Acoustic Mode Conversion Research Articles

Multimode photoacoustic characterization of subsurface damage in ground thin wafers

A multimode photoacoustic method has been applied for evaluating the degree of subsurface damage (SSD) in the ground silicon wafer. Mode conversion of laser-generated broadband guided waves is caused by wafer thinning. Bayesian inversion is applied to quantify the effective elastic constants of the SSD layer together with the corresponding uncertainty estimates from the experimental dispersion data of multimode guided waves. Laser ultrasonic experiments have been performed on the polished and ground silicon wafers with different thicknesses to determine the ability of the method to characterize SSD and the effect of acoustic mode conversion on the quantization results. The experimental results show that the mode of the laser-generated broadband guided waves will gradually change from Rayleigh wave to Lamb wave as the wafer is thinned. Both the Rayleigh mode and the A0 Lamb mode can accurately estimate the effective elastic constants of SSD, but the A0 Lamb mode leads to longer confidence intervals, which has been theoretically explained.

Acoustic wave propagation mechanism in the local stiffened structure of space station cabin using finite element method

Acoustic-based leakage detection and location in a spacecraft has attracted many researchers’ interest. Due to the fact that wave propagation through the stiffened structure is too complicated to be comprehensively understood, current leakage detection technologies have low-quality performance. Using numerical simulations, this paper parametrically investigates wave propagation across stiffened structure under different configurations. First, wave propagation through the stiffened structures with different stiffener heights is analyzed, showing that acoustic propagation features are different from Reusser’s model. Second, the stiffened structures with different widths are analyzed; meanwhile, the two cases of too large and too small stiffened ratios were compared. Acoustic wave propagation paths become complicated due to acoustic mode conversions. Third, different configurations of structure types are addressed. When the stiffener’s height–width ratio was large, the acoustic wave first reached the bottom of the stiffened block and then traveled up to the top of the stiffened block. When the stiffener’s height–width ratio was small, the acoustic wave reached the incident side at the bottom of the stiffened block and then propagated separately.

Tonal Fan-Noise Radiation From Aero-Engine Bypass With Serrated End Treatments

Abstract Chevrons, which are also known as serrations, are initially developed to suppress jet noise radiating from aero-engine nozzles. The associated fluid mechanics are already well known. Compared with jet noise, turbomachinery fan noise has become relatively more important along with the ever-increasing bypass ratio. However, it is still unclear whether the trailing-edge chevrons on the bypass duct would attenuate fan noise and, if the answer is yes, what is the associated mechanism. In this work, we first use a theoretical model based on the Wiener–Hopf method to rapidly conduct parametric studies across a number of different setups. The results from such a theoretical model suggest that the chevrons are also effective in the reduction of fan noise scattering. Next, we perform high-fidelity computational fluid and acoustic simulations for a realistic aero-engine with some representative setups, and the results further confirm the effectiveness of chevrons. Both analytical and numerical results show the associated noise control mechanism, that is, chevrons would induce acoustic mode conversion (especially from low modes to high modes), which shall further result in evanescent waves in the radial direction and the final noise reduction at various radiation angles. The findings may find applications in the next-generation low-noise aero-engine design.

Robust modeling of acoustic phonon transmission in nanomechanical structures

The transmission of acoustic phonons is an important element in the design and performance of nano-mechanical devices operating in the mesoscopic limit. Analytic expressions for the power transmission coefficient, T, exist only in the low-frequency limit, in cases described by thin-plate elasticity theory, and for well-defined elastic waveguiding geometries. We compare two numerical techniques based on finite-element computations to determine the frequency dependence of T for arbitrary phonon scattering structures. Both methods take into account acoustic mode conversion to acoustic and optical modes. In one case, the phase and amplitude of complex-valued reflected waves are determined and related to transmission through a Fresnel equation, while in the other, the magnitude of the transmitted mechanical power is directly calculated. The numerical robustness of these methods is demonstrated by considering the transmission across an abrupt junction in a rectangular elastic beam, a well-known problem of considerable importance in mesoscopic device physics. The simulations presented extend the standard results for acoustic phonon transmission at an abrupt junction, and are in good agreement with analytic predictions in the long-wavelength limit. More generally, the numerical methods developed provide an effective tool for calculating acoustic mode energy loss in nano-mechanical resonators through mode conversion and heat transfer in arbitrary mesoscopic structures.

Air-Coupled Lamb and Rayleigh Waves for Remote NDE of Defects and Material Elastic Properties

Conventional air-coupled ultrasound (ACU) is a well-established tool for acoustic NDT and material characterization. Its major shortcoming is concerned with a week penetration into solid materials due to a severe impedance mismatch at the air-solid interface. A dramatic rise in acoustic coupling is obtained by using acoustic mode conversion into plate and surface waves in slanted configurations. In our experiments, an increase of the ultrasound amplitude by up to one order of magnitude was observed in various materials (metals, wood, concrete, composite) under phase matching conditions. On this basis, fully air-coupled configurations are developed and applied for non-contact NDT. The methods based on this principle enable precise measurements of fibre directions and quantification of in-plane anisotropy in composites and natural materials, elastic depth profiling, drying of coatings, advanced imaging of cracked defects and delaminations.

Generation and Detection of Shear Acoustic Waves in Metal Submicrometric Films with Ultrashort Laser Pulses

We present experimental and calculational results demonstrating the thermoelastic generation of shear acoustic waves using femtosecond laser pulses in submicrometric isotropic aluminum films. We show that the generation of the shear waves is correlated to the reduction of the width of the optoacoustic source on the surface. The presence of shear waves is related to acoustic diffraction and acoustic mode conversion at the thin film interfaces.

Analysis of coupled oceanographic and acoustic soliton simulations in the Yellow Sea: a search for soliton-induced resonances

The area south of the Shandong peninsula has been of great interest to both oceanographers and underwater acousticians due to the measurements of Zhou et al. [J. Acoust. Soc. Am. 90 (1991) 2042]. Over a period of years, in the summer, they observed anomalous losses in shallow water acoustic signals. They attributed the losses to acoustic mode conversions produced by solitary internal waves (solitons). Their hypothesis remains unsubstantiated due to a scarcity of oceanographic measurements in this area. In this simulation study the non-hydrostatic, 2.5-dimensional Lamb model was used to generate solitons in an adjacent, deeper area than that examined by Zhou et al. [J. Acoust. Soc. Am. 90 (1991) 2042]. Topographic variations and semi-diurnal tidal strength magnitudes were obtained from digital atlases and published data. For summer conditions, the Lamb model simulations showed that the existing semi-diurnal tidal flow over the topographic variations could lead to the formulation of internal bores and solitons. SAR observations of solitons near this region were used to select parameters and initialize the Lamb model. The resulting soliton simulations were comparable to the two-dimensional surface spectra seen in the SAR data. Soliton amplitudes of 2–7 m were indicated. Acoustic studies were made using the highly accurate finite element parabolic equation (FEPE) acoustic model applied to the initial soliton state data generated by the Lamb model. Mode decomposition of the acoustic fields in the deeper region showed that the mode conversions necessary for anomalous signal losses were present. These findings are consistent with the soliton hypothesis made by Zhou et al. [J. Acoust. Soc. Am. 90 (1991) 2042].

Multiple focused-waves production by parametric mixing in a layer with hysteretic quadratic nonlinearity

Nonlinear scattering of a weak probe wave by a strong pump wave in a layer with nonlinearity hysteresis is analysed. It is demonstrated that a spherical probe wave from a point source after nonlinear process of difference frequency excitation is scattered in several foci. Different foci correspond to different frequencies of the transformed wave spectrum. In the case of a plane pump wave and under the condition that nonlinear scattering proceeds without acoustic mode conversion wave front reversal is possible. To achieve wave front reversal in a layer with hysteretic quadratic nonlinearity the pump wave should have a frequency equal to the frequency of the probe wave or to any of its subharmonics.

Dynamic coupling of primitive equation soliton models and acoustic propagation models via acoustic mode analysis

Simulation studies of acoustic mode conversions resulting from underwater acoustic fields propagating through solitons in the Strait of Messina have indicated a novel way of coupling oceanographic models with ocean acoustic models using dynamic feedback from the acoustic analysis. A weakly nonhydrostatic primitive equation model was used to generate simulations of oceanographic soliton fields in the Strait of Messina. The soliton fields were used to study acoustic mode conversions as the acoustic field passed through the solitons and propagated upslope. Conversion into modes having higher bottom loss have shown resonance effects similar to that observed in the Yellow Sea. Oceanographic parameters were adjusted so that amplitude and phase of the first two wavelengths of the simulated soliton fields were in agreement with measured soliton data. However, identical acoustic signals propagated through the simulated and measured soliton fields produced different acoustic mode structures. Oceanographic parameter values were varied until the differences in acoustic predictions were minimized. Thus, a feedback approach was used to dynamically couple the oceanographic and acoustic models. Initial results and the suitability of a two-layer oceanographic model for detailed acoustic studies will be discussed. [Work supported by ONR/NRL and by a High Performance Computing DoD grant.]

Results from a simulation study of mode coupling due to acoustic wave-soliton interactions

There is a strong interest in understanding the interactions between ocean acoustic waves and shallow water solitons. Central to these interactions are the acoustic mode conversions caused by the solitons. Such mode conversions potentially occur each time the acoustic wave and soliton interact; however, at certain combinations of acoustic-oceanographic wavelengths, the interactions can produce large losses in acoustic signal. Mode coupling results will be discussed from simulation studies that examined the signal loss due to interactions with solitons in the Strait of Messina. Coupled mode formulations were avoided so that interpretations of results were not dependent upon any particular mode coupling scheme. Rather, the full-wave, two-way, finite-element model, FFRAME, was used to produce numerically accurate predictions of the complex pressure field as it passed through the soliton. This complex field was then decomposed into its component wave numbers via an FFT to determine the mode structure. This simulation included the entire acoustic field (forward propagation, backscatter, multiple scatter, etc.). The results were realistic and difficult to interpret. Results will be shown at various points along the way as the acoustic wave passed thorugh the soliton. [Work supported by ONR/NRL and by a High Performance Computing DoD grant.]

A concept for remote acoustic monitoring of solitons generated in straits

A search algorithm for resonance anomalies (SARA) has been developed to predict possible resonance frequencies of shallow-water soliton packets as they travel through straits. The algorithm relies on characteristics that accompany large losses due to acoustic wave-soliton interactions: (a) acoustic mode conversions; (b) large signal losses within a narrow band of acoustic frequencies; and, (c) large transmission losses due to strong couplings between lower-order propagation modes and higher-order, bottom-interacting modes. The SARA algorithm has been verified using oceanographic data from the Strait of Messina. As a remote acoustic sensor, the SARA algorithm could be used in an ‘‘inverse mode’’ to predict key oceanographic parameters (e.g., predominant horizontal spatial wave numbers and travel speeds) of those soliton packets that produce large acoustic losses. The parameters would initialize a primitive shallow-water soliton model that generates soliton simulations. The SARA algorithm could be used in a forward fashion to validate the soliton simulations. The concept exploits the unique sloping bathymetry of straits, where natural mode stripping can occur around the sills that generate the solitons. Details will be discussed and computer simulations will be presented that illustrate the feasibility of the approach. [Work supported by ONR/NRL and by a High Performance Computing DoD grant.]

A search algorithm to predict the resonance frequency of a shallow-water soliton packet

Recently a number of researchers have performed acoustic simulations of shallow-water regions that contain large amplitude internal waves (solitons). Their results have confirmed earlier findings: (a) mode conversions, due to acoustic interactions with soliton packets, can produce a large loss in acoustic transmission; (b) this loss occurs within a narrow band of acoustic frequencies (about the resonance frequency); and, (c) the transmission loss is most prominent when there is strong coupling between the lower-order (water-borne) propagation modes and the higher-order, very lossy (bottom interacting) modes. Using this knowledge, development of a search algorithm has begun to predict the resonance frequency of a shallow-water soliton packet. In this proof-of-concept work, a shallow-water soliton model was used to generate replicas of the soliton fields in the Strait of Messina. The allowable acoustic mode conversions were calculated and correlated with the dominate spatial wave numbers of the soliton packet. This was used to predict the resonance frequency. The resonance frequency was later confirmed by making hundreds of acoustic runs. The search algorithm will be discussed and examples shown. [Work supported by ONR/NRL and by a Federal High Performance Computing DoD grant.]

Modeling backscatter from a shallow-water soliton: Relationship to the anomalous resonance effect observed in Yellow Sea data

Computer simulations have confirmed that large amplitude shallow-water internal waves (solitons) can effect transmission loss. Zhou etal. [J. Acoust. Soc. Am. 82, 287–292 (1987)] first hypothesized that solitons caused the anomalous transmission loss observed in acoustic data taken in the Yellow Sea. Others [Chin-Bing etal., Math. Model. Sci. Comput. 4, (1994); King etal., Theoret. Comput. Acoust. 2, 793–807 (1994)] confirmed via computer simulations that interactions of acoustic energy with shallow-water solitons resulted in coupling between the lower-order propagation modes and the very lossy, higher-order modes. Furthermore, a single soliton packet could transfer sufficient acoustic energy from lower-order modes to higher-order bottom-attenuated modes to produce the anomalous transmission loss effects observed in the Zhou data. Recently this simulation study has been extended to include a wave-number analysis of the backscattered field from the soliton. Examples of the backscattered field from the soliton will be presented that include the frequency interval where the forward-field acoustic mode conversion (and corresponding anomalous transmission loss) occur. [Work supported by ONR/NRL and by a Federal High Performance Computing DoD grant.]

Determination of the Mechanical Properties OP Integrated Circuit Packaging Materials using a Focused Acoustic Beam Technique

ABSTRACTA novel method for the measurement of the mechanical properties of integrated circuit interconnection and packaging materials using a conventional acoustic microscope lens, is presented in this paper. This technique allows the simultaneous generation and detection of longitudinal (LW) and shear (SH) acoustic waves in a material system based upon acoustic mode conversion principles at a water-solid interface. Experimental results obtained using a 50 MHz acoustic lens are presented for a range of ceramic materials and compared with published data. Acoustic ray propagation theory is employed to analyse the experimental results.

Photoacoustic generation and detection of 10-ns acoustic pulses in solids

We describe the generation and detection of short acoustic pulses (∼10-ns widths) in opaque plates, using weak laser pulses (less than 1-mJ energy) for excitation. The longitudinal, shear, and surface acoustic waves generated by a single laser pulse can be time resolved by using a thin-film piezoelectric transducer, and acoustic mode conversions at surfaces are clearly observable. This new photoacoustic material testing technique is useful for fast ultrasonic measurements or for detecting deep subsurface flaws.