= Wave action
{disambiguate=fluid dynamics}
{title2=$\mathcal A=E/\widehat\omega$}
= Wave action
{synonym}
For a small-amplitude conservative wave packet in a slowly varying <fluid flow>, its intrinsic <energy> divided by <intrinsic frequency> is transported along its <group velocity>, with the basic <advection> added in the laboratory frame. A stationary background conserves laboratory <angular frequency>, but a changing mean <velocity> changes the intrinsic one through the <Doppler effect>; intrinsic <energy> alone need not be conserved. Geometrical spreading and dissipation enter the action-flux balance.
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