Apr 26, 2009
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Poiseuille’s Principle says that the flow rate is proportional to (radius of the pipe)^4, while the continuity equation says that the flow rate is proportional to (radius of the pipe)^2. They seem to give two different relationships on flow rate and radius of the pipe.

Which leads me to my question…when do you use Poiseuille’s principle and when do you use flow rate? Do you use Poiseuille’s Principle when there is viscosity/friction within a fluid, while you use the continuity equation for ideal flow?
 

PhilIvey

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Poiseuille’s Principle says that the flow rate is proportional to (radius of the pipe)^4, while the continuity equation says that the flow rate is proportional to (radius of the pipe)^2. They seem to give two different relationships on flow rate and radius of the pipe.

Which leads me to my question…when do you use Poiseuille’s principle and when do you use flow rate? Do you use Poiseuille’s Principle when there is viscosity/friction within a fluid, while you use the continuity equation for ideal flow?
If they want poiseulle it will be hinted at. Poiseuille is for real fluids. If you see the words viscosity as you stated or real fluids then it's most likely poiseuille

Also, the continuity utilizes simple math. In order to get volume/s you take area m^2*m/s= m^3/s. So, yes, it's radius^2.

What Poiseulles law shows is that in reality, Radius has a much bigger effect on flow rate than by the simple manipulation shown above. The difference is that pressure is directly involved. It's a different manner of representing the same idea.

P(R^4)/(Lv) Since in reality, the length and viscosity must be taken into account. Whereas the continuity doesn't account for these. They'll will be clues to hint at which they want.