Arrays of dry-coupled thickness-shear transducers are often employed in the guided wave sector to inspect pipelines and plate-like structure. The dry coupling permits to dismiss any coupling material between the transducer and the waveguide, but as a drawback a preload must be applied on the transducers to guarantee an effective coupling between the two surfaces. Although the influence of the preload on the natural frequencies is studied in the literature, the frequency response function of a transducer relating the input voltage to the displacement output is not present in the literature. Moreover, the distribution of force on the backing mass and the effect of the preload on the uniformity of vibration of the transducers are still missing. A natural frequency analysis and a forced analysis are then computed numerically with finite element analysis to quantify the influence of the preload on a thickness-shear transducer. Furthermore, these results are compared with experimental results obtained with a Laser Vibrometer. It is then shown how the geometrical layout of the transducer coupled with the preload influences the vibration of the transducer.

We are interested in the vibration prediction for a finite flexural plate lying on a semi-infinite soil, whose surface is free, except under the plate. Both the plate equation and the Navier equations are solved, using their bidimensional spatial Fourier Transforms. A cousin problem is the one of the acoustic radiation of the unbaffled plate, a one velocity problem. In this soil problem, two velocities are taken into account, the soil shear and dilatation velocities, considered as a visco-elastic homogeneous medium. Finally, expanding the plate displacement on its modes, linear systems in plate displacement amplitude are solved. As for the unbaffled acoustic radiation problem, equivalent vibratory radiation impedances set is proposed, totally new, describing the modal coupling between the plate modes and the soil. It is shown, contrary to the acoustic one velocity problem that the sign of the imaginary part of the complex vibratory radiation terms is negative at very low frequency, and positive above, meaning that the soil adds stiffness to the plate at low frequency and mass above. The soil effect on the plate vibration is of first importance, highly decreasing the plate vibration by more than 30 dB even for thick concrete plates.

Inside micro cavities, specific dissipative mechanisms influencing acoustic wave propagation occur due to viscous and heat-conducting nature of the fluid. This work focuses on a possible extension of the so called “Low Reduced Frequency” model for acoustic wave propagation in a thermoviscous fluid. This extension is built starting from geometrical and physical assumptions (boundary layer theory, straight waveguides) and consists in the incorporation of a stationary laminar and subsonic mean flow. The resulting equivalent fluid model provides a new damping coefficient which depends on the Mach number, the shear and thermal wave numbers and the cross-sectional profiles of axial velocity and temperature. The main application area is the study of acoustic attenuation within automotive catalytic converters or also thin fluid layers like cooling systems in small electronic devices. This formulation has been implemented for a simple one dimensional thin tube. Convergence to the original model in the absence of mean flow has been reached and comparisons with variational solutions given by Peat show good agreements.

It has been proposed that microphones mounted flush on the ground surface should measure reduced wind noise levels since the microphone does not disturb the flow and the wind velocity at the ground is small. However, there are pressure fluctuations induced on the ground by the interaction of the turbulent fluctuations above the ground with the average wind velocity gradient. Model calculations of the pressure spectrum under different wind velocity profiles have been developed based on the theoretical work of Kraichnan. 1 Kraichnan calculated the turbulence-shear contribution to pressure fluctuations under boundary layer turbulence with an exponentially varying flow velocity profile. The calculations presented here incorporate measured outdoor spectra of the turbulence and different models of the velocity profile to investigate pressure fluctuation measurements outdoors at the ground surface. The predicted power spectral densities of the pressure are compared to each other and to measurements. The ultimate purpose of this research is to develop means to minimize wind noise interference in acoustic measurements. [Research supported by the U.S. Army TACOM-ARDEC at Picatinny Arsenal, NJ.]

Lock-in occurs between many different types of flow instabilities and structural-acoustic resonators. Factors that describe the coupling between the fluid and structure have been defined for low flow Mach numbers. This paper discusses how different flow instabilities influence lock-in experimentally and analytically. A key concept to the lock-in process is the relative source generation versus dissipation. The type of fluid instability source dominates the generation component of the process, so a comparison between a cavity shear layer instability with a relatively stronger source, for example wake vortex shedding from a bluff body, will be described as a coupling factor. In the fluid-elastic cavity lock-in case, the shear layer instability produced by flow over a cavity couples to the elastic structure containing the cavity. In this study, this type of lock-in was not achieved experimentally. A stronger source, vortex shedding from a bluff body however, is shown experimentally to locks into the same resonator. This study shows that fluid-elastic cavity lock-in is unlikely to occur given the critical level of damping that exists for a submerged structure and the relatively weak source strength that a cavity produces. Also in this paper, a unified theory is presented based on describing functions, a nonlinear control theory used to predict limit cycles of oscillation, where a self-sustaining oscillation or lock-in is possible. The describing function models capture the primary characteristics of the instability mechanisms, are consistent with Strouhal frequency concepts, capture damping, and are consistent with mass-damping concepts from wake oscillator theory. This study shows a strong consistency between the analytical models and experimental results.

This Part-2 paper applies Part 1’s theory of sheared rapid distortions to compute broadband noise from flow over large roughness elements, and compares those calculations to recent wind-tunnel measurements. The calculations suggest that shear effects are subdominant in the sound-production process. Post-processing of computed results brings out key features of the theory’s non-equilibrium distorting turbulence. A follow-up analysis makes possible the physical interpretation of the measured acoustic spectral densities in terms of the kinematics of the spatially non-uniform carrier flow.

This is the first of a two-part paper that lays out a theory of broadband noise from rough surfaces and uses it to interpret recent experimental data. The analysis in Part 1 is based on an application of Rapid Distortion Theory to an incident field of correlated micro-velocities defined upstream of the roughened region. Those velocity disturbances are linked to the frequency-wavenumber spectrum of wall pressures from a standard model of turbulent boundary-layer flow. The field of incident microvelocities distorts as it is convected irregularly around and over the roughness elements and thereby generates the predicted broadband sound. Computed results in the Part-2 paper will gauge the role of a boundary layer’s mean shear in the noise-production process relative to the rapid distortions carried by an artificially irrotational mean flow (for reasons to be described in Part 2, shear effects turn out to be insignificant for the application of immediate interest). Calculations in support of recent measurements will be presented for a range of operating conditions and for their associated set of dimensionless scaling parameters.