= Lagrangian perturbation of a fluid variable
{c}
{title2=$\Delta q=\delta q+\xi\cdot\nabla q$}
= Fluid Lagrangian perturbation
{synonym}
The Lagrangian perturbation compares a variable on the same moving fluid element before and after a displacement. Taylor expansion of the background value at the displaced position gives the displayed relation to an Eulerian perturbation. For an adiabatic <ideal gas>, $\Delta(p/\rho^\gamma)=0$ even when the equilibrium <entropy> varies spatially.
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