Textured surfaces & aero - 2012

[easygeez10]

Mod edit - There were three 2012 threads on the subject of textured surfaces influencing aero so I've merged them here.

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Does anyone have any ideas or theory on the effect stone damaging has to the aerodynamic performance of the aero parts?

I've been noticing quie a bit of damage lately to certain areas of the cars following races. I think that as well as being caused by stones from gravel traps following the drivers detours when missing the odd apex, is also caused by the large amount of 'pick-up' and rubber which seems to shed off the tyres.

There doesn't seem to be a lot of articles/papers/information about this on the net and wondered what peoples opinions were.

I know one theory, which is a little questionable, is that it actually improves the 'sliperyness' (don't know if that is actually a word!) of the aero parts - such as a golf ball effect or some of the latest swim suits which are being develoed to imitate shark skin.

I would have thought that it would create quite a lot of turbulence and swirling? Therefore breaking up the boundary layer creating a premature transition point thus feeding 'dirty' turbulent air into the rest of the car.

Intersted to hear what people think!

[furious_g]

I know one theory, which is a little questionable, is that it actually improves the 'sliperyness' (don't know if that is actually a word!) of the aero parts - such as a golf ball effect or some of the latest swim suits which are being develoed to imitate shark skin.

If this theory was correct, the teams would deliberatly produce a roughend surface.

A rough surface can indeed produce aero gains on some things (such as golf balls), but it's uses are quite specific and as I understand, not suited to things such as cars / aircraft etc.

[Lycoming]

The point of the dimples on golf balls is to cause the laminar boundary layer to transition into a turbulent boundary layer. While for many situations this is not ideal, turbulent boundary layers can delay flow separation.

That said, I don't think random rock hits are likely to have the same effect as the dimples on a golf ball. I have also never seen any damage from rocks that did much worse than scratch the paint.

[thisisatest]

dimples and roughened surfaces are generally only useful on bluff objects. with complete design freedom, one would just make the object aerodynamic in shape.

[skeatesy]

DIMPLES OR NOT TO DIMPLE THATS THE QUESTION ..IT all depends on the reynolds numbers whether it is less drag or more drag .. lower reynolds numbers involve viscosity and higher reynolds numbers involve inertia so dimples in the lower reynolds numbers may stick the air to the surface more but then add viscus drag where as higher reynolds numbers allow the molecules to zip by easier but then they may separate the boundry layer earlier ..so its all a power band of speed verses cord width and wetted area so the dimples may work at the front of the cars wings but turbulate on the rear wings or visa versa depending on the width of the wing cord and the speed of the airflow and the time it takes to go by and the laminar effect and its separation point which might help down the track as a turbulator effect or then again it may be too catastrophic and upset every thing blanketing the advantages where smooth surface is needed ..also the heated air has a different reynolds number than cool air because the v/2 of the air changes ...hence you cant tell until it happens and observe it ..come see come saa ...skeatesy.com =D>

[mx_tifoso]

Some previous discussions on the golf ball dimples in case you want some more info on it...

Golf ball aerodynamics in F1

Golf Ball Aerodynamics

[garygph]

I have searched for this on previous topics and have not found it but apologies if this is old.

I have been told that Red Bull have found it beneficial finish off the underside of the rear wing with 3000 water paper and not polish it. Apparently something they picked up from observations when running in the wet. Could this slightly rougher surface help delay seperation? Does water behave more uniformally on that surface than a polished one where it may form streams? Although I find that hard to imagine at the speeds they do but hell every time I think I have a grasp of aero something pops up and I realise I know zip.

source: an employee of Red Bull and so obviously I cannot say who and it is your choice to believe it or not.

[garygph]

Further thoughts on surface finish is the question of how that would influence the coanda effect? Positively I would guess. Would it actually help it and if so would you not then use it on the surface near the exhaust outlets etc?

[hardingfv32]

3000 grit paper gives pretty much a polished surface. A 3000 grit wet and dry sandpaper finish can be hand rubbed with polishing compound and wax with little effort. 1200 grit is the normal last paper you use when finishing a high quality paint job.

Brian

[garygph]

@ Harding

I agree as I have worked with water paper on finishing products and I wonder if my friend has not got an extra zero added on as a 300 grit would make a noticable differance. It must have been an obvious differance in surface finishing for it to be sent back from quality control untill it was explained that it was not an error i.e. the rougher unpolished finish on the bottom of the wing

[mep]

That must have something to do with the boundary layer.
A lot of research is done in that field. Super computers are used for CFD simulations of very small sections. Even though I don't know if F1 teams do that simply because they don't have the resources for that.

[superdread]

I have searched for this on previous topics and have not found it but apologies if this is old.

I have been told that Red Bull have found it beneficial finish off the underside of the rear wing with 3000 water paper and not polish it. Apparently something they picked up from observations when running in the wet. Could this slightly rougher surface help delay seperation? Does water behave more uniformally on that surface than a polished one where it may form streams? Although I find that hard to imagine at the speeds they do but hell every time I think I have a grasp of aero something pops up and I realise I know zip.

source: an employee of Red Bull and so obviously I cannot say who and it is your choice to believe it or not.

Water droplets have less adhesion on a rough surface, but then they could just buy a can of Lotus-Effect spray ("roughness" through nanoparticles).

[strad]

That must have something to do with the boundary layer.
A lot of research is done in that field. Super computers are used for CFD simulations of very small sections. Even though I don't know if F1 teams do that simply because they don't have the resources for that.

That is exactly right..a paper presented by marcush said as much..

believe that the front diffuser scenario is related to surface finish, and hence boundary layer characteristics rather than a pure scale effect. The pressure coefficients under the throat are the highest on the car, and super sensitive to the surface finish. A mirror smooth diffuser will always give much higher peak suctions and hence more leverage

Front diffuser or rear wing it's the same.

[olefud]

It’s been awhile but I did a boundary layer literature search for another concern. As I recall, a 300 micron “rough” surface produced a boundary layer 300% thicker than a “smooth” surface under more or less standard conditions.

My objective was different, but the boundary layer is sort of a glue that sticks the slipstream to the aero surface. At high velocities and with scouring vortices, a better attached and thicker boundary layer can maintain attachment –which is pretty much what is said above.

[garygph]

It’s been awhile but I did a boundary layer literature search for another concern. As I recall, a 300 micron “rough” surface produced a boundary layer 300% thicker than a “smooth” surface under more or less standard conditions.

My objective was different, but the boundary layer is sort of a glue that sticks the slipstream to the aero surface. At high velocities and with scouring vortices, a better attached and thicker boundary layer can maintain attachment –which is pretty much what is said above.

This makes the observation make sense then as maintaining attachment under the wing is where the challenge is and if it assists in wet weather as well, as mentioned in a previous post, then that is an added bonus. To me the reason air follows ,the designers hope, the contour under the wing is in part a contribution by the coanda effect. This then brings me back to one of my original querie/thoughts, would a "rough" surface aid the coanda effect elsewhere? A thicker boundary layer due to slowing the air down close to the surface and aiding attachment just screams out counda effect aid to me. In fact where there is a need to guide the air stream/flow or help initiate a vortice I could imagine areas of smooth and rough surfaces strategically placed could help to a small extent....but that is F1 isn't it? Every little bit adds up

Your thoughts on this would be great. Love learning from the well informed contributors here.