= Aerodynamics
<Aerodynamics> studies the <motion> of <gases> and their interaction with moving or stationary bodies. The resulting surface <pressure> and <viscous stress> determine <lift> and <drag> on an aircraft, a turbine blade or a moving vehicle.
A useful model depends on dimensionless comparisons. The <Reynolds number> compares inertial and viscous effects; the <Mach number> compares flow speed with the <speed of sound>. Low <Mach number> can justify an approximately <incompressible flow>, whereas transonic and supersonic motion may produce <shock waves>. A wing's lift follows from the complete pressure and momentum balance, rather than a requirement that air parcels passing above and below reunite simultaneously.
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