Drag

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Figment
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Drag

Post by Figment » Fri Nov 11, 2005 12:30 pm

I suppose I should be the one to get the Nerdy ball rolling.

This morning's discussion of propellor drag has blown some of the dust off of my old fluid dynamics studies, but I'm still pretty dusty. I'm trying to remember the principal components of the drag equation...

Frontal area
viscosity of fluid
velocity
surface area, or is that part of the drag/prismatic coefficient??????

I know I'm missing something. I also know that I could just look it up already, but what fun is that?

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Post by Tim » Fri Nov 11, 2005 1:23 pm

Figment wrote:I know I'm missing something.
I'm missing something too, but I think it's a link.
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Re: Drag

Post by Matt B. » Fri Nov 11, 2005 2:36 pm

Figment wrote:I suppose I should be the one to get the Nerdy ball rolling.

This morning's discussion of propellor drag has blown some of the dust off of my old fluid dynamics studies, but I'm still pretty dusty. I'm trying to remember the principal components of the drag equation...

Frontal area
viscosity of fluid
velocity
surface area, or is that part of the drag/prismatic coefficient??????

I know I'm missing something. I also know that I could just look it up already, but what fun is that?
I can't believe I remember this...

D (force of Drag) is determined by the

"reference area" (frontal area, basically)
density, not viscosity, of fluid
velocity
drag coefficient

So if we replace viscosity with density, you had everything.

I can't quite remember the equation, although it would take me less than a minute to find it. It's something like

D = .5pvACd

where

p = density of fluid
v = velocity
A = reference area
Cd = drag coefficient

And of course there's all the fun of the drag coefficient - no way to calculate it, you have to determine it by experiment.

(OK, I just cheated and looked it up. I was almost right - velocity (v) should be squared.)
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Post by heartofgold » Fri Nov 11, 2005 9:19 pm

Matt wrote:D = .5pvACd
You know, somehow I missed that when I was reading my Advanced Naval Architecture book last night...
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Post by Matt B. » Sun Nov 13, 2005 12:33 am

heartofgold wrote:
Matt wrote:D = .5pvACd
You know, somehow I missed that when I was reading my Advanced Naval Architecture book last night...
The advantages of an aerospace degree. <grin>

The equation's the same, the density of the fluid's just higher.
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Post by bcooke » Sun Nov 13, 2005 10:41 am

Two questions.

How is density measured? By that I mean what is a 'unit' of density?

By frontal area, do you mean the total area of the propeller 'disk' or is it some fraction thereof? I wonder since the propeller is actually moving if the frontal area is something less than the total area the propeller swings through?

Do propellers create any 'induced' drag as well when they are being dragged through the water?

Just curious...

-Britton

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Post by bcooke » Sun Nov 13, 2005 10:42 am

Oh, you said reference area. I suppose that answers my second question. So I wonder how the reference area is determined?...

-Britton

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Post by bcooke » Sun Nov 13, 2005 10:43 am

You know, somehow I missed that when I was reading my Advanced Naval Architecture book last night...
That is why you should go back and read those books over and over and over and...

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Post by Matt B. » Sun Nov 13, 2005 3:13 pm

bcooke wrote: How is density measured? By that I mean what is a 'unit' of density?
Density is mass/volume. Across the pond (and, therefore, in most textbooks) density is referred to as "specific gravity". Standard units are g/cm^3.

Distilled water, by definition, has a density of 1. Salt water is denser than fresh, which is roughly the same as distilled. Sea water is usually 1.03.
bcooke wrote: By frontal area, do you mean the total area of the propeller 'disk' or is it some fraction thereof? I wonder since the propeller is actually moving if the frontal area is something less than the total area the propeller swings through?
Some fraction thereof. The drag of a spinning prop will be less than the drag of a flat plate of the same diameter.

This is one of the "sticky bits" about determining drag, because not only does the motion of the prop matter, but the thickness of the prop, the curve of the blade, the velocity of the prop's rotation, the amount of air in the water, the turbulence of the water's flow - all of those make a difference in the final answer.

I've never calculated the drag of a boat prop before, and even aircraft props are something I did only as an exercise. (I didn't do planes, my aerospace degree was emphasis on the space.) But in real life, engineers use rounded figures and determine a drag coefficient by experiment - in short, build one, put it in the water, and determine what it is. Then use that number to determine the calculated drag for varying conditions on the assumption the coefficient won't change. (Which is actually not the case, hence one of the reasons why engineers drink.)

If it's critical to know the correct answer at all operating regimes - such as for a spacecraft, or a fighter jet - you spend millions on years of simulation and calculation, and when you're done you still do a thousand real-world tests and change the answers to match.
bcooke wrote: Do propellers create any 'induced' drag as well when they are being dragged through the water?
Yes.

Overall, the drag of a prop is not something I would really spend much time worrying about. Even for a racer, I'm willing to bet that the difference in drag between a locked prop inline with the keel and a locked prop perpendicular to it is less than the difference in performance of an extra inch in the foot or leech of a sail. To me it's like the idea of keeping the bottom of the hull glass-smooth; it's less about the performance of the boat and more about the discipline and dedication of the crew.

Hence the reason why I don't sail competitively...
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Post by Matt B. » Sun Nov 13, 2005 3:24 pm

bcooke wrote:Oh, you said reference area. I suppose that answers my second question. So I wonder how the reference area is determined?...
That's the other reason engineers drink...

First, frontal area where the angle of attack is 90 degrees. Think of a flat plate laid across the front of the prop and cut to match wherever the prop touches the plate. That's the frontal area.

Now, you have to consider the rest of the prop. The blades go back at some angle, so it's not correct to just add their area to the frontal area; you need some fraction of it, and that's a complicated equation I couldn't begin to remember, I'd have to look it up. And it's an approximation anyway. <grin> But for something like a prop I'd imagine it would be something like .75 of the area.

Then the prop is spinning or not... if spinning, you increase the reference area, again by some calculated amount. Less than the area of a flat plate of the same size, how much depends on the speed of rotation, density of the water, etc.

And again, you determine the answer as best you can by measurement and calculation - then you put the prop in the water (would we call it a current tunnel instead of a wind tunnel?) and measure it, put it on a scale and see how much force it exerts as a result of drag. Divide the measured answer by the calculated answer, and presto - you have the drag coefficient.

In other words, the definition of a drag coefficient, if we're going to be completely honest, is "the correction factor applied to make our numbers match reality". Which doesn't mean it's not useful; for something like a boat prop, the drag coefficient is a constant across the range of possible operating conditions. (Sea water versus fresh, cold versus warm, dead slow versus top speed). So by measuring it once, you can then use that number confidently to calculate a correct answer for any conditions. As long as you don't go for too many decimal places, that is.
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Post by catamount » Mon Nov 14, 2005 6:33 pm

Matt B. wrote:
bcooke wrote: How is density measured? By that I mean what is a 'unit' of density?
Density is mass/volume. Across the pond (and, therefore, in most textbooks) density is referred to as "specific gravity". Standard units are g/cm^3.

Distilled water, by definition, has a density of 1. Salt water is denser than fresh, which is roughly the same as distilled. Sea water is usually 1.03.
Specific Gravity is actually a non-dimensional quantity, as it is the ratio of two densities -- the density of the stuff of interest relative to the density of "pure" water (or other reference material).

Also, note that the density of water depends not only on its composition (i.e. salinity), but also on its temperature. The density of pure water is about 1 g/cm^3 only at about 4?C -- at temperatures higher (or lower) than 4?C, the density of pure water is somewhat less than 1 g/cm^3...

:-)
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Post by Matt B. » Mon Nov 14, 2005 8:32 pm

catamount wrote: Specific Gravity is actually a non-dimensional quantity, as it is the ratio of two densities -- the density of the stuff of interest relative to the density of "pure" water (or other reference material).
<head smack> I knew that. At some point, anyway...
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Post by Figment » Mon Nov 14, 2005 8:45 pm

The heart swells.

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Re: Drag

Post by Figment » Fri Dec 08, 2017 10:48 am

I was reminded of this thread this morning.

LONG LIVE THE NERDERY!!!

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