# I [Fluid mechanics] Navier-Stokes and Hagen-Poiseuille

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1. Mar 2, 2016

### ChaoY

Can the Hagen-Poiseuille equation be used for a vertical flow in the water tab or any flow that has circular cross-sections with varying diameters? If not, how can the Navier-Stokes equations or any other equations be used to describe a viscous incompressible free-falling vertical jet?

2. Mar 2, 2016

### Staff: Mentor

What are your thoughts on how to approach this problem? What is the order of magnitude of the viscosity you are considering?

3. Mar 2, 2016

### ChaoY

I honestly don't know what viscosity we should use yet. We're preparing a little experiment to show that Bernoulli equation can be used to calculate velocity for liquid with low viscosity. We also want to calculate the flow rate of the jet this time with viscous liquid. The idea is to show that the Bernoulli equation becomes less effective. Now we're wondering if we could use the Hagen-Poiseuille equation to do the job instead this time considering the viscosity. Do you know if this equation still works if the circular cross section varies from two points?

4. Mar 3, 2016

### Staff: Mentor

If the fluid is in free fall, the hagen poiseulle equation doesn't apply because there is no stationary solid pipe wall.