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Starting flow past an accelerating body in superfluid helium
Blaha, Jiří ; La Mantia, Marco (advisor) ; Chagovets, Tim (referee)
An airfoil with a significant angle of attack is accelerated at various rates from rest in superfluid helium. Starting vortices, shed from the trailing and leading edge of the airfoil, are studied by visualization methods. The vorticity field is approximated by Lagrangian pseudovorticity, which makes it possible to determine the position and relative strength of the vortices in time and compare them with a classical theory. It was found that the trailing edge starting vortex moves considerably faster than predicted by the theory and apart from a possible short initial period does not follow the analytically obtained scaling laws. The leading edge vortex was found to be ill-defined in the present experimental setup, but the results also hint at differences from the theory. The disparity is attributed mostly to viscosity effects. A possible explanation is proposed for the observed phenom- ena, where the vortex may after a short period escape the airfoil's attached flow and move with an approximately constant velocity. Overall, the study supports the idea of similarity between viscous flows and large scale, mechanically driven flows of He II. 1
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Visualization of particle motions in superfluid helium flows
Švančara, Patrik ; La Mantia, Marco (advisor) ; Chagovets, Tim (referee)
Flows of normal and superfluid 4 He (He I and He II, respectively) are investigated experimentally. Relatively small particles of solid hydrogen and deuterium are suspended in the experimental volume and their motions are tracked in both mechanically and thermally driven flows. A statistical study of the particle velocity and velocity increment distributions is performed at scales smaller and larger than the mean distance between quantized vortices, the quantum length scale of the investigated flows. We show that, at small scales, the observed particle dynamics in He II is greatly influenced by that of quantized vortices. We, additionally, report that this behavior is independent of the imposed large-scale flow. Instead, at large scales, we observe that particle motions are quasiclassical, that is, very similar to those reported to occur in turbulent flows of viscous fluids. The study reinforces therefore the idea of close similarity between viscous flows and large-scale (mechanically-driven) flows of He II, and simultaneously highlights the small-scale differences due to the presence of quantized vortices in He II.
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