Visualization of viscous and quantum flows of liquid 4He due to an oscillating cylinder of rectangular cross section

The motions of micrometer-sized solid deuterium particles in liquid He-4, at temperatures between approximately 1.2 and 3 K, are visualized in the proximity of an oscillating cylinder of rectangular cross section (3 mm high and 10 mm wide). The cylinder is oscillating vertically, perpendicularly to its cross-section width, at frequencies between 0.05 and 1.25 Hz, and amplitudes of 5 and 10 mm, resulting in Reynolds numbers Re up to 10(5). The aim of the reported experiments is to investigate systematically the macroscopic vortical structures shed at the cylinder sharp edges, by tracking the deuterium particles. We find that large-scale, millimeter-sized vortices are generated in the surrounding fluid by the oscillating cylinder, both in viscous He I and superfluid He II. An estimate of the strength of the shed vortical structures reveals that, for Re > 10(4), the corresponding magnitudes are approximately equal in He I and He II if, in He II, the kinematic viscosity is suitably defined. For Re < 10(4), the strength of the large-scale vortices is smaller in He II than in He I. Although the outcome is partly affected by the larger scatter of the He I data and possibly also by the much larger heat conductivity of superfluid 4He, we argue that the fundamental physical reason for observing this difference is that, at these Reynolds numbers, the experimentally probed length scales in He II are smaller than the average distance between quantized vortices-the quantum length scale of the flow. The result strongly suggests that, similarly to thermal counterflow, both viscous and quantum features can be observed in mechanically driven flows of He II, depending on the length scales at which the quantum flow is probed.