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A unique new removable anechoic system and new acoustic treatment for the Virginia Tech Stability Wind Tunnel is described. The new system consists of a Kevlar-walled acoustic test section flanked by two anechoic chambers. In its new configuration the facility is closed aerodynamically and open acoustically, allowing far-field acoustic measurements with a flow quality comparable to that of a hardwalled wind tunnel. An extensive program of experiments has been conducted to examine the performance of this new hardware under a range of conditions, both to examine the effects of acoustic treatment on overall test-section noise levels and to ascertain the aerodynamic characteristics of the new test section. Noise levels in the test section of the anechoic facility are down by as much as 25 dB compared to the original hard-walled configuration. Lift interference corrections (for a baseline NACA 0012 airfoil) are less than half those expected in an open-jet wind tunnel. Acoustic measurements of airfoil trailing edge noise using a microphone phased array are compared to past experiments conducted on similar airfoils in an open-jet facility.

In this Letter, the tensorial nature of the nonequilibrium dynamics in nonlinear mesoscopic elastic materials is evidenced via multimode resonance experiments. In these experiments the dynamic response, including the spatial variations of velocities and strains, is carefully monitored while the sample is vibrated in a purely longitudinal or a purely torsional mode. By analogy with the fact that such experiments can decouple the elements of the linear elastic tensor, we demonstrate that the parameters quantifying the nonequilibrium dynamics of the material differ substantially for a compressional wave and for a shear wave. This result could lead to further understanding of the nonlinear mechanical phenomena that arise in natural systems as well as to the design and engineering of nonlinear acoustic metamaterials. DOI: 10.1103/PhysRevLett.116.115501 Nonlinear mesoscopic elastic materials [1] exhibit unique and interesting properties related to nonlinear and nonequilibrium dynamics that are relevant to various natural and industrial processes ranging in scales and applications, e.g., the onset of earthquakes and avalanches in geophysics [2][3][4], the aging of infrastructures in civil engineering [5,6], the failure of mechanical parts in industrial settings [7][8][9], bone fragility in the medical field [10][11][12], or the design of novel materials, including nonlinear metamaterials, for shock absorption, acoustic focusing, and energy-harversting systems [13]. These properties include the dependence of wave speed and damping parameters on strain amplitude [5,14,15], slow relaxation [16,17], and hysteresis with end-point memory [18][19][20]. Consolidated (see the work referenced previously) and unconsolidated granular media [21][22][23][24] are of particular interest for laboratory-scale experiments because they can provide reference measurements to study or engineer these properties. The latter, when consisting of disorded bead pack or granular crystal lattices, serves as a simplified paradigm for understanding the key mechanisms responsible for nonequilibrium dynamics whereas the former provides a more complex but faithful representation of realistic systems.In consolidated granular media, nonequilibrium dynamics is thought to originate from the microscopic-sized imperfections (e.g., microcracks, debonding at interfaces, grain contacts, etc.) in the "soft" bond system that connects together mesoscopic-sized "hard" elements (e.g., grains or crystals) [25], with experimental evidence recently presented for thermally damaged samples of concrete [26]. These micro-and mesoscopic features are typically distributed throughout the sample and affect its dynamic response at a macroscopic scale through a process of homogenization, thus offering a rich multiscale problem in material physics. Nonequilibrium dynamics has been quantified experimentally in nonlinear mesoscopic elastic materials, through the nonclassical nonlinear elastic parameter α, by resonant experiments in which a slender bar with free boundary conditions...

International audienceIn this letter, the time reversal mirror is used to focus elastic energy at a prescribed location and to analyze the amplitude dependence of the focus signal, thus providing the nonlinearity of the medium. By varying the frequency content of the focused waveforms, the technique can be used to probe the surface, by penetrating to a depth defined by the wavelength of the focused waves. The validity of this concept is shown in the presence of gradual and distributed damage in concrete by comparing actual results with a reference nonlinear measurement and X ray tomography images

We study theoretically and experimentally the mechanisms of nonlinear and nonequilibrium dynamics in geomaterials through dynamic acoustoelasticity testing. In the proposed theoretical formulation, the classical theory of nonlinear elasticity is extended to include the effects of conditioning. This formulation is adapted to the context of dynamic acoustoelasticity testing in which a low-frequency "pump" wave induces a strain field in the sample and modulates the propagation of a high-frequency "probe" wave. Experiments are conducted to validate the formulation in a long thin bar of Berea sandstone. Several configurations of the pump and probe are examined: the pump successively consists of the first longitudinal and first torsional mode of vibration of the sample while the probe is successively based on (pressure) P and (shear) S waves. The theoretical predictions reproduce many features of the elastic response observed experimentally, in particular, the coupling between nonlinear and nonequilibrium dynamics and the three-dimensional effects resulting from the tensorial nature of elasticity.