Aerodynamic Mirrors (fluid dynamics help?)

[Mighty Mira]

Originally Posted by edenstrang

I had a sassy retort for this one, until I realized that the rest of the post answered my questions exactly. I chose poor phrasing too, I'll admit immediately, and will watch my step from here on. I was trying to imply that I thought the taper (not the mystical void beyond, granted) on the body would aid a reduction in drag given the logistical compromise necessary to fit a flat mirror on the end, a hundred lame apologies.

No problems! I don't blame you, I blame journalists and marketers either out of their depth or dishonest. e.g. What are you going to buy, a snazzy looking, practical car, or a teardrop shaped auto with better fuel economy and top end speed? Can't decide? How about if I told you that you could get all the benefits of the teardrop shape with all the practicality/style of a Kamm back?

Because your controllables are Cd * A (as someone else said), if you stick the mirror at the back of a Kamm back type shape, you are making the Cd better but making the A bigger, as well as increasing skin friction. Ultimately, this provides negative benefit.

You have to fit the mirror at the point of widest frontal area, which is the mirror. This means the only thing you can do is fit some sort of transparent taper (either full or partial) after the mirror. There is one company that sells an exorbitantly expensive mirror that does exactly that. I can't find it at the moment, they sell it for racing not the FE set.

[JanGeo]

Hey a couple of points . . .
1 Do the mirrors have to be sticking out that far since you want to see what is behind you but more to the side you could put the mirror right along the edge of the door frame.
2 If you use the wake of the windshield along the side of the door and tuck the mirror in close to it you may actually improve the air flow coming off the windshield.
3 The surface area creates drag and a sphere has the least surface per volume and the best aero dynamic shape for the leading surface.

[lunarhighway]

1 Do the mirrors have to be sticking out that far since you want to see what is behind you but more to the side you could put the mirror right along the edge of the door frame.

would that be a good idea aerodynamically? you'd spoil the attached flow to the vehicle... a lot of mirrors on new cars i see are not directly attached to the vehicle but have some space between the body of the car and the mirror.

i might be wrong but it's my impression that as long as te attachment brace of a mirror is thin enough (it could be shaped like a wing) free standing mirrors add less drag

like this



maybe it's just for noise reduction (most mirror developement seems to be driven by this), but a lot of mirrors i see are freestanding... some even with the support running down to the door rather than sideways to the window

[jwxr7]

I just thought I'd put up a link in case you wanted to see what I ended up doing for an aero alternative to stock mirrors . (Edit: you can skip all the way down to the last posts of the first page to view some images.)http://www.gassavers.org/showthread.php?t=4504
I thought about making something but was too lazy so I bought the motorcycle mirror.

[edenstrang]

okay, back to the faired bubble after all. If it doesn't have a hard edge, or taper to a point, it shouldn't impede my vision except in bright sunlight. It's a compromise for useability, but should definitely be an improvement on what's mounted currently...

[edenstrang]

Okay, getting speculative, but since my mirror has a rounded tail for clarity, as opposd to a clean break, do you think it may benefit from some boundry layer acceleration device, i.e. trip wire et al? I know it's worked wonders for spheres, could the same propertys (EDIT: Prauperteeies) apply to an oblong tapered ellipse?

Tangentially, has anyone heard of the golf balls with faceted dimples? Apparently they travel much farther than the smoothly dimpled ones... I just googled it and Callaway had a patent published in July of this year...

[skewbe]

took me a while to find it, but here is co zx2's entry:
http://www.gassavers.org/showthread....1962#post61962

[Mighty Mira]

Originally Posted by JanGeo

2 If you use the wake of the windshield along the side of the door and tuck the mirror in close to it you may actually improve the air flow coming off the windshield.

That's a good point, you might have something there.

3 The surface area creates drag and a sphere has the least surface per volume and the best aero dynamic shape for the leading surface.

I think it may be possible to do better than that. Consider the front end of some torpedos:


There is less surface area than the sphere (less wetted area), it's still smooth, so may actually flow better.

The reason why you don't see this shape in things like cars and velomobiles is that they have to house things inline like motors, people, luggage etc, and if you have to encase such things, you may as well minimize the wetted area.

But if all we are concerned about housing is a small plane such as a mirror, I suspect that a more blunted end (but still smooth and with a plane parallel to the direction of the car's motion at the mirror's edge) would be (slightly) better.

[Mighty Mira]

Originally Posted by edenstrang

Okay, getting speculative, but since my mirror has a rounded tail for clarity, as opposd to a clean break, do you think it may benefit from some boundry layer acceleration device, i.e. trip wire et al? I know it's worked wonders for spheres, could the same propertys (EDIT: Prauperteeies) apply to an oblong tapered ellipse?

In a word, no.

I like your mirror design, it's very similar to what I thought of when I considered how to do my mirrors (but didn't because there is a lot of lower hanging fruit than the mirror). And it's very similar to those racing mirrors I mentioned.



As to boundary layer separation, the reason why those trip wires on a sphere help is that they enable the boundary layer to cling to a more sharply raked trailing edge. This means the area of low pressure at the back of the sphere is smaller. Unless your trailing edge is beyond about the 15-17 degree region, you don't need to trip the boundary layer.

[Mighty Mira]

Originally Posted by skewbe

took me a while to find it, but here is co zx2's entry:[/IMG]

Thanks for digging that up! Interesting that someone here tried it, I'm sure that most smart people concerned with vehicle aerodynamics have considered the obvious transparent boattail on the mirror idea, and then probably dismissed it after thinking about visibility concerns (dust, warping, etc).

I wonder how the visibility is?