Improvement of beam finite element for ship structural analysis

U radu su opisane formulacije grednog konačnog elementa: hibridna, obična ekscentrična i modificirana ekscentrična koje se najčešće upotrebljavaju u analizi odziva brodskih konstrukcija. Ispitana je njihova točnost kod proračuna čvornih pomaka i normalnog aksijalnog naprezanja u odnosu na model s finom mrežom te je dana i njihova međusobna usporedba. Uočeni su problemi kod proračuna unutrašnje aksijalne sile i s njom povezanih naprezanja kod običnog ekscentričnog grednog elementa te je predložena korekcija. Izveden je i gredni konačni element sa sedam stupnjeva slobode te su pokazani njegovi nedostaci. Na kraju rada je opisan još i ljuskasti element koji je, uz modificirani ekscentrični gredni konačni element, ugrađen u program PLO2X te je prikazan detaljni proračun čvornih pomaka, unutrašnjih sila i naprezanja za promatranu konstrukciju.Hybrid, native eccentric and modified eccentric beam finite element formulations, most commonly employed in the analysis of the strength of ship structures, are described and discussed in this work. Their accuracy in terms of the nodal displacements and normal axial stresses is tested against a fine mesh model and they are also compared to each other. Existence of error in calculation of internal axial force and related stresses when native eccentric beam element is used was demonstrated and certain correction is proposed. A seven-degrees-of-freedom element is presented as well as the negative aspects of its use. At the end shell element implemented in PLO2X software together with modified eccentric beam element is described as well as detailed calculation of nodal degrees of freedom, internal forces and stresses for structure considered in this work

Testni primjer za strukturnu optimizaciju

U ovom radu se razmatraju različite formulacije grednog konačnog elementa koji se upotrebljava u metodi konačnih elemenata. Na početku je dana kratka teorijska osnova svakog od tri razmatrana elementa: hibridnog grednog konačnog elementa te dva ekscentrična gredna konačna elementa (obični ekscentrični gredni konačni element i modificirani ili kompatibilni gredni konačni element). Potom je izrađen jednostavni teorijski model s tri skupine rubnih uvjeta, tri različita slučaja opterećenja te tri različite gustoće mreže konačnih elemenata. Za svaku kombinaciju slučaja opterećenja i rubnih uvjeta prikazani su rezultati proračuna naprezanja, vertikalnih pomaka i parametara podobnosti. Isti proračun je izvršen i na modelu stvarnog broda (Ro-Pax) s prikazanim rezultatima proračuna za jedan uzdužnjak i sponju odabrane palube tog broda. \Na kraju su sve tri formulacije međusobno uspoređene u smislu izračunatih naprezanja, vertikalnih pomaka i parametara podobnosti

Unapređenje grednog konačnog elementa za analizu brodske konstrukcije

U radu su opisane formulacije grednog konačnog elementa: hibridna, obična ekscentrična i modificirana ekscentrična koje se najčešće upotrebljavaju u analizi odziva brodskih konstrukcija. Ispitana je njihova točnost kod proračuna čvornih pomaka i normalnog aksijalnog naprezanja u odnosu na model s finom mrežom te je dana i njihova međusobna usporedba. Uočeni su problemi kod proračuna unutrašnje aksijalne sile i s njom povezanih naprezanja kod običnog ekscentričnog grednog elementa te je predložena korekcija. Izveden je i gredni konačni element sa sedam stupnjeva slobode te su pokazani njegovi nedostaci. Na kraju rada je opisan još i ljuskasti element koji je, uz modificirani ekscentrični gredni konačni element, ugrađen u program PLO2X te je prikazan detaljni proračun čvornih pomaka, unutrašnjih sila i naprezanja za promatranu konstrukciju

Current State of Development of Ship Structural Design and Optimization Methods

This paper presents state-of-the-art methodologies and methods used in the rationally-based structural design of ships and offshore structures, namely design support system, structural optimization, surrogate modelling and sensitivity analysis. It demonstrates their application in structural design of a platform support vessel. It ends with a list of benefits that a structural designer may expect when the presented methods/methodologies are used. It also shows the obstacles to their full implementation in the engineering practice

Current State of Development of Ship Structural Design and Optimization Methods

This paper presents state-of-the-art methodologies and methods used in the rationally-based structural design of ships and offshore structures, namely design support system, structural optimization, surrogate modelling and sensitivity analysis. It demonstrates their application in structural design of a platform support vessel. It ends with a list of benefits that a structural designer may expect when the presented methods/methodologies are used. It also shows the obstacles to their full implementation in the engineering practice

Gaia Focused Product Release: A catalogue of sources around quasars to search for strongly lensed quasars

Context. Strongly lensed quasars are fundamental sources for cosmology. The Gaia space mission covers the entire sky with the unprecedented resolution of $0.18$" in the optical, making it an ideal instrument to search for gravitational lenses down to the limiting magnitude of 21. Nevertheless, the previous Gaia Data Releases are known to be incomplete for small angular separations such as those expected for most lenses. Aims. We present the Data Processing and Analysis Consortium GravLens pipeline, which was built to analyse all Gaia detections around quasars and to cluster them into sources, thus producing a catalogue of secondary sources around each quasar. We analysed the resulting catalogue to produce scores that indicate source configurations that are compatible with strongly lensed quasars. Methods. GravLens uses the DBSCAN unsupervised clustering algorithm to detect sources around quasars. The resulting catalogue of multiplets is then analysed with several methods to identify potential gravitational lenses. We developed and applied an outlier scoring method, a comparison between the average BP and RP spectra of the components, and we also used an extremely randomised tree algorithm. These methods produce scores to identify the most probable configurations and to establish a list of lens candidates. Results. We analysed the environment of 3 760 032 quasars. A total of 4 760 920 sources, including the quasars, were found within 6" of the quasar positions. This list is given in the Gaia archive. In 87\% of cases, the quasar remains a single source, and in 501 385 cases neighbouring sources were detected. We propose a list of 381 lensed candidates, of which we identified 49 as the most promising. Beyond these candidates, the associate tables in this Focused Product Release allow the entire community to explore the unique Gaia data for strong lensing studies further.Comment: 35 pages, 60 figures, accepted for publication by Astronomy and Astrophysic

Pulsations in main sequence OBAF-type stars

CONTEXT: The third Gaia data release provides photometric time series covering 34 months for about 10 million stars. For many of those stars, a characterisation in Fourier space and their variability classification are also provided. This paper focuses on intermediate- to high-mass (IHM) main sequence pulsators (M ≥  1.3 M⊙) of spectral types O, B, A, or F, known as β Cep, slowly pulsating B (SPB), δ Sct, and γ Dor stars. These stars are often multi-periodic and display low amplitudes, making them challenging targets to analyse with sparse time series. AIMS: We investigate the extent to which the sparse Gaia DR3 data can be used to detect OBAF-type pulsators and discriminate them from other types of variables. We aim to probe the empirical instability strips and compare them with theoretical predictions. The most populated variability class is that of the δ Sct variables. For these stars, we aim to confirm their empirical period-luminosity (PL) relation, and verify the relation between their oscillation amplitude and rotation. METHODS: All datasets used in this analysis are part of the Gaia DR3 data release. The photometric time series were used to perform a Fourier analysis, while the global astrophysical parameters necessary for the empirical instability strips were taken from the Gaia DR3 gspphot tables, and the v sin i data were taken from the Gaia DR3 esphs tables. The δ Sct PL relation was derived using the same photometric parallax method as the one recently used to establish the PL relation for classical Cepheids using Gaia data. RESULTS: We show that for nearby OBAF-type pulsators, the Gaia DR3 data are precise and accurate enough to pinpoint them in the Hertzsprung-Russell (HR) diagram. We find empirical instability strips covering broader regions than theoretically predicted. In particular, our study reveals the presence of fast rotating gravity-mode pulsators outside the strips, as well as the co-existence of rotationally modulated variables inside the strips as reported before in the literature. We derive an extensive period–luminosity relation for δ Sct stars and provide evidence that the relation features different regimes depending on the oscillation period. We demonstrate how stellar rotation attenuates the amplitude of the dominant oscillation mode of δ Sct stars. CONCLUSIONS: The Gaia DR3 time-series photometry already allows for the detection of the dominant (non-)radial oscillation mode in about 100 000 intermediate- and high-mass dwarfs across the entire sky. This detection capability will increase as the time series becomes longer, allowing the additional delivery of frequencies and amplitudes of secondary pulsation modes

Improvement of beam finite element for ship structural analysis

U radu su opisane formulacije grednog konačnog elementa: hibridna, obična ekscentrična i modificirana ekscentrična koje se najčešće upotrebljavaju u analizi odziva brodskih konstrukcija. Ispitana je njihova točnost kod proračuna čvornih pomaka i normalnog aksijalnog naprezanja u odnosu na model s finom mrežom te je dana i njihova međusobna usporedba. Uočeni su problemi kod proračuna unutrašnje aksijalne sile i s njom povezanih naprezanja kod običnog ekscentričnog grednog elementa te je predložena korekcija. Izveden je i gredni konačni element sa sedam stupnjeva slobode te su pokazani njegovi nedostaci. Na kraju rada je opisan još i ljuskasti element koji je, uz modificirani ekscentrični gredni konačni element, ugrađen u program PLO2X te je prikazan detaljni proračun čvornih pomaka, unutrašnjih sila i naprezanja za promatranu konstrukciju.Hybrid, native eccentric and modified eccentric beam finite element formulations, most commonly employed in the analysis of the strength of ship structures, are described and discussed in this work. Their accuracy in terms of the nodal displacements and normal axial stresses is tested against a fine mesh model and they are also compared to each other. Existence of error in calculation of internal axial force and related stresses when native eccentric beam element is used was demonstrated and certain correction is proposed. A seven-degrees-of-freedom element is presented as well as the negative aspects of its use. At the end shell element implemented in PLO2X software together with modified eccentric beam element is described as well as detailed calculation of nodal degrees of freedom, internal forces and stresses for structure considered in this work

Improvement of beam finite element for ship structural analysis

U radu su opisane formulacije grednog konačnog elementa: hibridna, obična ekscentrična i modificirana ekscentrična koje se najčešće upotrebljavaju u analizi odziva brodskih konstrukcija. Ispitana je njihova točnost kod proračuna čvornih pomaka i normalnog aksijalnog naprezanja u odnosu na model s finom mrežom te je dana i njihova međusobna usporedba. Uočeni su problemi kod proračuna unutrašnje aksijalne sile i s njom povezanih naprezanja kod običnog ekscentričnog grednog elementa te je predložena korekcija. Izveden je i gredni konačni element sa sedam stupnjeva slobode te su pokazani njegovi nedostaci. Na kraju rada je opisan još i ljuskasti element koji je, uz modificirani ekscentrični gredni konačni element, ugrađen u program PLO2X te je prikazan detaljni proračun čvornih pomaka, unutrašnjih sila i naprezanja za promatranu konstrukciju.Hybrid, native eccentric and modified eccentric beam finite element formulations, most commonly employed in the analysis of the strength of ship structures, are described and discussed in this work. Their accuracy in terms of the nodal displacements and normal axial stresses is tested against a fine mesh model and they are also compared to each other. Existence of error in calculation of internal axial force and related stresses when native eccentric beam element is used was demonstrated and certain correction is proposed. A seven-degrees-of-freedom element is presented as well as the negative aspects of its use. At the end shell element implemented in PLO2X software together with modified eccentric beam element is described as well as detailed calculation of nodal degrees of freedom, internal forces and stresses for structure considered in this work