شبیه سازی اندرکنش سازه- سیال و ارتعاش ناشی از جدایش گردابه در استوانه های دایروی و بریده شده

در مطالعه حاضر ارتعاشات ناشی از گردابه در اندرکنش سازه سیال استوانه‌های دایروی و بریده شده مورد بررسی قرار گرفته است. بدین منظور، جریان آرام حول یک استوانه (دایروی و بریده شده) دو درجه آزادی که می‌تواند آزادانه در جهات جریان اصلی و عمود بر آن حرکت کند، در نظر گرفته شده است. به منظورحل کردن معادلات پیوستگی و مومنتوم برای جریان تراکم ناپذیر، غیر دائم و دوبعدی، روش عددی حجم محدود براساس الگوریتم سیمپل به کار برده شده است. علاوه بر آن، جهت شبیه سازی اندرکنش سازه سیال، حل‌گر معادلات حرکت جسم صلب در جهت جریان و عمود بر آن با حل‌گر دینامیک سیالات محاسباتی کوپل شده است. به منظور اعتبار سنجی روش عددی به کار گرفته شده در بررسی اندرکنش سازه سیال، نتایج بدست آمده برای نوسانات عرضی استوانه دایروی و بریده شده در اعداد رینولدز مختلف با نتایج سایر مطالعات مورد مقایسه قرار گرفت و انطباق بسیار خوبی مشاهد گردید. همچنین اثر زاویه بریده شده پشت استوانه بر کاهش ارتعاشات ناشی از جدایش گردابه مورد ارزیابی قرار گرفت. نتایج نشان می‌دهد با افزایش عدد رینولدز جریان از 80 تا 85 ، ارتعاش استوانه‌های بریده شده وارد ناحیه قفل شدگی شده و پرش شدیدی در جابجایی عرضی آنها دیده می‌شود. تفاوت مهم در دامنه جابجایی استوانه‌های بریده شده با استوانه دایروی در نیمه سمت راست ناحیه قفل شدگی است. با افزایش زاویه بریدگی، استوانه‌ها زودتر این ناحیه را ترک می‌کنند و به عبارتی عرض ناحیه قفل شدگی کاهش می‌یابد.

Numerical simulation of fluid-structure interaction and vortex induced vibration of the circular and truncated cylinders

Introduction Vortex induced vibration is a wellknown phenomenon in the engineering applications involving the fluid/structure interaction. Especially, it has been observed in various ocean engineering applications such as offshore risers, deep water bridge piers and oil pipelines. In the flow around bluff bodies such as marine risers, in a specific range of Reynolds numbers, the asymmetric vortex shedding at the bluff body wake results in periodic hydrodynamic forces on the riser and consequently the vortexinduced vibration. When the vortex shedding frequency is close to the natural frequency of the structure, the cylinder tends to dramatically vibrates in transverse direction which is commonly termed as the &lockin& phenomenon. Since vortex induced vibration is one of the most important causes of fatigue damage and structural instability in marine risers, exploring efficient ways to reduce or suppress vortex induced vibrations, has attracted the attention of many ocean engineering researchers. In the present study, twoway fluid/structure interaction simulation of vortex induced vibration of the circular and truncated cylinders are conducted. For this purpose, laminar flow around an elastically supported two degree of freedom cylinder (circular or truncated), which can freely vibrate in streamwise and transverse directions, is considered. Methodology To solve the governing equations of twodimensional, unsteady and incompressible flow over circular and truncated cylinders, a finite volume technique is employed. Moreover, the rigid body motion equations in streamwise and transverse directions are incorporated into the computational fluid dynamics solver to treat the coupling which exists between the fluid flow and cylinder movement. To calculate the rigid body motion of cylinder and treat the fluidcylinder interaction, a UserDefined Function is used. In every time step, the temporal variation of hydrodynamic forces (lift and drag) determined by solving the mass and momentum equations are employed as the source terms in rigid body motion equations to compute the velocity and displacement of cylinders. Fluidstructure interaction is handled using the Fluent’s moving deforming mesh feature which deforms and remeshes cells during transverse and streamwise motions of the cylinders. The pressurebased solver with firstorder implicit unsteady formulation is employed to solve the discretized continuity and momentum equations. The coupling between pressure and velocity fields are handled by using computationally efficient fractional step method along with the noniterative timeadvancement algorithm for time matching strategy in computational fluid dynamics solver. To solve the governing equation for the velocity fields, one needs suitable boundary conditions at the inlet, outlet, lower and upper boundaries, and on the surface of cylinders. A uniform profile of freestream velocity is used at the inlet. At the outlet, the downstream boundary is located far from the cylinders such that the streamwise gradients for the velocity vectors could safely be set equal to zero. Along the upper and lower boundaries, the ycomponent velocity is considered to be zero while for the xcomponent velocity, the gradient in the ydirection is set equal to zero. At the cylinder’ walls, the noslip condition is imposed on both velocity components. Results and discussion In order to validate the numerical method used in the study of fluidstructure interaction, the results for the transverse oscillations of the circular cylinder and truncated one (with truncation angle of 45 degrees) at different Reynolds numbers are compared with the results of Kumar et al. (2018). It is noteworthy that the obtained results in the present study are in good agreement with those of Kumar et al. (2018) and the numerical model accurately predicts the maximum amplitude of transverse vibration and the width of the lockin region. Moreover, the influence of the truncation angle (behind the cylinder) on the vibration suppression of truncated cylinders is evaluated. The results show that as the Reynolds number increases from 80 to 85, the vibration of the truncated cylinders enters the lockin region and experiences a sharp jump in their transverse displacement. Also, in this region, the truncation angle does not have a significant effect on the transverse vibrations of the cylinders and merely reduces their inline vibration. However, changing the structural design of the cylinder (making a truncation at the back of the cylinder) has a substantial effect on the vibration reduction in the right half of the synchronization region. At Re = 100 (Reynolds number corresponding to the lockout region), when the truncation angle increases from zero to 60 degrees, the transverse vibration of the cylinder is reduced by about 66%. Conclusion In summary, it is concluded that the significant difference in the oscillation amplitude of the circular and truncated cylinders is in the right half of the lockin region. When the truncation angle increases, the width of the lockin region decreases.