How does air react to a surface with a lower coefficient of friction

[tmh556]

Hi,
Does air flow along a surface with a lower coefficient of friction more easily or does it not make a difference?

 

[FredGarvin]

In this case, the terminology you are looking for is not COF, but surface roughness. COF implies two surfaces/objects in contact. Surface roughness simply is a statement of the average variation of a surface. In the case of airflow, a smoother surface definitely helps. Look at any aircraft wing. The rivets are as flush as possible and are kept very clean.

 

[tmh556]

What if you applied a lubricant to a surface? Would that lubricant reduce the aerodynamic drag of the surface? I would think it would be similar to the effect achieved with a lubricant for a solid on another solid.

 

[tmh556]

any thoughts

 

[FredGarvin]

Again, back to wings... aircraft are regularly waxed or have some form of surface treatment for this purpose.

http://www.cleanwingdetail.com/exterior-detail-services.html
http://www.wingwaxing.com/index.html

 

[Andy Resnick]

Hi,
Does air flow along a surface with a lower coefficient of friction more easily or does it not make a difference?

That's a good question: strictly speaking, air (or any fluid) in contact with a solid surface moves at the speed of the surface; the phrase 'air flow along a surface' usually refers to the boundary layer, the thickness of which is arbitrarily defined. The connection between the boundary layer flow and macroscopic (far-field) flow can be quite complex.

Surface coatings, as Fred mentioned, are used to affect the boundary layer in order to create a particular "far-field" flow pattern; microroughness treatments are a characteristic type of treatment which, IIRC (I'm not an expert), thin the boundary layer, move the separation point downstream, and restrict far-field turbulent wakes. This reduces drag.

 

[rcgldr]

In the case of gliders, the wing surfaces are sometimes "roughed" up a bit with fine (600) grit sandpaper to better control the transition from laminar to turbulent flow that "detaches" from the wind during the transition. Do a web search for "glider oil flow test" to see example of this.

Note that a golf ball's dimples are there to reduce Magnus effect (how much the ball curves due to it's spin and forwards speed).

 

[Ranger Mike]

Having been Product Manager for a Metrology Instrument Company, I can clear this up. There is NO such thing as Surface Roughness. Per ANSI B 46.1 Standards, Surface Texture is made up of Flaws ( blow holes, cracks etc...) Lay, or direction of the grain of the material, Waviness and Roughness. Above topics is about Roughness. There are over 120 parameters to describe or qualify Roughness. The most popular is Ra or Roughness Average. Not to go into too much detail but Ra is obtained by a diamond stylus on a motorized drive that moves across a surface and calculated the average up and down stylus movement to determine and average.

From a racers prospective, you want the top of the rear wing to be super slick or smooth and the bottom side of the wing to be " rough" so we spray the top with gloss paint and the bottom of underside of the wing primer paint. It disrupts the flow and slows the air. Thus Bernoulli's Theorem is in effect. Works.

 

[Phrak]

Note that a golf ball's dimples are there to reduce Magnus effect (how much the ball curves due to it's spin and forwards speed).

Jeff. I was shooting the breeze with an aeronautical engineer this afternoon and this very topic came up. The dimples promote a turbulent boundary layer that stays attached to the surface further along the profile of the ball than a laminar flow. This reduces the size of the turbulent wake--a little cone shaped region at the leeward side of the ball. This is a low pressure region that slows the ball in flight. So, with less wake, the ball flies further.

Interestingly the turbulent boundary layer has greater surface drag than laminar flow, but this is more than offset by the reduction in wake size, be it a golf ball or a wing.

 

[Phrak]

There is on such thing as Surface Roughness. Per ANSI B 46.1 Standards,...

Sure there is, and it existed a long time before ANSI. If ANSI has dropped the term in favor of others, that is the concern of those who use ANSI standards.

 

[Ranger Mike]

All I can say is that ANSI recognized the need to explain the properties of what the layman calls "surface finish"," surface roughness" and the like.
Measurement of these parameters began in 1938. Believe it or not, some of the Big Three Auto makers are using terms dating to 1956!

 

[rcgldr]

From a racers prospective, you want the top of the rear wing to be super slick or smooth and the bottom side of the wing to be " rough" so we spray the top with gloss paint and the bottom of underside of the wing primer paint. It disrupts the flow and slows the air. Thus Bernoulli's Theorem is in effect.

That's Coanda effect, not Bernoulli. The rough surface on the bottom, combined with friction and viscosity of the air, cause the air to follow a convex surface, resulting in the air accelerating "inwards", with a corresponding reaction force "outwards".

 

[FredGarvin]

Having been Product Manager for a Metrology Instrument Company, I can clear this up. There is NO such thing as Surface Roughness. Per ANSI B 46.1 Standards, Surface Texture is made up of Flaws ( blow holes, cracks etc...) Lay, or direction of the grain of the material, Waviness and Roughness. Above topics is about Roughness. There are over 120 parameters to describe or qualify Roughness. The most popular is Ra or Roughness Average. Not to go into too much detail but Ra is obtained by a diamond stylus on a motorized drive that moves across a surface and calculated the average up and down stylus movement to determine and average.

Given the continued use in industries, I would argue that point. I don't know of anyone that uses the term "Surface Texture." Granted, you are probably correct in following the letter of the law per ANSI, but if you look at most drawings, etc... you will see a surface roughness called out as some number Ra. I'm sure it's a legacy thing, but it is still widely used.

 

[FredGarvin]

Jeff. I was shooting the breeze with an aeronautical engineer this afternoon and this very topic came up. The dimples promote a turbulent boundary layer that stays attached to the surface further along the profile of the ball than a laminar flow. This reduces the size of the turbulent wake--a little cone shaped region at the leeward side of the ball. This is a low pressure region that slows the ball in flight. So, with less wake, the ball flies further.

Interestingly the turbulent boundary layer has greater surface drag than laminar flow, but this is more than offset by the reduction in wake size, be it a golf ball or a wing.

That is absolutely correct. It forces a turbulent BL and the reduction in wake is very dramatic.

 

[Phrak]

That is quite a difference. Now if they could just put the dimples on the leading edge of the spinning ball, and leave the rest nice and smooth...

That is absolutely correct. It forces a turbulent BL and the reduction in wake is very dramatic.

Hmm. I wasn't completely correct, apparently. I'd thought that the cross-section of wake reduced in size and eventually closed-off--a cone shape. But the turbulence in the wake doesn't just disappear, but diminishes in amplitude over time.

 

[Ranger Mike]

That's Coanda effect, not Bernoulli. The rough surface on the bottom, combined with friction and viscosity of the air, cause the air to follow a convex surface, resulting in the air accelerating "inwards", with a corresponding reaction force "outwards".

Thank you.. I always wondered what the correct term was. I should be using Bernoulii regarding the wing shape and Coanda regarding surface TEXTURE of said surface.
I stand corrected..

 

[Ranger Mike]

Given the continued use in industries, I would argue that point. I don't know of anyone that uses the term "Surface Texture." Granted, you are probably correct in following the letter of the law per ANSI, but if you look at most drawings, etc... you will see a surface roughness called out as some number Ra. I'm sure it's a legacy thing, but it is still widely used.

Thank you. I can agree on the point and it is unfortunate that most people ( present company excluded) take the easy path and go with the flow. Regarding the "surface roughness call out" above...If one were to look at the ANSI Y14.36M Standard on Geometric and Dimensional Tolerances, there are 10 different "Surface Texture Symbols for specifications and drawings. None of these are surface roughness. I know that most of you guys know this but for those who do not..This ANSI Y14 Standard is what designers use to "draw" the part..still relative in the CAD world today..
enuff on beating this dead horse about terminology...