# What is the normal viscous force(stress)?

• Makveger
In summary, normal viscous force is a type of force that occurs when two surfaces are in contact and moving relative to each other. It is caused by the viscosity of the fluid between the surfaces and is different from other types of forces as it is a shear force. The units of measurement for normal viscous force depend on the system of units being used and it can be calculated using the formula F = ηA(dv/dx). The magnitude of normal viscous force is affected by the fluid's viscosity, surface area of contact, velocity gradient, and temperature.
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!

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.

## 1. What is the definition of normal viscous force?

The normal viscous force, also known as normal stress, is a type of force that occurs when two surfaces are in contact and moving relative to each other. It is caused by the viscosity, or thickness, of the fluid between the surfaces.

## 2. How is normal viscous force different from other types of forces?

Normal viscous force is different from other types of forces, such as friction or gravity, because it is a result of the fluid's viscosity rather than the properties of the surfaces themselves. It is also a shear force, meaning it acts parallel to the surfaces in contact.

## 3. What are the units of measurement for normal viscous force?

The units of measurement for normal viscous force depend on the system of units being used. In the SI system, it is measured in newtons per square meter (N/m^2), while in the CGS system it is measured in dynes per square centimeter (dyn/cm^2). In both systems, it can also be expressed in terms of pressure (force per unit area).

## 4. How is normal viscous force calculated?

The formula for calculating normal viscous force is F = ηA(dv/dx), where F is the force, η (eta) is the dynamic viscosity of the fluid, A is the area of contact between the surfaces, and dv/dx is the velocity gradient (change in velocity over change in distance) between the surfaces.

## 5. What factors affect the magnitude of normal viscous force?

The magnitude of normal viscous force is affected by several factors, including the viscosity of the fluid, the surface area of contact between the surfaces, and the velocity gradient between the surfaces. It can also be affected by temperature, as higher temperatures can decrease the viscosity of some fluids.

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