Development And Application Of Supermesh In Composite Grid Interface

As one of the main methods to solve the problem of multi-block grid coupling,grid interface is widely used in the CFD simulation of ship hydrodynamics.Flux interpolation is the key factor to determine the accuracy of flow field information transfer between different computational domains.According to whether the integral values of physical quantities before and after interpolation are equal,interpolation algorithms can be divided into two types: conservative and non-conservative type;Relatively speaking,the conservative interpolation algorithm has higher accuracy,less dependence on meshing,and can ensure the conservation of physical quantities before and after the interface.Aiming at the requirement of complex CFD simulation such as ship self-navigation,this paper develops a conservative interpolation algorithm based on supermesh for typical complex surface mesh interfaces,and reconstructs the interface flux based on gradient calculation and area weighting to realize the conservative transfer of flow field data between multiple grids.Firstly,we introduces the supermesh algorithm and process of the planar interface,including a series of algorithms such as donor cell searching and selection,spatial projection transformation,polygon intersection,etc.and focuses on the analysis of the donor cell search algorithm that has the great impact on the computational efficiency of the supermesh method.Then we select the alternating digital tree(ADT)algorithm;At the same time,the multi-block coupling calculation method is briefly introduced.Then,aiming at the complex surface interface formed by the common typical surfaces such as spherical surface,cylindrical surface,conical surface and their intersection/ combination/difference combination,a series of algorithms such as the identification of the surface and type of the mesh element,and the three-dimensional space projection transformation of the surface-plane are developed.While maintaining the calculation accuracy and efficiency of the plane polygon intersection algorithm,the influence of the non-conformity between the surface mesh interfaces is eliminated,The supermesh algorithm is extended to typical surfaces and complex surfaces formed by their combination.Then,the supermesh algorithm was systematically tested: a series of test examples were designed under different grid division conditions,including plane,sphere,cylindrical surface,conical mesa and complex plane-surface composite interface shape formed by intersection/combination/difference.The geometric error generated by the supermesh was tested,and the accuracy and conservation of the interface interpolation algorithm were tested by three typical distribution functions.Finally,the developed supermesh algorithm is combined with the independent CFD solver to apply to the numerical evaluation of the energy saving effect of the installation of stern tube in the international standard model JBC of bulk carriers.Firstly,based on the calculation results of ship model resistance and wake flow field with different grid numbers and turbulence models and the comparison with the model test results,the grid division and turbulence model for subsequent CFD calculation are determined;Subsequently,the CFD simulation of propeller open water and ship model self-propelled is carried out using conservation and non-conservative interpolation algorithms respectively,and based on the simulation results,the ship model self-propelled factor and the energy saving effect of stern tube are predicted.The comparative analysis of the results of the two interpolation algorithms shows that the calculation and prediction results based on the conservative interpolation algorithm are closer to the model test results,and the mesh sensitivity is lower;The comparison and analysis of the calculation results of the flow field in front of the propeller shows that the CFD calculation based on the conservative interpolation algorithm can better simulate the influence of the stern tube on the flow field in front of the propeller under the self-propelled state,and thus can more accurately predict the energy-saving effect.The research and development work in this paper is innovative in the supermesh generation algorithm of typical surfaces and complex plane-surface composite interfaces formed by their intersection/combination/difference,which will provide technical support for the autonomous CFD solver’s multi-block mesh high-precision coupling calculation.