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Gloss or Matte ?

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zfennell
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Re: Gloss or Matte ?

Postby zfennell » Fri May 09, 2014 8:07 pm

Peter_Frank wrote:Hi

I dont know...

What I DO know is, that this has been discussed since the early days of sailing, surfing, windsurfing, kitesurfing, hydrofoiling and so on.

....
Does anybody have some tests confirming one or the other ?

....... 8) PF



correct peter.
no way to know for sure without knowing all of the details.

there has certainly been an extensive amount of research in this area with documented results under 'controlled' conditions. It's interesting to note that you will find few experimentalists that endorse a rough surface as opposed to a smooth one. Riblets are typically 'smooth channels' aligned with the flow. ' theoretically' they are intended to minimise the lateral spread of turbulence in the boundary layer.
Its my assumption that riblet technology is similar to sanding. However, I have no specific reference you would consider controlled enough to draw definitive conclusions.

A revealing reference is a compilation of experiments edited by Dennis Bushnell and Jerry Hefner;
"Viscous Drag Reduction in Boundary Layers";
Most book sites (Amazon, Google etc) will give you a preview of any section of interest.
details related to riblets are, as you surmised, dependant upon
reynolds number,
pressure gradient,
width of groove,
depth of groove.
surface roughness of groove interior.
laminar or turbulent flow.

the list goes on, but they do work, if applied correctly.
results exceeding 5% skin friction reduction were reported

the list of 'competing' approaches to viscous drag reduction is extensive.
'shark skin'
"whale turbucles'
hydrophilic surfaces
hydrophobic surface.
gas injection
liquid suction
boundary layer heating
particle injection
LEBUs
and....
but the best all time is polymer injection. (>30% skin friction reduction in sea water with polyOX)

devil is in the details.
-bill

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Re: Gloss or Matte ?

Postby dblass » Fri May 09, 2014 11:05 pm


zfennell
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Re: Gloss or Matte ?

Postby zfennell » Sat May 10, 2014 1:29 pm

again,
details will be the deciding factor for most personal opinions.

but if the comparison is just sanded vs polished, this experiment is a bit more definitive
http://www.dtic.mil/dtic/tr/fulltext/u2/a575300.pdf

abstract below:
An experimental investigation has been carried out to document and relate the frictional
resistance and roughness texture of painted surfaces smoothed by sanding. Hydrodynamic
tests were carried out in a towing tank using a flat plate test fixture towed at a Reynolds
number ~ReL ! range of 2.8310625.53106 based on the plate length and freestream
velocity. Results indicate an increase in frictional resistance coefficient ~CF! of up to
7.3% for an unsanded, as-sprayed paint surface compared to a sanded, polished surface.
Significant increases in CF were also noted on surfaces sanded with sandpaper as fine as
600-grit as compared to the polished surface

in this case Rougher is Slower
-bill

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Re: Gloss or Matte ?

Postby zfennell » Sat May 10, 2014 1:51 pm

Peter_Frank wrote:Hi

.........Does anybody have some tests confirming one or the other ?

......But besides water tunnel tests, there should also be real world tests head to head, repeatedly - to confirm the differences.
With real reynolds numbers (not taken from another sport that is).

.....
PF



the 'real world' head to head test is likely to be the most difficult to deliver
especially when you are talking about a few percentage points of something that is only a partial contributer to the total drag of the system.

if my two two option were to
A) sand my favorite foil to my heart's content
or
B) let Johnney Heiniken ride any old piece of junk

i'll go with option B

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Re: Gloss or Matte ?

Postby skerrj » Sat May 10, 2014 8:51 pm

does a wing, or fin, need to have a certain amount of "adhesiveness" to it's environment to work?

-scott

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Re: Gloss or Matte ?

Postby Peter_Frank » Sat May 10, 2014 9:28 pm

skerrj wrote:does a wing, or fin, need to have a certain amount of "adhesiveness" to it's environment to work?

-scott


Not sure I understand your question ?

But no, "adhesiveness" is not needed....

8) PF

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Re: Gloss or Matte ?

Postby zfennell » Sat May 10, 2014 10:27 pm

scott,
if you are referring to friction in the boundary layer, the term that describes friction in a fluid is viscosity.

i believe that as long as the wing is able to redirect the flow , it will generate lift.
with or without friction.

potential flow describes the flow field around a body without the boundary layer caused by friction.
the mathmatecians that invented this 'frictionless' fluid needed to come up with a few tricks to make the math represent what they observed in real life. So they included vorticity without any real justification other than it worked.
(i was never good at math)
:)

-bill

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Re: Gloss or Matte ?

Postby skerrj » Sun May 11, 2014 12:51 am

not really sure what i'm asking either.... sorry for the confusion.

peter mentioned something about different surfaces needing different finishes... has something todo with that.

how do superhydrophobic coatings, like neverwet, fit into all this?:

- i now for sure they wouldn't work for fins and foil wings. why is this?

- would they work for bottoms of boards, like windsurfers and kite race boards, something that just skips on top of water? or would they just create more drag?

that's kind of where my "adhesion" question came from.

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Re: Gloss or Matte ?

Postby Tone » Sun May 11, 2014 9:54 am

my board with the matt finish feels like it gets up and goes faster but yet to really get the right angle off the wind for a proper go.

only managed 34.33 yesterday over 500m, had a few peaks of nearly 40 but couldn't hold it for the full 500. The course was too tight. Today we have 30 knots and the wind is much broader in relation to the sandbar so hope to see some faster times from the kite surfers.

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Re: Gloss or Matte ?

Postby flying grandpa » Sun May 11, 2014 10:25 am

skerrj wrote:how do superhydrophobic coatings, like neverwet, fit into all this?:

- i now for sure they wouldn't work for fins and foil wings. why is this?

- would they work for bottoms of boards, like windsurfers and kite race boards, something that just skips on top of water? or would they just create more drag?


IMO hydrophobics increase the surface tension of water, promoting the air into the uderpressure area = ventilation of fins and foils => spinouts.

Surface tension force may be very high, and we can imagine how destructive it can be to the boundary layer thickness,
if the flow runs above the surface unevenly covered by neverwet.

Maybe that explains a bit of the phenomenon?


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