Quote:
Originally Posted by Phase
Can you go into detail about the VGs with the smaller side mirrors? I replaced my super silent factory side mirror with a hand crafted mini side mirror, but the wind noise is insane from it and really annoying at higher speeds.
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Hey Phase! Sure.
First of all, I bought the mini side mirrors. They came with already curved backs, and were 5 sq inches in area, which was exactly what I was looking for. So I don't know what can be done about your hand crafted mirrors except to shroud them with a smooth manufactured shaped.
As for the vortex generators, VGs are used to re-attach flow separation to a surface, like behind a mirror or to a back windshield / trunk that's concave to the airflow. They "inject" higher pressure air into a low pressure "shadowed" zone. Both airflows are turbulent, but when properly used the VG causes earlier re-attachment and thus better flow.
Each VG comes with a characteristic length L and width W. They are usually designed so the two are comparable. You want the turbulent unattached zone to be between 1L - 3L behind the VG, and if large enough, to place multiple VGs around 1W-2W apart. (Placing them closer than 1W at best does nothing and at worst interferes with each other.) Also note that we want at least 2L flat surfaces
ahead of the VG for effectiveness, but often we can't get that near a mirror. (It's much easier on a roof, say.)
To give you real numbers, the gaspods I'm using have L = W = 3 inches. So I want to place them 3"-9" ahead of the separation point from the mirror, and around 3"-6" apart.
To find the separation point one can use tuft testing / smoke injection / look at the mirrors in a rainstorm / anything else that visualizes airflow around the mirrors. If you know the W and L of your VGs, place the tufts around 2L apart horizontally and 1.5W apart vertically. See where the flow becomes separated and put the VGs ahead of it.
You can see videos of me tuft testing here:
https://www.instagram.com/p/Ce13TiGFm0_/
and I'll take a pic of my current setup and post it in a moment.
Note that mirrors are notoriously finicky and you shouldn't use my setup as a template. I've seen other cars where the optimal placement was below the mirrors, they needed 3 VGs to cover the whole separation, they were placed differently, etc. Also note this isn't the optimal placement even for my car; it should be an inch ahead and both two inches lower. But there wasn't nothing to stick them to, so I had to compromise putting them on the mirror plate.
Be aware that VGs can also make a whistling noise. I originally used the gaspods on the front edge of my spoiler, where it met the rear window. It gave me a whopping +7-11% FE, depending. For an already optimized car, that's astounding. I took them off a week later, because they caused a heinous low level shriek. I'm an audiophile, and the improved mileage wasn't worth going batty from the noise.
Last but not least, if you are custom crafting your mirror, then I've seen other VG variants that might be more suitable for you. One had very tiny L = W ~ 1 inch VGs along the outside rim of the mirror. It directly injected air into the shadow (instead of onto the side of the car like I'm doing). Another is a flat flap used like a winglet on an airplane wing. That has the dual use of being a vortex stopper and a VG. Anyhoo, those might be appealing options.
Hope this helps!