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Re: [PSUBS-MAILIST] motor thought

The animation posted could be manipulated to illustrate either flow effect. Assuming subsonic fluid flow (as I think it is safe to assume for all watercraft), nozzles are flow transitions from greater to lesser diameter, while diffusers are flow transitions from lesser to greater diameter. Kort "nozzles" are perhaps misleadingly named, as they can actually be either type - accelerating (a nozzle), or decelerating (a diffuser). On tugs, they are typically decelerating "nozzles" (actually diffusers), intended to increase torque. Your example of putting your finger over the end of the hose, analogous to water jet style propulsion, creates a high velocity fluid stream at the expense of internal pressure within the fluid flow. This is the classical definition of a nozzle. In this case, you are converting that flow energy due to pressure to kinetic energy of the fluid stream, generating thrust.

Not really relevant to subs, but interesting to note, is that at supersonic velocities, the functions of diffuser and nozzle actually reverse, such that an increasing diameter becomes a nozzle and a decreasing diameter becomes a diffuser. This is why the nozzles on the space shuttle engines look the way they do, with diameter increasing. The exhaust gas flow is occuring at supersonic velocities, and the effect is the reverse of that of a low velocity flow.


riskeverything21@aol.com wrote:

I watched the animation, and I see where the misunderstanding is. My "nozzle" is SMALLER than the pipe leading to it. Think of it as putting your finger over the end of the hose.

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