 Bogoliubov sound speed and Landau bound

The contact-gas spectrum $E_k=\sqrt{{ϵ}_k({ϵ}_k+2g{n}_0)}$ gives $\min E_k/(\hslash k)=\sqrt{g{n}_0/m}$. This energetic homogeneous-flow bound need not equal a measured critical speed when vortices or boundaries enable other dissipation.