The immersed boundary method(IBM)is one of important numerical methods for solving the interaction between flexible structure with large deformation or rigid structure and viscous fluid. The advantages of IBM are high computational efficiency, suitable for complicated shape and simple grid structure which means a conformal mesh can be avoided and an adaptive mesh technique can be implemented easily. The original IBM is suitable for the flexible body and was updated with feedback force by Lai and Peskin to approximately deal with a rigid boundary problem. In this paper the unsteady flow around a tandem cascade of Francis turbine is simulated by this updated method. Particularly, the cartesian adaptive mesh refinement and finite difference method are applied in the whole flow field. The unsteady characteristics of the flow field, the non-uniformity indexes for pressure, velocity and vorticity in the guide vane wake are attained. These results are helpful to study on the effects of uneven flow field on fluid-induced blade vibration in a hydro-turbine in future.
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