Academic literature on the topic 'Fluid Viscous Damping'

The cluster structure is visualized and the physical properties of new two types of nano MR fluids are measured in the applied magnetic fields. Correlating to these measurements, the damping characteristics of an oscillating flat plate immersed in two types of nano MR fluids such as damping amplitude, phase difference, viscous damping coefficient and viscous drag force acted on a flat plate are experimentally clarified, comparing with those of commercial magnetic fluid from the fluiddynamic points of view. It is shown that the resonance of damping amplitude and phase difference are very sensitive to the applied magnetic field, and further the damping effect of MR fluid is about ten times stronger than that of the commercial magnetic fluid even in low magnetic fields of 50–100 Gauss due to the robust cluster formation.

Squeeze film damping in systems employing micro-plates parallel to a substrate and undergoing small normal vibrations is theoretically investigated. In high-density fluids, inertia forces may play a significant role affecting the dynamic response of such systems. Previous models of squeeze film damping taking inertia into account do not clearly isolate this effect from viscous damping. Therefore, currently, there is no simple way to distinguish between these two hydrodynamic effects. This paper presents a simple solution for the hydrodynamic force acting on a plate vibrating in an incompressible fluid, with distinctive terms describing inertia and viscous damping. Similar to the damping constant describing viscous losses, an inertia constant, given by ρL 3 W / h (where ρ is fluid density, L and W are plate length and width, respectively, and h is separation distance), may be used to accurately calculate fluid inertia for small oscillation Reynolds numbers. In contrast with viscous forces that suppress the amplitude of the oscillation, it is found that fluid inertia acts as an added mass, shifting the natural frequency of the system to a lower range while having little effect on the amplitude. Dimensionless parameters describing the relative importance of viscous and inertia effects also emerge from the analysis.

Recent rapid advances in microelectromechanical system have facilitated the applications of various oscillating microstructures, where fluid damping plays a critical role and becomes a main cause of energy dissipation. In the present study, we have used lattice Boltzmann method to investigate the Stokes' second problem for non-equilibrium effect.

Damping coefficient and damping exponent play a dominant role on the displacement constraint effect and the damping force of the fluid viscous damper. In this paper, parametric sensitivity study of the fluid viscous damper is carried out by using nonlinear dynamic time history method based on a long suspension bridge. Acceptable damping force and reasonable displacement between stiffening girder and tower are set as the optimization objectives of the damping parameters. It is shown that: as to long suspension bridge, bigger damping coefficient can lower the displacement but increase the force of the dampers, and the smaller damping exponent can aggravate the trend. Fluid viscous damper can play the biggest role of displacement constraint effect while with little additional seismic force is introduced in the whole structure with suitable damping parameters.

Bi-directional-cylinder viscous fluid dampers are studied numerically. Based on the power-law characteristic of silicone oil，the flow pattern of silicone oil is analyzed, the non-Newtonian fluid power-law model’s influence on the energy dissipation characteristic and the damping resistance of viscous dampers is discussed, and the calculation formulas are presented. The damping force of the viscous damper samples is simulated by professional CFD software based on the dynamic mesh model, Based on comprehensive analysis of the results, the experiential design method of large-tonnage-damping-force viscous fluid dampers is finally established. On the other hand, the numerical simulation curves are in good agreement with experimental curves, which validates the feasibility of numerical method in the preliminary design of bi-directional-cylinder viscous fluid dampers. The numerical method helps to enhance the engineering precision, guide the design, and shorten the development cycle.

Vibrations are unwanted in most of the day today usage especially in mechanical machines, civil structures, aerospace industries and automotive parts. It is usually to get the rid of these vibrations by using magneto rheological fluids. A magneto rheological fluid provides viscous damping which gets added up when magnetic field is applied. The damping properties of fluid get multiplied and natural frequency of the body under observation also changes, using this technique the three layered MR fluid sandwich beam was fabricated and tested it for undamped and damped conditions. The controllability of variations in the various dynamic parameters like natural frequencies, vibration amplitudes and damping factors was observed. Keywords: Magnetorheological fluid, MRFluid sandwich Beam, Natural frequency, Damping factor, Damping coefficient.

We consider internal gravity waves in a stratified fluid layer with rigid horizontal boundaries and periodic boundary conditions on the sides at constant temperature with a small constant viscosity, modelled using the incompressible Navier-Stokes equations. Using operator-theoretic methods to study the damping rates of internal waves we prove there are non-oscillatory wave modes with arbitrarily small damping rates. We provide an asymptotic approximation for these non-oscillatory modes. Additionally, we find that the eigenvalues for damped oscillations are in an explicitly describable half-ring.

We theoretically study small-amplitude oscillations of permeable cylinders immersed in an unbounded fluid. Specifically, we examine the effects of oscillation frequency, permeability and shape on the effective mass and damping coefficients, the latter of which is proportional to the power required to sustain the vibrations. Cylinders of circular and elliptical cross-sections undergoing transverse and rotational vibrations are considered. The dynamics of the fluid flow through porous cylinders is assumed to obey the unsteady Brinkman–Debye–Bueche equations. We use a singularity method to analytically calculate the flow field within and around circular cylinders, whereas we introduce a Fourier-pseudospectral method to numerically solve the governing equations for elliptical cylinders. We find that, if rescaled properly, the analytical results for circular cylinders provide very good estimates for the behaviour of elliptical ones over a wide range of conditions. More importantly, our calculations indicate that, at sufficiently high frequencies, the damping coefficient of oscillations varies non-monotonically with the permeability, in which case it maximizes when the diffusion length scale for the vorticity is comparable to the penetration length scale for the flow within the porous material. Depending on the oscillation period, the maximum damping of a permeable cylinder can be many times greater than that of an otherwise impermeable one. This might seem counter-intuitive at first, since generally the power it takes to steadily drag a permeable object through a fluid is less than the power needed to drive the steady motion of the same, but impermeable, object. However, the driving power (or damping coefficient) for oscillating bodies is determined not only by the amplitude of the cyclic fluid load experienced by them but also by the phase shift between the load and their periodic motion. An increase in the latter is responsible for the excess damping coefficient of vibrating porous cylinders.

A viscous damping model is proposed based on a simplified equation of fluid motion in a moonpool or the narrow gap formed by two fixed boxes. The model takes into account the damping induced by both flow separation and wall friction through two damping coefficients, namely, the local and friction loss coefficients. The local loss coefficient is determined through specifically designed physical model tests in this work, and the friction loss coefficient is estimated through an empirical formula found in the literature. The viscous damping model is implemented in the dynamic free-surface boundary condition in the gap of a modified potential flow model. The modified potential flow model is then applied to simulate the wave-induced fluid responses in a narrow gap formed by two fixed boxes and in a moonpool for which experimental data are available. The modified potential flow model with the proposed viscous damping model works well in capturing both the resonant amplitude and frequency under a wide range of damping conditions.

This thesis presents the results of a study that uses Incremental Dynamic Analysis to assess the seismic performance of steel moment-resisting frames with fluid viscous dampers subjected to earthquake ground motions. The study systematically investigated the effects of linear and nonlinear dampers on the response of steel moment-resisting frames to earthquakes that varied in intensity and type. Both near-field and far-field motions were considered. Two different types of nonlinear dampers were investigated; one had a hardening and the other had a softening force-velocity relationship. The nonlinear dampers were calibrated to the linear dampers so that there was a basis of comparison. Maximum damper displacement is one of the parameters of the calibration, and it was varied to investigate its effect on structural response. Several nonlinear inelastic time history analyses were performed to obtain responses, such as peak base shear, peak interstory drift, or residual displacement index, which were plotted versus earthquake intensity to create individual IDA curves. Sets of related IDA curves provide a useful summary of the structural behavior for a wide range of variables. IDA curves for the tests with different damping types are presented. The results show that for both near-field and far-field ground motions the nonlinear dampers with a hardening force-velocity relationship are best suited to reduce undesirable drifts and residual displacements; however, these reductions come at the cost of high base shear forces.
Master of Science

This thesis presents the details of a study regarding both the use of linear viscous fluid dampers in controlling the interstory drift in steel moment frames, and the use of incremental dynamic analysis as a method of visualizing the behavior of these moment frames when subjected to seismic load effects. Models of three story and nine story steel moment frames were designed to meet typical strength requirements for office buildings in Seattle, Washington. These models were intentionally designed to violate seismic interstory drift restrictions to test the ability of the linear viscous fluid dampers to reduce these drifts to the point of code compliance. Dampers were included in one bay of every story in each model. These devices were used to produce total structural damping ratios of 5%, 10%, 20%, and 30% of critical. Undamped, traditional stiffness controlled models of both three stories and nine stories were also created for comparison purposes. Incremental dynamic analysis was used to subject these models to ten ground motions, each scaled to twenty incremental levels. Two new computer applications were written to facilitate this process. The results of these analyses were studied to determine if the linear viscous fluid dampers were able to cause compliance with codified drift limits. Also, incremental dynamic analysis plots were created to examine the effects of the dampers on structural behavior as damping increased from inherent to 30% of critical. It was found that including linear viscous fluid dampers in steel moment frame design can satisfactorily control interstory drift, and incremental dynamic analysis is a beneficial tool in visualizing dynamic structural behavior.
Master of Science

At the moment of writing, an expansion of the Swedish railway network has started, by constructions of new lines for high-speed trains. The aim is to create a high-speed connection between the most populous cities in Sweden - Stockholm, Göteborg and Malmö, and the rest of Europe. Thereby, the likelihood of faster, longer and heavier foreign trains crossing the Swedish lines is increased. However, this could be problematic since the dynamic response in railway bridges and, consequently, the risk of resonance increases with increasing train speeds. Bridges are usually designed based on contemporary conditions and future requirements are rarely considered, due to e.g. cost issues. Prospectively, the dynamic performance of existing bridges may become insufficient. Hence, the current expansion of the high-speed railway network results in an increased demand of innovative design solutions for new bridges and cost-efficient upgrading methods for existing lines. The aim of the present thesis is to propose a vibration mitigation strategy suitable for new and existing high-speed railway bridges. The main focus is a retrofit method with fluid viscous dampers installed between the bridge superstructure and the supports, which is intended to reduce the vertical bridge deck acceleration below the European design code limits. Furthermore, the intention is to investigate the efficiency of such a system, as well as to identify and analyse the parameters and uncertainties which could influence its functionality. In order to examine the applicability of the proposed retrofit, case studies, statistical screenings and sensitivity analyses are performed and analysed. Two different models, a single-degree-of-freedom system and a finite element model, are developed and compared. From the different models, it is possible to study the influence from the damper parameters, the variability of the material properties and different modelling aspects on the bridge response. After the installation of the fluid viscous dampers, it is found that the acceleration level of the bridge deck is significantly reduced, even below the design code requirements.
I skrivande stund har en utbyggnad av det svenska järnvägsnätet initierats. Målet är att skapa en höghastighetsanslutning mellan de folkrikaste städerna i Sverige - Stockholm, Göteborg och Malmö, och vidare ut i Europa. Därmed ökar sannolikheten att snabbare, längre och tyngre utländska tåg korsar de svenska järnvägslinjerna. Dock kan detta bli problematiskt i och med att järnvägsbroars dynamiska respons och, följaktligen, risken för resonans ökar med ökad tåghastighet. Broar dimensioneras ofta utifrån nuvarande förutsättningar och hänsyn tas sällan till framtida hållbarhetskrav, exempelvis p.g.a. kostnadsbesparingar. Ur ett framtidsperspektiv kan därför det dynamiska beteendet hos befintliga broar komma att bli otillräckligt. Utbyggnaden av höghastighetsnätverket ökar därmed behovet av innovativa konstruktionslösningar för nya broar och kostnadseffektiva uppgraderingsmetoder för befintliga sträckor. Syftet med föreliggande avhandling är att föreslå en metod för att minska de vibrationsnivåer som kan uppstå i både nybyggda och befintliga järnvägsbroar för höghastighetståg. Huvudfokus är en eftermonteringsmetod med viskösa dämpare, som har installerats mellan brons överbyggnad och landfästen, för att minska brobanans vertikala acceleration under gällande europeiska dimensioneringskrav. Vidare avses att undersöka effektiveteten av ett sådant system, samt att identifiera och analysera de parametrar och osäkerheter som kan påverka dess funktionalitet. Fall- och parameterstudier, samt statistiska metoder används och utvärderas för att undersöka tillämpbarheten av den föreslagna vibrationsdämpningsmetoden. Två olika modeller, ett enfrihetsgradssystem och en finit elementmodell, har skapats och jämförts. Utifrån dessa modeller kan påverkan av dämparens parametrar, variabiliteten hos materialegenskaperna och behandlingen av olika modelleringsaspekter studeras. Från resultaten är det tydligt att brobanans accelerationsnivå avsevärt reduceras efter monteringen av viskösa dämpare, till och med under dimensioneringskraven.

QC 20170425

Ce travail de recherche traite du couplage entre un liquide incompressible, irrotationnel et son contenant : une structure élastique. Cette interaction fluide-structure est traitée dans le cadre des petites déformations autour d'un état d'équilibre.Dans un premier temps, on présente une méthode d'introduction des sources dissipatives visqueuses dans le liquide à partir des équations du système couplé conservatif en s'appuyant sur une approche de type fluide potentiel généralement utilisée pour traiter les problèmes de couplage fluide-structure linéarisés non amortis. Un modèle d'amortissement diagonal est alors choisi pour le liquide et les effets dissipatifs de celui-ci sont pris en compte en calculant les coefficients d'amortissement modaux. Seuls les effets dissipatifs liées à la viscosité du liquide sont alors pris en compte. Le système couplé dissipatif obtenu possède une matrice d'amortissement non symétrique. Une résolution de ce système à amortissement non classique est alors présentée et les expressions des réponses fréquentielle et temporelle linéarisées sont données pour différents types d'excitations.Dans un deuxième temps, le liquide est supposé non visqueux et les forces de tension surfacique sont prises en compte. Cette configuration concerne principalement les satellites où le système couplé est en situation de microgravité. Une formulation du problème conservatif permettant de prendre en compte l'incompressibilité du fluide, la condition de continuité à l'interface fluide structure, les effets de capillarité du fluide ainsi que les effets éventuels de précontraintes statiques est alors établie. On se propose pour cela d'utiliser une méthode énergétique via le Principe de Moindre Action. La démarche est alors décomposée en deux étapes : une étude statique afin de déterminer la position de référence, puis une étude dynamique linéarisée autour de cette position d'équilibre. Cette formulation forme notamment une base pour l'introduction des sources dissipatives liées aux effets de capillarité via la méthode précédemment introduite.

El presente trabajo, consiste en la implementación de estos disipadores fluido-viscosos en una edificación con predominación de muros estructurales de 5 niveles que presenta un comportamiento torsional. El diseño de estos disipadores, parten con el objetivo de diseño de daño moderado y bajo un sismo de 475 años de periodo de retorno, es decir que la deriva objetivo necesario para el diseño es 0.58%. Se analiza bajo la colocación en diagonal para amortiguadores lineales y no lineales. La colocación se hace de manera uniforme, y de manera que compense el movimiento torsional. Se colocaron un total de 40 disipadores en todo el edificio. Entre los principales resultados, la fuerza fuerzas en los disipadores fueron en un orden de 213 ton-f y comportamiento torsional debido a zonas frágiles de la estructura fueran reducidas en 80%. La implementación de los disipadores fluido-viscosos permiten reducir la deriva en 60% y todas las derivas se mantienen por debajo de 0.58%, en teoría. En un futuro, en el Perú, será necesario implementar una normativa para el diseño y el aporte de amortiguamiento en el edificio.
The present work consists of the implementation of these fluid-viscous dissipators in a building with a predominance of structural walls, of 5 levels that presents a torsional behavior, as well as fails to comply with the permissible drift limit established by Norma Técnica E.030. The design of these dissipators starts with the design objective of moderate damage and under an earthquake of 475 years of return period, whose corresponding objective drift assumes a value of 0.58%. It is discussed under diagonal placement for linear and nonlinear dampers. The placement is done uniformly, and in a way that compensates for torsional movement. A total of 40 heatsinks were placed throughout the building, 8 per floor. Among the main results, the forces in the dissipators were in the order of 200 ton-f and torsional behavior due to flexible areas of the structure were reduced by 80%. The implementation of fluid-viscous heatsinks allows the drift to be reduced by 60%, and all drifts are kept below 0.58%, that is, both linear and non-linear devices meet the target drift, even though the latter have a higher drift, given their lower C, therefore, lower force, less drift control, even so, they are efficient, both structurally and economically, given their lower strength. In the future, in Peru, it will be necessary to implement a regulation for the design and the cushioning contribution in the building.
Trabajo de investigación

碩士
國立嘉義大學
土木與水資源工程學系研究所
98
Nonlinear fluid viscous dampers are the most widely used seismic energy dissipation devices currently. The increasing of damping is to prevent excessive displacement. The purpose of the study is to discuss the accuracy of the equivalent damping ratio of nonlinear viscous dampers, and it is focused on the modification of the existing design equation of equivalent damping ratio. Conventional equation of equivalent damping ratio is obtained by harmonic loads that are deterministic, but the real external force is the earthquake loads that are random. Thus, this research is to modify the equation of equivalent damping ratio according to the real ground motions recorded in California. Using SAP2000 to calculate the maximum displacement of single-degree-of-freedom (SDOF) systems and comparing it of equivalent linear system with it of nonlinear systems. The results show that the equivalent damping ratio overestimate the maximum displacement of the short-period structure, while it underestimate the maximum displacement of the long-period structure. Therefore, this study will modify criterion of computing equivalent damping ratio. Damping coefficient of the nonlinear system is adjusted by multiplying a modification factor to get better equivalent damping ratio. Then, regression equations of damping modification factors are obtained by nonlinear regression analysis. The study show that the proposed equations of equivalent damping ratio for various site classes and α values acceptable and stable.

Στην εργασία αυτή διερευνάται η δυναμική απόκριση κρυογενικών δεξαμενών υγροποιημένου φυσικού αερίου υποκείμενων σε δράση σεισμού. Επειδή οι εν λόγω δεξαμενές αποτελούν κρίσιμα στοιχεία για τη λειτουργία ενός συστήματος διανομής φυσικού αερίου και επειδή ενδεχόμενη καταστροφική αστοχία τους θα είχε δυσβάστακτες κοινωνικές επιπτώσεις, ιδιαίτερα αυστηρές απαιτήσεις ισχύουν σχετικά με τον αντισεισμικό σχεδιασμό τους. Για το σχεδιασμό έναντι σεισμού προδιαγράφονται συνήθως ένα σεισμικό γεγονός με μέση περίοδο επαναφοράς της τάξης των 500 ετών, κατά το οποίο οι δεξαμενές απαιτείται να παραμένουν πλήρως λειτουργικές, και ένα σεισμικό γεγονός με μέση περίοδο επαναφοράς της τάξης των 5000 έως 10000 ετών, κατά το οποίο απαιτείται η εξασφάλιση της ασφαλούς διακοπής της λειτουργίας των δεξαμενών, ενώ ελάχιστη μόνο βλάβη είναι αποδεκτή στα δομικά στοιχεία τους. Οι εν λόγω δεξαμενές αποτελούνται από δύο κελύφη: Ένα εξωτερικό κυλινδρικό κέλυφος από προεντεταμένο σκυρόδεμα με θολωτή στέγη από οπλισμένο σκυρόδεμα και ένα εσωτερικό κέλυφος από κρυογενικό χάλυβα, εντός του οποίου αποθηκεύεται το προϊόν. Τα δύο κελύφη εδράζονται σε κοινή πλάκα βάσης. Στην εργασία αυτή εξετάζονται δύο εναλλακτικοί τρόποι έδρασης της πλάκας βάσης: α) απλή έδραση επί του διαμορφωμένου εδάφους και β) εφαρμογή κάποιας διάταξης σεισμικής μόνωσης επί της οποίας εδράζεται η πλάκα βάσης. Ειδικότερα, εξετάζονται τα ακόλουθα συστήματα σεισμικής μόνωσης: α) μόνωση με ελαστομεταλλικά εφέδρανα με ελαστομερές υψηλής απόσβεσης, β) μόνωση με ελαστομεταλλικά εφέδρανα με πυρήνα μόλυβδου, και γ) μόνωση με συνδυασμό γραμμικών ελαστομεταλλικών εφεδράνων και μη γραμμικών αποσβεστήρων. Στις ιδιαιτερότητες του δυναμικού προβλήματος περιλαμβάνονται, εκτός από την επίδραση της μόνωσης, η δυναμική αλληλεπίδραση των δύο κελυφών, η δυναμική αλληλεπίδραση υγρού-δεξαμενής, ο κυματισμός της ελεύθερης επιφάνειας του υγρού και η δυναμική αλληλεπίδραση εδάφους-δεξαμενής. Τα φαινόμενα αυτά λαμβάνονται υπόψη στην ανάλυση. Για τη μελέτη του κυματισμού της ελεύθερης επιφάνειας χρησιμοποιείται η γραμμική θεωρία κυμάτων. Για τη μελέτη της αλληλεπίδρασης υγρού-δεξαμενής εφαρμόζεται μια προσέγγιση τύπου Lagrange με χρήση πεπερασμένων στοιχείων υγρού. Για τη μελέτη της αλληλεπίδρασης εδάφους-δεξαμενής χρησιμοποιούνται συγκεντρωμένα στοιχεία δυσπαραμορφωσιμότητας και απόσβεσης. Επίσης, συγκεντρωμένα στοιχεία δυσπαραμορφωσιμότητας και απόσβεσης χρησιμοποιούνται για την προσομοίωση των συστημάτων μόνωσης. Τα χαρακτηριστικά των παραπάνω συγκεντρωμένων στοιχείων για την προσομοίωση της αλληλεπίδρασης εδάφους-δεξαμενής και των συστημάτων μόνωσης επιλέγονται με βάση ρεαλιστικές υποθέσεις για τις δυναμικές ιδιότητες του εδάφους θεμελίωσης και τα χαρακτηριστικά των συστημάτων μόνωσης. Δύο περιπτώσεις υφιστάμενων δεξαμενών υιοθετούνται προς ανάλυση. Οι δύο δεξαμενές διαφέρουν κυρίως ως προς τη χωρητικότητα και το λόγο ύψους προς ακτίνα. Και οι δύο δεξαμενές έχουν μελετηθεί και κατασκευαστεί χωρίς σεισμική μόνωση. Ένα συνθετικό επιταχυνσιογράφημα συμβατό με το φάσμα απόκρισης σχεδιασμού του Ευροκώδικα 8 για λόγο απόσβεσης 5%, για την αντίστοιχη κατηγορία εδάφους και για μέγιστη εδαφική επιτάχυνση λίγο μεγαλύτερη από αυτή για την οποία είναι σχεδιασμένες οι δεξαμενές περιγράφει τη διέγερση της κατασκευής στην οριζόντια διεύθυνση. Για την ανάλυση αναπτύσσεται ένα συνολικό παραμετρικό προσομοίωμα πεπερασμένων στοιχείων το οποίο περιλαμβάνει τα δύο κελύφη και το αποθηκευμένο υγρό. Στο προσομοίωμα συμπεριλαμβάνονται, ανάλογα με την περίπτωση που κάθε φορά αναλύεται, τα κατάλληλα στοιχεία για την προσομοίωση της αλληλεπίδρασης εδάφους-δεξαμενής ή των διαφόρων συστημάτων μόνωσης. Το προσομοίωμα χρησιμοποιείται για τον υπολογισμό των ιδιομορφών και την πραγματοποίηση δυναμικών γραμμικών και μη γραμμικών αναλύσεων με ολοκλήρωση στο πεδίο του χρόνου. Οι μη γραμμικές αναλύσεις είναι αναγκαίες για να ληφθεί υπόψη με ρεαλιστικό τρόπο η έντονα μη γραμμική συμπεριφορά των συστημάτων μόνωσης. Στην περίπτωση αυτή η μη γραμμικότητα είναι συγκεντρωμένη στα στοιχεία που προσομοιώνουν τη μόνωση ενώ για την υπόλοιπη κατασκευή γίνεται σε κάθε περίπτωση η παραδοχή γραμμικής συμπεριφοράς. Η παραδοχή αυτή προκύπτει από το γεγονός ότι για τις δεξαμενές αυτού του τύπου η αποδεκτή βλάβη, και επομένως μη γραμμικότητα στη συμπεριφορά, των δομικών στοιχείων είναι ελάχιστη. Παρουσιάζονται επιλεγμένα αποτελέσματα των αναλύσεων που πραγματοποιήθηκαν με χρήση του προσομοιώματος πεπερασμένων στοιχείων. Τα αποτελέσματα αυτά αναφέρονται κυρίως στην τέμνουσα βάσης, τη ροπή ανατροπής, τις οριζόντιες μετακινήσεις της κατασκευής σε διάφορες στάθμες και το ύψος του κυματισμού. Έμφαση δίνεται στην ποσοτικοποίηση της επίδρασης της αλληλεπίδρασης εδάφους-δεξαμενής και της σεισμική μόνωσης στα ανωτέρω μεγέθη. Η εργασία τελειώνει με γενικά συμπεράσματα.
In the present work the dynamic response of liquefied natural gas cryogenic tanks subjected to earthquake action is investigated. Since these tanks are critical elements for the function of a natural gas distribution system and because a potential failure of them could lead to a major disaster, very severe requirements concerning the seismic design of these tanks are imposed. For the design against earthquake action two seismic events are generally considered: an event with a mean return period of 475 years during which the tanks are expected to remain fully operational and an event with a mean return period of the order of 5000 to 1000 years during which the safe shut down of the tanks is to be ensured, while minimum damage is accepted at the structural parts of the tanks. The type of tanks under consideration is constituted of two shells: an outer one made of prestressed concrete and an inner one made of cryogenic steel in which the product is stored. Both shells rest on a common base slab. In these work two alternative ways for the support of this slab are examined: a) the slab lays directly on the ground and b) the slab lays on a number of devices which provide seismic isolation. The following isolation systems are examined. a) high damping rubber bearings, b) lead core rubber bearings and c) low damping rubber bearing in conjunction with non-linear viscous dampers. The examined structural dynamic problem is characterised by certain particularities, besides the one of the implementation of an isolation system. The most important of them are sloshing of the free surface, dynamic fluid-structure interaction, dynamic soil-structure interaction and interaction between the inner and outer shell. These phenomena are taken into consideration in the preformed analyses. For the modeling of the free surface effect the realistic assumption of small wave height is made and the linear wave theory is employed. For the modeling of the fluid-structure interaction a Lagrangian approach is applied using finite elements for the modeling of the liquid and the solid domain. The soil-structure interaction is simulated by using concentrated stiffness and damping elements. Concentrated stiffness and damping elements are used for the modeling of the isolation systems as well. Two cases of existing tanks are adopted for analysis. The two tanks differ mainly in capacity and in the height to radius ratio. Both tanks are designed and constructed without seismic isolation. A global parametric finite element model is developed for the analyses. In this model the appropriate elements are incorporated for the modeling of the soil-structure interaction and the seismic isolation system, depending on the analysed case. The model is used for the calculation of eigenmodes and eigenfrequencies and for performing linear and non-linear transient analyses in time domain. Non-linear analyses are necessary in order for the highly non-linear behavior of the isolation devices to be properly simulated. In these cases the non-linearity is concentrated at the elements which simulate the seismic isolation system while the rest of the structure is considered elastic in all cases. This consideration is justified by the fact that for these tanks the acceptable damage, and consequently non-linearity in the behavior of the structural part, is minimum. Selective results of the performed analyses are presented. These results refer mainly to base shear forces, overturning moments, horizontal displacements at different levels of the tank and sloshing heights. Emphasis is on the quantification of the impact of the soil-structure interaction and the implementation of the examined seismic isolation systems.

Civil structures are subjected to various types of loading, which induce severe damage to the structures. Many techniques have been developed for structural rehabilitation; one of the emerging technologies is the use of energy dissipation systems such as fluid viscous dampers (referred to hereafter by FVD). In this chapter, the effect of these devices on the dynamic behavior of an RC building is investigated, with an optimal choice of the linear FVD parameter (i.e., damping coefficient), using a simplified and effective approach. It was found that the maximum inter-story drift of the analyzed retrofitted structures can be significantly reduced compared to the original ones.

Wind has a great impact on civil structures. It is considered a dynamic and random phenomena and it plays an important role in the design of tall structures. Existing buildings with certain height must resist wind effect. Many researchers have developed theories and schemes that consider more thoroughly wind components and the influence of its turbulence on buildings. It is known that any structure inherently dissipates and absorbs energy due to external loads thanks to its inherent damping. In order to improve this capacity and limit structural damage, fluid viscous dampers are commonly used for structural protection; they have confirmed their efficiency and reliability. Many researchers have investigated their effect by inserting them in the structure; some of the optimization methods for the design of these dampers previously used will be discussed. Finally, an effective method for optimal design of additional dampers will be illustrated by an example and discussion.

LIGA (from German, LIthographie, Galvanoformung und Abformung) is one of the most viable manufacturing techniques for creating precise micro-mechanical components. Due to their monolithic construction, LIGA structures lack the most important contributor to energy dissipation; the joints. In shock and vibration environments, the low damping capacity can cause functional and structural failures. As a means of increasing the energy dissipation of LIGA structures, this paper experimentally investigates coupled fluid-structural dynamics of LIGA structures when submersed into fluids. The experiments are conducted by base-exciting a rectangular cross-section cantilever LIGA beam with a shaker, and measuring the response through a laser Doppler vibrometer. The paper also evaluates simple fluid dynamics models from the literature for their effectiveness in capturing the experimentally-observed behavior.

Abstract Flexible and skyscraping frame-structural tower is sensitive to dynamic loads, that might oscillate intensely under seismic and wind loads. Thus, vibration suppression system is required and fluid viscous damper (FVD) is implemented to suppress such vibration in this study. The vibration behavior of frame-structural tower with FVD (F-FVD) is investigated by an equivalent model combined with experiment under seismic and wind loads, respectively, and the vibration suppression effect is evaluated by the displacement and energy distribution. The equivalent model of the frame-structural tower with FVD is primarily established, of which FVD is considered through Kelvin model. The parameters of FVD are supplemented to the damping term through a time-dependent damping matrix based in Rayleigh damping, and to the stiffness term through a stiffness matrix based in stiffness-equivalent simply-supported beam. Van der Pol wake oscillator is considered as the wind force term. Several conventional numerical integration methods for solving the equivalent model which belongs to the nonlinear system are compared and Newmark-β algorithm is adopted. The vibration suppression regularity of normalized stiffness coefficient Kr, damping coefficient Cr and velocity exponent α is analyzed via the equivalent model. There is a balance between the stiffness effect and energy dissipation effect of FVD, and the sensitiveness to α depends on excitation and stiffness of F-FVD system. The optimal parameters of fluid viscous damper, Kr = 1.30∼2.0, Cr = 1.65∼3.30 and α = 0.4∼0.8, are obtained eventually. It is indicated that the vibration suppression by fluid viscous damper is effective. And this study can be conducive to vibration suppression design of such flexible and skyscraping frame-structural tower.

Digital hydraulics have attracted attention towards fast switching valves and the increased focus on reliable fluid power entail that the lifetime of such valves is of great concern. An inherent feature of most valves for digital hydraulics is that of a mechanical end stop. Consequently, the squeeze film damping associated with end stops of switching valves is an interesting topic. This damping effect is perceived as beneficial for high lifetime and low impact sound, as the consequence of lowering the impact velocity at the mechanical end stops. In this paper the squeeze film damping effect is reviewed with a focus on maximum surface stresses. Using the Barus relation for viscosity-pressure dependency and different film geometries, the classical lubrication theory is applied together with the equation of motion, to obtain the gap height motion equation, both for the iso-viscous and piezo-viscous case. In consequence, this enable insights concerning the influence of piezo-viscosity on this damping effect. These models are used to investigate the loads, which the approaching surfaces experiences. Based on Hertzian theory, comparisons of impact loads and the dynamic squeeze loading are performed, whereby the relation between design parameters and the relative severity of these occurrences are analyzed.

Abstract Analysis of a nonlinear friction damping mechanism in a fluid-structure interaction system is performed by combining a generalized averaging procedure with a recently developed identification algorithm based on the Hilbert transform. The system considered includes a nonlinear restoring force and a nonlinear dissipation force incorporating both viscous and structural damping. Frequency and damping response backbone curves obtained from simulated data are compared with analytical and approximate solutions and are found to be accurate. An example large scale experiment exhibiting viscous and Coulomb damping is also analyzed resulting in identification of system parameters.

In many fluid-structure-interaction problems, the “added mass”, “inertial mass”, “virtual mass”, “carried mass” or “induced mass” is one of important interests. In the present study, the authors propose a simple and efficient method to specify fluid-force coefficients of an oscillating object in stationary fluid. In this method, the authors consider incompressible-and-viscous fluid under the assumption of an infinitesimal oscillation amplitude of a three-dimensional object, and properly modify the Navier-Stokes equations into linear equations, namely, the Brinkman equations. The solving method is based on a discrete singularity method, one of the boundary methods. Furthermore, the authors conduct experiments, in addition to computations. In order to confirm the method’s effectivity and validity, the authors compute a sphere, comparing our computations to theories and experiments which show good agreement.

Fluid viscosity is known to influence hydrodynamic forces on a floating body in motion, particularly when the motion amplitude is large and the body is of a bluff shape. While these hydrodynamic force or force coefficients have been predicted traditionally by inviscid-fluid theory, much recent advances had taken place in the inclusion of viscous effects. Sophisticated RANS (Reynolds-Averaged Navier Stokes) software are increasingly popular. However, they are often too elaborate for a systematic study of various parameters, geometry or frequency, where many runs with extensive data grid generation are needed. The Free-Surface Random-Vortex Method (FSRVM), developed at UC Berkeley in the early 2000, offers a middle-ground alternative, by which the viscous-fluid motion can be modeled and yet allowing vorticity generation be either turned on or turned off. The heavily validated FSRVM methodology is applied in this paper to examine how the draft-to-beam ratio and the shaping details of two-dimensional cylinders can alter the added inertia and viscous damping properties. A collection of four shapes is studied, varying from rectangles with sharp bilge corners to a reversed-curvature wedge shape. For these shapes, basic hydro-dynamic properties are examined, with the effects of viscosity considered. With the use of these hydrodynamic coefficients, the motion response of the cylinders in waves are also investigated. The origin of viscous damping is clarified. It is a pleasure and honor for the authors to contribute to the Jo Pinkster Symposium, held in his honor in OMAE-2011.