Question about centrifugal force and Bernoulli's law.

What I want to do is maintain a gas at very high pressure and
temperature for propulsion purposes. The problem is containing a gas at
high pressure requires a thick walled tank, and the resulting mass
would obviate the advantage of having the gas at high pressure.
So what I'm considering is moving the gas at high speed within the
tank. By the Bernoulli principle the tank should see the pressure on
its walls as much less.
However, if the gas is moving around a cylindrical tank or a toroidal
tank, then by centrifugal force that would seem to indicate it should
increase the pressure(!)
Which one wins out?
Here's one possible way to address this problem. You could have the
tank consist of long straight portions on either side but be curved at
the top and bottom. Then you would only have to keep the portions at
the top and bottom to be thick walled to maintain the pressure increase
due to centrifugal force. If the straight portions are much longer than
the curved parts at top and bottom, you could get a large volume
without having to add too much to the tank mass.


Bob Clark

"Robert Clark" wrote in message
ps.com...
What I want to do is maintain a gas at very high pressure and
temperature for propulsion purposes. The problem is containing a gas at
high pressure requires a thick walled tank, and the resulting mass
would obviate the advantage of having the gas at high pressure.
So what I'm considering is moving the gas at high speed within the
tank. By the Bernoulli principle the tank should see the pressure on
its walls as much less.
However, if the gas is moving around a cylindrical tank or a toroidal
tank, then by centrifugal force that would seem to indicate it should
increase the pressure(!)
Which one wins out?
Here's one possible way to address this problem. You could have the
tank consist of long straight portions on either side but be curved at
the top and bottom. Then you would only have to keep the portions at
the top and bottom to be thick walled to maintain the pressure increase
due to centrifugal force. If the straight portions are much longer than
the curved parts at top and bottom, you could get a large volume
without having to add too much to the tank mass.


Bob Clark


why not store the gas in a much smaller solid form, then ignite it so it
changes state from a solid to a gas ?

Robert Clark wrote:
What I want to do is maintain a gas at very high pressure and
temperature for propulsion purposes. The problem is containing a gas at
high pressure requires a thick walled tank, and the resulting mass
would obviate the advantage of having the gas at high pressure.
So what I'm considering is moving the gas at high speed within the
tank. By the Bernoulli principle the tank should see the pressure on
its walls as much less.
However, if the gas is moving around a cylindrical tank or a toroidal
tank, then by centrifugal force that would seem to indicate it should
increase the pressure(!)
Which one wins out?
Here's one possible way to address this problem. You could have the
tank consist of long straight portions on either side but be curved at
the top and bottom. Then you would only have to keep the portions at
the top and bottom to be thick walled to maintain the pressure increase
due to centrifugal force. If the straight portions are much longer than
the curved parts at top and bottom, you could get a large volume
without having to add too much to the tank mass.


Bob Clark

This won't work. The way Bernoulli's law works is that it has to be
applied along a streamline. Then an increase in speed along that
streamline results in a reduction in the pressure.
What I was envisioning is that if you have a flow at steady speed and
it flows horizontally over a surface, while maintaining the same
constant speed, then the pressure applied to the surface will be less
than the pressure far from that surface. This is not how Bernoulli
works. It only applies when the velocity is different between two
points in the flow.


Bob Clark

wrote in message
oups.com...
| Robert Clark wrote:
| What I want to do is maintain a gas at very high pressure and
| temperature for propulsion purposes. The problem is containing a gas at
| high pressure requires a thick walled tank, and the resulting mass
| would obviate the advantage of having the gas at high pressure.
| So what I'm considering is moving the gas at high speed within the
| tank. By the Bernoulli principle the tank should see the pressure on
| its walls as much less.
| However, if the gas is moving around a cylindrical tank or a toroidal
| tank, then by centrifugal force that would seem to indicate it should
| increase the pressure(!)
| Which one wins out?
| Here's one possible way to address this problem. You could have the
| tank consist of long straight portions on either side but be curved at
| the top and bottom. Then you would only have to keep the portions at
| the top and bottom to be thick walled to maintain the pressure increase
| due to centrifugal force. If the straight portions are much longer than
| the curved parts at top and bottom, you could get a large volume
| without having to add too much to the tank mass.
|
|
| Bob Clark
|
| This won't work. The way Bernoulli's law works is that it has to be
| applied along a streamline. Then an increase in speed along that
| streamline results in a reduction in the pressure.
| What I was envisioning is that if you have a flow at steady speed and
| it flows horizontally over a surface, while maintaining the same
| constant speed, then the pressure applied to the surface will be less
| than the pressure far from that surface. This is not how Bernoulli
| works. It only applies when the velocity is different between two
| points in the flow.
|
|
| Bob Clark


Sounds like a jet engine to me.
http://wings.avkids.com/Book/Propuls...ponents_01.jpg

Robert Clark wrote:
What I want to do is maintain a gas at very high pressure and
temperature for propulsion purposes. The problem is containing a gas at
high pressure requires a thick walled tank, and the resulting mass
would obviate the advantage of having the gas at high pressure.
So what I'm considering is moving the gas at high speed within the
tank. By the Bernoulli principle the tank should see the pressure on
its walls as much less.
However, if the gas is moving around a cylindrical tank or a toroidal
tank, then by centrifugal force that would seem to indicate it should
increase the pressure(!)
Which one wins out?
Here's one possible way to address this problem. You could have the
tank consist of long straight portions on either side but be curved at
the top and bottom. Then you would only have to keep the portions at
the top and bottom to be thick walled to maintain the pressure increase
due to centrifugal force. If the straight portions are much longer than
the curved parts at top and bottom, you could get a large volume
without having to add too much to the tank mass.

Offhand this doesn't sound promising, but let's be analytical.

gears grind

One problem with your scheme, ahem, is that the pressure term in
Bernoulli's equation is the real internal pressure of the gas. No
matter what your set up, low gas pressure is low gas pressure, not high
gas pressure -- you are not getting compression associated with a high
internal pressure while only seeing the effects of a low pressure on
the vessel.

Use a bolloon inside a ballon inside the tank? Inside balloon holds
moving gas while outside balloon regulates pressuue along length of the
tank?

Edward Green wrote:
Robert Clark wrote:
What I want to do is maintain a gas at very high pressure and
temperature for propulsion purposes. The problem is containing a gas at
high pressure requires a thick walled tank, and the resulting mass
would obviate the advantage of having the gas at high pressure.
So what I'm considering is moving the gas at high speed within the
tank. By the Bernoulli principle the tank should see the pressure on
its walls as much less.
However, if the gas is moving around a cylindrical tank or a toroidal
tank, then by centrifugal force that would seem to indicate it should
increase the pressure(!)
Which one wins out?
Here's one possible way to address this problem. You could have the
tank consist of long straight portions on either side but be curved at
the top and bottom. Then you would only have to keep the portions at
the top and bottom to be thick walled to maintain the pressure increase
due to centrifugal force. If the straight portions are much longer than
the curved parts at top and bottom, you could get a large volume
without having to add too much to the tank mass.

Offhand this doesn't sound promising, but let's be analytical.

gears grind

One problem with your scheme, ahem, is that the pressure term in
Bernoulli's equation is the real internal pressure of the gas. No
matter what your set up, low gas pressure is low gas pressure, not high
gas pressure -- you are not getting compression associated with a high
internal pressure while only seeing the effects of a low pressure on
the vessel.

What I really want is the high speed. So perhaps I can just inject the
fuel into a tank at the speed I want. The problem is you want it to
have the exhaust speed of a rocket, thousands of meters per second. So
again you have to be concerned with the high pressure that would be
created by the centrifugal force.
How do you calculate this for a *compressible* gas?


Bob Clark

Robert Clark wrote:

What I really want is the high speed. So perhaps I can just inject the
fuel into a tank at the speed I want. The problem is you want it to
have the exhaust speed of a rocket, thousands of meters per second. So
again you have to be concerned with the high pressure that would be
created by the centrifugal force.
How do you calculate this for a *compressible* gas?

I thought you wanted to store gas at high pressure and cheat on wall
thickness by applying Bernoulli's principle? Which won't work, for
reason I indicated.

So you just want to know the contribution to pressure as a high speed
gas rounds a corner? Roughly, Newton's second law should work: dP/dt =
net force around the elbow or U; average the force over the projected
wall area; pressure. Compressibility doesn't enter on this level.