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Old 11-01-2020, 07:19 PM   #1 (permalink)
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Will these strakes cause more drag ?

After doing a crude tuft test of my car, I saw a problem area behind the rear wheels.
There was a lot of detached flow.
At first, I thought the detached flow was due to the angle of the side strake.
I had seen how touchy this area can be on some of the videos Darin has posted.

However, ater the test was over, I could see part of the problem was my crude fit and finish.
Air was tripping over the protruding side strake just behind the wheel.
Air was also smooth across the plastic light cover I had on the first test. ( It is at the same angle as side strake; so the angle of the side strake is apparently not the issue.
During the test, the area was also open,but this shouldn't have affected the air on the strakes ( correct ? )

Besides adding some curved wheel gap fillers ( here mocked up in cardboard) I also wondered if mini "air fences " would help to direct flow like the do on old fighter jets ( a good example being the MiG -15 )

From what I recall, fences don't work on cars.
However, you DO see mini "fences" a lot on tailights at the edges of the vehicle. ( Prius C has them on the lights ) and on mirror stalks to act as a vortex generator.

So just guessing, would you think these strakes help or hinder the drag on the rear of the vehicle ?

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Old 11-01-2020, 08:00 PM   #2 (permalink)
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It's impossible to say without testing, but I'm doubtful that any technique we have of measuring drag changes is sensitive to show whether or not fences like that are helping. Have you tuft-tested with the fences in place?
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Old 11-01-2020, 08:11 PM   #3 (permalink)
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I have not tested with fences.
I plan to eventually do tuft testing on my car using a used GoPro or cheaper alternative with a suction cup mount, but that is a want, not a need at this point, so it can wait.

For now, the best I can do is an oil drip test ( does not reqire a second and third person to drive the chase vehicle and film )
I haven't had much luck with the accuracy of the drip tests so far though.
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Old 11-01-2020, 08:34 PM   #4 (permalink)
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Yeah, I'm in the same boat--at some point I'll be buying a GoPro and mount so I can record tuft behavior on non-transparent surfaces.
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Old 11-02-2020, 01:00 AM   #5 (permalink)
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The GoPro with a suction cup mount is the way to go.

Maybe spray-mist some sort of water-diluted salt or calcium on the surface and go for a ride ? Once dried, it might leave a residue on the surface which might reveal the flow pattern.

I say that because we use a lot of salt on the roads here, in the winter months, and our cars are always coated with salt and calcium residue. Often times, you can see streaks which reveal the airflow.
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Old 11-02-2020, 01:30 AM   #6 (permalink)
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From the book Modifying the Aerodynamics of Your Road Car (paraphrased pg 56):

Spray a film of water onto the area of interest until it is damp. Then dusted with clay that sticks in a thin film to the wet panel (bentonite is suggested). Then spray with some more water (there should be an even coating of clay) and drive the car. The process is repeated until the airflow pattern is revealed in the clay.

Also says this method is really good for identifying separation and reattachment areas. There are pictures, but unless I scan or take a picture of the picture with my phone I don't know how to share them.

I haven't tried it before.

I am going to try and replicate this (see video) to make a camera mount for tuft testing on my car.

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Old 11-02-2020, 02:09 AM   #7 (permalink)
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Quote:
From what I recall, fences don't work on cars.
However, you DO see mini "fences" a lot on tailights at the edges of the vehicle.
Fences are used on the roof in NASCAR to kill lift when the car goes sideways.

Mercedes used ribs on their taillights to induce vortexes that clean dust off the lens.
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Old 11-04-2020, 12:56 PM   #8 (permalink)
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strakes / drag

Without full wheel skirts, it seems like the lower two fins/ strakes/ vanes would just be embedded within turbulence.
If the flow onto the upper structure is oblique, vorticity may be introduced, which otherwise might not be present.
Drag is a function of pressure.
The panel that the 'strake' is mounted on is capable of altering pressure, however, the 'strake' itself cannot perform that function unless it somehow enhanced the performance of the panel by 'correcting' some spanwise anomaly.
I too believe that it would be hard to measure any effect outside of a laboratory setting.
Structures on mirrors are probably there to correct an aero-acoustic 'noise' issue.
Nissan LEAF headlights ARE 'turning vanes.'
Taillight forms are there to straighten flow and provide Fachsenfeld's 'burst' edge.
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Old 11-21-2020, 11:57 AM   #9 (permalink)
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I just made a short, fairly low speed 50 mph drive just now.
It's misting, and sure enough, it appears as though the strakes did nothing at all, and perhaps even made things worse.
If you look at the images, you can clearly see the wet trail from the tips of the tufts as they fluttered.
I had the wheel well open ( no wheel gap fairing installed yet - see last picture. )

I still plan to test them further when i install the whell gap fairings.

I just finished watching one of Julians' videos, and was surprised to see a 1970s BMW racing coupe that had strakes at the top of the fenders alongside the hood of the car.

So can strakes indeed be used to direct airflow without causing a large amount of drag ? ( I'd love your advice on this Julian )

How does such a thing affect drag on an aircraft by comparison ? Examples being early MiG aircraft, as well as along the back of Grummans' F-14 Tomcat.

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Old 11-21-2020, 12:23 PM   #10 (permalink)
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Quote:
Originally Posted by Cd View Post
...a 1970s BMW racing coupe that had strakes at the top of the fenders alongside the hood of the car.
What video/car? It sounds like a mod I was thinking about trying.

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