Dissipative shock waves generated by a quantum-mechanical piston

Abstract

The piston shock problem is a prototypical example of strongly nonlinear fluid flow that enables the experimental exploration of fluid dynamics in extreme regimes. Here we investigate this problem for a nominally dissipationless, superfluid Bose-Einstein condensate and observe rich dynamics including the formation of a plateau region, a non-expanding shock front, and rarefaction waves. Many aspects of the observed dynamics follow predictions of classical dissipative—rather than superfluid dispersive—shock theory. The emergence of dissipative-like dynamics is attributed to the decay of large amplitude excitations at the shock front into turbulent vortex excitations, which allow us to invoke an eddy viscosity hypothesis. Our experimental observations are accompanied by numerical simulations of the mean-field, Gross-Pitaevskii equation that exhibit quantitative agreement with no fitting parameters. This work provides an avenue for the investigation of quantum shock waves and turbulence in channel geometries, which are currently the focus of intense research efforts.

Dynamics of photon fluid flows driven by optical pistons

“…A similar effect has been predicted when the piston is schematised as a moving potential in the defocusing NLSE [53]. A recent experiment in BEC, however, was found to deviate qualitatively from this scenario, rather exhibiting, in the long term, signatures of non-undulatory shock waves of the viscous type [54].…”

Vectorial dispersive shock waves in optical fibers

“…It then turns out that the exchange of momentum during the shock process is here induced by the XPM coupling term which acts as a sharp defocusing potential for the fluid. In a sense, our experiment constitutes the 1-D counterpart of the "blast-wave" or double piston shock problem already studied in quantum superfluid and induced by a laser beam 9,11,31,32 . The background density which sets the reference speed of sound is here dictated by the intensity level of the CW landscape, while strong density perturbations (here the high-power pump beam) naturally correspond to supersonic sources 11,23 .…”

“…Because of this issue, a significant part of the physical information encoded on the CW landscape is hidden by the intense pump pulse itself, preventing its examination. Therefore, neither blast-waves (sudden disturbances creating a sharp area of supersonically expanding pressure or density), nor pistons or DSW collision problems can be emulated [6][7][8][9][30][31][32][33][34] . To overcome these issues, one has to dissociate the DSW formation from the initial high intensity pump in such a way as to imprint the dispersive hydrodynamics phenomenon only on the CW landscape.…”

Starting Flow Past an Airfoil and its Acquired Lift in a Superfluid

“…In classical fluids, a dissipative shock is formed where supersonic flow occurs. On the contrary, reaching the compressibility condition in numerous superfluid models leads to the shedding of vortices [7,20,21]. We use this phenomenological criterion to predict the number of vortices that will nucleate.…”

“…Accelerated hydrofoils and wings have recently been used to create vortices of arbitrary shape in classical fluids [26,27], a technique which might generalize to superfluids, offering a potentially powerful new procedure in superfluid manipulation, vortex generation, and observation of quantized lift -a measurement originally attempted in 4 He by Craig & Pellam [28] to demonstrate the quantization of circulation, later detected by Vinen using a different setup [29]. Among the various superfluid experimental realizations, some have recently started to address questions on vortex nucleation and manipulation using moving obstacles including cold atomic gases [21,23,[30][31][32][33] and quantum fluids of light [34,35]. Details of each experimental realization will differ: 3d effects need to be considered for non quasi-two-dimensional BECs, the rotons' emission instead of vortex shedding might be important in 4 He, and out-ofequilibrium exciton-polariton systems will require modelling to consider intrinsic forcing and damping terms.…”