ME5002 Thermofluids II — Chapter 1: Fluid Mechanics Foundations

A set of vocabulary-style flashcards covering the fundamental concepts of fluid mechanics, including fluid properties, flow types, and dimensionless numbers as presented in ME5002 Chapter 1.

Fluid

A substance that continuously deforms when a shear stress is applied; it cannot sustain shear stress without flowing.

System

A fixed quantity of mass.

Control volume

A chosen region in space through which mass may flow, often used for pipes, pumps, and ducts.

Continuum assumption

The assumption that fluid properties (density, pressure, temperature, velocity) are smoothly defined at every point despite the fluid's molecular nature.

Velocity field

A description of flow at fixed locations giving the velocity vector at every position and time: $\vec{V}(x, y, z, t) = u\hat{i} + v\hat{j} + w\hat{k}$.

Streamline

A line tangent to the velocity field at an instant.

Pathline

The actual trajectory of a single fluid particle.

Streakline

A line that joins particles that have all passed through a specific fixed point.

Timeline

A marked line of neighboring particles observed as it deforms over time.

Steady flow

A flow where properties at a fixed location do not change with time, and streamlines, pathlines, and streaklines coincide.

Unsteady flow

A flow where at least one property at a fixed location varies with time.

Stress

Force per unit area, categorized as either normal stress (acting perpendicular) or shear stress (acting tangentially).

Viscosity

A fluid's resistance to shear deformation or internal friction.

Dynamic viscosity ($\mu$)

The property relating shear stress to velocity gradient in Newtonian fluids via $\tau = \mu \frac{du}{dy}$.

Kinematic viscosity ($\nu$)

The ratio of dynamic viscosity to density, defined as $\nu = \frac{\mu}{\rho}$.

Newtonian fluid

A fluid that has a linear relation between shear stress and shear rate ($\tau = \mu \frac{du}{dy}$).

Non-Newtonian fluid

A fluid whose apparent viscosity changes with shear rate; examples include shear-thinning, shear-thickening, and Bingham-plastic behavior.

Specific gravity ($SG$)

The dimensionless ratio of the density of a substance to the density of water at $4^{\circ}C$: $SG = \frac{\rho}{\rho_{H_2O(4^{\circ}C)}}$.

Surface tension ($\sigma_s$)

A force per unit length at a liquid interface caused by molecular attraction, making interfaces behave like stretched membranes.

Contact angle ($\theta$)

The angle between a liquid interface and a solid surface; wetting occurs if $\theta < 90^{\circ}$ and non-wetting if $\theta > 90^{\circ}$.

Capillary effect

The rise or fall of a liquid in a narrow tube due to surface tension and wetting, calculated as $h = \frac{4\sigma \cos(\theta)}{\rho g D}$.

Reynolds number ($Re$)

The ratio of inertial forces to viscous forces; it determines if flow is dominated by inertia (high $Re$) or viscosity (low $Re$).

Incompressible flow

Flow in which density changes are negligible; most liquids are treated this way.

Compressible flow

Flow in which density changes are significant; gases are usually considered compressible when the Mach number exceeds $0.3$.

Internal flow

Flow that is bounded by walls, such as flow in a pipe or duct.

External flow

Flow occurring over or around a body, such as air over a wing or cylinder.

Laminar flow

Smooth and orderly flow; in pipes, this usually occurs when $Re < 2300$.

Turbulent flow

Flow containing chaotic eddies and strong mixing; in pipes, this usually occurs when $Re > 4000$.

No-slip condition

The requirement that fluid velocity at a solid surface must equal the surface velocity (zero velocity at a stationary wall), which leads to the formation of boundary layers.