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Old 08-13-2014, 05:42 PM   #21 (permalink)
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..the front of a current model Fiat 500 & Panda...
Oh dear ...

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Old 08-13-2014, 06:13 PM   #22 (permalink)
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streamlined spokes

Quote:
Originally Posted by serialk11r View Post
I understand the inside of the wheel itself being a big void probably contributes more drag than the spokes, but say you had streamlined spokes instead of the typical rectangular chunks, how much of a difference would that make? Would you be better off with a partial cover (a full cover missing a circle in the middle)?
The concept of a streamlined spoke is kinda oxymoronish if you think about them.
*In a calm situation,the spokes are shielded inside the wheel from the oncoming flow.
*They are churning within an environment of sequestered air.
*Their boundary layers and individual wakes travel in a circular train with the wheel.
*If their next to an open engine bay,we'd probably have transverse flow,moving through the turning spokes,creating a pulsation perturbation which destroys the flow.
*If in a crosswind,some of the spokes may remain shielded,while others are attacked by the free-stream entering the void.
*If the spokes were airfoil sections,they'd perhaps have a minimum windage drag in the context of their peculiar flow,but it would still be intermittent flow.
*You can't model this in CFD yet.
*It would cost perhaps a $100,000 to experiment with them in a tunnel.
*Low drag concepts use full wheelcovers (or full disc wheels) inside a full skirt,if not with an inner fender as with Ford's Probe-IV.
*As far as aesthetics and aero go,the closer you get to a full disc the better the drag situation,but then you lose the 'look.'
*If you were to cover partially,you'd cover the center and leave your opening out close to the rim.
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Old 08-14-2014, 12:42 AM   #23 (permalink)
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Quote:
Originally Posted by aerohead View Post
The concept of a streamlined spoke is kinda oxymoronish if you think about them.
*In a calm situation,the spokes are shielded inside the wheel from the oncoming flow.
*They are churning within an environment of sequestered air.
*Their boundary layers and individual wakes travel in a circular train with the wheel.
*If their next to an open engine bay,we'd probably have transverse flow,moving through the turning spokes,creating a pulsation perturbation which destroys the flow.
*If in a crosswind,some of the spokes may remain shielded,while others are attacked by the free-stream entering the void.
*If the spokes were airfoil sections,they'd perhaps have a minimum windage drag in the context of their peculiar flow,but it would still be intermittent flow.
*You can't model this in CFD yet.
*It would cost perhaps a $100,000 to experiment with them in a tunnel.
*Low drag concepts use full wheelcovers (or full disc wheels) inside a full skirt,if not with an inner fender as with Ford's Probe-IV.
*As far as aesthetics and aero go,the closer you get to a full disc the better the drag situation,but then you lose the 'look.'
*If you were to cover partially,you'd cover the center and leave your opening out close to the rim.
Thanks for the insight.

Why would you want to cover the center with a partial cover though? I can't wrap my head around this...Aren't the spokes moving faster at the edge of the rim?

On the same note, I noticed that some cars have the wheels flush to the side of the car, but others (such as my car, and other Toyotas from the same time period) have the fender poking out more at the top of the wheel and receding towards the bottom, would this possibly "isolate" the air being churned by the wheels better?
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Old 08-14-2014, 03:17 AM   #24 (permalink)
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Quote:
Originally Posted by serialk11r View Post
Why would you want to cover the center with a partial cover though? I can't wrap my head around this...Aren't the spokes moving faster at the edge of the rim?
A full cover on a rim with holes will trap rain, dirt etc.
The holes in my steel rims were small, but even in 2 weeks time my full covers trapped a lot of dirt.
If you don't seal the gaps on the rim you better have the hub cap open at the edge.

Aerodynamically it is almost impossible to make the transition from tire/rim edge to hub cap completely flush. (*)
If you are to have a gap in your wheel cover you better have it where the air flow is disturbed anyway - close to the rim edge.

(*) The very same minute that I posted this, Niky showed how it can be done:

http://ecomodder.com/forum/showthrea...tml#post440055
Thanks Niky
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Old 08-14-2014, 04:17 AM   #25 (permalink)
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Not so fast, Red. That's a rubber cap! In fact, had one fly off. So much pressure built up under the cap that the tape alone couldn't hold it... the spokes were simply too thin.

Good thing it wasn't plywood!

I guess, however, you can still use duct tape to fill the edges for a smooth transition to the tire with a pizza pan.
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Old 08-14-2014, 07:18 AM   #26 (permalink)
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One of the underbody panels on my Insight has a strange dimple. It does not look aerodynamic at all.
From 2012 onwards Honda mounts a flush dimpleless panel...

I covered the dimple with some thick plastic foil and glued and taped it with duct tape about a year ago.
The tape on the front has withered and I had to tear the remnants off.
Duct tape won't last forever.
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Old 08-14-2014, 05:36 PM   #27 (permalink)
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why the center

Quote:
Originally Posted by serialk11r View Post
Thanks for the insight.

Why would you want to cover the center with a partial cover though? I can't wrap my head around this...Aren't the spokes moving faster at the edge of the rim?

On the same note, I noticed that some cars have the wheels flush to the side of the car, but others (such as my car, and other Toyotas from the same time period) have the fender poking out more at the top of the wheel and receding towards the bottom, would this possibly "isolate" the air being churned by the wheels better?
The inner face of the rim provides a duct 'wall' which is in the area unobstructed by the rotor and hub.Tranverse flow can move as freely as possible,through the spokes and out.
If you sealed this area and opened the center,you'd be asking the air to make a right-angled turn past a Cd 1.11 flat plate(rotor) within the wheel and flow towards and through an equally 'draggy'
vena contracta.

Here is a Borda tube with the vena contracta (in yellow).You can see how the flow is choked off even though it's an 'open' hole.
It's a torture chamber as far as airflow goes.
If you keep the flow near the rim you have more interior space to provide some radii inside the cover to kill the vena contracta with.Think of an inside-out velocity stack/outside curve of a donut inside the cover.
This internal drag far outweighs any penalty due to the spokes.
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Old 08-14-2014, 09:06 PM   #28 (permalink)
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I learned a new word[s].

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Old 08-14-2014, 09:52 PM   #29 (permalink)
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Quote:
Originally Posted by RedDevil View Post
Duct tape won't last forever.
Mine didn't even last a day.

Granted, I should have wiped down the wheels with some really good cleaner, but there just isn't enough purchase on porous rubber for the duct tape (it's okay where I used it on the airdams, because it sticks to automotive finishes quite well.

Too well, in fact.

One thought... you can actually vulcanize thin sheets of rubber to the sidewall. Perhaps make a doughnut shape out of rubber, then attach it to the sidewall, leaving a hole in the center for a thin, flush-mounted aero cap.

Hmmm....
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Old 08-15-2014, 04:47 PM   #30 (permalink)
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vena contracta

I spent an entertaining but fruitless hour trying to find a jpeg I saved years ago. It was an early example of using 3D graphics to show anatomical details and was a chart of branches in arterial/veinal structures. As a general rule, sidebranches were at 60, not 90 and the main flow path was non-linear (i.e., hydro-formed).

The closest I have been able to find is this:

Possibility of Atherosclerosis in an Arterial Bifurcation Model

This is a model simplified for computational purposes; reality more closely follows Victor Shauberger. 3D printed induction/exhaust will rule.

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