What is the normal viscous force(stress)?

[Makveger]

Hello,

I understand the tangential shear stresses (or forces) acting on a fluid element due to friction between moving fluid layers, but I can't imagine how viscosity might cause a normal force (on a fluid element)...explain please!

 

[boneh3ad]

Who ever said that it did? Viscosity can only cause normal stresses in a compressible flow where the divergence of the velocity field is nonzero. This is, however, bulk viscosity rather than the typical dynamic viscosity.