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Fluid Mechanics

1.1 Fundamentals of Fluid Mechanics

2 min readLast Updated on July 19, 2024

Fluid mechanics is the study of how liquids and gases behave when they're still or moving. It's crucial for engineers working on everything from airplanes to water pipes. Understanding fluid mechanics helps us design better machines and systems that use or interact with fluids.

Fluids have unique properties that set them apart from solids. They can't hold their shape and flow when forces are applied. Density, viscosity, and compressibility are key characteristics that determine how fluids behave in different situations.

Introduction to Fluid Mechanics

Fluid mechanics in engineering

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  • Studies behavior of fluids at rest and in motion including liquids (water, oil) and gases (air, natural gas)
  • Investigates interaction of fluids with solid boundaries (pipe walls, airfoils) and other fluids (mixing, multiphase flow)
  • Applies to aerodynamics of aircraft (lift, drag) and vehicles (streamlining, wind tunnels)
  • Utilized in hydraulic systems (pumps, turbines) and machinery (excavators, brakes)
  • Essential for piping systems (water distribution, oil pipelines) and fluid transportation (tankers, pipelines)
  • Crucial in heat exchangers (radiators, condensers) and cooling systems (HVAC, refrigeration)
  • Fundamental to environmental engineering (wastewater treatment, air pollution control) and water resources management (dams, canals, rivers)

Properties of fluids

  • Fluids deform continuously under applied shear stress which acts parallel to the surface
  • Density ρ\rho represents mass per unit volume varies with temperature (thermal expansion) and pressure (compressibility)
  • Viscosity μ\mu quantifies resistance to deformation under shear stress higher values indicate more resistance to flow
  • Viscosity depends on temperature (decreases with increasing temperature) and pressure (increases with increasing pressure)
  • Compressibility measures change in density with respect to pressure gases are highly compressible while liquids are nearly incompressible

Fluids vs solids

  • Solids resist deformation under stress and strain maintaining their shape under external forces
    • Exhibit elastic deformation up to a limit followed by plastic deformation or fracture
  • Fluids deform continuously under shear stress conforming to the shape of their container
    • Flow under the influence of external forces like gravity or pressure gradients
    • Cannot sustain shear stress at rest due to lack of a fixed molecular structure

Continuum concept in fluids

  • Assumes fluid properties (density, velocity, pressure) vary continuously from point to point
    • Enables use of differential equations (Navier-Stokes) to describe fluid behavior
  • Valid when length scales of interest are much larger than the molecular mean free path
    • Mean free path represents average distance traveled by a molecule between collisions
  • Knudsen number KnKn compares molecular mean free path to characteristic flow length scale
    • Kn<0.01Kn < 0.01 indicates continuum hypothesis is valid (most engineering applications)
    • Kn>0.1Kn > 0.1 suggests continuum approach breaks down molecular effects become significant (rarefied gas dynamics, microfluidics)
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© 2025 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.

© 2025 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.