Side mirror drag & effect on fuel economy - quantified

[aerohead]

Originally Posted by AeroMcAeroFace

Was that because it is away from vortices from the A-pillar or because it created separation all down the side of the car in its wake?

The car's near-field flow interfered with the mirror, while the mirror's near-field flow interfered with the car.
By moving the mirror away from the body, to perhaps the initial outboard streamline's distance, it essentially began to 'fly in formation' with the car, neither affecting the flow of the car, or vice versa.
And this also relied on careful shaping of both the mirror housing and mounting strut.
An added side benefit was a lower acoustic signature ( noise-vibration-harshness ).

[Phase]

Originally Posted by aerohead

The car's near-field flow interfered with the mirror, while the mirror's near-field flow interfered with the car.
By moving the mirror away from the body, to perhaps the initial outboard streamline's distance, it essentially began to 'fly in formation' with the car, neither affecting the flow of the car, or vice versa.
And this also relied on careful shaping of both the mirror housing and mounting strut.
An added side benefit was a lower acoustic signature ( noise-vibration-harshness ).

Is that why companies put vortex generators near the mirror if it’s closer? So it controls the bigger and more wild vortex’s between the car and mirror?

[aerohead]

Originally Posted by Phase

Is that why companies put vortex generators near the mirror if it’s closer? So it controls the bigger and more wild vortex’s between the car and mirror?

Sometimes the very presence of the VGs alters the natural frequency of the mirror housing, eliminating some unwanted noise that couldn't be modelled in CFD, and which hadn't shown up until they got the car into an acoustic wind tunnel.
And since 'adding' the VGs is a simple 'additive' modification to the existing tooling, it costs very little for the good it does.
All VGs are 'BAND-AIDs'. If they exist at all, it's because there was something wrong with the original design. If the 'local' flow turns out to be a disappointment, there's only so much a VG would be able to mitigate.
And whatever they come up with has got to be able to stand up to a commercial car wash.

[AeroMcAeroFace]

Originally Posted by aerohead

And this also relied on careful shaping of both the mirror housing and mounting strut.
An added side benefit was a lower acoustic signature ( noise-vibration-harshness ).

Although that is assuming that the length and stiffness of the arm and resonant frequencies of the attaching and detaching flow don't create mirror wobble or transmitted buffeting through the arm. But I don't know if that is a realistic problem because the frequencies without design may be so far off that it is a non-issue.

[aerohead]

Originally Posted by AeroMcAeroFace

Although that is assuming that the length and stiffness of the arm and resonant frequencies of the attaching and detaching flow don't create mirror wobble or transmitted buffeting through the arm. But I don't know if that is a realistic problem because the frequencies without design may be so far off that it is a non-issue.

It came up with BMW. They published information from research they performed during the development of one of their passenger cars.
Only in their aero-acoustic wind tunnel did an un-wanted interaction of the side mirrors with the main body reveal itself.
From the cabin's vantage point, the mirrors produced a really undesirable flow disruption down both sides of the beltline.
Master clay modelers were able to shave and enhance the mirrors, succeeding in tuning out the unwanted turbulence.
In the United States, automakers are held to certain legal noise pollution constraints, in order to market their products. One cannot sell a vehicle if it exhibits 'whistling', 'buffeting', 'booming', as well as splash and spray hazards.

[CVTCivic]

My side mirrors are really huge:




What do you think, would it be an improvement to fold the mirrors like this?



Or does it look more like a flow disruption?

[aerohead]

You'd lose some of the field of view, a safety concern, compromising situational awareness.
The housing would no longer be streamlined.
What was the inner face, parallel with the flow, is now cocked outwards, as a wind-catcher, producing a jet where the flow is now nearly completely choked off.( pressure spike ) on the leading side, directed towards the glass.
The outer housing inwards curvature now exceeds the 23-degree threshold for attached flow, triggering turbulence behind this outer housing area.
Like it or not, the original housing was optimized for this area, and the separated flow-turbulence was helping to scour moisture from the mirror's surface.
As per Subaru's research, simply extending the stalks out, to create an 80mm separation between the mirror housing and side glass would 'solve' any drag issue.

[dremd]

If I did that, I’d put a stick on blind spot mirror on the glass, I lost a mirror on a trip (hit a bird) I stuck a large spherical mirror out there and honestly it was better in many ways than the factory.
Do listen to others about aero improvements, I’m not informed enough to advise you there.

[Phase]

I folded in my mirrors like that on a road trip. Didn’t noticed any mpg improvement. It could’ve been in my head, but it seemed like I was getting worse mpg than with them out, it’s hard to track mpg improvements with my Hyundais system. There was a lot more wind noise. It had a deep base drumming and flapping/pulsing sound and I eventually just put the mirrors back out during that experience. I’m assuming drag wasn’t improved if the wind noise got unbearable. Actually removing the mirror and putting a small cover over helped drag and absolutely no wind noise

[freebeard]

It had a deep base drumming and flapping/pulsing sound and I eventually just put the mirrors back out during that experience

The example at #126 exposes a leading edge that likely would induce interference drag with the main body.