For the unsteady simulation of compressible subsonic flows (Large Eddy Simulation orDirect Numerical Simulation), the proper handling of the inlet boundary is a challengingtask. Indeed, inflow generation through imposition of the velocity may lead to anon-physical reflection of the upstream acoustic waves. In the present contribution, amethod that allows both filtering of these waves and proper imposition of the requiredvariables is proposed. This method is based on identification of the roles of the temporalrate of change of wave amplitudes at the inlet in the low Mach number regime. Theformulation obtained is tested numerically on unsteady one-dimensional flows at low Machnumber for which the unsteady inlet velocity signal is purely harmonic or harmonic withthe superimposition of synthetic turbulence.

The transient dynamics of evacuated and fluid-filled circular elastic shells, submerged in an infinite fluid medium and subjected to an external weak shock wave,is considered in this paper. This circular shell/acoustic medium interaction problem has already been tackled with simplified thin shell models, based on the Love-Kirchhoff hypotheses for thestructural dynamics. In this case, the resulting radiated pressure field displays some discrepancies related to the A0/S0 waves when compared to the experimental data available inthe literature for the evacuated case. These drawbacks are overcome here by the use of an isotropic elastic model for the structural dynamics and an inviscid acoustic flow for the fluid dynamics,in a two-dimensional framework. The approach is based on the methods of Laplace transform in time, Fourier series expansions and separation of variables in space. For the fluid-filled case,the transient thick shell-weak shock wave interaction problem is explored and the radiated acoustic field described.

The main issue in the present paper is to comment and illustrate on new acoustic examples the method of analysis of modal series, termed “method of orthocomplement”, that has been recently proposed by the authors to improve the convergence of such series. The general method consists in a direct analysis and transformation of the remainders of ordinary series. It results in a family of “hybrid” modal representations involving an ordinary modal sum of order N, a “quasi-static” term based on the N first modes, and an “accelerated” modal series. Using the transformed modal formulae eliminates the Gibbs oscillations – that are attached in infinite dimensional models to modal boundary discontinuities – and also the consequences of such phenomena on finite element approximations. The method is applied in the present paper to plane waves in acoustic tubes and to 3D acoustic fields inside a car compartment, in view of the synthesis of acoustic receptances or impedances to be used in practical acoustic design. The main technical difficulty being the treatment of singular linear boundary problems or systems of linear equations that arise during the study of closed rigid cavities or tubes, a whole section of the paper had thus to be devoted to pseudo-inversion.

The purse-seiners of the Java Sea use light and fish aggregating devices (FAD) to concentrate and catch small pelagic fish (Decapterus sp., Amblygaster sirm, Sardinella gibbosa). This sets a question about the interaction between fish and fishing vessels. Relevant estimates of fishing mortality and fishing effort require an understanding of this interaction. To gain information on this relationship, acoustic surveys were performed on the fishing grounds and the present paper analyses these data. Using acoustic surveys, the fish density was measured and the number of schools was counted per sea surface unit. The fishing vessels were located by radar. Acoustic data were stratified into two groups, one where fishing vessels were present and the other where they were not. This stratification was tested for by resampling the data. Day and night sampled data were analysed in each strata separately. Variograms were computed to characterise spatial structure both for the fish and the purse-seiners. Contingency tables between number of schools and fish density were computed to characterise the relation between biomass and schools for the day and the night when seiners are absent or present. At night, in the absence of purse-seiners, the number of schools increase and fish rise from the bottom. In areas where purse-seiners are present this natural phenomenon is stronger. It also changes the spatial structure both of the fish density and the school abundance. The spatial structure of the vessels is similar to that of the fish density. The fishermen seem to be able to detect by day fish concentrations which will be fished later by night.

L'étude présente une application de la méthode géostatistique transitive à une dimension (1D) pour estimer la précision des évaluations de biomasse effectuées au Sénégal par écho-intégration. Le problème majeur rencontré dans cette étude a été la prise en compte de la variabilité temporelle dans l'estimation de la variance d'échantillonnage. Lors des campagnes acoustiques, l'écho-intégration se fait en continu le long de la route du navire. Lorsque le plan d'échantillonnage est constitué de transects parallèles, cette caractéristique permet de considérer chaque transect comme une unité d'échantillonnage. En faisant la somme des valeurs mesurées le long des transects, l'estimation à 2D est simplement ramenée à une estimation à 1D. Si de plus les transects sont équidistants, la méthode géostatistique transitive à 1D offre une solution correcte et simple au calcul de la variance d'estimation. La méthode a été appliquée à la série des campagnes acoustiques sénégalaises qui ont toutes été effectuées selon le même réseau de transects équidistants et parallèles d'orientation Est-Ouest. Les densités sont sommées le long des transects. Les variations latitudinales de la biomasse par transect sont présentées sous forme de profils à 1D. La biomasse, la répartition spatiale et l'autocorrélation sur ces profils présentent des différences importantes d'une campagne à l'autre. Les profils ont été regroupés en 4 catégories présentant des caractéristiques différentes. L'autocorrélation moyenne (structure) par catégorie a été estimée et modélisée. Une valeur moyenne de l'erreur relative d'estimation est fournie pour chaque catégorie. L'erreur relative d'estimation varie en fonction des différences de structures entre catégories et aussi en fonction de différentes hypothèses pour la modélisation de la structure aux petites distances. Deux cas ont été considérés : continuité de la distribution (absence d'un effet pépite) ou discontinuité (présence d'un effet pépite). L'erreur relative maximale estimée est d'environ 31 %. Le problème majeur rencontré à la modélisation de la structure 1D des biomasses par transect a été de choisir de prendre en compte ou non un effet de pépite. L'interprétation de cette pépite serait la variabilité temporelle à l'échelle de la radiale.

One of the main limitations in acoustic stock assessment is fish identification. We propose here a new method for identifying several communities of tropical fish within a surveyed area. This method, called "Acoustic Populations", consists in splitting a study area in systematic regular strata such as geographical rectangles, and calculating for each stratum a set of acoustic parameters that can be easily obtained with conventional acoustic equipment, and have a discriminant power, such as target strength, mean density, confidence interval and dispersion indexes. It is based on the fact that the biological (specific diversity, physiology) and ethological (gregarism, migrations, etc.) characteristics of fish communities have a particular influence on the echoes recorded, which become characteristical and thus are able to discriminate acoustically several populations. An example of application of the method is given for Eastern Venezuela, and the use of the results for mapping, evaluation and stratification of acoustic data is discussed.