Observation of a gravitational Aharonov-Bohm effect

Gravitational interference The Aharonov-Bohm effect is a quantum mechanical effect in which a magnetic field affects the phase of an electron wave as it propagates along a wire. Atom interferometry exploits the wave characteristic of atoms to measure tiny differences in phase as they take different paths through the arms of an interferometer. Overstreet et al . split a cloud of cold rubidium atoms into two atomic wave packets about 25 centimeters apart and subjected one of the wave packets to gravitational interaction with a large mass (see the Perspective by Roura). The authors state that the observed phase shift is consistent with a gravitational Aharonov-Bohm effect. —ISO

Dispersive MUSIC algorithm for Lamb wave phased array

By controlling the excitation time delay on each element, the conventional phased array can physically focus signals transmitted by different elements on a desired point in turn. An alternative and time-saving strategy is that every element takes turns to transmit the excitation and the remaining elements receive the corresponding response signals, which is known as the full matrix capture (FMC) method for data acquisition, and then let the signals virtually focus on every desired point by post-processing technique. In this study, based on the FMC, a dispersive multiple signal classification (MUSIC) algorithm for Lamb wave phased array is developed to locate defects. The virtual time reversal is implemented to back propagate the wave packets corresponding to the desired focusing point and a window function is adopted to adaptively isolate the desired packets from the other components. Then those wave packets are forward propagated to the original focusing point at a constant velocity. For every potential focusing point and all receivers, the virtual array focuses the signals from all transmitters so as to obtain the focusing signals. The MUSIC algorithm with the obtained focusing signals is adopted to achieve Lamb wave imaging. Benefiting from the post-processing operations, the baseline subtraction as well as the estimation for the number of the scattering sources is no longer required in the proposed algorithm. Experiments on an aluminum plate with three artificial defects and a compact circular PZT array are implemented and the results demonstrate the efficacy of the proposed algorithm.

Ultrasound Defect Localization in Shell Structures with Lamb Waves Using Spare Sensor Array and Orthogonal Matching Pursuit Decomposition

Lamb waves have multimodal and dispersion effects, which reduces their performance in damage localization with respect to resolution. To detect damage with fewest sensors and high resolution, a method, using only two piezoelectric transducers and based on orthogonal matching pursuit (OMP) decomposition, was proposed. First, an OMP-based decomposition and dispersion removal algorithm is introduced, which is capable of separating wave packets of different propagation paths and removing the dispersion part successively. Then, two simulation signals, with nonoverlapped and overlapped wave packets, are employed to verify the proposed method. Thereafter, with the proposed algorithm, the wave packets reflected from the defect and edge are all separated. Finally, a sparse sensor array with only two transducers succeeds in localizing the defect. The experimental results show that the OMP-based algorithm is beneficial for resolution improvement and transducer usage reduction.

Experimental study of perturbations modeled by a membrane in 2D and 3D boundary layers

The work is devoted to experimental studies of the dynamics of the development of perturbations introduced by a membrane under various conditions. The studies were carried out under conditions of a low and moderate degree of free-flow turbulence. It is shown that the impulsive motion of the membrane generates a localized longitudinal structure in the boundary layer, as well as wave packets at its fronts. A circular membrane generates wave packets consisting of forward and oblique waves, while a rectangular membrane generates predominantly forward waves. A moderate degree of turbulence inhibits the development of wave packets at the linear stage and intensifies at the nonlinear stage. The separation of the boundary layer stimulates an increase in the amplitude of the wave packets.

Passive and Active Slab Waveguide Mode Analysis Using Transfer Matrix Method

We present a general approach for numerical mode analysis of the multilayer slab waveguides using the Transfer Matrix Method (TMM) instead of the Finite Difference Frequency Domain (FDFD) method. TMM consists of working through the device one layer at a time and calculating an overall transfer matrix. Using the scattering matrix technique, we develop the proposed method for multilayer structures. We find waveguide modes for both passive and active slabs upon determinant analysis of the scattering matrix of the slab. To do this, we enhance the formulation of spatial scattering matrix to reach spatiotemporal scattering matrix. Our proposed technique is more efficient and faster than other numerical methods. Simulation results show either the spatial modes of inactive and hybrid spacetime modes of active planar waveguide. Also, spacetime wave packets can be seen using plane wave injection into the time-dependent slab waveguide instead of previously reported diffraction-free wave packets which have been obtained using the multifrequency input injection into the un-patterned inactive slab waveguides.