Mekanika Fluida FM01 (Lecture1: 2/4). Newtonian/Non-Newtonian Fluid

Summary

This lecture discusses fluid viscosity, explaining the differences between Newtonian and non-Newtonian fluids. It introduces concepts like shear stress and shear rate, using examples such as water, oil, honey, and mayonnaise. Newtonian fluids, like water, have a linear relationship between shear stress and shear rate, while non-Newtonian fluids, such as honey and mayonnaise, show more complex behavior. The lecture emphasizes the importance of understanding viscosity in fluid dynamics, with a focus on real-world applications like pump efficiency and fluid flow behavior. Students are also introduced to more advanced concepts that will be explored in higher studies.

Takeaways

• 😀 Viscosity is a property of fluids that indicates their resistance to flow and shear stress.
• 😀 Fluids can be categorized based on viscosity, with water, air, and oil having unique viscosity values.
• 😀 Fluids with higher viscosity, such as oil, are harder to move and require more force to flow through pipes.
• 😀 Fluids with lower viscosity, like air, are easier to move and require less force to flow.
• 😀 The concept of viscosity is illustrated through experiments where water’s speed of movement changes under shear stress.
• 😀 Toothpaste exhibits a paradoxical behavior, acting like a solid when low shear stress is applied, but flowing like a fluid under higher pressure.
• 😀 Non-Newtonian fluids, such as honey and mayonnaise, behave differently from Newtonian fluids, showing resistance to flow that changes with pressure.
• 😀 Newtonian fluids display a linear relationship between shear stress and velocity, with examples like water and air.
• 😀 Non-Newtonian fluids do not follow this linear behavior, and examples include honey, mayonnaise, and toothpaste.
• 😀 Viscosity is distinct from other concepts like density and should not be confused with them.
• 😀 Understanding viscosity is essential in fields like engineering, as it affects fluid dynamics in various applications, from simple liquids to complex non-Newtonian substances.

Q & A

What is viscosity, and why is it important in fluid dynamics?

Viscosity is a property of fluids that measures their resistance to flow. It plays a crucial role in fluid dynamics because it determines how easily a fluid flows under applied forces. Fluids with high viscosity, like oil, are harder to move, while fluids with low viscosity, like water or air, flow more easily.

How does viscosity affect fluid behavior?

Viscosity affects how a fluid behaves when subjected to shear stress. For instance, fluids with higher viscosity, such as oil, require greater force to move, while fluids like water require less force. Viscosity can vary depending on the fluid type and temperature, impacting the energy needed for fluid movement.

What is the difference between kinematic viscosity and absolute viscosity?

Absolute viscosity refers to the internal resistance of a fluid to flow when subjected to shear stress. Kinematic viscosity, on the other hand, takes into account the fluid's density and is defined as the ratio of absolute viscosity to density. Kinematic viscosity is typically used when the fluid's density is considered important.

What is the relationship between shear stress and viscosity in fluids?

Shear stress in a fluid causes deformation, and the viscosity of a fluid determines how much resistance it provides to this deformation. For fluids like water, which have low viscosity, shear stress leads to easy movement. In contrast, for fluids with high viscosity, like honey or oil, shear stress results in less movement.

How does viscosity influence fluid flow in pipes?

Fluids with higher viscosity, such as oil, require more force to flow through pipes, and higher pressure is needed to maintain the same flow rate as lower viscosity fluids. In contrast, less viscous fluids like water require less pressure to flow through pipes at the same rate.

Can you explain the behavior of fluids like toothpaste and honey with respect to viscosity?

Toothpaste and honey exhibit non-Newtonian behavior. Toothpaste behaves like a solid until a certain threshold of force is applied, at which point it flows like a fluid. Honey, on the other hand, requires a higher shear stress to flow but will flow more easily with increasing force.

What are non-Newtonian fluids, and how do they differ from Newtonian fluids?

Non-Newtonian fluids are those whose viscosity changes under different conditions of stress or strain, meaning their flow behavior is not linear. For example, honey and mayonnaise are non-Newtonian. In contrast, Newtonian fluids, like water and air, have a constant viscosity regardless of the applied stress.

Why is the concept of viscosity important in practical applications like pumping oil?

Viscosity is crucial when pumping fluids like oil because it determines the amount of force needed to move the fluid. High-viscosity fluids require more powerful pumps to achieve the same flow rate as lower-viscosity fluids. Understanding viscosity helps in designing systems that manage fluid flow efficiently.

What is the difference between shear-thickening and shear-thinning fluids?

Shear-thickening fluids become more resistant to flow as the applied shear stress increases, meaning they get thicker when agitated. Examples include substances like honey. Shear-thinning fluids, such as mayonnaise, become thinner and flow more easily with increased shear stress.

How does viscosity relate to the behavior of fluids in nature, like in the bloodstream or air?

In nature, viscosity plays a critical role in fluid behavior. For instance, the viscosity of blood affects circulation, with higher viscosity making blood flow more slowly through vessels. Similarly, the viscosity of air influences how easily it moves and spreads, affecting weather patterns and aerodynamics.