Joseph J. Niemela

Dynamics of thin vortex rings

As part of a long-range study of vortex rings, their dynamics, interactions with boundaries and with each other, we present the results of experiments on thin core rings generated by a piston gun in water. We characterize the dynamics of these rings by means of the traditional equations for such rings in an inviscid fluid suitably modifying them to be applicable to a viscous fluid. We develop expressions for the radius, core size, circulation and bubble dimensions of these rings. We report the direct measurement of the impulse of a vortex ring by means of a physical pendulum.

Dissipation of grid turbulence in helium II

The results of a number of recent experiments on high Reynolds number grid turbulence in helium II suggest that its flow on large length scales resembles that of a classical fluid. It has been known for some time that the effective kinematic viscosity of this turbulent fluid, describing energy flow in the inertial range of wave numbers, is of the order of η/ρ where η is the normal fluid viscosity and ρ is the total density of the liquid. However, dissipation must be strongly influenced by quantum processes, and it cannot be associated simply with the normal-fluid viscosity. The importance of quantum processes arises because the dissipation occurs at small length scales, comparable with the spacing of the quantized vortex lines that allow turbulent motion in the superfluid component. We report an analysis of experimental data that allows us to deduce experimental values of the effective kinematic viscosity, which we call ν′(≠η/ρ), to which theories of the quantum dissipative processes can be compared.

Heat transport in the geostrophic regime of rotating Rayleigh-Bénard convection.

We report experimental measurements of heat transport in rotating Rayleigh-Bénard convection in a cylindrical convection cell with an aspect ratio of Γ=1/2. The fluid is helium gas with a Prandtl number Pr=0.7. The range of control parameters for Rayleigh numbers 4×10^{9}<Ra<4×10^{11} and for Ekman numbers 2×10^{-7}<Ek<3×10^{-5} (corresponding to Taylor numbers 4×10^{9}<Ta<1×10^{14} and convective Rossby numbers 0.07<Ro<5). We determine the transition from weakly rotating turbulent convection to rotation dominated geostrophic convection through experimental measurements of the heat transport Nu. The heat transport, best collapsed using a parameter RaEk^{β} with 1.65<β<1.8, defines two boundaries in the phase diagram of Ra/Ra_{c} versus Ek and elucidates properties of the geostrophic turbulence regime of rotating thermal convection. We find Nu∼(Ra/Ra_{c})^{γ} with γ≈1 from direct measurement and 1.2<γ<1.6 inferred from scaling arguments.

Density and thermal expansion coefficient of liquid helium-4 from measurements of the dielectric constant

Measurements have been made of the dielectric constant of liquid helium from 1.15 K to 4.9 K. The density and thermal expansion coefficient have been derived from these measurements using the Clausius-Mossotti equation. Comparison is made to modern measurements. Formulae are presented representing the absolute density and thermal expansion coefficient from 0 K to 4.9 K. Difficulties and uncertainties in these results are discussed.

Electrostatic Charging and Levitation of Helium II Drops

Liquid Helium II drops, of diameter 1 mm or less, are charged with positive helium ions and subsequently levitated by static electric fields. Stable levitation was achieved for drops of order 100–150 micrometers in diameter. The suspended drops could be translated to arbitrary positions within the levitator using additional superimposed DC electric fields, and also could be made to oscillate stably about their average positions by means of an applied time-varying electric field. A weak corona discharge was used to produce the necessary ions for levitation. A novel superfluid film flow device, developed for the controlled deployment of large charged drops, is described. Also discussed is an adjustable electric fountain that requires only a field emission tip operating at modest potentials, and works in both Helium I and Helium II.

Effective kinematic viscosity of turbulence in superfluid 4He

Abstract Using superfluid helium II for the study of grid turbulence has many advantages. He II has the lowest kinematic viscosity of any known substance, enabling large Reynolds numbers to be achieved in a relatively small apparatus. In addition, the resolution of second sound attenuation measurements provides insights into the decay of turbulence behind a grid. From measurements of this decay, we report preliminary experimental observations pertaining to the effective kinematic viscosity of superfluid turbulence. We also provide a better interpretation of the relation between second sound attenuation measurements and vorticity.

Thermal Convection in Liquid Helium

Above about 2.2 K liquid helium is assumed to behave as an ordinary incompressible Newtonian fluid, free from the macroscopic quantum effects possessed by its lower temperature counterpart. Recent thermal convection experiments support this assumption.

A new low temperature device for high resolution, in situ measurement and control of submicron gaps

We have developed a device to investigate finite-size scaling of the thermal expansion coefficient in liquid helium near the lambda transition. Motivated by the need for a range of well known, uniform, sub-micron gaps with constant surface conditions, we have built a variable-gap parallel-plate capacitor that can be adjusted in situ. We measure the gap at three points using laser interferometry. A careful choice of the material and thickness of the reflecting surfaces results in asymmetric fringes, for which both exceptional sensitivity and a high contrast ratio can be achieved simultaneously. Both the gap size and its uniformity are actively controlled using voice coil actuators. This design has a number of additional benefits: the gap can be kept closed until low temperature use, data can be collected for all gaps with no thermal cycling, and no spacers are needed to establish the gap.

Vortex Nucleation and the Levitation of Charged Helium II Drops

Intrinsic nucleation of quantized vortices in Helium II can be studied by means of rotating freely suspended superfluid drops at angular velocities above some critical value. The motivation for doing so is described, as well as recent progress in the electrostatic levitation of Helium II drops charged with positive ions. To date, stable levitation has been achieved for drops of order 100–150 micrometers in diameter, with a surface charge density about a factor of ten smaller than Rayleigh limit, and a diameter a similar factor less than the maximum allowed in normal gravity. We discuss the possibility of rotating these drops via the surface charge density and discuss the advantages of a microgravity environment, including the attainment of significantly larger suspended drops. Recent efforts to find optical seed particles for angular velocity measurements are discussed.

An experiment to investigate the superfluid transition in a confined planar geometry

We present a progress report on our investigation of finite size effects on the superfluid transition. In our experiment, the helium is confined between the parallel plates of a carefully constructed capacitor. We measure the dielectric constant as a function of temperature, from which we can calculate the thermal expansivity. We discuss some of the following important features of our experiment: The capacitor, built in collaboration with the Chex experiment, gives us a simple, well defined planar geometry, extremely flat surfaces, and a sub-micron gap. A low temperature audio-frequency preamp improves the sensitivity of the capacitance measurement. A high resolution paramagnetic salt thermometer, also from Chex, enables us to work extremely close to the lambda point. We also present a novel design for a simple low temperature burst disc that is part of our apparatus.