Freely decaying Turbulence and Bose–Einstein Condensation in Gross–Pitaevski Model

We study turbulence and Bose–Einstein condensation (BEC) within the two-dimensional Gross–Pitaevski (GP) model. In the present work, we compute decaying GP turbulence in order to establish whether BEC can occur without forcing and if there is an intensity threshold for this process. We use the wavenumber–frequency plots which allow us to clearly separate the condensate and the wave components and, therefore, to conclude if BEC is present. We observe that BEC in such a system happens even for very weakly nonlinear initial conditions without any visible threshold. BEC arises via a growing phase coherence due to anihilation of phase defects/vortices. We study this process by tracking of propagating vortex pairs. The pairs loose momentum by scattering the background sound, which results in gradual decrease of the distance between the vortices. Occasionally, vortex pairs collide with a third vortex thereby emitting sound, which can lead to more sudden shrinking of the pairs. After the vortex anihilation the pulse propagates further as a dark soliton, and it eventually bursts creating a shock.