Water temperature and fin performance

surfingbymyself wrote:

Sorry, the snip above was an attempt on my phone to reposition the quote markers. My response to reynolds numbers is that the numbers used on your website are 4.5x10^5 and 7.5x10^5. 5x10^5 is transitionary and testing below that number is difficult, so validating model performance in that range is difficult. While I concede that 5x10^5 is not extremely low, it is still at the low end of the spectrum, and in any case, the original point wass not that the numerical value is low but rather that the models aren't super reliable in this region due to the difficulties of validating

Neither of the numbers are in the transition zone actually, and if you do some research you'll find that even at lower Reynolds numberse Xfoil works reasonably well.

The range of reynolds numbers in the example is certainly significant, what you are attempting to do is say that it isn't significant because of inaccuracies in model predictions. That's not a logical argument.

Would you try to argue that doubling the velocity of a foil has insignificant effects on flow just because model predictions of the effects are never 100% accurate? That's the equivalent of what you are doing here.

How about saying that the effect of a few litres of extra volume has no effect because it can't be modelled accurately... or the difference between sets of fcs fins doesn't exist because there's no accurate scientific data on the ride?

The surf industry abhors physics, it relies on marketing and unmeasurable team rider feedback... yet few object to the conclusions which are reached that way... i find it hypocritical ( not yourself personally, but in general) that the demand for peer reviewed studies etc etc come out only when someone makes a reasonable observation based on sound physics, with no big claims as to the result, and without carefully groomed marketing spiels.

That's a predicted increase in fin drag at higher angles of attack for the foil given in the original example, calculated by Xfoils.

It's just one of an infinite number of possible examples, all of which give different results.

It's easy to find an example where there's no significant difference in fin behaviour, either by increasing the chord length of the fin or by choosing another foil. Generally though the overall efficiency is then reduced.

Craig wrote:

Yes, thanks but do you know what the fin drag numbers are?

There's some relevant info here:

http://www.thereefjournal.com/files/7._Lavery_Foster_and_Carswell.pdf

http://eprints.qut.edu.au/16824/1/c16824.pdf

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

http://people.eng.unimelb.edu.au/imarusic/proceedings/15/AFMC00105.pdf

caml wrote:

This threads not easy to follow . Beyond me . I have never heard of surfers in cold or warm water say that they need different fins . But I have heard of lots of other things that a surfer may feel in equipment thats subtle or specific to a particular wave .

Hi Caml,

The most noticeable effect would be the stall point in other words when the fin spins out.

So, I'd expect that surfers who deliberately blow their fins out on a regular basis would find it easier to do so in very cold water... the fin should stall earlier.

surfingbymyself wrote:

I've acknowldeged where I overstretched, I haven't seen similar from you.

How about the part where I thanked you for pointing out that viscosity changes with temperature are non linear, where I had assumed that they are linear?

e wrote:

Your position on Xfoil and tip effects has changed, from no tip effects, to it accounts for spanwise flow and tip effects.

No it hasn't changed and I certainly didn't say that.

e wrote:

don't see how it can be both ways, if the model is of an infinite foil, then there is no spanwise flow, if its not infinte, then there are tip effects. But that is a distraction.

I agree on both counts.

e wrote:

my fundamental argument is that Xfoil or any number of modelling platforms are ultimately only validated through testing and testing in these Reynolds number regions, and with the flow characteristics of a fin (tip effects, spanwise flow, rapid changes in alpha and incidence) is difficult and extremely limited. ( Xfoil has been validated lots in applications that are more straightforward and gliding is a good example of that.) Until the model is validated in a particular context, it's useful in an academic sense, but has limited application beyond that and the information it yields needs to be considered in light of the limitations.

I'm not quite as pessimistic but understand your point.

By the way Caml,

Surfers could be experiencing the fin blow out change but thinking that it is something else, basically none of them are likely to think of water temperature having an effect unless it's suggested to them.

Surfingbymyself: I suspect from your posts that neither of us are trying to push one particular viewpoint as much as it might appear, it's natural for lines to be drawn when a debate starts but we are probably both just trying to figure this stuff out.

That's interesting Caml. I notice also ( kind of off topic) that you recommend smaller fins in smaller surf. that's the opposite of what one would expect since at higher speeds fins produce a lot more lift, suggesting smaller fins in big surf.

All's not always what it seems though. Maybe the smaller fins allow the board to feel looser in small waves i.e. you are deliberately changing the handling instead of trying to keep it exactly the same over the wave spectrum?