Friction Coefficient

Discussion in 'Hydrodynamics and Aerodynamics' started by jesdreamer, Sep 30, 2015.

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PI DesignSenior Member

Well, I’ve not managed to track down a copy of the 1957 proceedings but there are plenty of mentions in the literature that it is “just” a correlation line rather than a skin friction line. My concern is that without the original paper, eventually the subtleties of its application get forgotten.

Interestingly, not one single paper I have read on the subject provides a reference for the equation, it just gets mentioned as if it’s a physical law like F = ma when in fact it is a semi-empirical formula with limitations of applicability (which remain unknown to me).

In as much as any form factor, k, should remain constant between model and ship scale it seems that the ITTC ’57 line is bettered by the Grigson line anyway, and tracing a reference for that ought to be easier.

One final thing that annoys me slightly. I can see that it is done for ease of use, but it seems ugly. It is often written that:

Rt = Rf + Rr and therefore

Ct = Cf + Cr

where t = total, f = skin friction and r = residual resistance.

However, that is only true if the same area is used to non-dimensionalise the friction and residual resistance components – and standard practise is to use the wetted surface area.

It makes total sense to use the wetted surface area for the skin friction, but it is misleading to apply this to the residual resistance too.

Keeping things simple by assuming a deeply submerged submarine, hence no waves, Rr is a function of shape (form), not wetted surface area, and so it would surely be more appropriate to turn Rr into Cr by using some area that describes form – such as displaced volume / length, or volume * length / wetted area, length * beam * block coefficient, etc.

Whilst this does not allow you to add Cf and Cr together, it is a better way of cataloguing Cr values. Using WSA as the area means that two boats with the same residual resistance and same WSA have the same Cr, even if they are different shapes and lengths.

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DCockeySenior Member

Go to the link, click on 8th Conference (Madrid 1957), then click on Subjects 2-4: Skin Friction and Turbulence Stimulation. That opens the proceedings and discussion for that subject. (I'm using Windows 8.1 and Internet Explorer 11.)

Also look at the Skin Friction and Turbulence Stimulation proceedings of the 9th Conference (Paris 1960). In Appendix I which states in regard to the "ITTC 1957 model-shop correlation line" that "The Conference requested that should be clearly understood that this line is to be regarded as an interim solution to this problem for practical engineering purposes." Other appendices, formal discussion and informal discussion are also included.

The "interim solution" for "practical engineering purposes" has endured for 59 years.

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DCockeySenior Member

My recollection is Leo has previously discussed in other threads the 1957 ITTC curve and other correlation curves. Anyone interested in the topic may want to search those threads.

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daiquiriEngineering and Design

Yes, and those have been some of the best and most informative threads I've seen on this forum so far.

Here:
http://www.boatdesign.net/forums/design-software/skin-friction-formulas-31280.html

and here:
http://www.boatdesign.net/forums/hy...in-friction-lines-some-comparisons-46272.html

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PI DesignSenior Member

Ah thanks. I think it is an iPad thing will try the pc tomorrow.

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PI DesignSenior Member

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jesdreamerJunior Member

Back to aft sinkage

Very enlightening posts as I read this thread again. But I am still perplexed by the aft sinkage so clearly shown in referenced you tube videos of double ended paddle racing kayaks. DCockey post #42 claims this is "simple mechanics from the paddle force at it's distance below hull pivoting at CG somewhere within paddler's body" -- and I even agreed in my post #43. I have been unable to sleep since I am convinced both posts are wrong --

Assuming a clean straight down digging paddle entry and feathered exit, I see no force pivoting hull backward about an overall or net CG (somewhere within paddler's body). I visualize submerged paddle force as tending to rotate hull about CG in opposite direction (as with water in a bucket on a water wheel) -- sinking bow and raising stern. But resistance under forward motion is tending to add support at bow -- could this added bow support actually be yielding a "new effective pivot point" at bow or at least far forward of paddler, instead of at CG and thus causing aft sinkage resulting from each stroke's paddle force??

Real question relates to the increased drag due to greater wetted area with aft sinkage resulting upon each forward surge at each stroke -- But if bow sees increased support as postulated above and hull sees aft sinkage, don't we now also have hull trying to climb this trim angle, perhaps of even greater resistance than that of the increased wetted area aft??

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jehardimanSenior Member

In a case like this, always draw a free body diagram.

The propulsive paddle force is resolved into the rigid structure (i.e. the paddlers body in the hull) at the shoulder well above the CG as an aft working force and moment. These two forces can only be resisted by the stern sinking to provide a canceling bow down moment.

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jesdreamerJunior Member

Torque and moment arm analysis

YouTube videos seem to clearly show aft sinkage during each paddle stroke -- original question was for some analysis of how this can take place since simple mechanics seems to suggest bow should sink instead --

I understand the proposal of a free body diagram but question just where reaction force gets applied or shows up if we really have a "rigid structure"

A free body diagram tells me that with the "rigid structure" as described, a force toward rear applied some distance below CG would cause "clockwise rotation" or torque trying to sink bow and raise stern. I propose that forward motion increases bow support resisting bow sinkage & preventing rotation about CG -- and that this leads to an "effective" pivot well forward of CG -- in turn yielding aft sinkage with little or no bow sinkage. Applied force some 2 ft below CG but reaction force distributed along hull with reaction pivot far ahead of CG. Does this make sense??

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daiquiriEngineering and Design

Looks like the axiom behind your reasoning is that the free-body diagram (FBD) you have drawn is correct.
However, the empirical observations are contradicting the conclusions which you have drawn from the FBD.
Hence, I would start the debugging by re-examining the axiom.
Could you post a sketch of your FBD?

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jesdreamerJunior Member

Axiom correct but empirical observations contradict??

We may have a language problem here. Daiquiri, I can read your post and understand the words -- but don't understand just what you are trying to say

Do you mean that my moments analysis of the hull-paddler (assumed "rigid structure") that paddle pressure toward rear will tend to push bow down, is correct??

And if so, do you then mean that the observation of aft sinkage during stroke contradicts this theory??

If so, It might appear that you are agreeing with first phase of my analysis wherein I pose that a moments analysis yields bow sinkage -- but that observations (aft sinkage) contradict -- which leads me to a 2nd phase of further analysis and postulation of an "effective" pivot up toward bow (which has added dynamic support via forward motion, tending to reduce bow sinkage)

And so it seems that you agree with (1) my original moments analysis and that its result (2) is in contradiction with observed results --

And that you conclude that further analysis (3) is in order

Might you find my further analysis (3) postulating an explanation of aft sinkage observations to be reasonable??

Or are you suggesting that we re-examine the original axiom itself -- perhaps the assumption of a paddler/boat "rigid body" is wrong, etc, etc

Now if you feel re-examination of this original assumption is wrong, could you pose an alternative way to look at this phenomenon of aft sinkage during paddle stroke??

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jesdreamerJunior Member

2nd thoughts on the moments involved

DCockey in post #40 stated that the aft sinkage was a simple case of torques or moments applied to the boat -- Jehardiman #83 made similar comments

Daquiri #85 suggested re-examining the original axiom (concept where paddle force tried to sink bow which pivoted due to increased support by motion forward, yielding aft sink)

In #86 I gave more detail on the idea of a moment trying to sink the bow but dynamic support at bow yielding an "effective" pivot far forward of CG resulting in aft sinkage. Please note I feel this concept could well be wrong --

I think Daquiri may be correct that my original concept might be wrong (I had posted earlier that this was keeping me awake at night). We are considering paddler and boat to be a single rigid "unit" and my analysis per #86 was based on force provided by the paddle -- However, perhaps the reaction force is the right one to use for trim angle analysis.

We could think of the "single rigid unit" not in terms of a force it might apply from muscular exertion somewhere within that unit but of an outside force which might be applied to this "single rigid unit" -- namely the reaction force balancing that force applied by the paddle. If so we have a simple case of moment arms and forces -- This reaction force against back or working side of paddle will clearly tend to rotate the "single rigid unit" yielding aft sinkage

It may be academic since both approaches explain the aft sinkage so visible in referenced YouTube videos -- but It is really important to me to know which analysis is the correct one -- Is it the latter??

I had originally felt the sinkage was due to reduced hydrostatic support (Bernouli) during the sudden surge of speed during each paddle stroke. Might we really have 2 factors causing the aft sinkage (Bernouli as well as paddle reaction)?? And if so, might anyone have an idea as to relative contribution??

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jehardimanSenior Member

The issue will be where you place the paddle force. I assume full submergence of the paddle blade and applying the hull drag at the CB, so the aft pitching moment at the shoulder to resist the paddle force is larger than the forward pitching couple between the driving force and the drag. If the driving force is above the center of drag, the boat pitches bow down (like a square rigger driving her head under), if it is below the center for drag the boat pitches bow up (which is why shaft alignment on submerged bodies is very important).

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jesdreamerJunior Member

Which way is it?? aft sinks or bow sinks with paddle stroke

This post by jehardiman with it's 2 alternate explainations illustrates just why the question has been keeping me awake at night. I think aft sinkage is the result but I can come up with an explaination for bow sinkage just about as easily --

I believe we can consider the boat and paddler as being a single "fixed" mass or structure, including paddle blade some 2ft below hull -- Now if in some internal way the muscles within this fixed structure manage to apply a force toward rear at paddle blade, and if we can consider a pivot point to exist somewhere around CB, we would have a rotational moment tending to sink aft portion of the boat -- But that analysis involves a lot of "Ifs" and I think jehardiman's discussion of pivot at the paddler's shoulder also makes sence, so I can visualize his explaination as to why it might be the bow which sinks -- What do the other experts here at the forum have to say??

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