Flowvis questions

[raymondu999]

Hey folks - just wondering. How exactly do you "read" flowvis? Ok, so you've just done a test run. Or even just a straightline test. So you have a rorschach pattern on your rear wing endplate now. I'm assuming that the pattern is showing... well, the flow where the air goes.

But why doesn't all the flowvis get blown off? As in, if it's of a specific consistency that can be "pushed" to the trailing edge by the airflow; why doesn't all of it get pushed off? I hope my question makes sense.

[PNSD]

You've seen cars or even racecars when its rained, and often you can see the dried dirt trails;

http://www.mulsannescorner.com/mkiiic-4.html

I guess like this, the flow vis is made in such a way that it does leave trails and likely dries very quickly. I know stuff used in wind tunnels, UV paint stains clothes, and dries pretty quickly.

[marcush.]

what about boundary layer effects?
and dynamic behaviour? you slap on 3kg of flow vis and do a lap ..but your murals on the car develop over a out lap timed lap and an inlap ..where was the separation present and where not? It´s a bit like a temperature strip showing the max temp only..but how long or was a peak not even registered because of the slow reaction of the sticker?

[raymondu999]

Yes that was sort of what I was thinking too - unless of course you have a camera rig that is recording the thing as it goes...

[marcush.]

exactly ,but then you need a cameraa array to be able to record all those details at the same time ...I still wonder why teams do NOT design their cars completely as Friday testrigs ..although I think Mclaren has or had mounting points molded into the tub for their rake...Sure the friday sessions are most important to accumulate reliable data for in season development but teams still seem to focus more on setup....in fact a lot of teams seem to be comfortable understanding the car and tyres only after doing half the season ...

[Kiril Varbanov]

One of the main inferences of the flow vis. is to check whether the flow stays attached to the surface as expected or projected in the CAD / tested in wind tunnel - to correlate real air data on the track.

Then, the next thing to look for is any obvious generations of unwanted vortex patterns, and finally there's the direction (which is very obvious).

[Just_a_fan]

You've seen cars or even racecars when its rained, and often you can see the dried dirt trails;

I have some pictures of Audi LeMans cars taken at the Goodwood Festival of Speed. They are still dirty from the race and the flow viz effect is extremely interesting.

I'll see if I can find them...

[hardingfv32]

I wonder how fast the fluid dries up or takes a set?

You apply it in the pits, and has it flowed out in two or three turns after leaving the pits?

This is a representation of boundary layer activity, correct? Is that always a fair representation of what the flow is like above the surface, say 1-2 cm?

Brian

[n_anirudh]

http://scarbsf1.wordpress.com/2010/04/1 ... thing-new/

^^An earlier article from Scarbs...

since the oils are "heavier" would these flow viz mixture move after a particular speed??

I find it analogous to the surface oil lines used in experiments.

Brian,It is not always representative of BL, as you may have a vortex striking that area of the car...

[hardingfv32]

What does viewing under a special light bring to the table? Is there more detail available that what we are seeing under standard light?

Brian

[Dragonfly]

If there is a fluorescent agent in the paint, then the smallest tracks would be visible under UV light.

[Owen.C93]

What does viewing under a special light bring to the table? Is there more detail available that what we are seeing under standard light?

Brian

Other teams can't read it so well is one use. But yeah I think if you have the time to take proper pictures of it then can be more defined under a proper UV light.

With flo-vis you're looking at the pattern and flow lines, not necessarily the density of the residue because that can be affected by many things (for example how thickly you apply it etc). What they will do is simulate flo-vis in CFD and probably in the wind tunnel and just make sure the patterns match up and find out where the inaccuracies are.

It's not terribly exciting if you don't know what it should look like unfortunately.

[Mikey_s]

I have no specific expertise in flow-vis, but spent a lot of time in the lab working on fluid flow properties and would suggest that the paint is a pseudo-plastic liquid with a yield stress; think of Heinz Tomato ketchup, you can turn the bottle upside down and the ketchup doesn;t come out, but if you exceed the yield stress (e.g. by smacking the bottom of the bottle) the ketchup will flow out onto your fries. Thereafter the lquid should cease to flow, thereby leaving a nice 'lump' of ketchup on the fries and not running off (like cheaper ketchup!) all over the bottom of the plate.

As such, the paint should not slump after being applied (picture evidence suggests it takes a while to regain it's gel properties as there are clearly some dribbles as it is applied), then when the car is moving at a speed high enough for the air flow to overcome the yield stress, the paint flows. Thereafter the car slows down, the paint gels again and the pattern is fixed on the skin of the car.

Just my thoughts...

[j2004p]

I have no specific expertise in flow-vis, but spent a lot of time in the lab working on fluid flow properties and would suggest that the paint is a pseudo-plastic liquid with a yield stress; think of Heinz Tomato ketchup, you can turn the bottle upside down and the ketchup doesn;t come out, but if you exceed the yield stress (e.g. by smacking the bottom of the bottle) the ketchup will flow out onto your fries. Thereafter the lquid should cease to flow, thereby leaving a nice 'lump' of ketchup on the fries and not running off (like cheaper ketchup!) all over the bottom of the plate.

As such, the paint should not slump after being applied (picture evidence suggests it takes a while to regain it's gel properties as there are clearly some dribbles as it is applied), then when the car is moving at a speed high enough for the air flow to overcome the yield stress, the paint flows. Thereafter the car slows down, the paint gels again and the pattern is fixed on the skin of the car.

Just my thoughts...

A non-newtonian liquid I believe

[kilcoo316]

Hey folks - just wondering. How exactly do you "read" flowvis?

Very badly.

It is nigh on useless - to be honest I don't why they even bother. Marcush has alluded to some of the more pertinent points. I suppose it might* tell you if your correlation from CFD to tunnel to track is completely f**ked, but beyond that, you'll learn very little as you'll have no faith in the accuracy of the paint streaks.


*emphasis on might.


The pitot-static rake on the other hand - much more useful.