Materials for Dimple Surfaces (reduced separation)

[chillsworld]

Originally Posted by freebeard

Isn't this the archetypal story arc for every discussion thread on the internet, ever?

This is true sir, so very true. This is just my first taste of it on ecomodder haha

How about "You can hope in one hand and poop in the other and see which one fills up first"?

I had a random drunk guy tell me that in a bar once, then he proceeded to throw up in the bathroom, then he proceeded to hit on the hottest woman in the bar, then they left together... And I was left sitting at the bar wondering what the heck just happened

Which thread is that, and more importantly did I repost this:


exploder ii concept 02 by criarpo d5b2ct2

/That's rhetorical, I can find the thread

That's pretty slick, and no I don't think that got posted in the other thread , but you did contribute heavily to my VW baja thread

~C

[aerohead]

Originally Posted by rumdog

So connecting the dots here...

For the ultimate drag reduction, build a VW XL-1.*
...with dimples!

So it looks to me like the dimples (or grooves) create a subtle turbulence within the layers close to the body. As opposed to VGs (vortex generators) which create an aggressive local and downstream mixing action using a vortex.
Does the non smooth surface (NSS!) also reduce the thickening of the boundary layers?
It is interesting how the turbulence (rear bluff rotating eddie) is reduced by the NSS [figure 8] and it almost looks like a vortex develops on each side... disapating energy as vortex instead of rear eddie. Or what?

I think that other body shapes may not be so lucky. Or just need engineering too.

*The same year Song/Zhang/Wang/Hu published,Martin Olsson's Masters Thesis was also published at Chalmers University,Goteberg,Sweden.
Olsson stated that at the time,only Direct Numerical Simulation could accurately predict 3-D flow of an automobile,and that it was "not today possible (lack of computer power)."
Olsson spoke of "incertainties,deviations,underestimations,overest imations,suspicious results,rerun (results) totally different.
He summed up,"due to the uncertainties in the CFD simulations of a few drag counts which make it impossible to trust all findings."
*'actual ' results did not correlate with CFD results.
FLUENT solves the Reynolds-averaged Navier-Stokes (RANS) which cannot predict all turbulence values.
Olsson was getting 45-drag count deviations (13%) which exceed the drag claims of 10.3% for the non-uniform surface.
Olsson recommended full-scale wind tunnel investigations so as to actually learn something.
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Nathan A. Williams basically came to the same conclusion about CFD in his Masters Thesis of 2003.'desktop' CFD gives one insight and graphical tools to think about to steer the actual wind tunnel work.
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I don't think Song et all knew what they didn't know and were carried along with presumptions of accuracy.They didn't even have the presence of mind to investigate the viability of the software they were using.
Personaly,I'm gonna wait for bigger dogs to weigh in before I can embrace what's being passed off for knowledge.

[freebeard]

chillsworld -- I like the design because it is an inverted wing which gives it high approach and departure angles, with downforce. And the pulled-in beltline reduces frontal area and improves sightlines.

I'm thinking on a Citroen 2CV chassis with electric motors front and back.


And dimples, of course.

[chillsworld]

Originally Posted by freebeard

chillsworld -- I like the design because it is an inverted wing which gives it high approach and departure angles, with downforce. And the pulled-in beltline reduces frontal area and improves sightlines.

I'm thinking on a Citroen 2CV chassis with electric motors front and back.


And dimples, of course.

Go all out, hubless wheels with electric motors IN each wheel... Lower center of gravity with more torque, but you would need to be sealed or waterproof or something if it's to be used off road

Hooray for being off on tangents haha!!

EDIT: actually you mentioned dimples, totally still on track!

~C

[chillsworld]

Originally Posted by aerohead

*The same year Song/Zhang/Wang/Hu published,Martin Olsson's Masters Thesis was also published at Chalmers University,Goteberg,Sweden.
Olsson stated that at the time,only Direct Numerical Simulation could accurately predict 3-D flow of an automobile,and that it was "not today possible (lack of computer power)."
Olsson spoke of "incertainties,deviations,underestimations,overest imations,suspicious results,rerun (results) totally different.
He summed up,"due to the uncertainties in the CFD simulations of a few drag counts which make it impossible to trust all findings."
*'actual ' results did not correlate with CFD results.
FLUENT solves the Reynolds-averaged Navier-Stokes (RANS) which cannot predict all turbulence values.
Olsson was getting 45-drag count deviations (13%) which exceed the drag claims of 10.3% for the non-uniform surface.
Olsson recommended full-scale wind tunnel investigations so as to actually learn something.
-------------------------------------------------------------------------
Nathan A. Williams basically came to the same conclusion about CFD in his Masters Thesis of 2003.'desktop' CFD gives one insight and graphical tools to think about to steer the actual wind tunnel work.
--------------------------------------------------------------------------
I don't think Song et all knew what they didn't know and were carried along with presumptions of accuracy.They didn't even have the presence of mind to investigate the viability of the software they were using.
Personaly,I'm gonna wait for bigger dogs to weigh in before I can embrace what's being passed off for knowledge.

Well crap...



Maybe the plane people will have good results and inspire real world testing on automobiles some day.

~C

[aerohead]

These vehicles are dominated by surface friction drag.
Automobiles operate in a fully-turbulent boundary layer,their skin friction is already minimized.
The automobiles drag is dominated by pressure drag which is a function of separation.It is the turbulent boundary layer which allows automobiles to achieve as low a drag as they'll attain.
The premise of aerodynamic streamlining is to reduce or eliminate separation.This is a function of shape.
Virtually all attempts to apply non-automotive streamlining tricks to automobiles have failed.They're a completely different animal.
After reading Song et al's paper I have no confidence that they accomplished anything at all.I believe that their work is extremely flawed.

[aerohead]

Originally Posted by chillsworld

Well crap...



Maybe the plane people will have good results and inspire real world testing on automobiles some day.

~C

Aeronautical engineer,Gary O.Wheeler,inventor of the Wheeler Vortex Generator has been active in automotive drag reduction since the 1990s,offering his VGs for those with notchback cars.There's an article about him in my photo archive.
As long as notchbacks are produced,there will always be an opportunity for drag reduction.Whether or not it makes it into production remains to be seen.
There are manufacturing considerations and aesthetic considerations.The average car buyer might never be ready for VGs.
The trend appears to be,to follow the organic forms of a hundred years ago,which don't produce separation over their body length.
Here's a 2016 HONDA FCV Concept as an example

2015 Buick Riviera Concept

An earlier 2004 HONDA IMAS,Cd 0.20

[chillsworld]

Originally Posted by aerohead

These vehicles are dominated by surface friction drag.
Automobiles operate in a fully-turbulent boundary layer,their skin friction is already minimized.
The automobiles drag is dominated by pressure drag which is a function of separation.It is the turbulent boundary layer which allows automobiles to achieve as low a drag as they'll attain.
The premise of aerodynamic streamlining is to reduce or eliminate separation.This is a function of shape.
Virtually all attempts to apply non-automotive streamlining tricks to automobiles have failed.They're a completely different animal.
After reading Song et al's paper I have no confidence that they accomplished anything at all.I believe that their work is extremely flawed.

Originally Posted by aerohead

Aeronautical engineer,Gary O.Wheeler,inventor of the Wheeler Vortex Generator has been active in automotive drag reduction since the 1990s,offering his VGs for those with notchback cars.There's an article about him in my photo archive.
As long as notchbacks are produced,there will always be an opportunity for drag reduction.Whether or not it makes it into production remains to be seen.
There are manufacturing considerations and aesthetic considerations.The average car buyer might never be ready for VGs.
The trend appears to be,to follow the organic forms of a hundred years ago,which don't produce separation over their body length.

Vortex generators are used on airplanes. They are capable of creating a 50% decrease in drag, as per NASA studies in the 90's. These same vortex generators are used on trucks, cars, etc with success (which varies according to who you ask). You state a plane has surface friction and is streamlined, where as a automobile has turbulent flow creating minimal surface friction on a non-streamlined object. Yet the vortex generator has a (generally) positive impact on drag in both scenarios, depending on placement and size. They accomplish the same goal, lessening separation, under what you describe as dissimilar situations. So I guess I don't understand why something else couldn't work in both scenarios, and I guess this is why I never became and engineer But since the most specific, and data filled study I could find, is apparently flawed... I guess I got no choice but to go along with the flow (get it, it's a pun )

Thanks for taking the time to read the study, and to post accordingly

~C

[aerohead]

Originally Posted by chillsworld

Vortex generators are used on airplanes. They are capable of creating a 50% decrease in drag, as per NASA studies in the 90's. These same vortex generators are used on trucks, cars, etc with success (which varies according to who you ask). You state a plane has surface friction and is streamlined, where as a automobile has turbulent flow creating minimal surface friction on a non-streamlined object. Yet the vortex generator has a (generally) positive impact on drag in both scenarios, depending on placement and size. They accomplish the same goal, lessening separation, under what you describe as dissimilar situations. So I guess I don't understand why something else couldn't work in both scenarios, and I guess this is why I never became and engineer But since the most specific, and data filled study I could find, is apparently flawed... I guess I got no choice but to go along with the flow (get it, it's a pun )

Thanks for taking the time to read the study, and to post accordingly

~C

*Looking at the NASA micro-VG applications,there is a context in which the claims are made.
*The micro-VGs which produced the 50% drag reduction are not present to the air during trimmed,cruise flight.
*They are hiding atop the flaps while stowed forward into the wings trailing edge.
*During take off and landing the flaps are extended for high-lift and STOL and the 'hidden' VGs offer a 50% drag reduction compared to fixed VGs attached to the wingtop ahead of the flap gap that they replace.
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Olsson mentioned R&D for the Lockheed C-130 Hercules high drag aft fuselage underside up-sweep (Hucho calls this a pseudo-Jaray profile).
*VGs were being evaluated to help with reattachment to this portion of the belly,as the US Air Force was complaining of high fuel costs.
*In the end,'stubs' were chosen over three different types of VGs for the lowest drag performance.
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For automobiles,since they are immersed in a turbulent boundary layer,the TBL will remain attached to the aft-body as long as the pressure rise is not too great (the profile is not too fast ).
Here is Paul Jaray's Cd 0.13 'pumpkin seed' of 1922.It is essentially incapable of producing separated flow

This 1983 Mercury Cougar with its formal roofline has massive separation and Cd 0.40.It could have used VGs.

Using their new 'aeroback' on the 1983 Thunderbird,Ford gets Cd 0.35 for the same car as the Cougar,just changing the roof.VGs could probably help it too if they were sized perfectly.

By 1985,Ford had relaxed the roofline of their Probe-V concept car closer to Jaray's profile of 1922 and scored Cd 0.137.VGs wouldn't have much to do on a car like this.

Here you can see where Mitsubishi is offering a 'Sportback' for their Lancer,which used VGs on their notchback to do the same thing with less interior space.

There is a place for VGs on any car with a 'fast' roofline.They won't be there to reduce skin friction though,just to help reattachment.

[freebeard]

I wonder if the sunvisor on a Beetle could be narrowed 4 inches and put over the backlight.


[that's the good rust, which is orange; not the bad rust which is dark brown]