Question 1

What is viscosity and why does it matter in fluid mechanics?

Accepted Answer

Viscosity measures how a fluid resists flowing due to internal friction between layers moving past each other. High-viscosity fluids like honey flow slowly, while low-viscosity fluids like water flow easily. Understanding viscosity is crucial for engineering applications such as pipeline design and engine lubrication, where proper fluid behavior under varying conditions ensures system efficiency and performance.

Question 2

What is the difference between dynamic viscosity and kinematic viscosity?

Accepted Answer

Dynamic viscosity, denoted by the Greek letter mu (μ), measures the force needed to move one fluid layer over another. Kinematic viscosity is the ratio of dynamic viscosity to fluid density, describing how the fluid spreads under gravity. Both are essential for characterizing fluid behavior, but kinematic viscosity accounts for the fluid's mass, making it useful for comparing flow rates across different fluids.

Question 3

How do Newtonian and non-Newtonian fluids differ in their viscosity behavior?

Accepted Answer

Newtonian fluids like water and air maintain constant viscosity regardless of applied force, exhibiting a linear relationship between shearing stress and shearing strain. Non-Newtonian fluids such as ketchup, toothpaste, and quicksand change their viscosity depending on the force applied. For example, ketchup flows more easily when the bottle is shaken, demonstrating how non-Newtonian fluids respond dynamically to applied stress.

Question 4

How does temperature affect the viscosity of liquids and gases differently?

Accepted Answer

Higher temperatures decrease viscosity in liquids because intermolecular forces weaken, allowing the fluid to flow more easily. Heated syrup pours more readily than cold syrup. Conversely, higher temperatures increase viscosity in gases as molecular activity and momentum exchange between layers rise. Warm air in a sauna feels thicker, illustrating this opposite temperature effect in gases compared to liquids.

Question 5

Why is understanding viscosity important for designing pipelines and engines?

Accepted Answer

In pipelines, oil must flow smoothly at varying temperatures, requiring careful consideration of how viscosity changes with temperature to prevent flow restrictions. In car engines, correct oil viscosity ensures proper lubrication and efficiency. Accurately predicting fluid behavior under temperature variations is essential for engineers to design and operate systems effectively, maintaining performance across diverse operating conditions.

Question 6

What causes viscosity in fluids at the molecular level?

Accepted Answer

Viscosity results from internal friction between layers of fluid moving relative to one another. When fluid layers slide past each other, molecular interactions create resistance to flow. This internal friction is why stirring honey requires more force than stirring water. The strength of intermolecular forces and molecular motion directly determines a fluid's resistance to deformation and flow.

Question 7

How does shear stress relate to viscosity in fluid flow?

Accepted Answer

Shear stress is the force applied to move one fluid layer over another, and viscosity quantifies the fluid's resistance to this stress. For Newtonian fluids, the relationship between shearing stress and the rate of shearing strain is linear and constant. This predictable relationship allows engineers to calculate the force needed to move fluid layers and design systems where controlled fluid motion is essential.

流体の粘度