Sandwich plates are certain specific type of composites, which are widely used in modern engineering. Due to their complicated multi-layered structure, in literature one can find many different modelling approaches: from a simple double-plate system to a complex theories based on complicated deformation hypothesis. In this paper the modelling of vibrations of sandwich plates is considered, as a brief resume of assumptions of several models along with their governing equations of motion are presented. In the calculation example the free vibrations of a certain sandwich plate strip are investigated with the use of all described models and additionally with the use of FEM model. Eventually, basing on the presented comparison of the obtained results, the discussion on the correctness of each modelling assumption is made.
Assessment of Baltic herring abundance can be carried out using acoustic techniques. Analysis of the relationship between the Baltic herring individual target strength, TS, and the total fish length, L, important for the acoustic assessment, showed the relationship to be dependent on the location of the study area. This finding motivated a detailed analysis of the relationship for the herring occurring in the southern Baltic ICES subdivisions 24, 25, and 26, in which Poland is responsible for the acoustic herring abundance assessment. The paper addresses the numerical analysis of the backscattering properties of the southern Baltic herring. The modelling approach used to study the backscattering was based on the Modal Series-Based Deformed Cylinder Model approximation. Details of the individual herring swimbladders were taken into account. Morphometric data necessary for the analysis was obtained from X-ray images of herring from the Polish coastal zone (ICES subdivision 26).
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