Intermittency enhancement in quantum turbulence in superfluid He-4

Intermittency is a hallmark of turbulence, which exists not only in turbulent flows of classical viscous fluids but also in flows of quantum fluids such as superfluid He-4. Despite the established similarity between turbulence in classical fluids and quasiclassical turbulence in superfluid He-4, it has been predicted that intermittency in superfluid He-4 is temperature dependent and enhanced for certain temperatures, which is in striking contrasts to the nearly flow-independent intermittency in classical turbulence. Experimental verification of this theoretical prediction is challenging since it requires well-controlled generation of quantum turbulence in He-4 and flow measurement tools with high spatial and temporal resolution. Here we report an experimental study of quantum turbulence generated by towing a grid through a stationary sample of superfluid He-4. The decaying turbulent quantum flow is probed by combining a recently developed He*(2) molecular tracer-line tagging velocimetry technique and a traditional second-sound attenuation method. We observe quasiclassical decays of turbulent kinetic energy in the normal fluid and of vortex line density in the superfluid component. For several time instants during the decay, we calculate the transverse velocity structure functions. Their scaling exponents, deduced using the extended self-similarity hypothesis, display nonmonotonic temperature-dependent intermittency enhancement, in excellent agreement with a recent theoretical and numerical study