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Old 01-21-2016, 05:06 PM   #1 (permalink)
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another vortex generator paper

I ran across this online.
http://iawe.org/Proceedings/8APCWE/K.Selvakumar.pdf
*They use a 1:10-scale model
*They need a 327.86 km/h air velocity for dynamic similarity
*Their highest test velocity is 12-m/s,when they need 91.07 m/s
*No where do they mention Reynolds number.

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Old 01-21-2016, 07:23 PM   #2 (permalink)
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Here's what I get out of it too, they're starting with a Cd of .56 to .63........I've always said if you have a brick to begin with than anything is likely to help.

I can just hear the other shoe dropping when people take a car with a .30 Cd stock and think they can get it down to .22 because the experiment with the brick took it from .63 to .41.....

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Old 01-21-2016, 07:50 PM   #3 (permalink)
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Quote:
Originally Posted by aerohead View Post
I ran across this online.
http://iawe.org/Proceedings/8APCWE/K.Selvakumar.pdf
*They use a 1:10-scale model
*They need a 327.86 km/h air velocity for dynamic similarity
*Their highest test velocity is 12-m/s,when they need 91.07 m/s
*No where do they mention Reynolds number.
Since they were comparing a digital model to a wind tunnel model both being 1:10 they don't need to scale the air speed.

Also since the model(both digital and wind tunnel) was pretty much a joke when it comes to representing a real car, this was more an exercise in CFD software validation.

If you want to do a real car and get results within about 5% you have to have rotating wheels and have flow through the grill taken into consideration.
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Old 01-23-2016, 12:24 PM   #4 (permalink)
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don't/grille/wheels

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Originally Posted by ConnClark View Post
Since they were comparing a digital model to a wind tunnel model both being 1:10 they don't need to scale the air speed.

Also since the model(both digital and wind tunnel) was pretty much a joke when it comes to representing a real car, this was more an exercise in CFD software validation.

If you want to do a real car and get results within about 5% you have to have rotating wheels and have flow through the grill taken into consideration.
*If they want meaningful results,then both CFD and wind tunnel testing must be conducted at supercritical Reynolds number.
*Yes,a cooling system would be nice,but I'm not certain that the effect of the VGs would be remarkably impacted by its presence.
*There's scientific evidence which would dispute the need for rotating wheels to obtain meaningful results.

--------------------------------------------------------------------------
For me,the takeaway from the article is that,the investigators don't understand boundary layer theory,and consequently,their testing methodology and results have zero scientific merit.
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Old 01-24-2016, 05:03 PM   #5 (permalink)
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Quote:
Originally Posted by aerohead View Post
*If they want meaningful results,then both CFD and wind tunnel testing must be conducted at supercritical Reynolds number.
*Yes,a cooling system would be nice,but I'm not certain that the effect of the VGs would be remarkably impacted by its presence.
*There's scientific evidence which would dispute the need for rotating wheels to obtain meaningful results.

--------------------------------------------------------------------------
For me,the takeaway from the article is that,the investigators don't understand boundary layer theory,and consequently,their testing methodology and results have zero scientific merit.
Ummm... the fact that the vortex generators show a reduction in drag prove that it was supercritical. Bumps corners and curves tend to create more turbulence than a flat plate. If the flow was entirely sub critical all VGs would add is drag.

Unless they faked data the test shows validity. I think you need some time working with a real CFD package or in a wind tunnel or at least objectivly look at the data befor you make claims it has zero validity.
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Old 01-25-2016, 04:37 PM   #6 (permalink)
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prove

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Ummm... the fact that the vortex generators show a reduction in drag prove that it was supercritical. Bumps corners and curves tend to create more turbulence than a flat plate. If the flow was entirely sub critical all VGs would add is drag.

Unless they faked data the test shows validity. I think you need some time working with a real CFD package or in a wind tunnel or at least objectivly look at the data befor you make claims it has zero validity.
All numerical or wind tunnel scale model testing must be conducted at supercritical Reynolds number or else the output data is meaningless.
There is not a published test which exists in the last 100-years which demonstrates otherwise.
If one doesn't understand dynamic similarity,there's no point pursuing testing of any kind.
If one doesn't understand boundary layer theory they'll never win a technical argument.
I'm in my 42nd year of objective observation.It's my responsibility to throw a caution flag whenever I see rule violation on the field.
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Old 01-26-2016, 10:11 AM   #7 (permalink)
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Would this one satisfy your standards? Or there is nothing to convince you that they work?

http://bpi-us.com/papers/vortexgenerators.pdf
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Old 01-26-2016, 11:30 AM   #8 (permalink)
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Keep throwing the caution flags, but throw them at the right thing.

An Indian grad student's paper showing empirical verification of theoretical modeling only proves that CFD makes testable predictions with a degree of accuracy. This isn't a VG paper, they just use them as a matter of convenience, as their properties are well known. Standard academic procedure.

The Mitsubishi paper is interesting, as it's clearly aimed at performance improvements. Those familiar with car aerodynamics (or at least with Hucho) know this test is rigged. The Lancer rear window is too steep to maintain attached flow past the roof. 14 degrees is the transition, IIRC. Mitsu's doing good engineering here.

VG's work wonders in certain applications, as on airplane wings. Not so much elsewhere; throw the flag at the offender, not the tool.

Frank
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Old 01-26-2016, 05:29 PM   #9 (permalink)
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Lancer

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Would this one satisfy your standards? Or there is nothing to convince you that they work?

http://bpi-us.com/papers/vortexgenerators.pdf
The Mitsubishi paper is probably the gold standard for automotive-related VG application.As an aeronautical company (you may remember their products from Pearl Harbor,December 7,1941) Mitsubishi is well acquainted with all things aerodynamic/hydrodynamic/gas dynamic.
Their science is water tight.
This paper is part of my library.
Unfortunately,some under-graduate,and graduate research on VGs,is being passed along without a jury peer review at their respective university,and would never pass the scrutiny of an SAE Congress,where most research papers are presented each year.
Too bad! These 'leaks' don't reflect favorably on the reputation of the college or university.
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Old 01-26-2016, 05:34 PM   #10 (permalink)
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offender

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Originally Posted by fbov View Post
Keep throwing the caution flags, but throw them at the right thing.

An Indian grad student's paper showing empirical verification of theoretical modeling only proves that CFD makes testable predictions with a degree of accuracy. This isn't a VG paper, they just use them as a matter of convenience, as their properties are well known. Standard academic procedure.

The Mitsubishi paper is interesting, as it's clearly aimed at performance improvements. Those familiar with car aerodynamics (or at least with Hucho) know this test is rigged. The Lancer rear window is too steep to maintain attached flow past the roof. 14 degrees is the transition, IIRC. Mitsu's doing good engineering here.

VG's work wonders in certain applications, as on airplane wings. Not so much elsewhere; throw the flag at the offender, not the tool.

Frank
The criticism was leveled directly at the testing methodology and ignorance on the part of the investigators.I've never said that VGs don't have a place,given specific context.Please reread my comments.I don't appreciate people putting words in my mouth.

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