Munday, Lydia and McClintock, Peter and Fisher, Shaun and Ihas, G. G. (2014) Quantum turbulence generated by moving grids. PhD thesis, Lancaster University.
Abstract
We present experimental results on quantum grid turbulence produced by moving grids within superfluid 4He, both at millikelvin temperatures, with an oscillating grid, and at temperatures above 1.4 K with a linearly moving grid. Floppy devices were used at millikelvin temperatures to produce quantum turbulence. We investigated the frequency dependence of the turbulent drag on an oscillating grid. At high velocities, the turbulent drag is independent of frequency and similar to what was measured in liquid helium-4 in its normal phase. We also present measurements of the inertial drag coefficient for grid turbulence, which is significantly reduced by turbulence produced in both superfluid and normal fluid 4He. To produce (approximate) homogeneous and isotropic grid turbulence in a quantum liquid, with little to no extraneous heating in the fluid, a new linear ‘control motor‘ has been designed and tested. The motor consists of a drive coil, surrounded by three control coils. A linear current ramp is passed through the drive coil, which lifts a superconducting armature placed in the centre of the solenoid. The control coils are designed, when a steady DC current is applied to them, to have constant magnetic field derivative. The control motor performs adequately, having smooth motions with no oscillations, and with peak velocities up to approximately 30 cm/s. The velocity is not, however, very uniform during the motion of the motor. Decaying turbulence is investigated using the attenuation of second sound. We produce turbulence inside a short channel totally submersedin liquid helium-II. The turbulence is produced by a superconducting, magnetically levitated linear motor, with a grid attached to the top of the armature. The theory applied, for calculating vortex line density decay from second sound attenuation, is taken from Stalp, S. (1998) Decay of Grid Turbulence in Superfluid Helium. Ph.D. Thesis. We investigate the effects of different grid meshes on the vorticity decay curve, in particular the time at which the turbulence becomes saturated. We present comparisons of three separate meshes. We observe a shorter saturation time, and therefore a longer inertial regime with a t−3/2 dependence, for the turbulent decay produced by the smallest mesh grid. It has been suggested elsewhere that there is a t−11/10 dependence at early times in the vorticity decay curves, we observe no such dependence. Finally, we present measurements of the effective kinematic viscosity, and saturation time and their dependence on grid mesh size.