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Mechanism of hydrodynamic aggregation of swimmers is an intriguing problem, which is usually studied by the immersed boundary (IB) method. In this paper, a loosely coupled partitioned algorithm is used to simulate this fluid-structure interaction (FSI) problem. Specifically, flow field is simulated by finite volume method, and structure field is approximated by solving the momentum equation and bending vibration equation of Euler-Bernoulli beam. First of all, this algorithm is validated through simulating collective behavior of two actively moving two-dimensional (2D) flexible plates in tandem configuration driven by the harmonic plunging motions of their leading edges with identical frequency and amplitude. Then, the hydrodynamic aggregations of multiple (two or three) 2D flexible plates in side-by-side configuration are studied. It is found that two different stable configurations (alternate-leading mode, AL and staggered-following mode, SF) are formed spontaneously in two parallel flexible plates, and are determined by the lateral spacing between them. In three parallel-plates scenario, the middle plate follows behind in SF mode, while the other two plates present AL and SF modes with different lateral spacings accordingly. Besides, the flow details in different configurations are illustrated and the corresponding propulsive properties (velocity, input power and efficiency) of plates are discussed. • Loosely coupled partitioned algorithm is adopted for hydrodynamic schooling problem. • Hydrodynamic aggregation of multiple 2D self-propelled flexible plates is simulated. • Flow details are discussed to explain mechanism by which collective modes are formed. [ABSTRACT FROM AUTHOR]

Hydrodynamic motion of rigid body in narrow space is a common phenomenon in nature and ocean engineering application. In this paper, a new simulation model based on Fluent is proposed to solve this kind of fluid-structure interaction problem in which added-mass effect is strong. In this new model, hydrodynamic force is decomposed, and a new six-degree-of-freedom rigid-body motion equation is formed with added mass force being added to both sides of the motion equation used by 6DOF solver in Fluent. First of all, numerical results of free-floating rigid body in open domain by 6DOF solver and experimental data of free-falling sphere in still water are presented for the validation of this new simulation model. Then, the scope of this model is studied. It is found that 6DOF solver in Fluent cannot solve the hydrodynamic motion of rigid body if added-mass effect is strong, while this new model is apt for this kind of problem and has good convergent characteristics. Finally, hydrodynamic behavior of rigid body in a changing domain is simulated by the new model. Acceleration and spatial displacement curves are obtained, and details of flow at some key moments are presented and analyzed. • A new simulation model improves Fluent in solving hydrodynamic behavior of rigid body with strong added-mass effect. • The scope of the new simulation model is introduced by comparative analyses with 6DOF solver in Fluent. • Added mass of rigid body in the changing domain is estimated. • The hydrodynamic motion of rigid body in the changing domain is analyzed. [ABSTRACT FROM AUTHOR]

Loosely-coupled scheme is extensively used to simulate fluid-structure interaction (FSI) problems due to its efficiency. However, it meets the numerical difficulty of instability in the strong added-mass scenario. In this paper, a loosely-coupled scheme is proposed for solving the flow-induced flapping problem of two-dimensional (2D) flexible plate with strong added-mass effect. Specifically, finite volume method is used to simulate the flow field and mode superposition method is applied to calculate the structure field. In order to avoid the added-mass induced numerical instability, governing equation of the flexible plate is modified with added mass being added to its both sides. First of all, this proposed scheme is validated through simulating hydrodynamic response of a single cantilever flexible plate in uniform flows. Then, to study the stability of the proposed scheme, the flow-induced oscillations of a cantilever flexible plate with various low mass ratios and strong added-mass effects are studied. The critical stability boundary of the flapping dynamics of a cantilever flexible plate is also presented. Finally, the proposed scheme is applied to simulate the hydrodynamic behaviors of two parallel plates in uniform flows. Two distinct flapping dynamics models (in-phase and out-of-phase oscillations) are obtained and their corresponding flow details are illustrated. • A loosely-coupled scheme is proposed for FSI problem with strong added-mass effect. • Flow-induced motion of a 2D flexible plate with various low mass ratios is discussed. • Two 2D flexible plates with different lateral spacings in uniform flows are studied. [ABSTRACT FROM AUTHOR]

The independent escape capsule, an important facility of submarine search and rescue (SAR) system, gains extensive attention recently. However, researches on its ejection performance are rare, not to mention the studies of structural design in terms of this theme. In this paper, the vibration-absorptive materials clinging to the inner wall of its housing are used, and hydrodynamic responses of the capsule at a particular orientation in steady, uniform currents under various design parameters such as rigidity, friction coefficient and thickness are simulated. In order to study this interdisciplinary problem of fluid-structure interaction and multi-body dynamics, a simulation model (loosely coupled scheme) based on Fluent (version 13.0.1) is adopted with complement of the contact force between the capsule and the vibration-absorptive materials. Specifically, Fluent is used to assess the hydrodynamic force acting on the capsule, distributed force mode and material mechanics method are adopted to obtain the contact force, and then fourth-order Runge-Kutta method is implemented to calculate the motion equations of the capsule. Discussion of motion characteristics of the capsule indicates that this vibration-absorptive design helps the ejection of the capsule. Mechanism of vibration-absorptive design is discussed and some design suggestions are given. • Vibration-absorptive design is proposed to improve the ejection performance of capsule. • Contact behavior between the capsule and vibration-absorptive materials is described. • Ejection process of the capsule using vibration-absorptive design is simulated. • Effects of design parameters of the vibration-absorptive materials on the ejection performance are discussed. [ABSTRACT FROM AUTHOR]

Hydrodynamic behavior of an independent escape capsule from a disabled submarine is a complex six-degrees-of-freedom motion, in which added-mass effect is strong and variable constraint conditions happen. In this paper, loosely (or weakly) coupled scheme is adopted to solve this fluid-structure interaction problem. Flow field is simulated by Fluent (version 13.0.1) and motion equations are solved by the fourth-order Runge-Kutta method separately. Unstructured dynamic mesh technology is used to trace the trajectory of capsule. Hydrodynamic force is decomposed into resistance and added mass force, and new motion equations are derived in order to handle the huge added mass. The surfaces of capsule and its chamber come into contact randomly. The variable constraint conditions are analyzed and their corresponding kinematical equations are introduced. This loosely coupled scheme is applied to study the hydrodynamic behavior of a two-dimensional capsule from a narrow space into an open domain under different ocean currents. Hydrodynamic force, motion characteristics and trajectory are obtained. The simulation indicates that the velocity and direction of flow strongly affect the motion of capsule, and the contact force leads to the jamming problem of capsule during its ejection process. • A loosely coupled scheme is proposed for fluid-structure interaction problem under variable constraint conditions. • Fluid-structure interaction problem with strong added-mass effect is studied. • Added mass of rigid body in finite fluid domain is calculated. • Motion characteristics and trajectory of an ejected capsule inside a chamber are analyzed. • The effect of ocean currents on ejection performance of capsule is discussed. [ABSTRACT FROM AUTHOR]