ME 341 Definitions

Fluid

A substance that deforms continuously when acted upon by a shearing force of any magnitude

B.G.

British Gravitational - force (lbf), mass (slugs), length (ft), Temp (ºR)

System

Fixed quantity of mass

System Boundaries

Separates system from surroundings; can be fixed or moveable; no mass can cross boundary

Control Volume

A volume in space through which a fluid flowsC

Control Surface (CS)

Geometric boundary of CV; may be real/imaginary

Infinitesimal Systems

For detailed behavior of fluid flow (e.g. pressure distribution on a wing surfa); use differential equations

Control Volumes

For bulk behavior of a flow device (e.g. overall lift a wing produces); use integral equations

Continuum Approximation

Fluids are continuous; properties are well-defined at very small volumes and they vary smoothly from point to point

Density (ρ)

mass/Volume [kg/m³]

Specific Weight (γ)

weight/Volume [N/m³] or [lbf/ft³]

Specific Gravity (SG)

γ/(γ_{H2O, 4ºC=39ºF}) or ρ/(ρ_{H2O, 4ºC=39ºF})

Ideal Gas Law

P = ρRT

Lagrangian Method

Analyze fluid flow by assuming fluid to be composed of a very large number of particles, tracking particle motion

Field Representation

Density (ρ = ρ(x, y, z, t)) and velocity vector (v = v(x, y, z, t) = uî + vjˆ + wkˆ) fields are functions given by spatial coordinates and time

Steady Flow

When properties at any point in the slope field do not change in time; dρ/dt = 0, dv/dt = 0

Unsteady Flow

When properties do change with time

Uniform Flow

When velocity is independent of all x, y, and z coordinates throughout entire field (v does not change with position)

Uniform Flow @ Given Cross-Section

Velocity is constant across any cross-section normal to the flow

No-Slip Condition

Fluids have zero relative velocity at a solid surface

Pathlines

Path traced out by a moving fluid particle

Streaklines

Li

Streamlines

Lines drawn in flow field so that, at a given instant, they are tangent to v (or direction of flow) at every point in the flow field