i know that fluid resistance varies with the square of speed. (to go twice as fast requires 4 times the power, 3 times as fast requires 9 times the power)

but i'm looking for a corolation between air and water resistance. i know that besides the fact that water fills a vessel to a level and air fills a vessel to an even pressure, there's virtually no differene in their behaviours. water is esentially VERY thick air.

can anyone find a relationship between the power required to travel a certain speed through air vs the power required to travel the same speed through water?

thanx,
speedfreak227

It's fluid dynamics in either case. So the only difference is the value used for the density of the material, the equation remains unchanged.

Air is typically taken as 1.225kg/m^3 (at 15 degrees centigrade at sea level)
Pure water is 1000kg/m^3 (sea water is nearer 1030kg/m^3)

Quote from Bob Smith :

It's fluid dynamics in either case. So the only difference is the value used for the density of the material, the equation remains unchanged.

doesnt the choice of equations depend on whether the fluid is compressible or incompressible ?
to be onest i dont know much of anything about fluid dynamics though ... wanted to attend a lectiure on it this semester for funsies but that didnt work out too well ...

At anything below Mach 0.7 you can treat air as incompressible, so therefore you can use the same equations. Above Mach 0.7 things get complicated.

Just how complicated?
No really, I've never found anything to read other than "it gets complicated".

LFS sensibly limits itself to Mach 0.6 to avoid such headaches. [/trivia]

Quote from speedfreak227 :

but i'm looking for a corolation between air and water resistance. i know that besides the fact that water fills a vessel to a level and air fills a vessel to an even pressure, there's virtually no differene in their behaviours. water is esentially VERY thick air.

Well, in both cases the liquid or the gas goes from higher pressure to smaller pressure. Any submarine engineers here?

ok this information is usefull but what i need is some science related website that i can use as a reference to introduce this material into my paper.

i'd prefer to not assume the relationship and just use one formula for both. the ideal situation would be a site relating the two directly.

my teacher probably wouldn't be too impressed if i cited, "smith, bob, live for speed forum:off topic, nov 23, 2005"

speedfreak227

Too bad you don't understand finnish I might have some material in pdf, but it goes deeper than you may want...

Quote from Bob Smith :

Just how complicated?

Very!

I'll dig up some of my notes on compressible flow tomorrow, and see what I can post here to give you a very brief insight...

Quote from speedfreak227 :

my teacher probably wouldn't be too impressed if i cited, "smith, bob, live for speed forum: off topic, nov 23, 2005"

Hehe, but it would make me feel good.

Quote from tristancliffe :

At anything below Mach 0.7 you can treat air as incompressible, so therefore you can use the same equations. Above Mach 0.7 things get complicated.

You can treat air as incompressible up till mach 0.3. Actually Air is never incompressible, but until mach 0.3 the error is smaller then 5%.

Mach 0.7 is a different boundary. If the free stream speed is at mach 0.7, the local speed somewhere on the body (mostly planes at that speed) can reach mach 1.0 or above. At that speed you get all kinds of additional effects, like shockwave. Hence the name 'transsonic' for free stream speeds between 0.7 and 1.0.


Also I think in most aerodynamic problems the buoyancy effect is neglected, while in water I dont think it is realistic to neglect that.

in case anyone cares, water has 3 times its regular density around 3000 psi. not exactly incompressible. check some basic steam tables to see.

speedfreak227

I've searched for some figures on the compressibility of water, and I found 0.46 GPa-1.

So if you apply a pressure of one gigapascal, the volume would decrease to 0.46 of its original value?

If so, applying a pressure of 3000psi (equals 20,684,271 Pascal), the volume would decrease (20,684,271 / 1,000,000,000) * 0.46 = 0.0095. So it shrinks by 0.95% if you apply a pressure of roughly 207 bar?

Quote from Frankmd :

I've searched for some figures on the compressibility of water, and I found 0.46 GPa-1.

So if you apply a pressure of one gigapascal, the volume would decrease to 0.46 of its original value?

If so, applying a pressure of 3000psi (equals 20,684,271 Pascal), the volume would decrease (20,684,271 / 1,000,000,000) * 0.46 = 0.0095. So it shrinks by 0.95% if you apply a pressure of roughly 207 bar?

i've only had a few hours of sleep and my eyes aren't clear enough to read your post yet, but the compresability of water isn't linear. in the last few hundred psi it compresses a LOT (near 3000psi) you'd have to look at steam tables to see what i mean. and saturated water isn't really water, i've heard scientists call it "stuff" in reference to supercritical boilers. there's no phase change anymore, there's only the "stuff" phase

speedfreak227

But we are talking about water as a liquid, not as a gas. I am sure pure steam would behave like a gas, but according to a lot of sources, liquid water is nearly incompressible.

Edit: here's a steam table.
http://kahuna.sdsu.edu/testcen ... les/tablesPC/TSatH2O.html

The properties of water change with temperature. The table doesnt show how it changes with pressure, but indeed water (or steam) changes a lot in density at high temperatures.

Also I think that the biggest difference between air and water resistance is the Reynolds number. The Reynolds number is defined as

( free streem speed * density * caracteristic length ) / viscosity

The viscosity of air is 55 times smaller then the viscosity of water, but the density of water is 816 times larger. So generally the Reynolds number is much higher for a body trough water (at the same speed and size) then trough air. You might find more info if you search for some info on Reynolds number.

Hmm, does this help:
http://en.wikipedia.org/wiki/Fluid_dynamics

Quote from Frankmd :

But we are talking about water as a liquid, not as a gas. I am sure pure steam would behave like a gas, but according to a lot of sources, liquid water is nearly incompressible.

Edit: here's a steam table.
http://kahuna.sdsu.edu/testcen ... les/tablesPC/TSatH2O.html

The properties of water change with temperature. The table doesnt show how it changes with pressure, but indeed water (or steam) changes a lot in density at high temperatures.

i was talking about the liquid in the boiler only though. steam tables give the liquid and vapour densities at temperatures and pressures (saturated of course)

speedfreak227

One thing you might want to be careful of is the change of Reynolds Number caused by the change of fluid.
The equation for aerodynamic (or hydrodynamic) drag is
F = 1/2 * rho * Cd * A * V^2
Moving from air to water changes the value of 'rho' from 1.225 kg/m3 to about 1000 kg/m3 but there might also be a change in the value of Cd.

Reynolds Number is defined as
Re = (rho * v * length) / mu
where 'mu' is the dynamic viscosity of the fluid. If you look at a graph of Cd vs Re for a sphere there is quite a variation due to the difference between laminar and turbulent flow and the resulting boundary layer behaviour. If the Reynolds Number is high enough then Cd is pretty much constant but at low Re it might vary by a large amount.
Oh...I see FrankMD has already mentioned this...ah well, since I've already written it I might as well post it
Incidentally, Frank is also correct about air being incompressible below Mach 0.3. By the time you get to Mach 0.7 air is very definately compressible.