= Fixed-body retarded acoustic loading formula
{title2=$p'\sim(n\cdot\dot F/c_0+n_i n_j\ddot Q_{ij}/c_0^2)/(4\pi r)$}
For a fixed impermeable object, the <Ffowcs Williams-Hawkings equation> has a volume <acoustic quadrupole> and a surface <acoustic dipole>, with no mass-injection term. If $F$ is the total force exerted on the fluid and $Q=\int T\,dV$ the integrated <Lighthill stress tensor>, the <acoustic compact-source approximation> in the <acoustic far field> gives the displayed pressure at retarded time. A force convention directed onto the body reverses the loading sign. The <retarded acoustic Green function> carries $1/c_0^2$ when the wave operator is $\partial_t^2-c_0^2\Delta$.
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