334 research outputs found
CFD Analysis of Helicopter Wakes in Ground Effect
The paper presents CFD results for the wake of a helicopter flying a low altitude at different advance ratios.
The wakes are assessed in terms of topology and velocity magnitudes. The structure of the wake near ground
changes rapidly with the advance ratio and its decay appears to be faster than what is suggested by theoretical
analyses. The results show clear the potential of modern CFD for use in helicopter safety and highlights the
need for detailed surveys of helicopter wakes using full-scale physical experiments
Josephson-Majorana cycle in topological single-electron hybrid transistors
Charge transport through a small topological superconducting island in
contact with a normal and a superconducting electrode occurs through a cycle
that involves coherent oscillations of Cooper pairs and tunneling in/out the
normal electrode through a Majorana bound state, the Josephson-Majorana cycle.
We illustrate this mechanism by studying the current-voltage characteristics of
a superconductor-topological superconductor-normal metal single-electron
transistor. At low bias and temperature the Josephson-Majorana cycle is the
dominant mechanism for transport. We discuss a three-terminal configuration
where the non-local character of the Majorana bound states is emergent.Comment: 6 pages, 4 figure
Perpendicular blade–vortex-interaction over an oscillating airfoil in light dynamic stall
An experimental and numerical study was performed to investigate the effects of perpendicular blade vortex interactions on the aerodynamic performance of an oscillating airfoil. The selected test cases studied the aerodynamic interaction of a stream-wise vortex impacting on a NACA 23012 airfoil oscillating in light dynamic stall regime, representing a typical condition of the retreating blade of a helicopter in forward flight. The analysis of particle image velocimetry surveys and time-accurate simulation results enabled to point out the different effects due to the blade pitching motion on the interacting flow field. Thus, numerical results enabled to achieve a detailed insight about the aerodynamic loads acting on the oscillating airfoil in the interacting cases. In particular, the comparison with the clean airfoil case shows that a severe loss of performance is produced by the interaction of the vortex during the airfoil downstroke motion, as the vortex impact triggers the local stall of the blade section
Local density of states in metal - topological superconductor hybrid systems
We study by means of the recursive Green's function technique the local
density-of-states of (finite and semi-infinite) multi-band spin-orbit coupled
semiconducting nanowires in proximity to an s-wave superconductor and attached
to normal-metal electrodes. When the nanowire is coupled to a normal electrode,
the zero-energy peak, corresponding to the Majorana state in the topological
phase, broadens with increasing transmission between the wire and the leads,
eventually disappearing for ideal interfaces. Interestingly, for a finite
transmission a peak is present also in the normal electrode, even though it has
a smaller amplitude and broadens more rapidly with the strength of the
coupling. Unpaired Majorana states can survive close to a topological phase
transition even when the number of open channels (defined in the absence of
superconductivity) is even. We finally study the Andreev-bound-state spectrum
in superconductor-normal metal-superconductor junctions and find that in
multi-band nanowires the distinction between topologically trivial and
non-trivial systems based on the number of zero-energy crossings is preserved.Comment: 11 pages, 12 figures, published versio
Experimental Investigation of the Rotor-Wing Aerodynamic Interaction in a Tiltwing Aircraft in Hover
The hovering performance and the lifting capability of tiltrotor aircraft are strongly affected by the aerodynamic interaction between wing and rotors. The tiltwing concept represents a promising technology to increase the hover performance by reducing the wing-rotor interference. The present work describes an experimental activity carried out on a 1/4 scaled tiltwing aircraft half-model to achieve a detailed insight about the main issues characterising the aerodynamic interaction between wing and rotor in hover. The results of the experimental campaign, including both force measurements and Particle Image Velocimetry surveys, enabled to evaluate both the aircraft performance for different configurations of the tilting wing and to achieve a detailed insight about the flow physics of the rotor wake in the interaction with the wing. The test activity provided a comprehensive experimental database that was obtained over a not confidential aircraft configuration
Experiments on the Helicopter-Obstacle Aerodynamic Interference in Absence of External Wind
The rotor-obstacle interaction has become a challenging research topic in the last few years.
In the present paper a comprehensive experimental survey carried out at University of Glasgow is
described, taking advantage of two di↵erent rotor rigs and several experimental techniques. The
results are then compared with those already obtained for a similar investigation at Politecnico
di Milano. The experimental database comprises load measurements on the rotor (in order
to assess the rotor performance for di↵erent positions with respect to a cubic obstacle), Laser
Doppler Anemometry (LDA) measurements of the rotor inflow and Stereoscopic Particle Image
Velocimetry (SPIV) measurements in the region between the rotor and the obstacle. Despite a
few slight di↵erences in geometry and test conditions, the two databases show several similarities
that are analysed in the paper
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