  We study an experimentally realizable system containing stable black
hole-white hole acoustic horizons in toroidally trapped Bose-Einstein
condensates - the quantum de Laval nozzle. We numerically obtain stationary
flow configurations and assess their stability using Bogoliubov theory, finding
both in hydrodynamic and non-hydrodynamic regimes there exist dynamically
unstable regions associated with the creation of positive and negative energy
quasiparticle pairs in analogy with the gravitational Hawking effect. The
dynamical instability takes the form of a two mode squeezing interaction
between resonant pairs of Bogoliubov modes. We study the evolution of
dynamically unstable flows using the truncated Wigner method, which confirms
the two mode squeezed state picture of the analogue Hawking effect for low
winding number.
