Grade 12 Physics: Fluid Dynamics

Comprehensive vocabulary flashcards covering key definitions, flow regimes, ideal fluid assumptions, equations, surface tension, capillary action, and viscosity principles from Chapter 3: Fluid Dynamics.

Fluid

A collection of molecules that are randomly arranged and held together by weak cohesive forces and by forces exerted by the walls of a container, encompassing both liquids and gases.

Fluid Dynamics

The branch of physics that studies the mechanics and behavior of fluids in motion.

Laminar Flow

The flow of a viscous fluid in which particles move in parallel layers, each with a constant velocity relative to neighboring layers, exhibiting smooth paths with little or no lateral mixing.

Turbulent Flow

Fluid motion characterized by chaotic changes in pressure and flow velocity, irregular streamlines that change over time, and unsteady vortices or eddies that cause lateral mixing.

Laminar and Turbulent Flow in a Pipe

An illustration comparing laminar flow (left), where fluid layers slide smoothly past one another along parallel paths, with turbulent flow (right), where colliding paths form swirling eddies and vortices.

Reynolds Number

A dimensionless parameter characterizing fluid flow that determines the threshold velocity at which flow transitions from laminar to turbulent, depending on the fluid's viscosity, density, and channel dimensions.

Boundary Layer

A very thin sheet of fluid (such as air) lying over the surface of an object (such as an aircraft wing) that adheres to the surface because of viscosity.

Non-viscous Fluid

An ideal fluid in which internal friction is neglected, meaning an object moving through the fluid experiences no viscous retarding force.

Steady Flow

A flow regime in which every fluid particle arriving at a given point in space has the exact same velocity over time.

Incompressible Fluid

A fluid whose density ($\rho$) remains completely constant throughout its motion.

Irrotational Flow

Fluid flow in which the fluid has no angular momentum about any point, such that a small paddle wheel placed anywhere in the fluid does not rotate about its center of mass.

Equation of Continuity

The mathematical expression of mass conservation in steady flow, stated as $\rho_1 A_1 v_1 = \rho_2 A_2 v_2$, which reduces to $A_1 v_1 = A_2 v_2$ for incompressible fluids.

Flow Rate (Volume Flux)

The volume of fluid crossing a given cross-sectional area per unit time, defined mathematically as the product of cross-sectional area and fluid speed ($Q = Av$).

Bernoulli's Equation

A statement of energy conservation applied to ideal fluids along a streamline: $p + \frac{1}{2}\rho v^2 + \rho gy = \text{constant}$, indicating that pressure decreases as fluid speed or elevation increases.

Surface Tension ($\gamma$)

The tendency of liquid surfaces at rest to shrink into the minimum surface area possible; defined quantitatively as the force per unit length acting in the plane of the interface ($\gamma = \frac{F}{l}$).

Cohesive Forces

Attractive intermolecular forces acting between molecules of the same type.

Adhesive Forces

Attractive intermolecular forces acting between molecules of different types, such as between liquid molecules and the walls of a solid container.

Contact Angle ($\phi$)

The angle between a solid surface and a line drawn tangent to the liquid surface at the contact edge; it is less than $90^\circ$ when adhesive forces dominate (wetting) and greater than $90^\circ$ when cohesive forces dominate (non-wetting).

Wetting Agent

A chemical substance, such as a detergent, added to a liquid to reduce the contact angle to less than $90^\circ$, allowing the liquid to thoroughly contact and penetrate a surface.

Capillary Action

The spontaneous process of a liquid rising or falling in a narrow space or tube without external assistance from forces like gravity, governed by surface tension and adhesive versus cohesive forces.

Capillary Rise and Depression

A diagram demonstrating capillary rise (top), which occurs when adhesive forces exceed cohesive forces ($\phi < 90^\circ$), and capillary depression (bottom), which occurs when cohesive forces exceed adhesive forces ($\phi > 90^\circ$).

Capillary Height Formula

The formula that determines the equilibrium height $h$ to which a liquid rises or depresses in a tube of radius $r$: $h = \frac{2\gamma \cos(\phi)}{\rho g r}$.

Viscosity ($\eta$)

A physical property of fluids measuring internal friction or resistance to deformation and relative motion between adjacent fluid layers, with SI units of $\text{Pa}\cdot\text{s}$ or $\text{N}\cdot\text{s}\,\text{m}^{-2}$.

Inviscid Fluid

An ideal, frictionless fluid that has zero viscosity.

Fluidity

The physical property defined as the reciprocal of viscosity ($\frac{1}{\eta}$).

Poiseuille's Law

The principle describing steady laminar flow through a cylindrical pipe, giving volume flow rate as $Q = \frac{\pi \Delta p\, r^4}{8\eta l}$, which is directly proportional to the pressure gradient and the fourth power of the radius, and inversely proportional to viscosity and length.