flow-induced vibration
در نشریات گروه مکانیک-
The vortex shedding from a bluff body can provoke structural vibrations known as flow-induced vibrations (FIV), which characterize an intrinsic phenomenon in the design of cylindrical structures. There are numerous passive and active methods to suppress FIV, among which suction and/or blowing on a cylinder surface is one of the most common approaches. In this work, different configurations of simultaneous suction and blowing are considered at two different Reynolds numbers corresponding to the galloping and frequency synchronization ranges. The parameters studied include the number of slots for suction and blowing, their length, and the velocity of the sucked and blown flow. The design of experiments method (DOE) is used to find the required simulation elements. The simulation results show that the dominant parameter in the reduction of galloping and vortex-induced oscillations and mass flow rate is the flow velocity in the areas of blowing and sucking. In addition, regression analysis is used to derive a relationship between various influencing parameters and performance parameters.
Keywords: Flow-Induced Vibration, Vortex-Induced Vibration, Galloping, Lock-In, Fluid-Solid Interaction (FSI), Suction, Blowing, Square Cylinder -
ارتعاش القاشده توسط جریان از عوامل موثر تخریب مکانیکی سازه هایی است که در معرض جریان سیال قرار دارند. در این مطالعه با روش گردابه تصادفی-المان مرزی، جریان آرام (رینولدز200) دوبعدی، حول چهار سیلندر استوانه ای یک و دو درجه آزادی در چیدمان مستطیلی، شبیه سازی و ضرایب نیروهای هیدرودینامیکی، خطوط جریان و جابجایی سیلندرها رسم شده است. میدان چرخشی به تعدادی گردابه عددی منفصل و تغییرات آن در هر گام زمانی، با تعقیب گردابه ها در دیدگاه لاگرانژی، تحت اثر دو مکانیزم جابجایی و نفوذ، همچنین ارضای شرط مرزی سرعت صفر روی جدار هندسه، با خلق گردابه ی صفحه ای صورت می گیرد. سیلندر متحرک با سیستم جرم، فنر و دمپر مدل سازی شده است. نتایج نشان داد تغییرات متوسط ضریب پسای سیلندر یک و دو درجه آزادی نسبت به سیلندر ساکن به ترتیب 0/84 و 0/97 برابر می باشد. دامنه ی ارتعاش سیلندرهای عقبی از سیلندرهای جلویی کمتر بود. دامنه ی عرضی نوسان، سه برابر دامنه طولی مشاهده شد. بیشینه دامنه ی عرضی ارتعاش سیلندرهای یک درجه آزادی 1/51 برابر سیلندرهای دو درجه آزادی بدست آمد. مقایسه حل جریان حول تک سیلندر با بکارگیری روش المان مرزی (و عدم احتیاج به نگاشت همدیس یا درنظر گرفتن تصاویر گردابه ها) با نرم افزار انسیس فلوئنت % 25 کاهش زمانی و % 2/3 افزایش دقت محاسبات را نشان داد.
کلید واژگان: ارتعاش القاشده توسط جریان، گردابه تصادفی، المان مرزی، چهار سیلندر استوانه ای، جریان آرامFlow-induced vibration is the effective factor in mechanical destruction of structures that are exposed to fluid flow. In this study, random vortex- boundary element methods (RVM-BEM), is used to simulate two-dimensional laminar fluid flow around four one/two-degrees-of-freedom cylindrical cylinders in a rectangular arrangemen. Hydrodynamic force coefficients, streamlines and cylinder displacements were plotted. The vorticity distribution is separated into blob-vortexes and its changes are studied by tracking these vortexes in the Lagrangian approach by considering two mechanisms of convection and diffusion in each time step. Satisfying no-slip boundary condition, vortex sheets were created on boundary. The cylinder vibrations were modeled as a system of mass, spring, and damper. The results showed that for 1 and 2DoF compared to the stationary cylinder, the average drag coefficient changes are 0.84 and 0.97, respectively. The rear cylinders vibrations amplitude were less than in the front cylinders. The y-amplitude was three times larger than x-amplitude. The maximum x-amplitude vibration of 1DoF cylinders was 1.51 times larger than 2DoF ones. Solving flow over 2DoF single cylinder by BEM(no need for homodis mapping or considering the vortexe images) with a similar solution in Ansys-Fluent software, showed 25% reduction in runtime and 2.3% increase in calculations accuracy.
Keywords: Flow-induced Vibration, Random vortex, boundary element, four cylindrical cylinders, laminar flow -
As one of the essential components of the conventional island in a nuclear power plant, the ejector supplies cooling water to the reactor core in an accident state. It needs serious maintenance for its structural stability. The flow-induced vibration of an ejector in service was numerically examined in this research while taking the cavitation phenomenon into account. To achieve this goal, a bidirectional fluid–structure interaction simulation based on the ANSYS platform was run. In our lab, an experimental loop was also set up to validate the fluid model. Then, under specific circumstances, it was possible to monitor the cavitation revolution process, pressure variation, and ejector vibration. According to the numerical results, the distribution of the vapor phase is largely found in the mixing and diverging portions, and it changes over time. In the ejector, a significant wideband excitation was observed. Additionally, the von Mises stress and flow-induced vibrational features of the ejector structure were investigated.Keywords: Numerical simulation, Flow induced vibration, Fluid-structure interaction, Ejector system, Cavitation
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The vibration induced by the cooling fluid flow around the fuel rods in the fuel Assembly of nuclear reactors causes the rods to be destroyed and eventually leak due to the fretting wear in the place of contact with their supports for a long time. In this paper, the vibration caused by axial fluid flow around a specific fuel rod under elastic supports is numerically simulated. In this study, the fluid flow is modeled using the Large Eddy Simulation (LES) turbulence model in the FLUENT software. The fluid-structure interaction is also modeled using the ANSYS coupling system. To validate the implemented numerical model, the test results of the reported brass rod vibration similar to the studied problem in this research are used. Due to the long execution time of the two-way fluid-structure interaction simulations with a high grid number, the one-way fluid-structure interaction method is proposed. The results of simulations show that the one-way fluid-structure interaction method can be used in cases where the vibration amplitude of the structure is less than the height of the viscous sub-layer. Also, this method reduces the simulation time by 80%. Finally, the results of the flow-induced vibration simulation of the fuel rod show that the vibration range of the fuel rod will increase by 20 times if the contact of the elastic supports with the rod is lost, which will lead to the intensification of the wear caused by the rod oscillation. Also, the main natural frequency of the rod decreases when the rod loses contact with the supports and falls within the range of the reactor excitation frequency, i.e. 0 to 50 Hz, which should be avoided.Keywords: Flow-Induced vibration, One-way fluid-structure interaction, LES turbulence model, Fuel rod, Elastic supports
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ناپایداری های ناشی از جریان یکی از چالش های اساسی در زمینه طراحی سازه های دریایی و زمینی در معرض جریان سیال به شمار می رود. روش های کنترلی غیرفعال و فعال بسیار متعددی برای کاهش نوسانات ناشی از جریان استوانه با مقطع مربعی به عنوان هندسه پایه ای برای سازه های دارای گوشه های تیز استفاده شده است. روش مکش و دمش جریان یکی از روش های اثربخش برای کنترل جریان و به تبع آن کاهش ارتعاشات حاصل از نیروهای آیرودینامیکی به حساب می آید که تا به حال برای استوانه مربعی قرار گرفته بر روی بستر الاستیک که آزادانه در جهت عرضی نوسان می کند، اعمال نشده است. بنابراین در مقاله حاضر جداگانه از روش های مکش و دمش برای سرکوب نوسانات ناشی از جریان بهره گرفته شده است. در روش مکش یک شیار در سطح جلویی استوانه و در روش دمش یک شیار در سطح پشتی استوانه جای گذاری شده است. در مطالعه حاضر اثر سرعت های مکش و دمش و هم چنین اثر طول شیارهای مکش و دمش بر کارآمدی روش به کار گرفته شده، مورد ارزیابی قرار گرفته است. با توجه به شبیه سازی های تعامل جریان- سازه ای مشاهده می شود که روش کنترل جریانی اعمال شده به خوبی توانسته است نوسانات ناشی از جریان را کاهش دهد.
کلید واژگان: ارتعاشات ناشی از جریان، ریزش گردابه، کنترل جریان، گالوپینگ، هندسه مربعیFlow-induced instabilities are one of the major challenges in the design of aerospace structures. Numerous passive and active control methods have been used to reduce the flow-induced oscillations of square cross-section cylinder as the basic geometry for structures with sharp corners. The suction and blowing flow control method are one of the most effective approaches for controlling the flow and consequently reducing the vibrations resulting from the aerodynamic forces, which so far are not considered for an elastically-mounted square cylinder which fluctuates freely in the transverse direction. Therefore, in this paper, suction and blowing flow control methods have been used separately to suppress flow-induced oscillations. In the suction-based method, a slut is placed on the front surface of the cylinder and in the blowing-based method another slot is inserted on the back surface of the cylinder. In the present study, the effect of suction and blowing velocities as well as the effect of suction and blowing slut lengths on the efficiency of the flow control approach was evaluated. According to the flow-structure interaction simulations, it is observed that the applied flow control method has been able to reduce flow-induced vibrations appropriately.
Keywords: Flow-induced vibration, Vortex Shedding, Flow Control, Galloping, Square geometry -
In this paper, numerical investigations of the harnessed power from Flow-Induced Vibrations of a new modified circular cylinder are performed. The proposed cylinder modification consists in adding two slots located on the front surface of the cylinder, instead of the baseline configuration, usually applied, which consists of a Passive Turbulence Control in form two straight strips. The computations are based on the solution of the Unsteady Reynolds- Averaged Navier-Stokes equations (URANS) coupled with the dynamic equations system describing the cylinder motion, where turbulence is modeled using the two-equation SST k – ω model. The harvested and the harnessed powers are thereafter calculated according to the amplitude and the frequency of the cylinder oscillatory motion. The numerical results show that the slots lead to shift the flow separation point toward the leading edge, which involves higher hydrodynamic instabilities resulting in higher oscillations amplitudes, and thereby a significant enhancement of the harnessed power is noticed.Keywords: CFD, Flow-Induced vibration, Elastically mounted cylinder, Turbulence, Energy harvesting
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در این مقاله یک استراتژی کنترلی فعال بر مبنای نوسانات چرخشی اجباری به منظور کاهش ارتعاشات ناشی از جریان بر روی یک استوانه دایره ای دو درجه آزادی که آزادانه در جهات طولی و عرضی حرکت می کند، در نظر گرفته شده است. معادلات حاکم بر میدان جریان، معادلات ناویر استوکس تراکم ناپذیر دو بعدی می باشد که به روش حجم محدود گسسته سازی شده است. نسبت فرکانسی f_rot/f_n، و نرخ چرخش α، دو پارامتر مهم در نوسانات چرخشی استوانه بوده که می بایست بگونه ای تنظیم شوند که فرکانس جدایش گردابه ها بر روی فرکانس نوسانات چرخشی اجباری قفل شده و در نتیجه آن، دامنه ارتعاشات عرضی و طولی استوانه کاهش پیدا کند. بر اساس شبیه سازی های جامع انجام گرفته در این مقاله، سه سیستم کنترل فعال حلقه باز منتخب برای سرعت های کاهش یافته واقع در ناحیه قفل شدگی فرکانسی با پارامترهای ورودی (برای)، (برای)، و (برای)، به منظور کاهش موثر دامنه ارتعاشات استوانه انتخاب شده اند. این سیستم ها توانسته اند بیشینه دامنه پاسخ عرضی استوانه را به ترتیب به میزان 88، 92 و 92 درصد نسبت به حالت کنترل نشده کاهش دهند در حالیکه این مقادیر برای دامنه پاسخ طولی استوانه به ترتیب برابر با 93، 90 و 82 درصد می باشند.کلید واژگان: ارتعاشات ناشی از جریان، پدیده قفل بر، نوسانات چرخشی، کنترل فعال ارتعاشات، تعامل جریان-سازهIn this paper, an active control strategy based on the cylinder forced rotary oscillation is considered to reduce the flow-induced vibration of an elastically mounted two-degree-of-freedom circular cylinder free to vibrate in both transverse and in-line directions. The fluid flow governing equations are two-dimensional incompressible Navier-Stokes model which discretized by means of the finite volume method. The frequency ratio〖 f〗_rot/f_n, and rotation rate α, are two important adjustable parameters which must be selected in such a way that the vortex shedding frequency locked on associated rotational forcing frequency, and the cylinder transverse and in-line vibrations will be suppressed accordingly. Based on comprehensive simulations accomplished in this paper, three different active open-loop control systems is selected in order to effectively reduce the cylinder vibrations for reduced velocities in synchronization region with the following input parameters: (for V_r=5: α=1,f_rot/f_n=1.1), (for V_r=6: α=1,f_rot/f_n=1.3), and (for V_r=7: α=1,f_rot/f_n=1.5). These three control systems are found to decrease the maximum transverse cylinder vibration amplitudes by 88%, 92%, and 92% while the corresponding in-line vibration amplitudes decrease by 93%, 90%, and 82%, respectively.Keywords: Flow-induced Vibration, Lock-on Phenomenon, Rotary Oscillation, Active Vibration Control, Fluid-solid interaction
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Journal of Theoretical and Applied Vibration and Acoustics, Volume:5 Issue: 1, Winter & Spring 2019, PP 69 -84An adaptive fuzzy sliding mode controller (AFSMC) is adopted to reduce the 2D flow-induced vibration of an elastically supported square-section cylinder, free to oscillate in stream-wise andtransverse directions in both lock-in and galloping regions. The AFSMC strategy consists of a fuzzy logic inference system intended to follow a sliding-mode controller (SMC), and a robust control system designed to retrieve the variance between the sliding mode and fuzzy controllers. The sprung square cylinder first experiences vortex-induced vibrations with increasing Reynolds number, and then, after passing the critical flow velocity, it confronts high-amplitude and low-frequencyvibrations of galloping owning to its sharp corners. A co-simulation platform is considered by linking the AFSMC system modeled in Matlab/Simulink to the plant model implemented in Fluent, aiming at the calculation of opposite control force needed for comprehensive annihilation of the cylinder motions. Based on the performed numerical simulations, it becomes clear that the utilized active control system has successfully mitigated the two-degree-of-freedom vibrations of a square cylinder in both the lock-in region and galloping zone. Here, the vibration amplitudes in the transverse andstreamwise directions have decreased by 93% and 94%, for the lock-in region and 93% and 99%, for the galloping zone, respectively.Keywords: Galloping, Flow-induced vibration, Square-section cylinder, Intelligent controller, active controller
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In many industrial equipment such as boilers and heat exchangers, the cylindrical tubes are exposed to the gas- liquid two phase flow. For any immersed body in flow field vortex shedding is created with a frequency that may be constant or variable, according to conditions such as flow rates, geometry of body, and etc. The failure will happen in the equipment, when this frequency is close to one of the natural frequencies of them. This can cause noise and flow induced vibration problem which is one of the main defects in the heat exchangers. Therefore considering these flows can play a significant role in long-term reliability and safety of industrial and laboratory equipment. In this study EulerianEulerian approach is employed to simulate two-phase flow around the cylindrical tube. Since the Reynolds Stress Model (RSM) accounts for the effects of streamline curvature, swirl, rotation, and rapid changes in strain rate in a more rigorous manner than other turbulence models, it has greater potential to give accurate predictions for complex flows. So in this study the RSM is used to recognize behavior of vortex shedding in the flow. Drag coefficient, Strouhal number, vortex shedding behind cylinder, void fraction and pressure coefficient distribution were investigated in air-water two phase flows. In order to verify validity of CFD model, inlet void fraction was set to zero, Strouhal number and its relation with low Reynolds number (100-3000) in single phase flow were compared with experimental and numerical result of available literatures. The results show a good agreement between them. Having reasonable judgment of fluctuating lift force frequency, fast Fourier transform (FFT) was applied to lift coefficient of cylinder. The FFT produces averaged spectral coefficients that are independent of time and are useful to identify dominant frequencies in a signal. Some remedies were introduced to reduce vortex frequency of cylindrical tube subjected in gas-liquid two phase flows and protect tube from hard vibrations. For this purpose, some geometrical modifications were applied and results showed that in all cases drag coefficient and Strouhal number reduce.Keywords: Fast Fourier Transform, Flow Induced Vibration, Reynolds Stress Model, Strouhal Number, Two Phase Flow, Vortex Frequency
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