Gas phase diagnostics by laser-induced gratings I. theory

Electrostriction and collisional thermalization of absorbed laser energy are the two dominant mechanisms leading to the formation of laser-induced gratings (LIGs) in the gas phase. In this article the results of the theoretical investigations that have been achieved in the past ten years at the Paul Scherrer Institute on this issue are summarized and yield a comprehensive understanding of the underlying physical concepts. Furthermore, a study of the influence of various parameters, such as the alignment and the spatial intensity profile of the beams on the generated electrostrictive and thermal signal is presented for the first time to the authors' knowledge. The variations of the refractive index responsible for the appearance of laser-induced gratings have been theoretically described by solving the linearized hydrodynamic equations. The contributions from electrostriction, as well as from instantaneous and slow relaxation of the absorbed radiation energy into heat is obtained. These expressions are employed for analysis of experimental data presented in the companion paper [1] which is devoted to the application of the technique for diagnostic purposes in the gas phase. Much effort has been undertaken in order to allow a straightforward physical interpretation of the experimental findings of the expressions presented her

On the modeling of sound propagation over multi-impedance discontinuities using a semiempirical diffraction formulation

Several approximate extensions of the semi-empirical De Jong model [ De Jong et al., J. Sound and Vib. 86, 23–46 (1983) ] are considered for the prediction of sound propagation over multiple impedance discontinuities. A limitation in the original formulation of the De Jong model is highlighted and a modified form that overcomes this limitation is derived based on reciprocity. This leads to the development of a model for multiple impedance discontinuities that can be used for the investigation of sound-wave propagation above a mixed, striped soft ground that is created by either porous absorbent strips, embedded grooves, or wells with different depths. The accuracy of the model is validated against the boundary element method. It is then used to evaluate the importance of the imaginary part of the admittance of the ground strips on sound attenuation along welled surface. It is shown that the attenuation performance of a surface with multiple impedance discontinuities is high when the imaginary part of the average admittance is large and negative (with an −iωt convention), but the magnitudes of the attenuation peaks are also substantially affected by diffraction from the impedance discontinuities. © 2006 Acoustical Society of Americ

A frequency-independent boundary element method for scattering by two-dimensional screens and apertures

We propose and analyse a hybrid numerical-asymptotic $hp$ boundary element method for time-harmonic scattering of an incident plane wave by an arbitrary collinear array of sound-soft two-dimensional screens. Our method uses an approximation space enriched with oscillatory basis functions, chosen to capture the high frequency asymptotics of the solution. Our numerical results suggest that fi�xed accuracy can be achieved at arbitrarily high frequencies with a frequency-independent computational cost. Our analysis does not capture this observed behaviour completely, but we provide a rigorous frequency-explicit error analysis which proves that the method converges exponentially as the number of degrees of freedom $N$ increases, and that to achieve any desired accuracy it is sufficient to increase $N$ in proportion to the square of the logarithm of the frequency as the frequency increases (standard boundary element methods require $N$ to increase at least linearly with frequency to retain accuracy). We also show how our method can be applied to the complementary "breakwater" problem of propagation through an aperture in an infinite sound-hard screen

Prediction and reduction of rotor broadband noise

Prediction techniques which can be or have been applied to subsonic rotors, and methods for designing helicopter rotors for reduced broadband noise generation are summarized. It is shown how detailed physical models of the noise source can be used to identify approaches to noise control

Transonic blade-vortex interactions noise: A parametric study

Transonic Blade-Vortex Interactions (BVI) are simulated numerically and the noise mechanisms are investigated. The 2-D high frequency transonic small disturbance equation is solved numerically (VTRAN2 code). An Alternating Direction Implicit (ADI) scheme with monotone switches is used; viscous effects are included on the boundary and the vortex is simulated by the cloud-in-cell method. The Kirchoff method is used for the extension of the numerical 2-D near field aerodynamic results to the linear acoustic 3-D far field. The viscous effect (shock/boundary layer interaction) on BVI is investigated. The different types of shock motion are identified and compared. Two important disturbances with different directivity exist in the pressure signal and are believed to be related to the fluctuating lift and drag forces. Noise directivity for different cases is shown. The maximum radiation occurs at an angle between 60 and 90 deg below the horizontal for an airfoil fixed coordinate system and depends on the details of the airfoil shape. Different airfoil shapes are studied and classified according to the BVI noise produced

Piezoelectric Electromechanical Transducers for Underwater Sound, Part II

The book presents a broad-scope analysis of piezoelectric electromechanical transducers and the related aspects of practical transducer design for underwater applications. It uses an energy method for analyzing transducer problems that provides the physical insight important for the understanding of electromechanical devices. Application of the method is first illustrated with transducer examples that can be modeled as systems with a single degree of freedom, (such as spheres, short cylinders, bars and flexural disks and plates made of piezoelectric ceramics). Thereupon, transducers are modeled as devices with multiple degrees of freedom. In all these cases, results of modeling are presented in the form of equivalent electromechanical circuits convenient for the calculation of the transducers’ operational characteristics. Special focus is made on the effects of coupled vibrations in mechanical systems on transducer performance. The book also provides extensive coverage of acoustic radiation including acoustic interaction between the transducers. The book is inherently multidisciplinary. It provides essential background regarding the vibration of elastic passive and piezoelectric bodies, piezoelectricity, acoustic radiation, and transducer characterization. Scientists and engineers working in the field of electroacoustics and those involved in education in the field will find this material useful not only for underwater acoustics, but also for electromechanics, energy conversion and medical ultrasonics. Part II contains general information on vibration of mechanical systems, electromechanical conversion in the deformed piezoceramic bodies, and acoustic radiation that can be used independently for treatment transducers of different type