Viscous liquid between two circular discs.

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Discussion Overview

The discussion revolves around a homework problem involving the behavior of a viscous liquid between two circular discs. Participants explore the equations of motion related to velocity and pressure, as well as methods for calculating the supported weight and force exerted by the fluid on the discs.

Discussion Character

  • Homework-related
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • One participant expresses difficulty in starting part a of the problem but indicates they can find the velocity profile and average velocity for part b.
  • Another participant describes the problem as related to creeping motion in fluids, suggesting that the pressure gradient relates to the Laplacian of the radial velocity and viscosity.
  • A participant questions whether it is possible to find the supported weight without first determining the pressure drop and completing part b.
  • There is a suggestion to integrate the shear stress Tau(rz) over the area of the top plate to find the force exerted by the fluid.
  • A participant seeks clarification on the definition of Tau(rz) and confirms it refers to shear stress.
  • One participant acknowledges a misunderstanding regarding the integration of stresses and recognizes the need to consider both the change in pressure and normal stress over the disc area.

Areas of Agreement / Disagreement

Participants express varying levels of understanding regarding the integration of stresses and the relationship between pressure and supported weight. There is no consensus on the best approach to find the force acting on the disc.

Contextual Notes

Participants mention the need for integration of different stress components and pressure changes, indicating potential limitations in their current understanding of the problem's requirements.

guitar24
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Homework Statement



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That is the problem statement. I can't seem to get started on part a.

For part b I can find the velocity profile of the viscous liquid form the equations of motion relating velocity and change in pressure, find the avg velocity, and find the change in pressure from that.

Any hints ??

Thank you!
 
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This is a creeping motion fluids problem like something you experience in a bearing. In this case the pressure gradient in the radial direction equals the Laplacian of the radial velocity times the viscosity. You indicate you can solve for the pressure and velocity profiles so you already are aware of the equation involved. If you integrate the pressure over the area of the disk, that should give you the weight that can be supported.
 
Thank you for your reply.

So there is no way I can find the weight that can be supported without finding the pressure drop and doing part b first?

I was thinking about integrating the stress Tau(rz) over the area of the top plate to find the force of the fluid on the solid. Would this be correct?
 
Thank you, I understand it a little better now and the velocity profile I got is the same. But that reference still doesn't mention anything about a force acting on the disc. Regarding to what I said earlier, can I integrate Tau(rz) over the area of the disc to find the force on the plate by the liquid?
 
How do you define Tau(rz). Is it shear stress?
 
Yes but nvm that isn't correct. I am sure I have to integrate the change in pressure + Tau (zz) (normal stress, but this is 0 for any fluid solid interface) over the disc area. I see what you were saying in your initial comment and you were right. Thank you!
 

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