Another look at Bernoulli

General Discussion
1

There has already been some discussion on the application of Bernoulli in surfboard design - http://www2.swaylocks.com/forums/hydrofoils-and-lift. However my interpretation (I think) is a different way of looking at it - however I am not as academic as a lot of you. Notably I am not attempting to understand Daniel Tomo’s application of Bernoulli theory.

Its been said that Bernoulli theory only applies to closed systems (eg pipes) but the Wikipedia and other sources shows that it has been used to explain airfoil lift which is a non-closed system. Even used to explain “squirting” from the open neck of a water filled balloon.

Many of us believe that water flow from under a board is diagonal, but my limited drawing skills make it easier for me to look at side-slip and forward travel in isolation. Also I think the concept of “vectors” allows us to look at side-slip and forward travel in isolation even though in reality both are combined to give diagonal forces.

My diagrams below, what do you reckon?

[img_assist|nid=1051837|title=Bernoulli theory in the role of side-slip forces|desc=|link=none|align=left|width=640|height=480]

 

 

the yellow area in the pic below is the cross section of board where it meets the water line. Due to the way a board tends to be thicker up front it will be foil shaped.

[img_assist|nid=1051838|title=Bernoulli theory in the role of forward travel with buried rail|desc=|link=none|align=left|width=640|height=469]

2

So you’re assuming the water accelerates when it comes in contact with the bottom of the board, and further accelerates as it wraps around the rail. This acceleration lowers the pressure within the fluid, creating lift.

First, I would argue that water may not accelerate at all as it flows across the bottom of the board and around the rail. In fact, I would guess (and I’m only guessing) that the water would become somewhat pressurized under the board, lowering its velocity, and creating drag. The rocker of the board is what’s creating lift… enduced by drag.

3

In your bottom picture you have the flow perpendicular to what really happens, unless I am misinterpreting something.

 

Lift is created when an object changes the relative direction of flow of a fluid.

4

My problem with your second drawing (if I’ve interpreted it right) is that, if you ever watch the water flow across a board, it almost never flows across the deck, certainly not enough to create lift.

Also, in the first drawing, when you’ve factored the vectors, more of the water is moving towards the outside rail. See Crafty’s water flow test-  http://www2.swaylocks.com/node/1010045

 

On the other hand, I started a thread while ago about “diagonals.”   I wanted to know if there was a way to see diagonal slices of boards using one of the computer assisted design programs.  I think only the bottom curve is necessary (again, because water doesn’t really flow over the deck).  It would be informative to see how the diagonal curves change with the addition of concaves or other bottom contours.  

 

It’s good to see someone trying to visualize and explain the theories, unlike Tomo who has not explained his APPLICATION of Bernoulli’s.

 

As I’m writing this I’m thinking the best person to invite into these discussions would be the “Epoxy Board Lady”-  Eva Hollman, who is also a licensed naval architect.  It would be interesting to hear her thoughts.  Anybody know her?  I’ll try to contact her and see if she’ll join in.

5

As i am mecanical teacher, i learn to my student that hypothesis one for application of Bernouilli’s theorem is laminar flow. It’s easiest to have a laminar liquid flow in pipe but in fact it’s hard to have a really laminar liquid flow. Flow accross the bottom of a surfboard don’t respect hypothesis of bernouilli’s theorem

sorry for my frenglish

6

Laminar flow is not needed for bernoulli.

You need to neglect the viscosity effects to use bernoulli and this is only accurate outside the boundary layer, no matter if the BL is laminar or turbulent.

As long as there is no flow seperation, you can use bernoulli.

 

I don’t understand what you’re saying. Where is that drag vector coming from?? and when planing, is there water flowing over the board?? I’ve only seen that on noseriders. And what are you trying to proof with your pictures?

7

word...

typically bernoulli's theorem is taught in hydrodynamic oceanography classes to describe laminar flow in an enclosed (cylinder) environment.

since air/gas is involved along with fluids passing along the board/water interface there's allot more complex interactions going on at the surface to fluid boundry layer than a direct fluid to surface interface. In my mind its more like the fluid interaction of water moving across a rocky river bottom on a molecular level than the mathematics of what bernoulli describes. Chaos theory mathematics seems more applicable

when I think of bernoulli i think something more of the consistancy of oil passing through a pipe or cylindrical vessel where the surface area pressure is constant in all dimensions so that acceleration of the media is measurable in all three dimensions.

But the again that was what I'm trying to remember from my oceanography studies over 30+ years ago.

I should keep my mouth shut though until I can build my own Fat Penguin to test this summer.

[img_assist|nid=1051854|title=Fat penguin|desc=|link=none|align=left|width=0|height=0]

 

 

speaking of laminar flow..

why does Tommy Peterson's bottom work?

[img_assist|nid=1051855|title=Tommy Peterson's Bottom|desc=|link=none|align=left|width=0|height=0]

 

 

 

8

I think Onuela you are right, mathematics doesn’t like chaos. Fluid mechanical model need laminar flow to be near of reality. In turbulent flow, tests are the key.

Sorry for my frenglish

9

     Aloha Bernie, 1 thing that isn't addressed is they don't show where your feet are on the deck and that will interupt the flow of water and would defintely restrict some of the flow. Just my 2 cents, Hope you are doing well these days and I would like to thank you for being my friend and there  I was in need. Aloha,Kokua

10

 

Oneula I take your point that there are all sorts of things going on in surfboard travel. My proposal is only attempting to show the ones explainable by Bernoulli.

I think there are some chaotic forces involved eg maybe the turbulence. However I disagree that the dominant forces involved in surfboard travel are chaotic. If that were the case then our surfboards would bounce around all over the place, but they don’t do that - when in trim and turning they behave predictably to within the limits of our balance and wave judgement. If that were not the case then we wouldn’t have this design forum, instead we may as well be randomly sliding down waves on inflated truck inner tubes.

Therefore I think that there are significant forces explainable by mathematics involved

11

some conflict there. The wikipedia says:
Bernoulli’s Principle can be used to calculate the lift force on an airfoil if you know the behavior of the fluid flow in the vicinity of the foil.

I put the words Bernoulli and airfoil into google and heaps of information came back.

12

Hans, I can’t prove anything with my diagrams and suggestions - they are just ideas as to how forces explainable by Bernoulli influence surfboard travel - the first diagram is about “rail grip” the second about “drive”.

My encyclopedia Brittanica has a brief section on airfoils - it says that lift is the force perpendicular to direction of travel and that drag is the force in the opposite direction of travel.

So in the side slip situation drag is sideways - I’m suggesting it is due to a low pressure area from fast wrapping water around the rail

 

13

NJ, yes thats what I’m proposing.

I think the water does accelerate around the rail - in my streamer experiment where I had taped a curly ribbon on the rail tuck part of the hull (actually 2 ribbons of different lengths) I always observed the ribbons pulled straight/tight when in trim or when it was wrapping the rail in a deep bottom turn. This suggests to me that the water accelerates as it exits/wraps the rail otherwise the ribbon would have been wavy at times.

I’m definately not claiming ownership of the truth on this though! I realise the ribbon is just a hint as to whats happening and not a complete illustration of all the water molecules.

[img_assist|nid=1050929|title=curly ribbon taped at hull rail tuck|desc=|link=none|align=left|width=640|height=480]

14

yes I credit Crafty’s experiment for the inspiration behind the ribbon experiment I did at the end of this thread: http://www2.swaylocks.com/forums/water-movement-behind-pic

most of the time the ribbon was exiting the rail diagonally although it did wrap in a deep bottom turn.

However I have another Bernoulli related theory on the situation where the deck has an air cavity when turning. Bernoulli’s equations incorporate density of the fluid and speed of the fluid - the main point is that faster speed is lower pressure - however rearranging some of the equations it appears to me that a higher density creates higher pressure. Air is lower density which means that the air travelling over the deck will create even more lift than a buried deck with water flowing over it?

However I’m not great at physics and hoping for some input on this suggestion

Below is some equations from the wikipedia  - I’ve probably picked the wrong ones - there are many - but I think that the idea of a higher density liquid at speed exerting more pressure is true?

[img_assist|nid=1051866|title=some Bernoulli equations|desc=|link=none|align=left|width=640|height=361]

15

MrJ, from your post:

“streamline” = laminar flow.  It’s the main hypothesis of validity of this bernouilli equation.

“incompressible fluid with constant densitie” the second hypothesis. Fluid around the board is sometimes, water, sometimes air and sometimes a mix of air+water emulsion.

Surfboards are to far of bernouilli model. To much unknown variable.

Sorry for my frenglish.

16

Hey MrJ.

You know the feeling...when paddling out, sometimes a big thick lip comes down right on top of you and pounds you really good right? Very powerfull isnt it? 

That pounding is a force known as pressure. Pressure is an effect which occurs when a force is applied on a surface (the surface of your body in the previous example). That powerfull pounding you feel has nothing to do Bernoulli.

Sorry, surfboard lift is caused by pressure. And physical pressure on a surfboard is explained by Newtonian mechanics.

Any object, traveling thru a fluid medium, with a large enough surface (enough to counter the effects of gravity) and a minimum angle of attack will experience lift.    

People who use Bernoulli to try to explain it are grasping at straws.

Issac Newton. The greatest scientific thinker man has ever known....by far.

17

Crafty and Lemat, I think I’ve taken my questioning as far as I can - I don’t have enough knowledge to either agree or disagree on subjects such as whether there is laminar flow or not or whether airfoils can have a Bernoulli effect but not hydrofoils. I get the impression that it is debated as to whether airfoils have lift/some lift explainable by Bernoulli or not?

18

pff removed it, this makes no sence.

19

fear not Mr J .

you are much closer than others would have you believe.

bernoulli and newton have no issues with each other.

a lot of the basics for this topic were established in the first thread. I'm not sure why crafty has chosen to ignore them this time.  I would think that MTB's position on the matter is quite in line with accepted/established theory and he cover most of the basics.

crafty i understand your reluctance to blindly accept what you do not understand.

however that does not change 200 years of practical observation.

mr bernoulli never said anything bad about you :)

rereading the first thread will still address all the points being addressed.

pitot tubes found on every airplane in the sky use bernoulli's principles quite effectively

its not pipe flow and its not laminar.

re: Mr J, the only issue i have with your approach is you seem to be trying to use bernoulli to predict the flow field around  your board. 

the truth is, he can't really do that. you'll need to apply the continuity equation or the navier-stokes equation to predict the flow.

the Savitsky diagram (first thread) is close to what you are looking for.

once you acknowledge the existance of a stagnation line you must also accept  the fact that the flow is innitially decelerated to zero ( with corresponding increase in static pressure) from there any acceleration is mostly to get back up to freestream conditions

however, once you have measured or calculated (or guessed) the flow, bernoulli is quite adept at  converting velocity to pressure to force  ( lift and drag) and back because it all relates to "conservation of energy"

the pressure field does describe the velocity field which does determine the net lift and drag forces.  and in the end, its the same as the force determined by calculating/measuring the change in momentum of the flow  ( forces required to redirect the flow)

it all works out to be the same

regards,

-bill

 

 

 

20

I think about this one all the time, and the more I think about it the more I come to similar conclusions:  Fins work like "wings" - to the extent they have low and high pressure sides, and those forces are part of the fins functioning...

When it comes to the bottom of the board, and what I take to be peoples' assertion that having the water flow faster over the bottom will create less pressure, thus "lift," thus more speed, or... 

But if "lift" is created on the bottom of the board, the part in the water, and the part that most looks like the top of "wing," it stands to reason the "lift" is actually lifitng the bottom of the board down into the water... the way the top of a wing helps lift it up into the air...