Evidence of superfluidity in a dipolar supersolid from nonclassical rotational inertia

A key manifestation of superfluidity in liquids and gases is a reduction of the moment of inertia under slow rotations. Non-classical rotational effects have also been considered in the context of the elusive supersolid phase of matter, in which superfluidity coexists with a lattice structure. Here we show that the recently discovered supersolid phase in dipolar quantum gases features a reduced moment of inertia. Using a dipolar gas of dysprosium atoms, we study a peculiar rotational oscillation mode in a harmonic potential, the scissors mode, previously investigated in ordinary superfluids. From the measured moment of inertia, we deduce a superfluid fraction that is different from zero and of order of unity, providing direct evidence of the superfluid nature of the dipolar supersolid.

A sensorless angular displacement measurement method for rotational oscillation generation in biomedical applications with Ros-Drill©

This paper presents a novel measurement method for angular displacement of an oscillation-assisted micro drill device, Ros-Drill©. The device is driven with a brushless dc motor (BLDC) which is desired to track a sinusoidal position reference. The measurement method is based on the principle of monitoring the back-emf voltage that is induced on the non-fed winding of the brushless motor. It offers sensorless analog measurement of the angular displacement of the oscillatory motion which is not possible with optical encoders. The measurement methodology and control algorithms are implemented utilizing a digital signal processor. Experiments reveal that the method is feasible for measuring angular displacements of rotational oscillations during cellular piercing operations. Furthermore, it presents fair performance for frequencies up to 1000 Hz and provides early diagnosis for a potential malfunction of the micro drill.

Flow-induced vibration control of a circular cylinder using rotational oscillation feedback

The effect of imposed rotation on a slender elastically mounted circular cylinder free to oscillate transversely to the incident flow has been studied experimentally in a free-surface water channel. Rotation rate and direction are imposed to be proportional to either the cylinder’s transverse displacement or the cylinder’s transverse velocity to determine the effectiveness of these rotation laws to control the dynamics of the cylinder, either to reduce or to enhance oscillations. The former can be of interest for energy harvesting purposes whereas the latter can be useful to avoid unwanted oscillations. In all cases, non-dimensional mass and damping are fixed ($m^{\ast }=11.7$, $\unicode[STIX]{x1D701}=0.0043$) so the analysis is focused on the role of the rotation law and the reduced velocity. The Reynolds number based on the diameter of the cylinder ranges from 1500 to 10 000. Results are presented in terms of steady-state oscillation characterization (say, amplitude and frequency) and wake-pattern topology, which was obtained through digital particle image velocimetry. Both laws are able to either reduce or enhance oscillations, but they do it in a different way. A rotation law proportional to the cylinder’s displacement is more effective to enhance oscillations. For high enough actuation, a galloping-type response has been found, with a persistent growth of the amplitude of oscillations with the reduced velocity that shows a new desynchronized mode of vortex shedding. On the other hand, a rotation law proportional to the cylinder’s transverse velocity is more efficient to reduce oscillations. In this case only vortex-induced-type responses have been found. A quasi-steady theoretical model has been developed, which helps to explain why a galloping-type response may appear when rotation is proportional to cylinder displacement and is able to predict reasonably the amplitude of oscillations in those cases. The model also explains why a galloping-type response is not expected to occur when rotation is proportional to the cylinder’s velocity.

Non linear model of rotational galloping of square, rectangular and bundle cylinder in cross-flow

The occurrence of rotational galloping for several geometries is assessed through two-dimensional flow simulations and fluid-structure interaction simulations. A finite element formulation specif­ically devised for fluid-structure interactions simulations has been used. Non-linear models aimed at predicting rotational galloping are determined based on cross-flow simulations around fixed cylinders for various angles of attack. The added torsional fluid damping coefficient is modelled based on the results of forced rotational oscillation simulations. The reliability of the models is assessed by confrontation with the results of free rotation simulations, where reduced velocity and maximum amplitude of galloping have been chosen as comparison criteria, when they can be measured. The quasi-steady model is shown to have strong limitations on stream­lined bodies. An extended quasi-steady model is introduced. This model does not consider history effects and considers damping up to second order. Contrarily to the previous model, this improved model can precisely predict unstable zones for a variety of section shapes. The torsional galloping of square, rectangular and bundle shape cross sections have been studied.