An Euler Solver Based Method For Effective Wake Calculation

In the propeller design practice of these days, the performance of a propeller in the ship wake inflow is evaluated by methods based on the potential flow theory. However, when the flow field around the propeller is analyzed by potential flow theory, the effect of the ship hull boundary layer interaction due to fluid viscosity is neglected. To avoid the inaccuracy caused by this potential flow assumption, propeller designers have introduced the notion of the effective wake. So the effective wake is the properly correction to the propeller analysis method based on potential flow theory which includes the interaction between the propeller and the inflow.The effective wake is one of the primary reasons of propeller cavitation and induced ship hull vibration. Because of the complexity of the three dimensional ship wake field, it is very difficult to determine the three dimensional effective wake correctly. Therefore, the prediction of effective has an important value in the propeller design stage. In addition, the requirements of vibration reducing and energy saving on modern ships are more and more high, then the design requirements of the propeller is also increasing, so more precise results of the effective wake distribution are required. Typically, the method of getting ship wake fraction is making specialized model test, but there exists some disadvantages of the model test, such as scale effect, result conversion errors, as well as large funding. While the theoretical computational method can make up for the shortcomings of the model test method, numerical calculations are indispensable in the prediction of effective wake.Based on the basic theories of computational fluid dynamics and commonly used numerical methods, the potential flow theory is adopted to prediction the effective wake in this paper. The Euler equations of incompressible and inviscid fluid are used for control equations of the stern flow field, and the propeller is computed by lift line theory. The nominal wake of model test measurement as computation region's inflow field is used to solve Euler equations.In view of the difficult of velocity and pressure coupling problem of the Euler solver, the SIMPLE algorithm is applied. The staggered grids and finite volume method (FVM) are applied for the discretization of momentum equations, while the continuity equation is handled by the pressure correction method. The discretization equations are solved by Tri-Diagonal Matrix Algorithm (TDMA) and SIMPLE (Semi-Implicit Method for Pressure-Linked Equation) algorithm. The propeller loads are converted into body forces distributed over the mesh cells, and included in the source term of momentum equations in order to consider the influence of the propeller on flow field. The induced velocity and loads of the propeller are calculated by lift line theory, and coupled with the Euler solver by iterations. The effective wake distribution will obtain when the iterations converged.The steady axisymmetric and non-axisymmetric effective wake fields are computed respectively by applying those methods stated above. But because of the limitation of the means and time and so on, the calculation results are not very ideal, it remains to be further research and improvement. Finally, the experiences and shortage from the present work are discussed, which should be paid more attention to in the future work.