# Wall shear stress question

fully developed flow liquid of dynamic viscosity 2.9x10^-3 Pa.s and relative density 0.9 in horizontal pipe of diameter 12mm, length of pipe=200mm and mean velocity is 0.5m/s

To find the wall shear stress, i used τ=μ.du/dy = (2.9x10^-3)(0.5/0.012) = 0.121Pa

The answer is 0.968Pa, where am i going wrong?

thanks

## Answers and Replies

What are the units of density?

Make sure you are dimensionaly consistent and use the kinematic viscosity which is absolute viscosity/density.

the density hasnt got any units, it just says the relative density. why should i be using the kinematic viscosity when the shear stress equation requires the dynamic viscosity, which is given