What is the normal viscous force(stress)?

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Viscosity primarily causes tangential shear stresses in fluids due to friction between moving layers. Normal forces in a fluid element can occur in compressible flows where the velocity field diverges, leading to bulk viscosity effects. This bulk viscosity is distinct from the typical dynamic viscosity associated with incompressible flows. The discussion clarifies that normal stresses are not a characteristic of viscosity in all scenarios. Understanding these distinctions is crucial for analyzing fluid behavior in different flow conditions.
Makveger
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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!
 
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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.
 

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