wouldn't take air from the base of the windshield.  Since you already have high-pressure air there, you would be losing some downforce by removing that high pressure area.  Sucking air from under the hood or from under the car would be ideal, but could be difficult to achieve.  I think you would also need a pretty big fan (I'm thinking multiple box fans) to maintain this pressure.  I'm not sure how much and how feasible it would be.

Not a real data point, but when driving in the highway I find opening front and rear side windows reduces turbulence in the cabin compared to just opening the front side windows. 

Of course most race cars are not sedans, so opening the rear side windows probably isn't an option. 

Modern cars are designed to have lower drag with the windows up as noted. Required windows down/open is more draggy and introduces flow to the cabin that generally creates a harmonic resonance (a low frequency noise most often). Try to imagine the air coming in at the back of the windows and swirling around toward the center and back toward the windshield, then out toward the a-post. Some air exists, a lot stays contained in the swirling vortex. The cabin pressure is lower than ambient due to high speed air flowing over the  a-post edge, this can be noted by the vents usually having some flow even with the fan off.

For drag reduction, you should try to deflect the air at the a-post further out board to reduce the inflow at the back of the window and keep it mostly attached to the sides. This will generate its own drag but best case scenario the increase is less than the swirling/vortex flows in the cabin. It will still be noisy and low pressure.

They did this on Shelby Mustangs ... made a noticeable increase in top speed .....(Race cars)

Adding to this thread to ask related questions.  My RX7 (1st gen) has open door windows per ChampCar rules.  Back windows are lexan.  We can put some holes in the back window to help with ventilation, which is ultimately what we are after.  I had some thoughts about putting a few 2-3" holes (whatever the rules allow) for ventilation.  

Now, is the best bet to put them high or low?  My thought is higher, especially in conjunction with the photo above.  

Related question:  what about using small gurneys on the back of the roof as vortex generators to help with airflow down the back of the car?  

I agree with the other comments that a high pressure builds inside the cockpit for open window race cars that helps to "push" incoming air out (to some extent).  I am not an aeronautics engineer, but it makes sense.  This topic was covered on an episode of Myth Busters TV show debating pickup trucks travelling with tailgate up or down, and with windows up or down.  

While I do not have experience with these, Hard Motorsports sell B-pillar wind deflectors to help push that incoming air out.  Maybe a conversation with Jack at Hard Motorsports can give more insight.

https://www.hardmotorsport.com/hard-motorsport-b-pillar-wind-deflectors-pair-bmw-e36/

jmabarone said:

Adding to this thread to ask related questions.  My RX7 (1st gen) has open door windows per ChampCar rules.  Back windows are lexan.  We can put some holes in the back window to help with ventilation, which is ultimately what we are after.  I had some thoughts about putting a few 2-3" holes (whatever the rules allow) for ventilation.  

Now, is the best bet to put them high or low?  My thought is higher, especially in conjunction with the photo above.  

Related question:  what about using small gurneys on the back of the roof as vortex generators to help with airflow down the back of the car?  

Missed your post, but wanted to give you some info. Putting holes in the top of the rear glass will draw the most air thru the cabin. It may hurt rear downforce if you have a spoiler or low mounted wing. The low preesure at the top of the rear glass is about as low as you will get on the upper body (front of roof is lower). Generally the air exiting the top of the rear glass can be used to help energize the flow down to the deck lid.

For hole shape, circular is the easiest but not the best. For max effect long slots across the width will be better. A slot with tapered edges is best. Think of it like cutting straight back to front (horizontal) instead of 90degrees to the glass. Leave some material every 10 to 12 inches to keep the lexan from flexing too much. 1/4 to 1/2 wide slots should be plenty. You can tune the opening area with tape if you have too much flow but probably wont need to.

An option if you go with round is adjustable vents used in side windows of small aircraft. Check for window vents at aircraft spruce or similar suppliers. They can be adjusted and rotated or closed.

Awesome!  Thanks for the info.  

Fantastic article on aero that addresses the open window question from the angle of "drag".

I cut and pasted the article below, and the hyperlink has more visuals to go along with the text.

https://occamsracers.com/2023/08/12/the-aerodynamics-of-open-windows/?fbclid=IwAR35if9W7evPWx6u0k5LvkXaCVUeEfsxtTS1h_q3qsa39gcBxwzZ9ovB3fI

In reply to ClassicCarPerformance :

I'm not the biggest fan of Mario... having spent a significant amount of time "debunking" his posts over the last couple years to some friends of mine... I tend to avoid using superlatives with his work.

That said, I don't think it's the right thing to do to pull his article onto here wholesale.  People can follow the link easily enough.

sleepyhead the buffalo said:

In reply to ClassicCarPerformance :

I'm not the biggest fan of Mario... having spent a significant amount of time "debunking" his posts over the last couple years to some friends of mine... I tend to avoid using superlatives with his work.

That said, I don't think it's the right thing to do to pull his article onto here wholesale.  People can follow the link easily enough.

Just sharing what I found.
Some hyperlinks don't work well with some browsers so I included the text.

Hmmmm. So window visors could be worth 1/10 ( +/- ) on a road course (where you run open windows)???   Seems worth investigating....   

I wonder if it is worth venting the rear cabin of the car? At Road America I generate enough cabin pressure on the front straight to pop my ears.

I'd say you need to retrofit some side vent windows. You could get fancy and pivot them in line with the A pillar. 

Side window deflectors might be worth a try

https://www.youtube.com/watch?v=cwkhi-ryyh8

I happened to notice some kind of deflector thing in front of the side window openings on some of Sierra's Pikes Peak entries.  I don't know what exactly it was supposed to do, or if it did it, or if anyone ever did any testing.  Crosskarts don't exactly put a high priority on drag reduction.

sleepyhead the buffalo said:

I'm not the biggest fan of Mario... having spent a significant amount of time "debunking" his posts over the last couple years to some friends of mine... I tend to avoid using superlatives with his work.

Mario is an amateur (at best) and requires regular "debunking". He writes for entertainment value, and I think this one was one of his best articles in that regard. 

I've been thinking about this lately because the removable hardtop on the Miata tends to create a high pressure area that pulls a lot of air into the cabin windows down and makes it difficult to record audio even at autocross speeds.

I presume there isn't much you could do at the A-pillar that would help that far back?

It's interesting that my 8th generation civic coupe, which has long doors and windows, and no opening back windows does not get the high speed buffeting and resonance that I have experienced in sedans and wagons.