Can air be forced into an ideal path using fences ?

[aerohead]

Originally Posted by Cd

I have read in the past that the fins on cars such as those in the above image resulted in large vortices. Much larger than if none were present.
Want to chime in on this Aerhead ?

We would want to take the cars on a case specific basis I suppose.
Alfa Romeo got some tasty Cds with their 3-car B.A.T.series of show cars.
Chrysler did some tests and reported directional stability benefits attributed to fins.The 'fin'/wing on the Daytona does add up to 36% added drag in strong crosswind.
Kamm was seemingly obsessed with fins and put them on all his own cars,up to his last project for the DKW/Audi notchback of the 1960s.
Some of the Citroens,due to their forward weight bias,are so stable without fins,that one may cruise all day at 127-mph with nary a care.
Morelli put them 'under' his banana car behind the rear wheels.
Ford claimed that the dorsal fin on their Probe-V,Cd 0.137 concept helped it achieve outstanding directional stability in gusts and crosswinds.
We could go on forever.
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The fin was a 'styling' gimmick for the most part.
When used as a palliative for stability issues,aerodynamicists figured that the car's body could be reconfigured so as not to create the situations needing 'fixing' with fins, as Lamborghini is doing today.

[ECONORAM]

Originally Posted by euromodder

Not just racing trucks, but just about all Euro cabover trucks.



And looking at the different truck models on the MAN website, it looks like they too have gone for blocked grilles

I understand the need for air fences on swept wing fighters, but as jtbo asked, what is the purpose of those on the front of Euro- cab-over tractors?

Originally Posted by RunningStrong

I've noticed the air fences on the front of trucks for a while, but was unsure as to whether they where aerodynamic or sacrificial plastic in case of bumps and scrapes.


Regarding fences in general I think in the vast majority of cases they are a waste of time, certainly when you are not actually using the air for anything and simply trying to maintain a clean flow.

Winglets on plane wings are functional but take 100s if not 1000s of hours in wind tunnels to perfect as they are very specific to each wing design. Earlier "bolt on" designs were of limited benefit economically as the added weight and cost actually gave little economical benefit over fuel savings but did allow airliners to appear more modern. Winglets that are integrated into new design (787 and A380) are far more effective from what I've been told in my studies.

Multiple fences on flat surface are simply increasing frontal area and can possibly cause more turbulence that they prevent. Don't forget you are creating further edges which will lead to further issues at the edge were the two separate air flows meet again.

And then finally there's the side profile, greatly increased but for what real gain?

That question crossed my mind, to a degree. Would there be anything to be gained with air fences running lengthwise? I would think little would change, except at the roof/door edges perhaps.

Which reminds me, air streaking down the sides of the vehicle has a pretty bumpy path, with wheel wells, door handles, window trim and all. Over the roof seems a little better, but that's just from looking. I guess some tuft testing would be in order...

[edit] as I stew on this more, I could see how these curved air fences may serve similar to a recent rear spoiler or Kammback design I saw here last week. It was shaped a bit like some wing flap airfoils, and was said to reduce drag. The page escapes me at the moment though...
Found it!! http://ecomodder.com/forum/showthrea...tml#post271093

[basjoos]

The wing fences on Mig fighters was an aero "band aid" installed to mitigate bad stall characteristics, not to reduce aero drag. The wing fences keeps the detached airflow that first appears at the wing root of swept wings as the Mig approaches stall speed from spreading out to the wing tip where it would hamper the roll response of the ailerons.

[ron22]

basjoos is currect. Wing fences are used on aircraft to get the wing to stall the way they want to. Nothing to do with drag.

[aerohead]

I'm going to take a stab at the structure on the tractor,and say that I believe that they are harvesting ram-air pressure from the stagnation footprint to create a 'jet' below the side glass and mirrors,to protect them in a rain from splash and spray,which would create a blind-spot for the driver.
There's already enough leading edge radius for attached flow,and concerning that,the turning vanes would be superfluous.

[gone.2]

While we're on the subject, what about the recessed rear windows with side extensions on cars from the 60s like the Charger? More or less drag than a flush window?

[user removed]

Compare that Charger to the Daytona version with a flush window. I thin kthey also made a 500 version that had a flush grilleand rear window before the Daytona version.

regards
Mech