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We perform numerical simulations using immersed boundary methods for flow over fish, single and in tandem arrangements, performing traveling wave like motion. We analyze nonlinear mechanics of hydrodynamic lateral side-force and drag/thrust coefficients. We compute their Fourier spectra and find that lateral side force carries fundamental and odd harmonics while drag is composed of even harmonics when single fish swim in free-stream. For tandem arrangement, we set the phase-speed of downstream fish as 0.75 times that of upstream fish. Fourier spectra of lateral force for both upstream and downstream fish are composed of forcing frequencies of both the fish. Even harmonics of forcing frequencies along with combined modes at sum and difference of these frequencies appear in their drag-spectra. We also investigate the symmetric or antisymmetric nature of the fundamental and combined modes using the conventional symmetry principles and proper orthogonal decomposition technique. We show that the results obtained from both of these techniques agree well with each other. However, we also find some anomalies in case of fish swimming in tandem configuration. [ABSTRACT FROM AUTHOR]

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]

Hydroelastic fluid-structure interaction (FSI) problems usually involve nonlinear free surface flows, large structure deformations and changing interfaces, which present great challenges for numerical modeling. To deal with violent FSIs, the authors proposed a multi-resolution smoothed particle element method (M-SPEM), which adaptively simulates local fluid regions with an improved smoothed particle hydrodynamics (SPH) method and models other solid/fluid regions with a smoothed finite element method (S-FEM). Both computational accuracy in local fluid domain and entire efficiency can be improved by M-SPEM compared with the conventional partitioned FSI solvers based on SPH-FEM coupling schemes. In this work, an improved M-SPEM is developed and extended to simulate the complex hydroelastic FSI problems with structural deformations considered in the numerical model. To solve the accuracy reduction problem in the multi-resolution simulations, we developed a velocity and position correction technique for the generated fluid and virtual particles. The computational results demonstrated that the new correction technique can effectively improve the computational accuracy of M-SPEM while the corrected schemes produce results very close to the reference ones. The positions of the generated particles should also be appropriately optimized during the element-particle conversion process. With both accuracy and efficiency improved, the improved M-SPEM is proven to be suitable to deal with some challenging hydroelastic FSI problems. • An improved M-SPEM approach is developed to model hydroelastic FSI problems. • The velocity and position correction technique is proposed to improve the computational accuracy. • The improved M-SPEM can greatly save computational cost compared to the general FEM-SPH coupling approaches. • The multi-resolution interfaces between elements and particles are handled by splitting ghost particles. • The M-SPEM is very effective for modeling challenging FSI problems involved with large structural deformations. [ABSTRACT FROM AUTHOR]