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We perform numerical simulations using immersed boundary method for flow over a single and two fish in tandem performing traveling wave like motion for a range of Strouhal numbers. We investigate the hydrodynamic performance of single- and tandem-fish configurations using unsteady profiles of lateral side-force and drag coefficients, their time-averaged values, and wake behind these bodies. We present the spectra of hydrodynamic forces and find that the nature of these forces for a single fish resembles to those of stationary/oscillating bluff bodies and oscillating airfoils. For tandem cases, we vary the phase speed of undulatory motion of the rear fish while keeping the free-stream velocity constant. We show that hydrodynamic forces of the upstream and rear fish contain harmonics which are produced by nonlinear interaction of the oscillation frequencies of both fish. We find that the wake and time-averaged drag of the upstream fish remain almost independent of the undulating frequency of the rear fish at a certain Strouhal number. We also relate this observation with the absence of oscillation frequency of the rear fish in the Fourier spectra of hydrodynamic forces of the upstream fish. For the complete range of parameters, it is inferred that swimming in a tandem configuration seems more beneficial for the upstream fish. It happens due to wake-splitting effect of the rear fish that causes an enhancement of pressure in its wake. For the rear fish, it gains an advantage of drafting under certain conditions and its performance deteriorates at Strouhal numbers greater than 0.40. [ABSTRACT FROM AUTHOR]

The nonlinearity and time-dependent nature of hydroelastic slamming along with violent fluid–structure interactions (FSIs) and evolution of free surfaces present great challenges to the numerical modeling. The inherent physical mechanisms of such a violent FSI involved with structural vibrations have not been well investigated so far. In this work, an adaptive element–particle coupling approach is extended to reveal the underlying mechanisms of hydroelastic slamming, which leverages the advantages of grid-based method in accurately modeling structural deformations and fluid flows with small deformations as well as particle method in effectively capturing moving interfaces. After well validating with experimental results, we focus on investigating the flow physics and hydrodynamic loads along with structural hydroelastic performances including wedge kinematics and dynamic responses. It is demonstrated that the plate deformation has minimal influence on the maximum hydrodynamic load in normal slamming, whereas it has significant effects in oblique slamming. Both rolling free surfaces and waves around the plate ends may affect the distribution of hydrodynamic loads on the elastic structure. For kinematics and dynamic responses of the structure under slamming loads, different configurations of deformation and stages of motion are identified. With increasing deadrise angle, the structural deformation becomes more pronounced and later gets smaller for a large structural flexibility. However, it may decrease in a direct manner with an increase in the deadrise angle when a smaller plate flexibility is adopted. Further, the important phenomenon of structural vibration is studied with typical influencing factors analyzed. A novel empirical formulation is proposed to predict vibrational characteristics of the wedge during hydroelastic slamming processes. This relation is unified and applicable for a broad of scenarios from normal to oblique slamming. [ABSTRACT FROM AUTHOR]