HIGH-RESOLUTION GEOMORPHOLOGICAL MAPPING OF THE SHALLOW CONTINENTAL SHELF WEST OF THE KAVALA BAY, NORTH AEGEAN

Σημαντικές γεωμορφολογικές δομές του ρηχού τμήματος της υφαλοκρηπίδας δυτικά του Κόλπου της Καβάλας χαρτογραφήθηκαν χρησιμοποιώντας τα δεδομένα από μια υδρογραφική αποτύπωση (τον Ιούνιο 2014) 320 ναυτικών μιλίων, η οποία περιελάμβανε υψηλής διακριτικότητας πολυδεσμική βαθυμετρική καταγραφή και διασκόπηση πυθμένα με σεισμική ανάκλαση. Αναγνωρίστηκε ένα σύστημα ρηγμάτων αποτελούμενο από ένα σετ δυο κυρίων κανονικών ρηγμάτων (καταγεγραμμένο μήκος και μετρημένο κατακόρυφο άλμα αυτών: 12 χλμ, 5 χλμ και > 40 μ, 25 μ, αντίστοιχα,) με έντονη επιφανειακή εκδήλωση στο θαλάσσιο πυθμένα, καθώς και τρία δευτερεύοντα ρήγματα νότια των κύριων ρηγμάτων, τα οποία φανερώνουν συνιζηματογενή τεκτονισμό. Η εντυπωσιακή διαφορά στις υφές των ιζημάτων που καλύπτουν αφενός το υποκείμενο ρηξιτέμαχος του βορειότερου κυρίου ρήγματος και αφετέρου την οροφή του νοτιότερου κυρίου ρήγματος δείχνει τη σημαντική επίδραση του τεκτονισμού στις ιζηματολογκές διεργασίες της περιοχής μελέτης. Όσον αφορά τις υπάρχουσες γεωμορφές, οι περισσότερο ενδιαφέρουσες είναι εκείνες των αμμωδών θινών στο βορειοανατολικό τμήμα της περιοχής μελέτης, ευρισκόμενες σε βάθη από 25 μ μέχρι τουλάχιστον 65 μ. Οι μεγάλες διαστάσεις τους καθώς και ο προσανατολισμός τους ως προς την ακτογραμμή υποδηλώνουν ως μηχανισμό σχηματισμού τους την δράση ισχυρών πυθμιαίων ρευμάτωνProminent geomorphological features of the shallow continental shelf west of the Kavala Bay (Loutra Eleftheron-Nea Peramos) were mapped using the data from a hydrographic survey (June 2014) of 320 nautical miles during which high resolution multibeam bathymetry and seismic-reflection subbottom profiling were carried out simultaneously. A fault zone comprised by a set of two primary sigmoidal gravity faults (recorded lengths and measured offsets: 12 km, 5 km and > 40 m, 25 m, respectively), with distinct expression on the seabed, and three other secondary gravity faults situated southern of the major faults, revealing synsedimentary tectonics, was identified. The striking difference between the texture of the footwall block sediments of the northern major fault and the texture of the sediments occupying the deep hanging wall block of the southern major fault emphasizes the impact of local tectonics on the sedimentary evolution of the study area. Concerning the observed bedforms, the most interesting were the sand dunes occurring at depths from 25 m to 65 m at least and occupying the northeast part of the study area. Their large dimensions and orientation in relation to the coastline position imply as a mechanism for their formation intense bottom-current activity

A parametric study on the dynamic response of planar multibody systems with multiple clearance joints

A general methodology for dynamic modeling and analysis of multibody systems with multiple clearance joints is presented and discussed in this paper. The joint components that constitute a real joint are modeled as colliding bodies, being their behavior influenced by geometric and physical properties of the contacting surfaces. A continuous contact force model, based on the elastic Hertz theory together with a dissipative term, is used to evaluate the intra-joint contact forces. Furthermore, the incorporation of the friction phenomenon, based on the classical Coulomb’s friction law, is also discussed. The suitable contact-impact force models are embedded into the dynamics of multibody systems methodologies. An elementary mechanical system is used to demonstrate the accuracy and efficiency of the presented approach, and to discuss the main assumptions and procedures adopted. Different test scenarios are considered with the purpose of performing a parametric study for quantifying the influence of the clearance size, input crank speed and number of clearance joints on the dynamic response of multibody systems with multiple clearance joints. Additionally, the total computation time consumed in each simulation is evaluated in order to test the computational accuracy and efficiency of the presented approach. From the main results obtained in this study, it can be drawn that clearance size and the operating conditions play a crucial role in predicting accurately the dynamic responses of multibody systems.Fundação para a Ciência e a Tecnologia (FCT

On the contact detection for contact-impact analysis in multibody systems

One of the most important and complex parts of the simulation of multibody systems with contact-impact involves the detection of the precise instant of impact. In general, the periods of contact are very small and, therefore, the selection of the time step for the integration of the time derivatives of the state variables plays a crucial role in the dynamics of multibody systems. The conservative approach is to use very small time steps throughout the analysis. However, this solution is not efficient from the computational view point. When variable time step integration algorithms are used and the pre-impact dynamics does not involve high-frequencies the integration algorithms may use larger time steps and the contact between two surfaces may start with initial penetrations that are artificially high. This fact leads either to a stall of the integration algorithm or to contact forces that are physically impossible which, in turn, lead to post-impact dynamics that is unrelated to the physical problem. The main purpose of this work is to present a general and comprehensive approach to automatically adjust the time step, in variable time step integration algorithms, in the vicinity of contact of multibody systems. The proposed methodology ensures that for any impact in a multibody system the time step of the integration is such that any initial penetration is below any prescribed threshold. In the case of the start of contact, and after a time step is complete, the numerical error control of the selected integration algorithm is forced to handle the physical criteria to accept/reject time steps in equal terms with the numerical error control that it normally uses. The main features of this approach are the simplicity of its computational implementation, its good computational efficiency and its ability to deal with the transitions between non contact and contact situations in multibody dynamics. A demonstration case provides the results that support the discussion and show the validity of the proposed methodology.Fundação para a Ciência e a Tecnologia (FCT