# Thread: MMD, I need to borrow your brain for a second

1. Join Date
May 2008
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Swiftly tilting planet
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## MMD, I need to borrow your brain for a second

And hopefully it won't take too much of your day.

Right to the point: I have no idea what Froude number and Reynolds number define and describe. Could you give me the Reader's Digest version of what does what and how?

Blight

2. ## Re: MMD, I need to borrow your brain for a second

http://www.diracdelta.co.uk/science/...er/source.html

You're going to have a hard time adjusting that one in any real life situation.

3. Join Date
May 2008
Location
Swiftly tilting planet
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## Re: MMD, I need to borrow your brain for a second

No, in this case, Google is my enemy. Well, not enemy--more like the clerk at the DMV who tells you you need a Form 1179 Stroke Zed when you've already waited in another line to get a Form 1179. Lots of information I can't quite make use of yet.

4. ## Re: MMD, I need to borrow your brain for a second

Fear is the enemy.
http://www.diracdelta.co.uk/science/...er/source.html

Reynolds Number:

This is a non-dimensional number (consistent units) which describes whether fluid flow is laminar, turbulent, or transitional between the two. It is significant in the design of a model of any system in which the effect of viscosity is important in controlling the velocities or the flow pattern of a fluid.
Now, what was the question?

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Nov 1999
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Broken Arrow, OK US
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## Re: MMD, I need to borrow your brain for a second

They are ratios of forces that tell you which terms are important in solving fluid flow problems.

Reynolds considered just the viscous force leaving us with a theory for high speed boats.

Froude considered just the gravity force leaving us with a theory for low speed boats.

6. Senior Member
Join Date
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## Re: MMD, I need to borrow your brain for a second

Froude number (Fn) is a technically correct (ie non-dimensional) speed length ratio. It tells you how fast a boat is going relative to her length, which is important when considering what the various components of resistance will be. Planing boats operate at high Fn, displacement boats low.

Reynolds number is again a non-dimensional thing. It is an indicator of the nature of a fluid flow (ie. laminar or turbulent) based on how fast it is going and its density and viscocity.

We use Froude number in determining and comparing resistance due to wave making. For example, when tank testing a model it is run at the same Froude number as the full size vessel - in effect it is a means of calculating a "scale speed".

Reynolds number is used in calculating skin friction resistance.

7. ## Re: MMD, I need to borrow your brain for a second

Well, it seems that I am late to my own party.

Captain, there are several good links and explanations above, and Ed Burnett's is particularly concise. Does this explain it well enough for you?

I'll see if I can find any other short, clear explanations to add to what is already here.

8. ## Re: MMD, I need to borrow your brain for a second

Keep in mind that the Reynold's number varies by fluid density and the velocity of the fluid.

The short story is that you're trying to avoid turbulent flow and flow separation as they area a huge consumer of energy. Turbulent flow is generally seen as downstream wake eddies that you're 'towing' along behind you and the flow separation is usually associated with stalled flow behind a wing due to large angle of attack. (loss of lift force...)

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