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 ρ represents mass per unit volume varies with temperature (thermal expansion) and pressure (compressibility)
Viscosity μ \mu μ 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 K n Kn K n compares molecular mean free path to characteristic flow length scale
K n < 0.01 Kn < 0.01 K n < 0.01 indicates continuum hypothesis is valid (most engineering applications)
K n > 0.1 Kn > 0.1 K n > 0.1 suggests continuum approach breaks down molecular effects become significant (rarefied gas dynamics, microfluidics)