E. Varga

Decay of counterflow turbulence in superfluid 4He

We summarize recent experiments on thermal counterflow turbulence in superfluid 4He, emphasizing the observation of turbulence in the normal fluid and its effect on the decay process when the heat flux is turned off. We argue that what is observed as turbulence in the normal fluid is a novel form of coupled turbulence in the superfluid and normal components, with injection of energy on the scales of both the (large) channel width and the (small) spacing between quantized vortices. Although an understanding of this coupled turbulence remains challenging, a theory of its decay is developed which accounts for the experimental observations.

Coexistence and interplay of quantum and classical turbulence in superfluid He 4 : Decay, velocity decoupling, and counterflow energy spectra

We report complementary experimental, numerical, and theoretical study of turbulent coflow, counterflow, and pure superflow of superfluid

Second-sound studies of coflow and counterflow of superfluid 4He in channels

^{4}\mathrm{He}

Transition to Quantum Turbulence and Streamwise Inhomogeneity of Vortex Tangle in Thermal Counterflow

in a channel, resulting in a physically transparent and relatively simple model of decaying quantum turbulence that accounts for interactions of coexisting quantum and classical components of turbulent superfluid

Quantum turbulence of bellows-driven4He superflow: Steady state

^{4}\mathrm{He}

Effective viscosity in quantum turbulence: a steady-state approach

. We further offer an analytical theory of the energy spectra of steady-state quantum turbulence in the counterflow and pure superflow, based on algebraic approximation for the energy fluxes over scales. The resulting spectra are not of the classic Kolmogorov form, but strongly suppressed by the mutual friction, leading to the energy dissipation at all scales, enhanced by the counterflow-induced decoupling of the normal and superfluid velocity fluctuations.

Energy spectrum of thermal counterflow turbulence in superfluid helium-4

We report a comprehensive study of turbulent superfluid 4He flow through a channel of square cross section. We study for the first time two distinct flow configurations with the same apparatus: coflow (normal and superfluid components move in the same direction), and counterflow (normal and superfluid components move in opposite directions). We realise also a variation of counterflow with the same relative velocity, but where the superfluid component moves while there is no net flow of the normal component through the channel, i.e., pure superflow. We use the second-sound attenuation technique to measure the density of quantised vortex lines in the temperature range 1.2 K ≲ T ≲ Tλ ≈ 2.18 K and for flow velocities from about 1 mm/s up to almost 1 m/s in fully developed turbulence. We find that both the steady-state and temporal decay of the turbulence significantly differ in the three flow configurations, yielding an interesting insight into two-fluid hydrodynamics. In both pure superflow and counterflow, ...

Quantum Turbulence of Bellows-Driven 4He Superflow: Decay

We report preliminary results of the complementary experimental and numerical studies on spatiotemporal tangle development and streamwise vortex line density (VLD) distribution in counterflowing

Intermittency enhancement in quantum turbulence.

^4