Friday 16 October 2020

What is the shear stress of a fluid?


One book defines the shear stress $\tau$ of a (Newtonian) fluid as



$$\tau = \eta \frac{\partial v}{\partial r} $$


where $\eta$ is the viscosity. There is not much context, so I've made some guesses. Are my following assumptions correct?



  • $v$ is the velocity of the flow line, parallel to the wall.

  • $r$ is the distance of the flow line from the wall.

  • the flow must be laminar for the above to hold. (Otherwise, what would $v$ mean?)

  • the "wall" must be a tube for the above to hold. (Otherwise, what would $r$ mean?)



Answer



Your assumptions are correct (but $r$ is often defined as the distance from the pipe centerline). However, this is a very specific case: laminar pipe flow.



In general, the stress will be a tensiorial quantity, defined as


$$ \tau_{ij}= \eta \frac{\partial u_i}{\partial x_j}$$


which is true for turbulent flow, in arbitrary geometries. Where $i,j$ are in the range ${1,2,3}$ for the $x,y,z$ components.


For your case, you only have velocities in the streamwise direction, and variations in the radial direction, which makes all other components zero.


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