Distributed Acoustic Sensing Based on Coherent Microwave Photonics Interferometry.

A microwave photonics method has been developed for measuring distributed acoustic signals. This method uses microwave-modulated low coherence light as a probe to interrogate distributed in-fiber interferometers, which are used to measure acoustic-induced strain. By sweeping the microwave frequency at a constant rate, the acoustic signals are encoded into the complex microwave spectrum. The microwave spectrum is transformed into the joint time-frequency domain and further processed to obtain the distributed acoustic signals. The method is first evaluated using an intrinsic Fabry Perot interferometer (IFPI). Acoustic signals of frequency up to 15.6 kHz were detected. The method was further demonstrated using an array of in-fiber weak reflectors and an external Michelson interferometer. Two piezoceramic cylinders (PCCs) driven at frequencies of 1700 Hz and 3430 Hz were used as acoustic sources. The experiment results show that the sensing system can locate multiple acoustic sources. The system resolves 20 nε when the spatial resolution is 5 cm. The recovered acoustic signals match the excitation signals in frequency, amplitude, and phase, indicating an excellent potential for distributed acoustic sensing (DAS).

Characterization of interface reflection coefficients using a finite-difference injection technique.

In this paper, a numerical wave field injection technique for characterizing the reflection coefficient of a planar medium interface is proposed. By injecting recorded wave field quantities into a three-dimensional (3D) finite-difference calculation, two key objectives are addressed: first, the recorded wave field is separated into its incident and reflected constituents without the need of spatial Fourier transforms or a temporal separation of incident and reflected parts in the recorded data. Second, the separated constituents are independently extrapolated to the location of the reflecting interface to determine its reflecting properties. The methodology is experimentally validated on 3D laboratory data consisting of reflections from the water-air interface in a water tank and is shown to give accurate results for incidence angles of up to 60°.

Mechanisms for agglomeration and deagglomeration following oblique collisions of wet particles.

Previous studies on wetted, particle-particle collisions have been limited to head-on collisions, but in many-particle flows, collisions are inherently oblique. In this work, we explore such oblique collisions experimentally and theoretically. Whereas in normal collisions particles rebound only due to solid deformation, we observe in oblique collisions a new outcome where the particles initially form a rotating doublet and then deagglomerate at a later time due to so-called centrifugal forces. Surprisingly, we discover the essential role of capillary forces in oblique collisions even when the capillary number (viscous over capillary forces) is high. This recognition leads to the introduction of a dimensionless number, the centrifugal number (centrifugal over capillary forces), which together with the previously established Stokes number characterizes the regime map of outcomes. Unexpectedly, we observe a normal restitution coefficient greater than unity at large impact angles, the mechanism for which may also be observed in other agglomerating systems.

Elegance of disordered granular packings: a validation of Edward's hypothesis.

We have found a way to analyze Edwards' density of states for static granular packings in the special case of round, rigid, frictionless grains assuming a constant coordination number. It obtains the most entropic density of single grain states, which predicts several observables including the distribution of contact forces. We compare these results against empirical data obtained in dynamic simulations of granular packings. The agreement is quite good, helping validate the use of statistical mechanics methods in granular physics. The differences between theory and empirics are mainly related to the coordination number, and when the empirical data are sorted by that number we obtain several insights that suggest an underlying elegance in the density of states.