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What is the normal viscous force(stress)?

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Old   March 10, 2012, 02:34
Default What is the normal viscous force(stress)?
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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), Doesn't it always have a tangential effect? .... explain please!
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Old   March 11, 2012, 15:20
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Quote:
Originally Posted by Emara View Post
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), Doesn't it always have a tangential effect? .... explain please!
are you talking about the relevance of the bulk viscosity? In general, a second viscosity coefficent increments the normal stress due to pressure, but often this contribution is disregarded within the Stokes hypothesis
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Old   March 12, 2012, 22:07
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I think gradients are harder to picture in the normal direction because we can really only experience a sliding type of friction when we a moving tangential to a surface, we can't really have a normal gradient in velocity because we are solids. It might help to imagine the friction in a tangential shear stress and rotate it into a normal stress. You still have particles sliding past each other just in a different and harder to understand way. You can also relate it to the diffusion of momentum in analogy with heat and mass transfer starting from ideal gas theory but that's a bit complicated to.
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