Marcel C. Remillieux

Calibration and Demonstration of the New Virginia Tech Anechoic Wind Tunnel

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.

Vibro-acoustic response of an infinite, rib-stiffened, thick-plate assembly using finite-element analysis

The vibration of and sound radiation from an infinite, fluid-loaded, thick-plate assembly stiffened periodically with ribs are investigated numerically using finite-element analysis. First, numerical simulations are compared to the analytical solutions presented recently for this particular problem [Hull and Welch, J. Sound Vib. 329, 4192-4211 (2010)]. It is shown that the solutions reported in this reference are partially incorrect because the number of modes was not chosen correctly. Subsequently, the numerical model is used to study the effect of repeated and equally spaced void inclusions on the vibro-acoustic response of the system.

Decoupling Nonclassical Nonlinear Behavior of Elastic Wave Types

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.

Resonant ultrasound spectroscopy for materials with high damping and samples of arbitrary geometry

Resonant ultrasound spectroscopy (RUS) is a powerful and established technique for measuring elastic constants of a material with general anisotropy. The first step of this technique consists of extracting resonance frequencies and damping from the vibrational frequency spectrum measured on a sample with free boundary conditions. An inversion technique is then used to retrieve the elastic tensor from the measured resonance frequencies. As originally developed, RUS has been mostly applicable to (i) materials with small damping such that the resonances of the sample are well separated and (ii) samples with simple geometries for which analytical solutions exist. In this paper, these limitations are addressed with a new RUS approach adapted to materials with high damping and samples of arbitrary geometry. Resonances are extracted by fitting a sum of exponentially damped sinusoids to the measured frequency spectrum. The inversion of the elastic tensor is achieved with a genetic algorithm, which allows searching for a global minimum within a discrete and relatively wide solution space. First, the accuracy of the proposed approach is evaluated against numerical data simulated for samples with isotropic symmetry and transversely isotropic symmetry. Subsequently, the applicability of the approach is demonstrated using experimental data collected on a composite structure consisting of a cylindrical sample of Berea sandstone glued to a large piezoelectric disk. In the proposed experiments, RUS is further enhanced by the use of a 3-D laser vibrometer allowing the visualization of most of the modes in the frequency band studied.

Novel Kevlar-Walled Wind Tunnel for Aeroacoustic Testing of a Landing Gear

which allowed aeroacoustic measurements to be carried out in the far field and in an environment with significantly less reflections. The model was a very faithful replica of the full-scale landing gear, designed to address the issues associated with low-fidelity models. A 63-element microphone phased array was used to locate the noise-source components of the landing gear from different streamwise positions, both in the near and far fields. The same landing-gear model was previously tested in the original hard-walled configuration of the tunnel with the same phased array mounted on the wall of the test section (i.e., near-field position). The new anechoic configuration of the Virginia Polytechnic Institute and State University wind tunnel offered a unique opportunity to directly compare data collected in hard-walled and semi-anechoic test sections, using the same landing-gear model and phased-array instrumentation. Through these tests, some of the limitations associated with testing in hard-walled wind tunnels were addressed. Nomenclature Cj = components of the steering vector d = distance from the array to the model f = frequency ffull-scale = full-scale frequency fmeasured = measured frequency k = wave number M = flow Mach number rj = distance traveled by an acoustic ray from the grid point with coordinates xn to array microphone j Rj = distance between the grid point with coordinates xn and array microphone j xn = coordinates of the grid point to which the array is being steered � = wavelength

Damage imaging in a laminated composite plate using an air-coupled time reversal mirror

We demonstrate the possibility of selectively imaging the features of a barely visible impact damage in a laminated composite plate by using an air-coupled time reversal mirror. The mirror consists of a number of piezoelectric transducers affixed to wedges of power law profiles, which act as unconventional matching layers. The transducers are enclosed in a hollow reverberant cavity with an opening to allow progressive emission of the ultrasonic wave field towards the composite plate. The principle of time reversal is used to focus elastic waves at each point of a scanning grid spanning the surface of the plate, thus allowing localized inspection at each of these points. The proposed device and signal processing removes the need to be in direct contact with the plate and reveals the same features as vibrothermography and more features than a C-scan. More importantly, this device can decouple the features of the defect according to their orientation, by selectively focusing vector components of motion into the object, through air. For instance, a delamination can be imaged in one experiment using out-of-plane focusing, whereas a crack can be imaged in a separate experiment using in-plane focusing. This capability, inherited from the principle of time reversal, cannot be found in conventional air-coupled transducers.

Improving the air coupling of bulk piezoelectric transducers with wedges of power-law profiles: A numerical study

An air-coupled ultrasonic transducer is created by bonding a bulk piezoelectric element onto the surface of a thick plate with a wedge of power-law profile. The wedge is used to improve the ultrasonic radiation efficiency. The power-law profile provides a smooth, impedance-matching transition for the mechanical energy to be transferred from the thick plate to the air, through the large-amplitude flexural waves observed in the thinnest region of the wedge. The performance of the proposed transducer is examined numerically and compared to that of a design where the piezoelectric element is isolated and where it is affixed to a thin plate of uniform thickness. The numerical analysis is first focused on the free-field radiation of the transducers. Then, time-reversal experiments are simulated by placing the transducers inside a cavity of arbitrary shape with some perfectly reflecting boundaries. In addition to time-reversal mirrors, the proposed concept could be integrated in the design of phased arrays and parametric arrays.

Probing material nonlinearity at various depths by time reversal mirrors

In 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.

Noise Reduction of a Model-Scale Landing Gear Measured in the Virginia Tech Aeroacoustic Wind Tunnel

The effectiveness of various fairings for landing gear noise reduction was measured in the Virginia Tech (VT) Stability Wind Tunnel. This wind tunnel was recently upgraded to an aeroacoustic facility, which allowed acoustic measurements to be carried out in the far-field, out of the flow, and in a low reverberant environment. The model was a very faithful replica of the full-scale landing gear, designed to address the issues associated with low-fidelity models. A 63-element microphone phased array was used to locate the noise source components of the landing gear in its baseline and streamlined configurations, and to measure the noise reduction potential of the fairings. Measurements were carried out from two far-field positions on the flyover path of the landing gear. Through a comparison between the noise levels of the landing gear with and without fairing, the noise reduction potential of each fairing could be estimated. The results from these experiments also showed that if phased-array measurements of the landing gear noise are carried out in the near-field, the noise reduction potential of the fairings could be largely overestimated.

Aeroacoustic Study of a 26%-Scale, High-Fidelity, Boeing 777 Main Landing Gear in a Semi-Anechoic-Wind-Tunnel Test Section

Experiments were conducted on a 26%-scale high fidelity Boeing 777 main landing gear in the Virginia Tech (VT) Stability Wind Tunnel. This wind-tunnel was recently upgraded to a semi-anechoic facility, which allowed aeroacoustic measurements to be carried out in the far-field and in an environment with significantly less reflections. The model was a very faithful replica of the full-scale landing gear, designed to address the issues associated with low-fidelity models. A 63-element microphone phased array was used to locate the noise source components of the landing gear on the flyover path, both in the near- and far-field. The same landing gear model was previously tested in the original hard-walled configuration of the VT tunnel with the same phased array mounted on the wall of the test section, i.e. near-field position. The new anechoic configuration of the VT wind tunnel offered a unique opportunity to directly compare, using the same gear model and phased array instrumentation, data collected in hard-walled and semi-anechoic test sections. Through these tests some of the limitations associated with hard-walled wind tunnels were discussed. The tests also allowed the noise source components of the landing gear on the flyover path to be identified. It was shown that noise from the landing gear on the flyover path cannot be characterized by only taking phased array measurement straight under the gear.