FloViz Interpretation

[Cam]

when I'm in doubt, I look to NASA, simply as they seem to have their 5hit in order and, well, if they can land a robot on mars, well, their info should be sound enough. Here's a nice paper titled 'A VISUAL INVESTIGATION OF TURBULENCE IN STAGNATION FLOW ABOUT A CIRCULAR CYLINDER'. You might be able to use some of this info to better decipher what's happening on and around that strut. The FlowViz is telling you what the air is doing - this should help to understand what air does in a similar instance, then you can join the two.

A visual investigation of turbulence in stagnation flow around a circular cylinder was carried out
in order to gain a physical insight into the model advocatedby the vorticity-amplification theory.

The visualization supplied striking evidence concerning: (1)the selective stretching of cross-vortex tubes which is responsible for the amplification of cross vorticity and, hence, of streamwise turbulence:(2) the streamwise tilting of stretched cross-vortex tubes: ( 3 ) the existence of a coherent array of vortices near the stagnation zone:( 4 ) the interaction of the amplified vorticity with the body laminar boundary laaynedr,: finally, (5) the growth of a turbulent boundary layer. These are basically the flow characteristics and events advanced by the vorticity-amplification theory. It is thus apparent that the amplification of turbulence in stagnation flow around a bluff body and its effects upon the body boundary layer are satisfactorily explained by the vorticity-amplification theory.

http://ntrs.nasa.gov/archive/nasa/casi. ... 025443.pdf

[hardingfv32]

From what I can find, the fence in the center of the main element which the strut is in contact with, is there to create a vortex to help maintain flow attachment to the wing surface at high AoA. So this explains the various sizes of V sections that we see in the photographs.

So, if we want to reduce the performance of this part of the wing (F duct system) then we need to reduce the strength of the vortex. Does bleeding flow into a vortex reduce it strength? How can the flow be used to kill the vortex?

http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA470052

Brian

[flynfrog]

has nothing to do with the vortex you blow the support to keep the flow attached.

[hardingfv32]

has nothing to do with the vortex you blow the support to keep the flow attached.

How does the duct flow get through the vortex? Why do you need the duct flow at this point if the vortex is already maintaining the attachment? The most common theory about the Lotus system is that you are blowing to stall the wing, F-duct type effect.

Note the 'extend fence' in front of the duct/strut. This is a classic example of an 'extended fence' which are very affective at forming a vortex.

Brian

[bhall]

Not for nothing, but the rules require the fence as a slot-gap separator. So, I don't know that it's fair to assume it's been placed to generate for a vortex.

I took gato azul's picture of last year's McLaren and flipped over the right half of the car to get a better overall impression of the flow patterns without the obstruction of Ferrari's pit stop light. My editing assumes symmetry, but I think that's ok. (I could be wrong, though, and it wouldn't be the first time.)



The MP4-26's two slot gap separators are placed at an angle, which means, relative to the air stream, the bottom part of the fence covers the top part of the fence when the flap is lifted to engage DRS. You can see the flow pattern created by the turbulence that causes. It looks identical to the flow pattern seen on the E20. Well, half of it anyway, and appearances can be deceiving.

Thoughts?

Keep in mind the McLaren flap pictured here has a much longer chord length than that found on the E20. That means it retained a relatively high AoA when lifted for DRS when compared to the Lotus.

[gato azul]

when I'm in doubt, I look to NASA, simply as they seem to have their 5hit in order and, well, if they can land a robot on mars, well, their info should be sound enough. Here's a nice paper titled 'A VISUAL INVESTIGATION OF TURBULENCE IN STAGNATION FLOW ABOUT A CIRCULAR CYLINDER'. You might be able to use some of this info to better decipher what's happening on and around that strut.

Nice pictures and buzz words again - VORTEX seems to be the "word of the month"
This strut mounting or duct, is nothing close to circular, so why would a study on flow around a circular cylinder have much bearing on the OP question?
NASA maybe a great organization, but they have not won to many races (no the one to the moon does not count in this context) on earth, so what makes them the be all end all when it comes to F1?

Cam, with all due respect, I really like you and your enthusiasm for most of the topics here a lot, but do you read any of the papers you post?
And if so, do you understand what the papers are about?
Or do you just post them because the contain the words NASA and Vortex, because this paper does next to nothing to explain anything related to the OP question.

[Richard]

Better to pause before flaming someone. Often posts seem completely wrong not because they are wrong, it is because we've read them in the wrong context.

Cam highlighted that NASA paper as a useful guide to interpreting flow viz on a curved surface. That would be useful for people wanting to understand the flow viz patterns on the back of the wing and other curved surfaces on the car.

Yes, I do realise that the wing is not a cylinder, and yes the graphic cam posted was for flow viz on the front face as opposed to the rear face. However, it is still a valid resource to illustrate how flow viz patterns indicate different flow conditions.

The OP was asking about interpreting flow viz patterns, does anyone else have sources to explain this especially on the rear face? We might even find examples of flow viz on ellipses.

[stez90]

About the size of the "V-shape" I think it is also correlated to the longitudinal size of the structure/duct and the yaw angle it reach against the flow direction..

[flynfrog]

not every vortex is a good vortex most of the time you try to avoid them.

[gato azul]

@ Cam

My unreserved apology to you.
Please continue with your very valuable and highly interesting contributions to the topic at hand.
I did not had the intention to "flame" you, just wanted to point out that some aspects of the experiment in the paper
you have linked to, should be considered carefully, before any conclusions to the OP topic are drawn. ( controlled vortices generated in front of the test object, Re number, free steam velocity, no Flow Viz paint used (smoke traces photographed and measured))
Keep up the good work

[gato azul]

Better to pause before flaming someone.

Yes, I do realise that the wing is not a cylinder, and yes the graphic cam posted was for flow viz on the front face as opposed to the rear face. However, it is still a valid resource to illustrate how flow viz patterns indicate different flow conditions.

Better to consider your own advice, and take the time to read the full paper.
There is no use of FlowViz paint whatsoever in this paper, and the experimental conditions (creation of controlled/defined vortices in front (upstream) of the test object, the Re numbers and free stream velocities (0.76 m/s)) used, are IMHO not representative of the conditions encountered at the rear wing of an F1 car.
I therefore questioned it's usefulness for the discussion at hand.
And I did say, that I respect him (cam) and appreciate his enthusiasm and eagerness for contributions, so it's beyond me
were the "flaming" aspect is.
Are we are not longer allowed to question someones data/contributions/"facts"?
I just ask him, to explain in better detail what the specific relevance of this paper is, and how it correlates to the interpretation of FlowViz paint on the surface of objects.

[hardingfv32]

Not for nothing, but the rules require the fence as a slot-gap separator. So, I don't know that it's fair to assume it's been placed to generate for a vortex.

The rules do not require a fence the full length of the underside of the main element. And it is completely logical that you would want a aero feature that improves attachment on a wing set at a high AoA.

Brian

[flynfrog]

Not for nothing, but the rules require the fence as a slot-gap separator. So, I don't know that it's fair to assume it's been placed to generate for a vortex.

The rules do not require a fence the full length of the underside of the main element. And it is completely logical that you would want a aero feature that improves attachment on a wing set at a high AoA.

Brian

You can clearly see that it is not helping to keep flow attached. It seems F1 Tech has been hit by Vortex fever once again.

[hardingfv32]

You can clearly see that it is not helping to keep flow attached. It seems F1 Tech has been hit by Vortex fever once again. #-o

While I agree that the FloViz does not represent what we think is 'attachment', it must be remembered that we do not have much, if any, knowledge about how FloViz should look like in these types of applications.

Using the McLaren red second element as an example:

1) Fact... Fences on wings with high AoA promote attachment.
2) This second element is at a high AoA.
3) What other reason could there be to add these fences?

If you therefore assume that the fences are there to promote attachment, then can we presume that the FloViz pattern (V shape) we see is that of a vortex promoting flow attachment? Is this logic flawed?

Brian



Brian

[gato azul]

1) Fact... Fences on wings with high AoA promote attachment.
2) This second element is at a high AoA.
3) What other reason could there be to add these fences?

If you therefore assume that the fences are there to promote attachment, then can we presume that the FloViz pattern (V shape) we see is that of a vortex promoting flow attachment? Is this logic flawed?

Where does the "fact" come from Brian? - from your linked paper? - no offense or flaming, just a question.
because, the application is a bit different.
The wing in your linked paper, does not have an endplate or winglet for example, so it's a different ballgame.

If you think/assume that "this is the pattern they wanted" (as you said in an earlier post) or that it would indeed promote attachment , why has about every team gone away from this design layout since last year?
Is attached flow not longer desirable in 2012?
Maybe not everything which looks like a "fence" is a fence in the way you describe them.
I think Flyn and bhallg2k called them differently, and that maybe hinds on their primary function and purpose.