Chapter 1–5 Fluid Mechanics Vocabulary

A concise set of vocabulary flashcards covering core concepts from the notes on fluids, viscosity, pressure measurement, buoyancy, and stability.

Fluid mechanics

The study of fluids (liquids and gases) and their behavior, either at rest (statics) or in motion (fluid dynamics).

Fluid

A substance that can flow; includes liquids and gases.

Density

Mass per unit volume, ρ = m/V.

Specific weight

Weight per unit volume, γ = w/V = ρg.

Specific gravity (sg)

Ratio of a fluid’s density (or specific weight) to that of water at a reference temperature. sg = ρ/ρwater or γ/γwater.

Surface tension

Work per unit area required to move molecules to the liquid surface; governs capillary effects.

Viscosity

A fluid’s internal resistance to flow; shear stress is proportional to velocity gradient.

Dynamic viscosity (μ)

Proportionality constant in τ = μ(dv/dy); units Pa·s (N·s/m²).

Kinematic viscosity (ν)

Dynamic viscosity divided by density, ν = μ/ρ; units m²/s.

Newtonian fluid

A fluid whose shear stress is proportional to shear rate with a constant viscosity.

Non-Newtonian fluid

A fluid whose apparent viscosity depends on shear rate and/or time.

Mass

Quantity of matter in a body, denoted m.

Weight

Force due to gravity on a mass, w = mg.

F = ma

Newton’s second law: force equals mass times acceleration.

Pressure

Force per unit area, p = F/A.

Absolute pressure

Pressure measured relative to a perfect vacuum; units Pa(abs) or psia.

Gage pressure

Pressure measured relative to atmospheric pressure; units Pa(gage) or psig.

Atmospheric pressure

Pressure exerted by the Earth's atmosphere at a location; ~101 kPa at sea level.

Barometer

Instrument that measures atmospheric pressure via a liquid column (often mercury).

Manometer

Device to measure pressure differences using a liquid column.

Pascal’s law

Pressure acts equally in all directions at a point in a fluid.

Buoyancy

Upward force on a submerged object equal to weight of displaced fluid; Archimedes’ principle.

Buoyant force

Fb = ρfluid g Vdisplaced.

Displaced volume

Volume of fluid displaced by a submerged body.

Center of buoyancy (cb)

Centroid of the displaced fluid; line of action of Fb passes through cb.

Center of gravity (cg)

Point where the weight of a body acts; location of gravitational force.

Metacenter (mc)

Intersection of the vertical line through cg with the vertical line through cb when the body is tilted.

Metacentric height (MG)

Distance between metacenter and center of gravity, MG = ymc − ycg.

Center of pressure

Point where the resultant hydrostatic force acts on a submerged plane area.

Piezometric head

Equivalent depth ha = pa/g representing pressure pa as a depth in the fluid.

Pressure–Elevation relationship

For liquids, Δp = γΔh or Δp = gh depending on notation; pressure changes with elevation.

Bulk modulus (E)

Measure of a fluid’s compressibility; Δp = −E(ΔV/V).

Absolute temperature

Temperature on an absolute scale (Kelvin); zero is absolute zero.

Kelvin (K)

SI unit of temperature; 0 K is absolute zero.

Rankine (°R)

Absolute temperature scale used with Fahrenheit-based measurements.

SI prefixes

Metric prefixes (T, G, M, k, m, μ, n, p) used with units.

Unit cancellation procedure

Six-step method to ensure correct dimensional consistency in equations.

Slug

Mass unit in the U.S. Customary System; quasi-equivalent to 32.174 lbm (with appropriate g_c).

gc (conversion constant)

gc = 32.2 (lbm·lbf)/(ft·s²); used to relate pounds-mass and pounds-force.

Mass vs. weight units (lbm vs lbf)

lbm is mass; lbf is force; conversions require g or gc depending on system.

Gage pressure (psig)

Pressure above atmospheric pressure.

Absolute pressure (psia)

Pressure above perfect vacuum.

Gage fluid

Fluid used in a manometer to measure pressure; typically water, mercury, etc.

Hydrostatic pressure

Pressure in a fluid at a point due to the weight of the fluid above.

Piezometric head (ha)

Equivalent depth corresponding to a given pressure above the free surface.

Pressure elevation (z)

Vertical coordinate measuring height in a fluid.

Bulk modulus vs compressibility (liquids vs gases)

Liquids are nearly incompressible with very high bulk modulus; gases are highly compressible.

Absolute zero (temperature)

Lowest possible temperature where molecular motion stops; 0 K.

Specific weight of water at 4°C

gw@4°C = 9.81 kN/m³; rw@4°C = 1000 kg/m³.

Specific gravity vs density at a reference

sg = ρ/ρwater or γ/γwater at reference temperatures.

API gravity and Baumé scales

Empirical scales for heavy liquids; API gravity uses water at ~60°F as reference.

Viscosity indices (VI)

Measure of how much viscosity changes with temperature; higher VI means less change.

Dynamic vs kinematic viscosity units

Dynamic: Pa·s; Kinematic: m²/s or cSt (mm²/s).

ISO VG (viscosity grade)

Industrial lubricant viscosity grade system; nominal kinematic viscosity at 40°C.

SAE viscosity grades

Oil viscosity ratings for engine oils; includes 0W, 5W, 10W, 20, 30, 40, etc.

Hydraulic fluids

Fluids used in fluid power systems (oil, water-glycol, HWBF, silicone, etc.).

Center of pressure (rectangular wall)

Point where the total hydrostatic force on a submerged vertical wall acts.

Center of centroid for a rectangle

Geometric center of a rectangular area; used in pressure calculations.

Center of curvature (curved surface)

Location on a curved surface where resultant force line of action passes.

Kinematic vs dynamic viscosity in practice

ν vs μ; used in various formulas and in viscosity measurements.

Viscosity measurement methods

Rotating-drum viscometer, capillary viscometer, falling-ball viscometer, Saybolt viscometer.

Hyper-brief on Newtonian/non-Newtonian polymers

Polymers can show time-dependent, thixotropic or rheopectic behavior.