Real-Time Signal Acquisition, High Speed Processing and Frequency Analysis in Modern Air Data Measurement Instruments

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Numerical approximation of scalar fields propagation in lossy media. Application to acoustic problems

This doctoral thesis deals with the use of the Finite-Difference Time-Domain (FDTD) method for solving problems of acoustic propagation and scattering. Since this method has been originally developed in the framework of electromagnetism as a stable and reliable numerical scheme for the solution of Maxwell’s equations and since little progress had been conducted towards the direction of applying this background in acoustic wave propagation, it is evident that a considerable need for an analogous technique in acoustics exists. This is the objective of the present work and the whole document is organized as follows. The first chapter constitutes an introductory note, which refers to the most important attempts made in the respective scientific domain as well as to the motivation for the selection of the specific research theme. There is also an analytical description of the content of every individual chapter. In the second chapter, the mechanism of the production and propagation of acoustic waves is presented in order to indicate the basic differences and similarities between electromagnetic and acoustic phenomena. The concept of the finite difference approximation is sufficiently described and finally the first FDTD scheme is properly built for the one-dimensional acoustic problem. This is the simplest case and provides the occasion for a first-hand evaluation of the method’s competence. The third chapter contains the development of the complete, three-dimensional, FDTD scheme in acoustics. In parallel, the crucial topic of the proper termination of the computational domain is discussed, and the technique of the perfectly matched layer (PML), the most effective boundary condition, is implemented in acoustics after the appropriate modifications. Taking into account all the individual characteristics of the acoustic propagation and scattering that could cause instabilities and other problems, we conclude to a stable and effective numerical scheme. The selection of specific problems, for which the exact analytical solutions are possible to be obtained, enables us to proceed to the comparison of the respective results and thus to the verification of the reliability of the suggested scheme. However, the aforementioned absorption technique of the PML does not work in cases where lossy media are concerned. This is of vital importance in acoustics as in most case losses due to thermal conduction and viscosity effects must be taken into account. The solution to this problem constitutes the content of the fourth chapter. Based on the idea of the stretched coordinates we introduce a generalized PML method that performs significant absorption of traveling acoustic waves for both lossless and lossy media. Numerical results support the suggested PML theory as well as reveal the proper modifications, which lead to the achievement of the optimum absorbing boundary condition. In the last chapter the proposed methodology is extended to more complicated problems, such as guided sound waves in ducts and rooms. The general characteristics of sound propagation in ducted domains are studied and the analytical solution in the simplest case of the rectangular duct with rigid walls is derived. The application of the developed FDTD scheme permits us to verify its effectiveness and to move further in cases with bent ducts or pipes of varying cross-section and Y junctions with flexible or rigid walls

Hydraulic Modeling of Drafi drainage basin (Attica) and calculation of flood hazard with model Mike 11 for differnet scenarios

114 σ.Εθνικό Μετσόβιο Πολυτεχνείο--Μεταπτυχιακή Εργασία. Διεπιστημονικό-Διατμηματικό Πρόγραμμα Μεταπτυχιακών Σπουδών (Δ.Π.Μ.Σ.) “Επιστήμη και Τεχνολογία Υδατικών Πόρων”Στη παρούσα διπλωματική εργασία μελετήθηκε η υδραυλική συμπεριφορά της περιοχής μελέτης και η επίδραση της οικιστικής ανάπτυξης και της κλιματικής αλλαγής στη πλημμυρική επικινδυνότητα. Επιλέχθηκε ως περιοχή μελέτης ένα κομμάτι του περιαστικού περιβάλλοντος της Ραφήνας, ο οικισμός του Ντραφίου, ο οποίος είναι οικοδομημένος στους νοτιαανατολικούς πρόποδες του Πεντελικού όρους. Η έντονη αστικοποίηση των τελευταίων ετών στην ευρύτερη περιοχή σε συνδυασμό με το φαινόμενο της κλιματικής αλλαγής έχουν ώς συνέπεια τη σταδιακή οικολογική υποβάθμιση της ευρύτερης περιοχής η οποία είναι επιρρεπής σε πυρκαγιές και πλημμύρες. Παρατέθηκαν ορισμένα χαρακτηριστικά παραδείγματα πλημμυρών που συνέβησαν στην Ευρώπη τα τελευταία χρόνια και είχαν ως αποτέλεσμα απώλειες ανθρώπινων ζωών και υλικές ζημιές σε κατοικίες και έργα υποδομών. Παρουσιάστηκε το Ευρωπαικό Θεσμικό Πλαίσιο γύρω απο τις πλημμύρες και τον τρόπο αντιμετώπισης αυτών με έμφαση στη πρόγνωση πλημμυρικών φαινομένων και στη πρόληψη των φυσικών καταστροφών στο ανθρωπογενές περιβάλλον. Τεκμηριώθηκε η επιλογή του λογισμικού Mike 11 by DHI προκειμένου να προσομοιωθεί υδραυλικά η λεκάνη απορροής του Ντραφίου και να εξαχθούν χάρτες πλημμυρικού κινδύνου όπου θα φαίνεται η χωρική κατανομή του βάθους του νερού. Έγινε σύγκριση λογισμικών που χρησιμοποιούνται ευρέως ως εναλλακτικά «εργαλεία» προσομοίωσης πλημμυρών και εν τέλει διευκρινίστηκε γιατί επιλέχθηκε το Mike 11 by DHI. Παρουσιάστηκε αναλυτικά η διαδικασία «στησίματος» και βαθμονόμησης του μοντέλου προσομοίωσης. Ακολούθησε η υδραυλική προσομοίωση της περιοχής και η κατάρτιση χαρτών πλημμυρικού κινδύνου για σενάρια πλημμυρών όπως υποδεικνύει η Κοινοτική Οδηγία 2007/60 (Τ= 10, 100, 1000 έτη). Τέλος, παρατίθενται τα συμπεράσματα καθώς και προτάσεις για περαιτέρω έρευνα. Η υδραυλική προσομοίωση σε συνθήκες σχεδόν μόνιμης ροής κρίθηκε αρκετά ικανοποιητική. Στους χάρτες πλημμυρικού κινδύνου παρατηρήθηκε ότι όσο αυξάνεται η «ένταση» του πλημμυρικού φαινομένου μεγαλώνει το βάθος του νερού στα κατακλυζόμενα πεδία ενώ η έκταση αυτών παραμένει σχεδόν η ίδια.Within the framework of this master thesis aim was to develop a methodology to quantify the flood risk in a suburban environment. Understanding the phenomenon of urban development and climate change in the suburban environment in relation to the flooding. Understanding the legal regime for the management of flood risk within the European Union. Selection of simulation model. Determination of case study. Setting up and calibration of the model. Creation of flood risk maps for different scenarios according to EU Directive 2007/60/EC. In this thesis we studied the hydraulic behavior of the study area and the effect of residential development and climate change on flood risk. Floods are causing more economic losses and affecting more human lives than any other natural disasters in Europe and around the world. The intention of the EU Flood Directive is to alleviate flood damages through systematic planning based on the best science and technology available. The chosen is the settlement of Drafi in Rafina Greece which is built at the southeast foot of Penteli Mountain. The intense urbanization in recent years in the region combined with the phenomenon of climate change have as result the gradual ecological deterioration in the region which is prone to fires and floods. The thesis presents examples of flooding that occurred in Europe in recent years and resulted in losses of lives and damage to housing and infrastructure projects. It also discusses the European Institutional Framework addressing flooding with an emphasis of flood forecast and prevention of natural disasters in human environment.Mike 11 was used to undertake hydraulic simulation of the catchment of Drafi and draw flood hazard maps showing the spatial distribution of the depth of the water. The choice of Mike 11 by DHI as the model of choice is discussed in comparison with other software packages that are widely used as alternative flood simulation tools. The thesis presents in detail the process of setting up and calibrating the simulation model. Then the flood risk maps for flood scenarios in accordance with the EU Floods Directive (2007/60). For return periods of T=10, 100 and 1000 years. Finally the conclusions and suggestions for further research were presented. Mike 11 is a part of the DHI software products based on the Mike Zero platform, comprising a fully Windows integrated Graphical User Interface, which conforms to the evolving standards for Windows based software. Mike 11 is a professional engineering software package for the simulation of flows, water quality and sediment transport in estuaries, rivers, irrigation systems, channels and other water bodies. Also is a user friendly, fully dynamic, one-dimensional modeling tool for the detailed analysis, design, management and operation of both simple and complex river and channel systems. Mike 11 comprises a number of different editors in which data can be implemented and edited independently of each other. As a consequence of the system of separated editor-files, no direct linkage exists between the different editors if they are opened individually. That is, it will not be possible to e. g. view the locations of cross-sections specified in the cross-section file in the Graphical view of the network editor (Plan plot) if these editors are opened individually. The integration and exchange of information between each of the individual data editors is achieved by use of the Mike 11 Simulation Editor. The Simulation Editor serves two purposes. It contains simulation and computation control parameters and is used to start the simulation. It provides a linkage between the graphical view of the network editor and the other Mike 11 editors as illustrated in the figure below. Mike 11 is a tool for Rapid flood mapping and flood zone delineation. Preparation of flood risk and hazard maps. Assessment of economic losses and environmental damage. Flood risk management plans. The study area is a part of suburban environment of Rafina, the settlement of Drafi, which is built at the southeast foot of Penteli Mountain. The thesis collected all those characteristics that are associated with the phenomenon under study. Displays an intense urbanization in the latest years combined with the effects of climate change which has lead to gradual environmental degradation of the region which is prone to fires and floods. In order to simulate the hydraulic behavior of the area we used the software Mike 11 by DHI and there was a hydraulic simulation of river basin of Drafi. The model was set up using data bases as described below. River topography (definition of river bed and river banks). Definition of cross-sections including the floodplain in several positions along the river. Time series of discharge as an upstream boundary condition. Regarding the roughness coefficient Manning’s n was used the default value of the software. During the model calibration the area was divided into four zones according to land cover properties and representative values for the Manning’s roughness coefficient n were selected for each cross-section (river bed and floodplain sides). Time series of discharge were introduced to the model in order to study the model performance for different scenarios. The selected scenarios were according to the indication of the E. U Flood Directive 2007/60. As a result of this simulation the following flood risk maps for return periods T=10, 100 and 1000 years respectively were exported. The hydraulic simulation in almost permanent flow conditions was considered quite satisfactory in terms of accuracy, simulation speed and simplicity in setting up the model. In flood hazard maps it can be observed that as the intensity of the flood phenomenon increases the water depth in floodplains is increased as well while the floodplain extent remains almost the same. Limiting factor in the visualization of the actual flood in some sections was the fact that considered a section width of 300 m. In order to be more accurate wider sections (extending over e.g. 400, 500 m) need to be inserted at certain locations. This was not the case in this thesis due to program license restrictions. The area of Drafi has not been affected recently by a flood phenomenon of similar scale to the examined flood scenarios. The results are indicative of the negative consequences that will occur in the floodplain area if some of the possible scenarios will take place.Απόστολος Δ. Κατσίμπα