Re: [PSUBS-MAILIST] motor thought

["Sean T. Stevenson" <cast55@telus.net>]

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.

-Sean


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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