Flow, in the context of fluid mechanics, refers to the movement of fluids (liquids, gases, or a mixture of solids and liquids) through a conduit, over a surface, or in open channels. It plays a crucial role in many engineering applications, such as pipelines, water treatment, HVAC systems, and hydraulic systems.

  • Laminar Flow:

Flow where fluid particles move along smooth, orderly paths or layers. It is characterised by low flow velocity and minimal mixing between the layers. Occurs at low Reynolds numbers (typically below 2000 for pipe flow). The flow is stable, with minimal turbulence. The velocity profile is parabolic in pipes, with the highest velocity at the centre.

Example: Flow of oil through a thin capillary tube.
Applications: Microfluidic devices, lubrication systems, blood flow in arteries.

  • Turbulent Flow:

Flow characterised by chaotic changes in pressure and flow velocity. It involves eddies and vortices, leading to increased mixing.
Typically occurs at high Reynolds numbers (above 4000 for pipe flow). The flow is unstable, with significant turbulence. The velocity profile is flatter compared to laminar flow.

Example: Flow of water through a large pipe at high velocities.
Applications: Piping systems, aerodynamics, rivers, and combustion chambers.

  • Transitional Flow:

Flow that exists between laminar and turbulent flow states. It shows characteristics of both.
Occurs in the Reynolds number range between 2000 and 4000. The flow fluctuates between laminar and turbulent, depending on conditions

Example: Water flows in a pipe with Reynolds number around 2500.
Applications: Fluid systems transitioning between laminar and turbulent regimes.

  • Steady Flow:

Flow where the fluid properties at any given point do not change over time. Involves constant velocity, pressure, and density at any given location. The streamline patterns remain fixed.
Example: Water flowing steadily through a constant diameter pipe.
Applications: Systems requiring consistent flow rates, such as hydraulic circuits.

  • Unsteady Flow:

Flow where fluid properties vary over time at any given point. The flow velocity, pressure, or other characteristics change with time, leading to changing streamlines.

Example: Tidal currents in coastal areas.
Applications: Transient flow situations like pump start-ups, ocean currents.

  • Compressible Flow:

Flow where the fluid density changes significantly within the flow field. Important in high-speed gas flows where compressibility effects like shock waves can occur.

Example: Airflow over a supersonic jet.
Applications: Aerospace engineering, high-speed gas flows.

  • Incompressible Flow:

Flow where the fluid density remains nearly constant throughout. Common in low-speed flows of liquids or gases where density changes are negligible.

Example: Water flow in pipes at low velocities.
Applications: Water distribution systems, cooling systems.

  • Rotational Flow:

Flow where the fluid elements exhibit angular motion around an axis. Fluid particles have a rotational component, leading to vorticity.

Example: Water swirling in a draining sink.
Applications: Rotating machinery, natural whirlpools.

  • Irrotational Flow:

Flow where fluid particles do not rotate about their own axes. No vorticity; typically associated with potential flow theory.

Example: Flow around an airplane wing in potential flow theory.
Applications: Aerodynamics, hydrodynamics.