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The precessing vortex core in the draft tube of Francis turbine is investigated using entropy production method. The high-fidelity Delayed Detached Eddy Simulation (DDES) method with SST k-ω turbulence model is used to acquire the entropy dissipative components of the flow, and the Q-criterion is selected to evaluate the vortex cores inside the draft tube under different operating conditions, separately. The results show that the numerical simulation is in good agreement with the experimental data obtained from the model test. It is found that the vortex motion, separation flow and shock phenomenon are considered to be hydrodynamic factors for the formation of entropy. Furthermore, draft tube accounts for the greatest proportion of entropy production, which is ascribed to the vortex rope. Vortex rope will rotate in the same direction as the runner, but the rotational speed is lower, resulting in low frequency pressure pulsation with large amplitude in the draft tube. It is concluded that the key reason why great energy dissipation is caused in the draft tube is that the vortex rope is transporting continuously towards the downstream, especially when the turbine deviating from optimum operating point.

In this paper, the hydrodynamic forces and vortex distributions of a riser subjected to a non-uniform flow generated by an upstream oscillating wing structure are simulated using high-precision code Incompact3d combined with immersed boundary method. The effects of in-line and transverse positions, oscillating frequency and amplitude on riser performance have also been discussed. It is found that with the increase of in-line spacing or the decrease of oscillation frequency and transverse positions, the time-averaged drag coefficient of riser changes from the negative values to positive values. The time-averaged lift coefficient of riser is generally insensitive to oscillation frequency. However, it shows a large shift when changing the transverse positions or the amplitude of the wing is larger than π/6. Moreover, for various in-line positions, oscillating frequency and amplitude, the vortex shedding of the rigid riser, indicated by the main frequency of lift force, is always locked by the oscillating wing. However, when the transverse position is changing, the ratio of main frequency of lift coefficient to the swing frequency does not still remain 1:1. More concretely, the maximum time-averaged drag coefficient and lift coefficient Root-mean-square value of 0.559 and 1.629 is obtained in the simulation respectively.

Fluid structure interaction (FSI) problems are often encountered in ocean science and technology. In this present, an implicit immersed boundary (IB) method combined with Lattice Boltzmann Method (LBM) and finite element method (FEM) is presented for the strong-coupled simulations of fluid structure interaction. Velocity field is solved rapidly by lattice Boltzmann-BGK equations without external force, and the deformation of a flexible structure was computed by FEM. Fluid structure coupling was processed by immersed boundary implicit forced method. The resulting combination gives an easy-to-use, inexpensive and accurate technique to solve FSI problems. Simultaneously, the body force on the immersed boundary can be calculated directly and acts as the load for structure solver, so the displacement of structure can be obtained at the same time, which is avoid the time lag between the fluid solver and solid solver. Several examples including flow over a static and deformable plate, and the flow problems of an oscillating and cantilever flexible plate were simulated and analyzed. The results agree well with the previous experimental and numerical investigations, which indicate the IB-LB-FEM proposed in this paper is an efficient and reliable fluid-structure interaction numerical simulation method.