جستجوی مقالات مرتبط با کلیدواژه "fluid–structure interaction" در نشریات گروه "عمران"

In this study, the seismic response and buckling of aboveground cylindrical steel liquid storage tanks subjected to horizontal components of earthquake ground motions is investigated using incremental dynamic analyses (IDA). A broad steel tank with diameter of 30 m and height to diameter (H/D) ratio of 0.40 was designed using API 650 standard. The incremental dynamic analyses of liquid storage tank were performed for seven real seismic ground motions, which were scaled for PGAs of 0.05g to 0.50g. To verify the accuracy of the propose finite element model of the tank-liquid system, natural periods of the tank-liquid system vibration modes computed from finite element analysis compared to those obtained by analytical solutions and other numerical study. Small difference between natural periods indicates the acceptance accuracy of the finite element model. The mean peak base shear and overturning moment of the steel tank are estimated using mass spring model and compared with those obtained by finite element model. The mean peak base shear and overturning moment from finite element model greater than those obtained by mass spring model for PGA less equal 0.20g and vice versa for PGA from 0.30g to 0.50g. The incremental dynamic analysis results show that buckling of tank shell occurred at a height of 2.8 m above the tank base. Also mean critical horizontal peak ground acceleration (critical PGA) and mean critical dynamic base shear force, which induces buckling at the bottom of the cylindrical shell, are estimated.

Water storage tanks are one of the vital and essential arteries whose immediate operational performance after an earthquake is of special importance and interruptions in their operation can indirectly increase the damage and losses caused by earthquakes. Due to the significant effect of earthquakes on structures located in the vicinity of faults and, also, the unknown effect of the simultaneous transitional and rotational components of these earthquakes, present study investigated the behavior of concrete elevated tanks with central shaft and lead rubber bearing base isolation, under simultaneous excitation of translational and rotational components of near- and far-field earthquakes using nonlinear time history analysis. Base shear force and normal stress of tanks were determined considering dynamic interaction of water and tank. The corresponding rotational component of each translational pair of accelerograms was generated using the classic formulation of the theory of elasticity and wave propagation simultaneously, taking into account the frequency-dependency of wave velocity. To model the base isolator in the present study, an equivalent simplified link and combination model, based on 3D model of reference [32], has been suggested and its performance accuracy has been evaluated. The results of dynamic analysis show that the presence of the base isolator in all the studied conditions causes a reduction of about in base shear and in normal stress of the elevated tanks. Increasing the tank volume will reduce the decreasing effect of the isolator on the normal stress of tanks, but will improve the performance of the seismic isolator in reducing the base shear. Also, increasing the angular acceleration of rotational component of near-field earthquakes weakens by about the decreasing effect of the base isolator on the seismic response of elevated-tanks. Thus, neglecting rotational component of the earthquakes in analysis of tanks located near the faults will magnify the decreasing effect of the base isolator on seismic demands and the poor design of structure. Therefore, use of the isolator to reduce the seismic response in these areas will not be economically justified.

In recent decades, different control strategies (i.e., passive, active, semi-active, or hybrid) have been extensively employed to suppress the response of offshore structures subjected to dynamic lateral loads. Jacket platforms are vital structures for oil-rich countries that are frequently subjected to dynamic harsh sea waves. Therefore, the vibration control of these structures is substantial to increase productivity, safety, and serviceability and prevent premature possible fatigue failure. In this study, the behavior of the Ressalat platform located in the Persian Gulf is evaluated under wave loads with two different return periods of 50 and 100 years. A simplified equivalent seven-degree-of-freedom lumped mass linear model is adopted for the Ressalat platform. In the Persian Gulf, wave loads are the dominant load in the designing procedure. Due to the stochastic nature of wave loads, random wave and constrained new-wave theories are utilized in the generation of the wave records. Then, Passive Tuned Mass Damper (PTMD) and Active Tuned Mass Damper (ATMD) are employed to suppress the platform vibration and subsequently, their control performances are compared. Various calculated normalized performance criteria (responses of controlled to uncontrolled structures) for passive and active controlled platforms including normalized maximum and Root Mean Square (RMS) of displacement, velocity, and acceleration of the deck level are calculated and compared. The uncontrolled and controlled platforms are modeled and analyzed using MATLAB and SIMULINK software. Moreover, the fuzzy intelligent algorithm with a triangular membership function is implemented to calculate the control force and the Harmony Search Algorithm (HAS) is examined to optimize the actuator power. Also, the Fluid-Structure Interaction (FSI), the effect of added mass due to the accelerated motion of the body in the fluid, and the saturation of the actuator are considered. The results show the effectiveness of the proposed control system by reducing 20% and 50% of the maximum and RMS of the platform deck acceleration, respectively, over time.

Inflatable marine structures are flexible cylindrical structures attached to a rigid base. These structures are generally cylindrical tubes, made of rubberized material, and inflated by air and/or water. Although these structures are permanently inflated, they have the advantage of being deflated and lie flat when not needed and then, inflated in a short period when required. They are relatively easy to install, do not corrode, require little maintenance, and have the capability to withstand extreme temperatures. Due to elasticity of the structure and continuous variation of its shape during operation, inflatable breakwater or sea-wall structural and hydraulic analyses are more complicated than the rigid types. Large deformation of the membrane due to the internal and external loads makes the governing equations of such problems non-linear and complex. In the present study, the behavior of an inflatable marine structure under loading by marine waves was simulated based on 2D numerical modeling. For this purpose, the deformed equilibrium geometry of the breakwater was calculated by solving the prevailing equations through the linear dynamic response of the system. The central difference method was employed to solve the governing equations of the linear dynamic response of the system of finite elements. According to the results of former studies, for 2D modeling of the aforementioned problem, the length of the tube was assumed infinity. Therefore, the effects of lateral supports and boundary conditions were neglected. This study carried out the numerical analysis of the inflatable marine structures for solving the flow-based problem associated with static and dynamic structural analyses. For this purpose, the two-dimensional fluid-structure interactions were analyzed numerically. It was shown that the equilibrium shape of the structure was a function of rubber thickness, elasticity modulus of the material, internal pressure, the dam foot width, and external loads. All the influential parameters of both flow and structure including internal pressure, water depth, wave height, wave period, etc. were attained based on the dimensional analysis. Accordingly, the results describing height and cross-sectional profile of the inflatable tubes loaded by marine waves were obtained.

Breakwaters are of great importance as coastal protection structures, but the double importance of breakwaters in our country is due to the creation of small and large shelters for different vessels. In addition, marine structures, including breakwaters, are subject to a variety of threats, including air, sea and submarine attacks. Therefore, attention to passive defense considerations is important in designing and operating breakwaters against threats and damage. One of the most important of these threats is the terrorist and explosive threats from the sea. In this research, the response of rubble mound breakwaters and their failure modes subjected to the underwater explosion was evaluated. Thus AUTODYN finite element software was used for the simulation and analysis of breakwater responses under the underwater blast loading. The used simulation and analysis method were the Coupling Euler-Lagrange process and explicit dynamical solving, respectively. For verification, first, the wave propagation from underwater explosion and its damaging effect on a concrete dam structure were evaluated. Then, The proposed model response of the breakwater using the software under the variation of explosive mass, water depth, event explosive depth, structure breakwater slope and size of armor layer blocks were investigated. The results showed, the greater the depth of the explosion and the shorter the scaled distance was less than 0.533 kg/m1/3, the geater the short-term damage would be. In the medium time simulation, was observed that by increasing the dimensions of concrete blocks of Armor layer from 2.4 m to 0.6 m, percentage failure decreased from 47% to 23%, and as well as for the structural slope of breakwater, with reducing the angle from 45 degrees to 26 degrees, the failure amount was reduced by about 50%. Generally, within the scope of this research, the critical explosion range was less than 8 meters from the structure.

Spillways are used in dam projects to convey flood flows and prevent destructive damages of downstream hydraulic systems, while the construction of other types of spillways is restricted, or there is a need to pass great values of flood discharge with a limited head. Siphon spillway is a kind of dam spillways employed in a number of dam reservoirs. This spillway is applied while the space for constructing other types of spillways is restricted, or when there is a need to pass great values of flood discharge with a limited head. One of the most important defects of this spillway is a complicated fluid-structure interaction due to the establishment of different flow regimes inside the spillway. This situation becomes more complicated when the spillway is not connected to the dam, working individually. Despite the importance of the operation of these hydraulic structures, there is a lack of comprehensive research to investigate the hydraulic behavior of these structures and the flow field around and inside them. In the present study, different unsteady flow regimes including sub-atmospheric flow, two-phase flow, and black-water flow and their effects on the hydraulic characteristics of siphon spillways were investigated. For this purpose, ANSYS CFX software was applied to simulate the flow field in such spillways. To validate the obtained flow regimes, via numerical modeling, experimental results of the former studies were used. Results indicated that the pressure fluctuations were mostly greater beneath the inlet and throat of the spillway compared to the other sections.

Transient flow in a pipe would cause interaction between the fluid and the pipe wall. This interaction can be studied by a mathematical model in which the pipe considered as a beam under an applied fluid pressure loading. This model is a fluid filled pipeline that is connected to a tank in the upstream and to a valve in the downstream and undergoes forces of water hammer when there is a sudden closure in the valve. The aim is to study the possibility of instability in this pipeline when there are large lateral displacements and in the meantime small constrains. As conventional dynamic analysis models in beams which are based on the infinitesimal constrain theories (ε=∂u⁄∂x) cannot reflect the effect of large lateral displacements in the governing equations. In the governing equations the axial stresses are modeled as linear stresses and constrains are modeled by so called von Karman constrains as nonlinear constrains. The resulting partial differential equations are solved by the method of finite elements in the time domain. On the other hand the linearized equation of lateral vibration of a pipeline becomes dimensionless and by drawing the dimensionless frequencies versus the dimensionless fluid velocity, the stability of the pipeline is studied in the frequency domain. The results show that in shortly supported spans the frequency domain criterion (dimensionless fluid velocity π and 2π for pinned-pinned and fixed-fixed configurations accordingly) determines the pipeline stability behavior and so does not the critical bulking pressure.

In this research, the behavior of the morning glory crest and throat were investigated in the event of flood. Spillway structure and dam reservoir fluid were respectively simulated using the finite element method and finite volume method in ANSYS software. The morning glory structure's behavior was performed in three scenarios: a) the static analysis scenario of the dam empty reservoir, b) the static analysis scenario of the full reservoir while there is no flow over the spillway crest, and c) the dynamic analysis scenario of structure-fluid interaction in two parts: 1- starting of the flood flow on the morning glory spillway, and 2- the steady state flood flow on the morning glory spillway. The concrete structure analysis was linearly done in all three scenarios. Based on the results, the most critical condition for the structure in terms of maximum tensile stress was 5.3 MPa in the first part of the third scenario. The maximum compressive stress was 12.3 MPa in the second part of the third scenario. Spillways are one of the most important and vital structures in the life of a dam. The efficiency and proper functioning of such structures requires a precise and responsible design. The morning glory spillways have a great deal of importance due to the standing structure inside the dam reservoir. The weight and hydrodynamic loading on the body of spillway concrete structure during a flood complicates the interaction analysis of the structure and the fluid. By using numerical methods in computational fluid dynamics and structural analysis, we will be able to predict the behavior of structure and fluid hydrodynamics parameters. In model's fluid domain, choosing the type of Navier---Stokes equations proportional to the efficient RANS mathematical model of turbulence, is very sensitive. Mainly one equation turbulence models such as the Spalart-Allmaras or two-equation models, such as K-Epsilon or K-Omega, are based on Boussinesq hypothesis. Boussinesq hypothesis explains how to estimate the components of the fluid stress tensor for the above-mentioned turbulence equations. This assumption, by simplifying the computation process, only a in some parts of the fluid domain, analyzes the turbulence as anisotropic.

Todays due to expansion of population and industry, the collection and storage of liquids has been paid attention more and more. Saving oil and chemical liquids require great care to maintain it. One of the most important and commonly structures for storing liquids is steel cylindrical above ground tanks. On the other hand, due to damage of tanks in past earthquakes, they persuade scientists to better understand the seismic behavior.As a consequence, this research has been studied examines the phenomenon of buckling under Bam, Golbaf, Firuzabad, Bojnurd, Manjil, Tabas and Sarein eartquakes with regard to the fluid and structure interaction. Tanks were selected with ratio of 0.5, 1.0, 1.5 and 2.0 with fixed and variable thickness in full mode. These Cases were analyzed with non-linear time history analysis by ABAQUS Software.Finally, Compare the results with American petroleum institute regulation (API), Iranian seismic design code for oil industries and forces show, both height and diameter increase to growth the risk of Elastoplastic buckling and also by increasing the height to diameter ratio reduces the risk of Elastoplastic Buckling. The amount of this damage has declined by increasing shell thickness and in the same vein designing these structure by define standards is not safe method.

One of the most important components of water supply systems is the liquid storage tanks. During an earthquake, the interaction of fluid and structure in the liquid storage tanks and the sloshing phenomenon has a significant effect on the response values of the structure. Regarding the importance of the effects of near-fault earthquakes and their effect on seismic behavior and structures loads, in this study, the sloshing height and vibration of 2D concrete rectangular tanks under near- and far-field earthquakes was investigated using numerical methods. The effect of tank's dimensions, depth of water and ground motion characteristics on the maximum sloshing height was taken into account. Therefore, 9 tank models and 10 near- and far-field ground motion records were considered. The results indicated that the median values of maximum sloshing height in the near-field records are significantly higher than those related to far-field records. The average of increase of sloshing heights in tanks with lengths 20, 40 and 60 meters is 65, 77 and 100 percentage respectively. Also, with increase of fluid depth and tank width, the median of the maximum sloshing height increases and decreases respectively. In tanks, subjected to far- and near-field earthquakes, sloshing height had the highest correlation with Arias Intensity and PGV respectively. According to the results of this research, correction coefficients for the relations presented in the codes can be proposed to consider the effects of near-field earthquakes in calculating the maximum sloshing height.

The present study deals with the seismic analysis of cylindrical liquid tanks, taking the rotational components of the earthquake into account for various ratios of reservoir height to diameters of tanks. While defining actual kinematic behavior of any point requires incorporating the rotational components of ground motions, as well as the translational components, however majority of studies ignore the effects of rotational components. Here we propose a methodology to evaluate the rotational components of the earthquake based on the translational components. Since the earthquake waves are mainly created by ground movements in fault direction and such kind of movements leads to creation of the shear waves, potential functions of SV and SH waves are used to evaluate the rocking and torsion components, respectively.
For this purpose, a transitional component of ground motion using frequency discrete Fourier transformed to discrete frequency and G value for each frequency determined. Then, the incident angle of the wave was calculated for each frequency. After determining the incident angle, Fourier spectrums of rocking and torsion components of ground motion were calculated. Finally, the inverse of Fourier conversion time histories of rocking and torsion components of ground motion were calculated. The methodology introduced to evaluate time history of rocking and torsion components of ground motion was coded in MATLAB software. In order to verify the proposed methodology, the rotational components of San Fernando earthquake were determined based on the proposed model and compared to Li and Liang's results. The results differed only for about 3% which could be attributed to the different wave velocities and incident angles. In Li's model, the incident angle and apparent wave velocity was supposed to be constant while in the present study the incident angle and apparent wave velocity were variable based on each frequency Then, the rotational components of San Fernando, Tabas and Taft earthquakes were calculated based on the proposed model and the results were used in dynamic analysis of the tanks. Finally, seismic analyses of cylindrical liquid tanks were presented to evaluate the effects of rotational components on the seismic response of the tanks. The time history method was employed for dynamic analysis of the structure considering fluid-structure interaction. The complete fluid structure interaction was considered in analyses taking account the compressibility of fluid . Fluid domain behavior implies small displacements of inviscid compressible fluid with irrotational motion. Water compressibility has a significant impact on the fluid-structure interaction for a wide range of ratios in natural frequencies of structure to fluid domain.
It is evident that the distribution of displacements and stress is very sensitive to the rotational components of the earthquake. Applying rotational components of earthquake may alter the maximum displacement values, as well as the pattern of displacements distribution in all three directions. Incorporation of rotational components will lead to decreases in the maximum stress and displacement. This reduction in responses is more obvious in the empty tanks. An increase in the height of the tanks can boost the effects of rotational components on the structure. Results indicate that it is necessary to take rotational components of the earthquake into account while designing and analyzing cylindrical liquid tanks.

Real accelerations and subsequent wall diaplecements appear as the main damage Factors in liquid storage tanks. Base-isolation systems, as the most appropriate earthquake engineering options in passive control, can be used to reduce these factors and following destruction of these special structures. In this paper, the dynamic behavior of base-isolated rectangular tanks is studied under different ground motions in which, two tanks (one broad and one slender) with fixed-base and LRB base-isolatiors including different stiffnesses, i.e. stiff and flexible, are considered. In the developed model, finite elements and boundary elements for the shell and liquid domains and nonlinear hysteretic elements for isolation system are respectively employed to perform the analysis of the flexible tank in frequency and time domains. Further, the effect of base-isolators on the seismic response of the whole system including natural frequency, liquid sloshing height, base shear and moment and dynamic pressure on the tank shell, under harmonic and real earthquake has been inspected.

In the present study, fluid, structure and soil interactions in a pressurized water tunnel were modeled by a two-dimensional finite element method, using the “ABAQUS” software. The tunnel lining was considered as a roughening coverage to decrease the roughness coefficient and to increase the flow velocity. The mechanical parameters of surrounding rock were assumed as weak as possible compared to the tunnel concrete lining. Water flow inside the tunnel was modeled, using acoustic elements that are capable of simulating the hydrodynamic forces. Accordingly, the mechanism of crack development in the surrounding rock as the most probable place to prone this destructive phenomenon is studied applying both steady state and transient flow conditions, considering fluid-structure and rock interactions. Steady state and transient flow analyses were performed using the “HAMMER” software. The resulting loads obtained from the hydraulic analyses by HAMMER software were transmitted to the ABAQUS software for structural analyses. The stress analysis of cracked elements of the surrounding rock was carried out based on Mohr-Coulomb fracture criterion. The least values of the overburden to prevent failure due to the hydraulic jacking were evaluated by imposing the flow pressure in both steady and transient flow states. Finally, the effects of increasing height of the overburden on the hydraulic jacking phenomenon were investigated considering transient flow conditions. Results indicate that, with respect to the positions of surrounding rock elements, providing an extra overburden over the tunnel is not essentially a safe solution for the predication of hydraulic jacking phenomenon in rock around a tunnel.

Rubber dams are flexible cylindrical structures, attached to a rigid base, and are inflated with air and/or water. Most of the rubber dams are permanently inflated, however, they have the advantages of deflating and being flat, when they are not needed, and then inflated in a short period of time when they are required. Large deformation of the membrane due to the internal and external loads, makes the governing equations of such problem to be non-linear and complicated. In the present study, three-dimensional behavior of the rubber dams with respect to the boundary conditions of dam and flow was simulated numerically. Dam geometry and flow hydraulics were modeled, using ANSYS software, CFX and transient structural in workbench environment, simultaneously. Flow hydraulic characteristics and deformation of the dam are obtained, considering fluid-structure interaction. Water free-surface was obtained, applying two-phase air-water flow interface. SST turbulence model in CFX was employed for modelling the separation of flow, downstream of the inflatable dams. Due to the flexibility of the structure, large deformation theory was used in the transient structural solution. Consequently, different features of the flow field, including flow streamlines, velocity and pressure profiles are obtained and compared with those of the rigid circular-crested weirs. Results indicated that the flow hydraulic characteristics over the equilibrium shape of the rubber dams is analogous to those of the rigid circular-crested weirs.