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Old 03-25-2015, 07:47 AM   #11 (permalink)
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I've been considering using these ebay rain guards for some time:

Universal Rear View Side Mirror Rain Board Shade Shield Water Guard | eBay

My reluctance is due to the fact that I won't be able to test the difference to see if they actually work.

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Old 03-25-2015, 08:56 AM   #12 (permalink)
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Looks like you could make a set from a plastic milk jug or a 2 liter soda bottle and attach it with some very high bond tape (3m VHB).
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Old 03-25-2015, 04:58 PM   #13 (permalink)
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talkin' about

Quote:
Originally Posted by Sven7 View Post
I can't help but wonder if this would make any appreciable difference on a modified car, or if it would even be remotely testable without a wind tunnel.

Of course, this begs the question, and I'm not trying to shut down this thread or anything: If something changes airflow too little to make a measurable difference, is it worth talking about?
If the DOT won't let us do the camera system replacement,then I suppose that there's gonna be a lot of dialogue about potential technologies and their savings with mirrors,and no stone will be left unturned.
Olsson,at Chalmers University did say that his scale model testing ought to be followed up with full-scale testing in order to optimize all the interactions.Something that can't be done at small scale.
The car/mirror must be evaluated as a system.We won't be able to attach mirrors to a wind tunnel wall or sting and learn anything.
What's worse,each car is different,so there isn't going to be any one-size-fits-all solution.
Since there are hundreds of millions of vehicles on the road,the energy department and EPA may be interested in mirror R&D to mitigate waste/air pollution.
The more streamlined the cars become,the more important the mirrors become.
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Old 03-25-2015, 05:16 PM   #14 (permalink)
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noise

In NASA's study of the silent flight of owls they mention the wings leading edge 'comb',the trailing edge 'fringe',and the downy feather sublayer which dampens turbulence.
In our modern aerocoustic wind tunnels we've got a great opportunity to tune out some noise as is happening already with side mirrors.
Also,I notice Ford SUV rear bumper fascias resonating as they go down the highway,beat up by the buffeting of the wake.If they'd run faster tunnel velocities,they'd see this and fix it before it makes it into production.
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Old 03-25-2015, 05:39 PM   #15 (permalink)
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length

Quote:
Originally Posted by serialk11r View Post
Hmmm that reminds me, some cars have much longer (in the sense that you used the word "length") mirrors than usual but some lowish drag cars have your typical flat brick shape mirror, does adding extra length help a lot? I'm guessing it's not more common because you have to pay attention to interference drag if the mirror is near the A pillar.
I don't have a pictorial to share,but here's some data to think about.
*Kach22i's prop spinner is about Cd 0.30 in a free stream of air.
*If you put 1-soup can(same outside diameter) behind it,it drops to Cd 0.233
*If you add a second soup can ,Cd 0.225
*Adding a 3rd can,Cd 0.215.
*Mair's boat tail wind tunnel model,with prolate ellipsoid nose and 3-soup cans was Cd 0.204.
*The 'GAVRE',Aberdeen,5-degree boat-tailed artillery projectile is Cd 0.16 in subsonic flight.

--------------------------------------------------------------------------
So length does matter,aerodynamically.
The 1996 Porsche 911 GT race car has very 'long' elliptical mirrors,far from the body.My opinion is that Porsche spent a lot of tunnel time developing these LeMans mirrors.
Only full-scale testing or DNS CFD is going to show us the way.
The A2 or DARKO tunnels would be perfect for us.They're both capable of 1-count accuracy and perfect repeatability.
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Old 03-25-2015, 06:41 PM   #16 (permalink)
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Quote:
Originally Posted by aerohead View Post
I don't have a pictorial to share,but here's some data to think about.
*Kach22i's prop spinner is about Cd 0.30 in a free stream of air.
*If you put 1-soup can(same outside diameter) behind it,it drops to Cd 0.233
Hmmm...0.3 is not bad considering that's only a little bit more than the average body drag right?

I noticed modern Porsche mirrors are 1 soup can longer than the typical brick shaped mirror, I wonder how much drag that cuts out. With the 911 coming in at Cd 0.27 with ginormous 275 width tires, they have to be doing something right.
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Old 03-25-2015, 06:56 PM   #17 (permalink)
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Porsche

Quote:
Originally Posted by serialk11r View Post
Hmmm...0.3 is not bad considering that's only a little bit more than the average body drag right?

I noticed modern Porsche mirrors are 1 soup can longer than the typical brick shaped mirror, I wonder how much drag that cuts out. With the 911 coming in at Cd 0.27 with ginormous 275 width tires, they have to be doing something right.
There's a lot of competition for Porsche customer dollars/euros/ etc.. and I suspect that Porsche owners have expectations that they have kinda leading -edge technology on their cars,including the mirrors.So I suspect that what we see is quite intentional.
I'll look at their mirrors next time.But I agree with your assumption.0.27 is really a coup,considering where they've been since 1939.
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Old 03-26-2015, 12:14 PM   #18 (permalink)
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Quote:
Originally Posted by aerohead View Post
I don't have a pictorial to share,but here's some data to think about.
*Kach22i's prop spinner is about Cd 0.30 in a free stream of air.............

So length does matter,aerodynamically.
Great information.

I posted for the wavy edge treatment, but got more.

For the record, I made the spinner cone as long as I could without infringing on the rudder vanes.

Also making the cone any longer than the duct might not be the best approach, but does pause for consideration.

The frictional skin drag losses from building a longer duct might negate benefits of straighten out the swilling trust flow.

However the added weight of a longer thrust duct and spinner, and weight so aft of the center of gravity are unforgiving realities on a hovercraft.

.................................................. .................

Back to side view mirrors; length, weight, and obscuring forward peripheral vision all must be weighed in the designer's mind. Just as other factors concerning the hovercraft tail cone.

.....................................

Saw-tooth or wavy edges may become a fashion at some point, just as with golf ball dimples.

We should be careful about what we wish for, right?
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Old 03-26-2015, 01:26 PM   #19 (permalink)
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Quote:
Originally Posted by aerohead View Post
*They could be modeled at 1:1 scale with DNS CFD on a $100,000,000 supercomputer.
Or on a PC if you spend a few hundred on a GPU, and do some programming :-)
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Old 03-26-2015, 04:01 PM   #20 (permalink)
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PC

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Originally Posted by jamesqf View Post
Or on a PC if you spend a few hundred on a GPU, and do some programming :-)
As I understand it,DNS can only be performed with a supercomputer,working in teraflops.
The mesh size and time steps are so vast that a PC cannot possibly handle the volume of information.
It is not a matter of a graphical processing unit.
The Full Navier-Stokes equation works in a spherical coordinate domain.It's millions of individual points in space,defined each by X-Y-and Z coordinates, which are interacting and solved for simultaneously in Eulerian differential calculus.All turbulence must be modeled,something only DNS can perform.
There is a RISC computing approach involving perhaps dozens of parallel computers operating together,and their cost,for DNS capability,as of 1992 was over $100,000,000.
DNS is quite old.It's only until recently that computers attained the speed and capacity to take on DNS.
Daimler-Benz' supercomputer requires over 48-hours continuous run time to analyze a single car.
The other thing about PCs is that,we need a digitized full-scale car with every,and I mean every small detail,down to the fraction of a millimeter.
I know of only Southwest Research Institute,in San Antonio,Texas which has the capability to do this.I've no idea what the cost would be,or if one could even gain access to their facility.
No doubt,Moore's Law will,over time,bring PC capacity up to otherwise 'impossible' levels,but I don't want to predict when they'd attain 'super' status.

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