The minimum cost of converting static pressure into dynamic pressure

kmarinas86

Can static pressure be induced to becoming dynamic pressure through the use of "right-angle forces" with respect to particle velocities? I imagine that one could accomplish this with an ionized gas and a magnetic field. This would reduce the amount of work that is need to accelerate the gas. Furthermore, what happens if one were to convert so much of the static pressure into dynamic pressure that the static pressure would fall below the ambient pressure of the atmosphere? Could the atmosphere itself do radial work on our system as soon as we relieve the dynamic pressure by allowing that dynamic pressure to do rotational work?

russ_watters

I'm not really sure what you mean, but all you need to convert static pressure to velocity pressure is a nozzle. And nozzles can be pretty efficient.

Also, you can use a high velocity airstream to induce additional flow. This is called induction.

kmarinas86

Are there any common mechanisms where one can convert static pressure into velocity pressure in a fixed volume (i.e. dV = 0)?

russ_watters

I can't make sense of that either. Fixed volume of what?

kmarinas86

= the volume occupied by the particles = no expanding or contracting walls, and all (would be) outlets are sealed

russ_watters

Velocity pressure is from air being in motion. How do you propose to have air in motion if there is nowhere for it to go? It is very unclear to me what you are asking. Could you please explain in more detail what you are hoping to accomplish?

kmarinas86

It could rotate.

I am trying to conceive of a way where one could take that static pressure inside a closed volume, and without any direct contact, through use of an external field, induce a net rotation leading to a conversion of the static pressure into velocity pressure corresponding to kinetic energy that can actually be tapped to do useful work in excess of the work incidental in converting the static pressure into velocity pressure.

russ_watters

How is that not an explicit violation of conservation of energy?

kmarinas86

The keyword "tapped" suggests something already exists which is "tapped" (suggesting that it was not created by the user doing the tapping) and then used for a purpose such as "to do useful work".

rcgldr

A magnetic field can accelerate ionized gas, but this would not directly result in a reduction of pressure. As mentioned above, once the ionized gas is accelerated, it could then be driven through a nozzle to increase it's speed and reduce it's pressure, but then that ionized gas has to end up somewhere. An ion rocket thruster uses this principle, but these are only practical in outer space. The wiki article explains the issues.

http://en.wikipedia.org/wiki/Ion_thruster

kmarinas86

Are you referring to static pressure or total pressure?

rcgldr

Any change in static or total pressure can not be determined with just the presence of a magnetic field and it's effect on an ionized gas. You need to know something about what the ionized gas is flowing through and the "reservoir" that the gas "exits" into, such as the case of a ion thruster in space where the gas exits into the vacuum of outer space, and any nozzle used for the ion thruster.