Fluids & Newtonian Fluids

Flashcards on fluids and Newtonian fluids, covering definitions, differences between fluids, and viscosity calculations.

What is a fluid?

A substance that deforms continuously when subjected to a shear stress. If shear stress is applied, it flows.

What is the key difference between fluid at rest and in motion?

At rest: no shear stress. In motion: shear stress develops due to velocity differences between fluid particles.

What causes shear stress in a moving fluid?

When adjacent fluid particles move at different velocities, it distorts the fluid and creates shear stress.

How do liquids and gases differ in compressibility and volume?

Liquids: Incompressible, fixed volume, may form free surfaces. Gases: Compressible, no fixed volume, fill containers completely.

What defines a Newtonian fluid?

A fluid with a linear relationship between shear stress (tau) and velocity gradient (du/dy), and a constant viscosity (mu).

Give examples of Newtonian fluids.

Water, air, alcohol, gasoline, oil.

What defines a non-Newtonian fluid?

A fluid where viscosity depends on the velocity gradient; the relationship between shear stress and velocity gradient is non-linear.

What are the types of non-Newtonian fluids?

• Bingham plastics: flow after exceeding a yield stress (e.g. toothpaste).

• Pseudoplastics: viscosity decreases with shear rate (e.g. milk, clay).

• Dilatants: viscosity increases with shear rate (e.g. quicksand).

What is Newton's law of viscosity?

tau = mu . (du/dy), where tau is shear stress, mu is viscosity, and du/dy is the velocity gradient.

How does temperature affect viscosity in Newtonian fluids?

Viscosity changes with temperature, but not with velocity gradient.

A fluid has a viscosity mu = 0.2 Pa.s and velocity gradient du/dy = 250 s-. What is the shear stress?

tau = mu . (du/dy) = 0.2 . 250 = 50 Pa

What's the shear stress if water (mu = 1.0x10-3 Pa.s) flows with a velocity gradient of 1500 s-?

tau = mu . (du/dy) = 1.0x10-3 . 1500 = 1.5 Pa