Water temperature and fin performance

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Roy Stuart started the topic in Tuesday, 13 Oct 2015 at 7:05am
udo's picture
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udo commented Friday, 30 Oct 2015 at 9:01pm

Very interesting Roy.

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Craig commented Friday, 30 Oct 2015 at 9:25pm

Interestng Roy, but surely insignifcant in the whole scheme of things?

IE Viscosity for water 1deg = 1.6438E-6  and 20deg = 9.7937E-7 are about the same order of magnitude.

And the Reynolds Number outputs of 400,000 and 700,000 are of the same order of magniutde, and hence insignificant as well yes?

Do you have the relevant change/loss in speed/friction etc.

Roy Stuart's picture
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Roy Stuart commented Friday, 30 Oct 2015 at 10:24pm

The changes in fin response are significant, but no rule of thumb can be used as it depends a lot upon the foil shape and chord length being used. There are in some cases large differences in the angle of attack capability of the foil.

Take a look at the graphs for the foil which i used as an example in the post. It's pretty obvious that there's a pretty big difference in aoa capability, and drag.

I thought that it might just be something to keep in mind, if trying to optimise fins.

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caml commented Saturday, 31 Oct 2015 at 1:00am

Where was this forum hiding for weeks ? Roy this is very interesting ! Except im going to have to decipher some words . Sometimes really wish I was smarter & could understand graphs other than swell graphs . Even without deciphering the words and as soon as reading the title I had to agree that fins would go slower in ice ! Craig its true , I can see it now ;" heated fins cuts thru like butter" rechargeable batteries ! And maybe combo with shark zapper ..

Roy Stuart's picture
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Roy Stuart commented Saturday, 31 Oct 2015 at 6:08am

Ok the the top graph shows how much lift is produced as the angle of attack changes when turning, in 20 degree water it's about 66% more than in 1 degree water. So in theory a smaller fin is needed in warm water, but it gets a little complex to apply that theory.

The second graph shows changes in drag versus angle of attack. The drag is much the same in cold or warm water, until the fin approaches the stalling or spinout zone... what's pretty dramatic is that the fin stalls at around 11 degrees angle of attack in cold water and about 14 degrees in warm water, that's obviously a big difference. One way to fix that in cold water is to increase the base length (chord) of the fin.

That's just an example, the results for different foil sections differ a lot... some have more difference, some less.

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blindboy commented Saturday, 31 Oct 2015 at 10:06am

Mildly interesting but pretty much useless in terms of practical design since these effects are going to be swamped by all the other variables. Trying to apply them without extensive tank testing would be pure guesswork.

Roy Stuart's picture
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Roy Stuart commented Saturday, 31 Oct 2015 at 10:41am

I disagree, for example a huge reduction in angle of attack plus a 66% increase in drag is highly significant.

The behaviour of known foils is well researched and the Xfoil computer modelling is by all accounts pretty good.... thus it is by no means'guesswork' to apply the concept.. in fact there's far less guesswork involved than there is in surfboard shaping.

In a world where countless surfboard design concepts are marketed and accepted as 'scientific' and proven simply on the basis of rider feedback or marketing hype, this concept stands out as being based on sound research, and I don't mean mine.

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Roy Stuart commented Saturday, 31 Oct 2015 at 10:26am
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Roy Stuart commented Saturday, 31 Oct 2015 at 10:47am

This study tested the range of angles of attack experienced by side fins, the highest positve aoa was around 12 degrees.

http://seit.unsw.adfa.edu.au/ojs/index.php/juer/article/viewFile/283/154

so, for the foil used in the example, the fin will spin out in cold water, but will not in warm water... that's more than just an insignificant difference which will be swamped by other variables.

I notice that the 'insignificant' comment is not applied to those who adjust fin cant by a degree, or who adjust board volume, length, width, rocker or thickness by a few percent.

We have differences in cold water : a 22 percent reduction in angle of attack capability and a massive 66 percent increase in fin drag when turning at higher angles of attack, with significant increases in drag throughout the aoa range.

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blindboy's picture
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blindboy commented Saturday, 31 Oct 2015 at 10:47am

As Craig pointed out the effects are small so 66% of not much is even less.

Roy Stuart's picture
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Roy Stuart commented Saturday, 31 Oct 2015 at 10:55am

blindboy wrote: As Craig pointed out the effects are small so 66% of not much is even less.

You are not making sense.

Please try to read and understand.

Fin drag is certainly significant, and a 66% increase is huge. The increase in total surfboard drag contributed by the fins in cold water for a typical mid length quad will be around 11 percent.

In terms of handling, when a fin spins out that's about as significant as it gets. Any fin which doesn't spin out in 1 degree water during turns is over designed for warm water.

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tim foilat's picture
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tim foilat commented Saturday, 31 Oct 2015 at 11:05am

reminds me of a quote I once read somewhere, "barley you really don't think I am going to waste my time responding to that do you? Go read something sometime. You just might learn that there are people who know more than you. Guess what? I' m one of them!"

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Roy Stuart commented Saturday, 31 Oct 2015 at 11:14am

Lol if I change my name to George Greenough then many people will think that I know more. ;)

A case in point is my decades old design axiom that the aerodynamic effects of the surfboard's nose can be significant and that rounded, thicker noses give better aerodynamic results. Most people thought that I was full of sh*t until George Greenough came out with the same theory a few years ago... now it's officially worth thinking about.

There's a lot more to my aerodynamicslly advantageous noses theory than George has come up with too.

For a different thread......

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tim foilat commented Saturday, 31 Oct 2015 at 11:30am

Love your informative threads/posts and checking in on your website it's an impressive body of work Roy!

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blindboy commented Saturday, 31 Oct 2015 at 1:09pm

11% ? Under what conditions? I mean if you look at the whole system a change in wind speed or direction could clearly be more significant than that. Then if a surfer goes from warm to cold water the mass of the entire system is increased by the wearing of a wetsuit. Surface conditions also have a considerable influence on total drag so your 11% sounds to me like a number calculated from dubious data. Rubbish in....rubbish out as they say.
Further since you seem to think the engineering perspective is important, what contribution has it ever made to design? Nil, nada, nothing at all in my experience. Surfboard design progresses by evolutionary methods in which boards which perform well are copied with minor changes. The best of them are also then copied and so on. This is a far more powerful and effective method of design than sticking dubious data into engineering equations which are usually based on simplistic assumptions which do not apply to surfing. It was the same story last time with your nonsense about beam theory......but you don't even believe in natural selection so this concept might be too challenging for you, which might explain why you are in a NZ backwater making over sized wooden prized for their appearance rather than their performance.

udo's picture
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udo commented Saturday, 31 Oct 2015 at 1:19pm

.

Roy Stuart's picture
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Roy Stuart commented Saturday, 31 Oct 2015 at 1:32pm

blindboy wrote: 11% ? Under what conditions? I mean if you look at the whole system a change in wind speed or direction could clearly be more significant than that. Then if a surfer goes from warm to cold water the mass of the entire system is increased by the wearing of a wetsuit. Surface conditions also have a considerable influence on total drag so your 11% sounds to me like a number calculated from dubious data. Rubbish in....rubbish out as they say.
Further since you seem to think the engineering perspective is important, what contribution has it ever made to design? Nil, nada, nothing at all in my experience. Surfboard design progresses by evolutionary methods in which boards which perform well are copied with minor changes. The best of them are also then copied and so on. This is a far more powerful and effective method of design than sticking dubious data into engineering equations which are usually based on simplistic assumptions which do not apply to surfing. It was the same story last time with your nonsense about beam theory......but you don't even believe in natural selection so this concept might be too challenging for you, which might explain why you are in a NZ backwater making over sized wooden prized for their appearance rather than their performance.

Not worthy of a reply, apart from the observation that your emotions are hanging out while the brain appears to be on the shelf.

Now's about the time when you mention the HOLOCAUST

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Roy Stuart commented Saturday, 31 Oct 2015 at 1:22pm

tim foilat wrote: Love your informative threads/posts and checking in on your website it's an impressive body of work Roy!

Thanks very much.

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Roy Stuart commented Saturday, 31 Oct 2015 at 1:34pm

How I see Blindboy... screwed in a bit too tightly from the outset...

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Roy Stuart commented Saturday, 31 Oct 2015 at 1:35pm

We do actual testing too...

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Roy Stuart commented Saturday, 31 Oct 2015 at 1:37pm

udo wrote:

full stop

New sentence.

Scissors paper rock Mr Udo?

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Roy Stuart commented Saturday, 31 Oct 2015 at 1:49pm

Sorry, can't be serious all the time

blindboy's picture
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blindboy commented Saturday, 31 Oct 2015 at 2:15pm

Hey Roy what happened to the discussion? Lost for an answer? Last time that happened you didn't show up for a couple of weeks. Why not make it months or years this time? Or are you going to get the police on to me for cyberbullying like you threatened last time .....oh and made the same threat to the web site.
Emotional? Not at all. Intolerant of bullshit? Absolutely.

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Roy Stuart commented Saturday, 31 Oct 2015 at 2:23pm

Tiresome fellow.

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blindboy commented Saturday, 31 Oct 2015 at 3:01pm

Truth hurts does it Roy? And since you raised the subject yep., I'll go there, what do you think about the holocaust? Or are your views illegal because not only are they wrong but they are also repulsive?

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caml commented Saturday, 31 Oct 2015 at 10:47pm

Seeing it seems ok to go off topic I want to ask how might this affect aerodynamics , like when airplanes go into below zero temps vs tropical climates

blindboy's picture
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blindboy commented Sunday, 1 Nov 2015 at 10:11am

I haven't looked at this in any depth but I think the only real issue is that under extreme heat the density of the air is so reduced, which directly reduces lift ,so that it may be unsafe to take off as the runway is too short.

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Roy Stuart commented Sunday, 1 Nov 2015 at 11:18am

As far as aircraft go, higher temperatures also reduce engine performance.

The kinematic viscosity in air at 20 degrees c is 13% less than it is at 0 degrees C.

For water the difference is 66%.

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wellymon commented Sunday, 1 Nov 2015 at 2:32pm

Roy Stuart wrote: We do actual testing too...

Thats funny as.
Not a soft landing, he must've done some damage for sure, Ouch.

Our brains are too small at the moment to comprehend the reality of what's happening in our forests . We're only just waking up so to speak . The big problem is we think we know everything, we are specks of dust on a timeline and we know nothing .

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Blowin commented Sunday, 1 Nov 2015 at 7:34pm

Hands up who surfs in 0 degree water.

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wellymon commented Sunday, 1 Nov 2015 at 8:16pm

So true Blowin haha.

Our brains are too small at the moment to comprehend the reality of what's happening in our forests . We're only just waking up so to speak . The big problem is we think we know everything, we are specks of dust on a timeline and we know nothing .

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Roy Stuart commented Sunday, 1 Nov 2015 at 8:42pm

The same difference will occur from 8 degrees to 28 degrees.

Zero to 20 is merely an example.

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caml commented Sunday, 1 Nov 2015 at 11:17pm

Roy thats right , (re 8-28) some of the words used are beyond me re scientific explanations

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surfingbymyself commented Monday, 2 Nov 2015 at 2:08pm

Roy, what flow conditions are those graphs based on? I assume they assume laminar flow across the whole foil? And typically foil Cl and Cd curves don't include tip effects?

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Roy Stuart commented Monday, 2 Nov 2015 at 5:04pm

The graphs are calculated by Xfoil, and don't assume laminar flow across the entire foil ( to the best of my knowledge anyway).

Tip effects depend upon the fin planshape, I don't think that Xfoil deals with that or spanwise flow at all, presumably it assumes a wing of infinite span.

http://web.mit.edu/drela/Public/web/xfoil/

"XFOIL is an interactive program for the design and analysis of subsonic isolated airfoils.
It consists of a collection of menu-driven routines which perform various useful functions such as:
Viscous (or inviscid) analysis of an existing airfoil, allowing
forced or free transition
transitional separation bubbles
limited trailing edge separation
lift and drag predictions just beyond CLmax
Karman-Tsien compressibility correction
fixed or varying Reynolds and/or Mach numbers
Airfoil design and redesign by interactive modification of surface speed distributions, in two methods:
Full-Inverse method, based on a complex-mapping formulation
Mixed-Inverse method, an extension of XFOIL's basic panel method
Airfoil redesign by interactive modification of geometric parameters such as
max thickness and camber, highpoint position
LE radius, TE thickness
camber line via geometry specification
camber line via loading change specification
flap deflection
explicit contour geometry (via screen cursor)
Blending of airfoils
Writing and reading of airfoil coordinates and polar save files
Plotting of geometry, pressure distributions, and multiple polars"

More reading http://www.mh-aerotools.de/airfoils/methods.htm

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sluggoes commented Monday, 2 Nov 2015 at 8:00pm

So, after a page of words, we now find out the curves are a computer model for aerofoils, not HYDROfoils?
I was a little skeptical about 1 degree C and freezing points for water, but no such problem for air.

So the curves are showing how your fin(s) might go when your board is on the roof of your car in the snow when you go around a corner?

Roy Stuart's picture
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Roy Stuart commented Monday, 2 Nov 2015 at 8:10pm

As a matter of fact the programme is also used for hydrofoils, and the conditions which surfboard fins experience is in the range for gliding aircraft.

As you can see, the programme includes the parameters for water, and the graphs shown have been carefully placed in the relevant reynolds number range for surfboard fins.

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surfingbymyself commented Monday, 2 Nov 2015 at 8:44pm

So I'm inclined to think that it's interesting but difficult to apply, and probably of very low impact, particularly given that those graphs are based on idealised flow and foils, and that accurate modelling at low Reynolds numbers is difficult. I think it's great that the question has been asked but I question the application of the results. For mine, relatively large variations in span wise flow and in angles of incidence, which occur continuously when surfing, (consider how much the flow over a fin varies during even a small mid face pump) are likely to be orders of magnitude more important to fin performance than water temp variations , yet they can't be modelled reliably (if at all) and thus make predicting actual performance based on model outputs extremely unreliable. I don't see how the information gained from reading those graphs can be extrapolated to confidently say that a fin will or won't spin out at a given temp.

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Blowin commented Monday, 2 Nov 2015 at 8:47pm

Do the factors involved remain valid even if the surfer isn't wearing a hot pink steamer ?

surfingbymyself's picture
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surfingbymyself commented Monday, 2 Nov 2015 at 9:33pm

Very difficult to model variations due steamer colour. Some NASA work from the 80s on flouro colours was 'discontinued' and there's been a world wide conspiracy of silence ever since

Roy Stuart's picture
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Roy Stuart commented Tuesday, 3 Nov 2015 at 5:04am

surfingbymyself wrote:

consider how much the flow over a fin varies during even a small mid face pump)

Ok, you tell me how much it varies.

e wrote:

likely to be orders of magnitude more important to fin performance than water temp variations

The statement above is ludicrous, since there is no scale with which we can measure importance.

As long as one gets to say 'orders of magnitude' though the troops will be impressed.

'Orders of magnitude' defines the number of orders as at least two, so that's at least 100 times more important... pease tell me how you are measuring importance. What is the unit of measurement?

Claiming to have measured importance while dismissing the known effects of water viscosity is extremely silly.

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Roy Stuart commented Tuesday, 3 Nov 2015 at 5:08am

surfingbymyself wrote: So I'm inclined to think.

This statement is revealing as it shows that you want to reach a certain conclusion.

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surfingbymyself commented Tuesday, 3 Nov 2015 at 7:00am

I see you've taken issue with my language but not the substance, fair enough. Here's the substance again then:
- I think it's interesting
- the models are not reliable for low Reynolds number flows
- the models are based on non span wise flow which doesn't relate to a fin moving through a turn on a wave face at all. There will be large components of span wise flow at times
- the models don't take into account tip effects
- the models are based on a continuous chord foil whereas fins are variable foil throughout
That makes me question any statement which confidently relates model results to real world performance. I'm not saying it may not be a factor but I am saying that until those other factors are able to be modelled, the idea isn interesting but not definitive.

Btw, the relationship between temperature a lift is obvious from the lift equation (L= 1/2 roe V^2 S Cl) if you consider that temp is a determinant of density (roe) then I would argue that the other terms, obviously V and also Cl will be much more significant contributors to final lift value, possibly even orders of magnitude, but I hesitate to use that phrase again, I was always taught that it's wrong to curry favour with the troops...

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surfingbymyself commented Tuesday, 3 Nov 2015 at 7:39am

I just did some googling; sea water at 14 degrees is 1026.xx kgm^3 and is 1029.xx kgm^3 at 24 degrees.
If I use those values in the lift equation, using the following values:
S=0.01m2
Cl=0.5
V=10ms

Then I get a L figure of 256.5N for 1026 and 257.25N for 1029. So a 10 degree temp variation in the representative range yields a variation of less than 1 N.
Whereas, if I vary V by only 1ms, then I get a L value of 311N. I acknowledge its not orders of magnitude, but it is a much much more significant variation and local variations in Chordwise (basis of L equation) V will be more significant but much much harder to model than temp variations.

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Roy Stuart commented Tuesday, 3 Nov 2015 at 7:59am

surfingbymyself wrote: I just did some googling; sea water at 14 degrees is 1026.xx kgm^3 and is 1029.xx kgm^3 at 24 degrees.
If I use those values in the lift equation, using the following values:
S=0.01m2
Cl=0.5
V=10ms

Then I get a L figure of 256.5N for 1026 and 257.25N for 1029. So a 10 degree temp variation in the representative range yields a variation of less than 1 N.
Whereas, if I vary V by only 1ms, then I get a L value of 311N. I acknowledge its not orders of magnitude, but it is a much much more significant variation and local variations in Chordwise (basis of L equation) V will be more significant but much much harder to model than temp variations.

You have the wrong end of the stick there, have you actually read what I've written?

The analysis is not based on density changes but on viscosity changes. I've already pointed out that density changes in water are insignificant.

Viscosity effects are not insignificant.

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Roy Stuart commented Tuesday, 3 Nov 2015 at 8:25am

surfingbymyself wrote:

- the models are not reliable for low Reynolds number flows

Do your homework please, the Reynolds numbers for typical fins at surfboard speeds are not low.

e wrote:

- the models are based on non span wise flow which doesn't relate to a fin moving through a turn on a wave face at all.

yes it does.

e wrote:

There will be large components of span wise flow at times

Not necessarily.

e wrote:

- the models are based on a continuous chord foil whereas fins are variable foil throughout

Polars can be generated for any chord length variation.

e wrote:

That makes me question any statement which confidently relates model results to real world performance.

There are many studies available which compare Xfoil predictions with tank and wind tunnel tests, and the results are good.

e wrote:

Btw, the relationship between temperature a lift is obvious from the lift equation (L= 1/2 roe V^2 S Cl) if you consider that temp is a determinant of density (roe) then I would argue that the other terms, obviously V and also Cl will be much more significant contributors to final lift value, possibly even orders of magnitude, but I hesitate to use that phrase again, I was always taught that it's wrong to curry favour with the troops...

as already mentioned you've completely missed what I've been saying.

I am not suggesting that lift changes due to density variations are significant, in fact I've stated several times that they are insignificant.

It is viscosity which I'm looking at, and changes due to viscosity are certainly significant.

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surfingbymyself commented Tuesday, 3 Nov 2015 at 8:35am

the above is a back of the envelope effort to quantify importance of temp variation, since you asked how I was measuring importance of terms. I can see from your original post on your site that viscosity is the basis of your argument but I also note that it doesn't vary linearly with temp which calls into question your statement that the same variation would occur from 8-28 degrees.
Again, so that what I'm saying is super clear, I think the premise is interesting, and I think the investigation is worthwhile, but for the reasons posted above, I find definitive statements about real world performance very hard to accept.

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Roy Stuart commented Tuesday, 3 Nov 2015 at 9:13am

surfingbymyself wrote:

I can see from your original post on your site that viscosity is the basis of your argument

Ok.

e wrote:

but I also note that it doesn't vary linearly with temp which calls into question your statement that the same variation would occur from 8-28 degrees.

That's correct. Thanks for pointing it out.

If we look at the change in viscosity over the lower twenty degree range it's around 66%, moving the range up so that it starts at eight degrees gives about 51% so it's still of a similar magnitude.

e wrote:

but for the reasons posted above, I find definitive statements about real world performance very hard to accept.

Unfortunately the reasons which you gave were mostly based on the erroneous assumption that we were discussing density, you kept the objections while changing the question, which doesn't work.

It's clear that the changes in Re are highly significant, this can't be refuted by making general complaints about the accuracy of any particular model. That's an error of logic known as the continuum fallacy.

Just to put the magnitude of the effect of water viscosity changes into perspective: We are looking at a doubling of the Reynolds number as the water temperature increases over the range given in the example.... that's the same (in terms of Re) as doubling the velocity of the foil. The effect on fin flow of doubling velocity would hardly be disputed, and it seems to me that the scepticism about water viscosity effects are therefore based less upon facts and more upon the idea seeming to be bizarre and new.

.

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surfingbymyself commented Tuesday, 3 Nov 2015 at 9:51am

Roy Stuart wrote:
surfingbymyself wrote:

- the models are not reliable for low Reynolds number flows

Do your homework please, the Reynolds numbers for typical fins at surfboard speeds are not low.

e wrote:
The reynolds numbers used on your website are 4.5x10^5 and 7.5x10^5. These are in the transitional region and certainly at the low end of the scale

- the models are based on non span wise flow which doesn't relate to a fin moving through a turn on a wave face at all.

yes it does.

e wrote: I disagree, unless the flow is perfectly perpendicular to the the LE at all times, you have spanwise flow. So even if the fin remains motionless with respect to the freestream, you will have increasing elements of spanwise flow as the fin curves back to the tip.

There will be large components of span wise flow at times

Not necessarily.

e wrote: see above. if you disagree, I'm keen to understand why, not just have the idea dismissed without being engaged with

- the models are based on a continuous chord foil whereas fins are variable foil throughout

Polars can be generated for any chord length variation.

e wrote: So did you vary chord and foil for your analysis?

That makes me question any statement which confidently relates model results to real world performance.

There are many studies available which compare Xfoil predictions with tank and wind tunnel tests, and the results are good.

e wrote:
Which studies? How relevant are they? The results might be good for wind tunnel and tank tests, but lets understand how the wind tunnel and tank tests relate to fins on a wave, which is what I mean by real world performance. Did the the wind tunnel and tank tests have relevant reynolds number flows? Were they done with infinite span foils ( for the wind tunnel)? Did they have chord and foil variation representative of fin shape?

Btw, the relationship between temperature a lift is obvious from the lift equation (L= 1/2 roe V^2 S Cl) if you consider that temp is a determinant of density (roe) then I would argue that the other terms, obviously V and also Cl will be much more significant contributors to final lift value, possibly even orders of magnitude, but I hesitate to use that phrase again, I was always taught that it's wrong to curry favour with the troops...

as already mentioned you've completely missed what I've been saying.

I am not suggesting that lift changes due to density variations are significant, in fact I've stated several times that they are insignificant.

It is viscosity which I'm looking at, and changes due to viscosity are certainly significant.

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Roy Stuart commented Tuesday, 3 Nov 2015 at 10:36am

surfingbymyself wrote:

The reynolds numbers used on your website are 4.5x10^5 and 7.5x10^5. These are in the transitional region and certainly at the low end of the scale

No they are not.

Low Reynolds numbers are less than 200,000, and it's not an arbitrary disntinction as it relates to the transition zone from laminar to turbulent flow.

e wrote:

- the models are based on non span wise flow which doesn't relate to a fin moving through a turn on a wave face at all.

yes it does.

e wrote: I disagree, unless the flow is perfectly perpendicular to the the LE at all times, you have spanwise flow. So even if the fin remains motionless with respect to the freestream, you will have increasing elements of spanwise flow as the fin curves back to the tip.

You have argued against here a proposition which i didn't make.

I didn't say that spanwise flow doesn't occur, I refuted your statement that the Xfoil predictions do not relate to surfboard fins since surfboard fins experience spanwise flow. All wings and foils which have tips experience spanwise flow, and yet the Xfoil predictions are widely used as a useful tool for predicting foil performance. you understand my point?

e wrote:

There will be large components of span wise flow at times

There will always be some, spanwise flow is dramatically reduced by using an upright fin with leading edge bumps.

Spanwise flow is a red herring in terms of viscosity effects.

e wrote:

see above. if you disagree, I'm keen to understand why

Answered above, the degree of spanwise flow will depend upon the planshape of the fin.

e wrote:

- the models are based on a continuous chord foil whereas fins are variable foil throughout

Polars can be generated for any chord length variation.

e wrote: So did you vary chord and foil for your analysis?

I offered one typical example. Changing the chord length is obviously one of the main ways in which the effects of water visosity changes can be dealt with.

e wrote:

That makes me question any statement which confidently relates model results to real world performance.

Have I made a prediction based on water temperature regarding the performance of any particular fin?

No I have not.

e wrote:

Did the the wind tunnel and tank tests have relevant reynolds number flows? Were they done with infinite span foils ( for the wind tunnel)? Did they have chord and foil variation representative of fin shape?

The bottom line is that computer modelling and tank/wind tunnel tests are sufficiently useful for design purposes that they are used all the time for air and water craft.

If surfboard fins are an exception then you'll need to say why.

You keep plugging this line that models have to have some pre determined but not specified level of accuracy or the effects of viscosity are insignificant. This is a highly illogical argument ( the continuum fallacy again).

Viscosity differences and their effects on flow are significant, happen whether we analyse them or not, and are not dependent on modelling. They are real world effects.

Likewise the effect of velocity on fin and wing performance is real regardless of whether or not it is modelled.

Furthermore general statements about the kind of effect which can occur based on reynolds number changes are legit, they are an 'abstract' which can lead to more modelling and testing.

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