Dynamic viscosity

Definition

Dynamic viscosity is a measure of a fluid's resistance to flow and deformation, defined as the ratio of shear stress to shear rate. This property is crucial in understanding how fluids behave under various conditions and is especially significant in the study of Newtonian fluids, which exhibit a constant viscosity regardless of the shear rate applied. It provides insights into the internal friction of fluids, impacting how they move and interact with surfaces and other fluids.

5 Must Know Facts For Your Next Test

1. Dynamic viscosity is typically represented by the Greek letter 'mu' (µ) and is measured in units of Pascal-seconds (Pa·s) or poise (P).
2. In Newtonian fluids, dynamic viscosity does not change with varying shear rates, making it easier to predict fluid behavior under different flow conditions.
3. Temperature significantly influences dynamic viscosity; as temperature increases, the viscosity of most liquids decreases while that of gases generally increases.
4. Dynamic viscosity plays a key role in various engineering applications, including lubrication, hydraulics, and the design of piping systems.
5. Different fluids can exhibit vastly different dynamic viscosities, influencing processes such as mixing, pumping, and flow through porous media.

Review Questions

• How does dynamic viscosity affect the behavior of Newtonian fluids in practical applications?
  • Dynamic viscosity directly influences how Newtonian fluids flow and interact with their surroundings. In practical applications such as lubrication systems or hydraulic machinery, understanding the constant nature of a Newtonian fluid's viscosity allows engineers to predict flow behavior under various conditions. For example, knowing the dynamic viscosity helps in selecting appropriate lubricants that minimize friction and wear in mechanical components.
• Discuss the relationship between dynamic viscosity, shear stress, and shear rate for Newtonian fluids and how this relationship impacts fluid flow.
  • For Newtonian fluids, dynamic viscosity is defined by the equation $$ au = u rac{du}{dy}$$ where $$ au$$ is the shear stress, $$ u$$ is the dynamic viscosity, and $$rac{du}{dy}$$ is the shear rate. This linear relationship means that if you increase the shear stress applied to the fluid, you will see a proportional increase in the shear rate. Understanding this relationship allows for better control over fluid behavior in processes like pumping and mixing.
• Evaluate how changes in temperature can affect the dynamic viscosity of different types of fluids and explain why this is significant for engineering applications.
  • Changes in temperature have a substantial impact on the dynamic viscosity of both liquids and gases. For most liquids, an increase in temperature leads to a decrease in viscosity due to reduced intermolecular forces, allowing molecules to move more freely. Conversely, for gases, increasing temperature typically raises viscosity because molecular collisions become more frequent. This understanding is critical in engineering applications since temperature fluctuations can alter flow rates and efficiency in systems like heat exchangers or chemical reactors, necessitating adjustments in design or operation to maintain optimal performance.

Related terms

shear stress: The force per unit area applied parallel to the surface of a material, which causes deformation.

shear rate: The rate at which adjacent layers of fluid move with respect to each other, typically expressed in reciprocal seconds.

Newtonian fluid: A type of fluid whose viscosity remains constant regardless of the shear rate applied, following Newton's law of viscosity.