Fluid Mechanics I - Properties, Statics, and Pressure Measurement

Vocabulary practice flashcards covering fundamental definitions, fluid classifications, fluid properties, manometers, hydrostatic laws, and forces on submerged surfaces.

Fluid

A substance that has no fixed shape and yields easily to external pressure, specifically encompassing gases and liquids.

Ideal (Inviscid) Fluid

A hypothetical fluid possessing no physical property other than density, encountering zero flow resistance or friction, and maintaining equal pressure in all directions at every point.

Real (Viscous) Fluid

A fluid that exhibits viscosity, surface tension, and compressibility in addition to density, where contacting layers experience both normal and tangential (shear) stresses.

Newtonian Fluid

A fluid with a constant dynamic viscosity that obeys Newton's law of viscosity, where shear stress is directly proportional to the velocity gradient ($\tau = \mu \frac{du}{dy}$).

Non-Newtonian Fluid

A fluid whose viscosity changes with applied force or rate of shear strain, exhibiting a non-linear relationship between shear stress and strain rate.

Viscoelastic Fluid

A class of non-Newtonian fluids that combines the properties of both solids and liquids, demonstrating partial elastic recovery following deformation.

Fluid System

A predetermined, identifiable quantity of fluid matter that may alter its shape, position, and thermal state, but consistently retains the exact same particles.

Control Volume

A designated region of fixed shape in space bounded by a control surface, used to analyze fluid flow entering and exiting an enclosure.

Body Forces

Forces that act distributed across fluid matter without requiring direct physical contact, quantified per unit mass (e.g., gravity, magnetic, and inertia forces).

Surface Forces

Forces acting on a fluid body resulting from direct physical contact with surrounding media or adjacent fluid layers (e.g., pressure force, friction, and surface tension).

Mass Density ($\rho$)

The mass of a fluid contained within a unit volume, expressed mathematically as $\rho = \frac{\text{Mass}}{\text{Volume}}$ with the SI unit of $\text{kg/m}^3$.

Specific Weight ($\gamma$)

The ratio of the gravitational weight of a fluid to its volume ($\gamma = \rho g$), expressed in units of $\text{N/m}^3$.

Specific Volume ($v$)

The volume of a fluid occupied per unit mass ($v = \frac{1}{\rho}$), expressed in $\text{m}^3\text{/kg}$, commonly applied in gas analyses.

Specific Gravity ($S$)

A dimensionless ratio comparing the specific weight (or mass density) of a fluid to that of pure water at $4\,^\circ\text{C}$.

Dynamic Viscosity ($\mu$)

The internal fluid resistance to the relative motion of adjacent layers, defined as the shear stress needed to sustain a unit rate of shear strain, with SI units of $\text{Pa}\cdot\text{s}$ or $\text{N}\cdot\text{s/m}^2$.

Poise ($\text{P}$)

A non-SI metric unit of dynamic viscosity equivalent to $0.1\,\text{N}\cdot\text{s/m}^2$ (or $\frac{1}{10}\,\text{Pa}\cdot\text{s}$).

Kinematic Viscosity ($\nu$)

The ratio of dynamic viscosity to mass density ($\nu = \frac{\mu}{\rho}$), with the SI unit of $\text{m}^2\text{/s}$.

Vapor Pressure

The equilibrium pressure exerted by evaporating vapor molecules within a closed space once the vapor reaches saturation above its liquid phase.

Bulk Modulus of Elasticity ($K$)

The ratio of an applied compressive stress (increase in pressure) to the resulting volumetric strain ($K = \frac{dp}{-d\forall / \forall}$).

Compressibility

A measure of the fractional change in fluid volume under external pressure, defined as the reciprocal of the bulk modulus of elasticity ($\frac{1}{K}$).

Surface Tension ($\sigma$)

The tensile force acting across an imaginary line on a liquid surface in contact with gas or another immiscible fluid ($\sigma = \frac{F}{L}$), expressed in $\text{N/m}$.

Capillarity

The vertical rise or depression of a liquid surface inside a narrow tube relative to the surrounding liquid level, expressed as $h = \frac{4\sigma \cos(\theta)}{\rho g d}$.

Convex Meniscus

A curved liquid surface formed when the internal cohesive forces among fluid molecules are stronger than the adhesive forces to the container walls (e.g., mercury in glass).

Concave Meniscus

A curved liquid surface formed when the adhesive forces between the fluid molecules and the container wall exceed the fluid's internal cohesive forces (e.g., water in glass).

Pascal's Law

The principle stating that pressure applied at any point in a static fluid acts equally in all directions and is transmitted undiminished throughout an enclosed container.

Fluid Static (Hydrostatic) Law

The fundamental law stating that static fluid pressure increases linearly with vertical depth according to the relation $P = \rho g h = \gamma h$.

Pressure Head ($h$)

The vertical height of a fluid column of specific weight $\gamma$ necessary to exert a given hydrostatic pressure ($h = \frac{p}{\gamma}$).

Atmospheric Pressure

The prevailing ambient pressure exerted by the surrounding air column, standardly measured as $101.325\,\text{kPa}$, $760\,\text{mm}\text{Hg}$, or $1\,\text{atm}$ at sea level.

Absolute Pressure ($P_{\text{ab}}$)

Pressure measured relative to a perfect absolute zero vacuum datum, computed as $P_{\text{ab}} = P_{\text{atm}} + P_{\text{gauge}}$.

Gauge Pressure ($P_{\text{gauge}}$)

Pressure measured relative to local atmospheric pressure, with the atmospheric reference baseline calibrated as zero.

Vacuum Pressure

The negative gauge pressure recorded in an enclosed space when the absolute pressure falls below local atmospheric pressure.

Barometer

An instrument designed to quantify atmospheric pressure by balancing an inverted column of liquid (usually mercury) against the ambient atmosphere.

Piezometer

A simple tube gauge consisting of a narrow vertical tube (typically $\approx 13\,\text{mm}$ diameter) where the rise of liquid column height directly measures pipe gauge pressure ($p = \gamma h$).

Differential Manometer

A U-tube instrument containing a manometric fluid used to determine the pressure difference between two separate points in a pipeline or between two distinct vessels.

Inverted U-Tube Differential Manometer

An inverted U-shaped manometer containing a light manometric liquid, used specifically to measure small differential pressures between two points.

Total Pressure ($F$)

The total normal force exerted by a static fluid against a submerged plane or curved surface, calculated as $F = \rho g A \bar{h}$.

Centre of Pressure ($h^*$)

The specific point of application on an immersed surface where the resultant total hydrostatic pressure force acts, positioned at depth $h^* = \frac{I_G}{A \bar{h}} + \bar{h}$.