The Improvement Of Coupling Software Of CFD-DEM Based On The Domain Decomposition Method

As the continuous development of modern industry, the application domain of CFD hasbecome more widely, extending from simple fluid analys to cross-industry, multi-functionalfluid simulation tools, and it has truely entered into the engineering fields to solve practicalproblems. As the same time, with the improvement of computer technology, a large amount ofCFD simulation softwares have emerged around the country and abroad. Not only can theyconveniently simulate the flow field in order to save great manpower, materials and time, butalso can analyze the simulation results to provide the theoretical direction to experiments.Nowadays, because of the application domain of CFD software expanding constantly and thedemand of industial design swelling rapidly, serial calculation process of CFD is not able tomeet the existing requirements in neither computational efficiency nor calculation time anymore, therefore, the feasible and efficient parallel methods of CFD are further researched bydomestic and foreign scientists, simultaneously it has also become a hot and frontier subjectall over the world.Starting from the parallel method of CFD, in this paper, the approach of domaindecomposition is applied to decompose the big promblem of the whole fluid computation intoseveral small problems which are easy to be solved. Thus, the computational scale is reduced,and the traditional serial SIMPLER algorithm is improved to be a parallel algorithm that issuitable for the implementation in parallel machine. The specific idea is as follows. First, thestructured and collocated grid is used to discrete the space of governing equations. Meanwhile,according to the geometric feature of flow field calculation region and the quantity of CPUs inparallel machine, the partially covered grid partition method that belongs to domaindecomposition method is selected to devide the whole3D flow field calculation region intoseveral sub regions in regular shape, and the sub region is called subfiled. Then, thecomputing task of each sub field is mapped to each computing node in the parallel machine,i.e., on every computing node the SIMPLER algorithm is implemented respectively to solvethe fluid. But before starting to calculate at each timestep, each computing node should firstexchange data with adjacent computing nodes depending on a pre-defined coupling rule, so asto ensure the convergence of the solutions. Finally, the calculated results on each computingnode are integrated to fulfill the solving process of the original problem. As for adjacent areasof the overlap part caused by partition, special treatment is needed to avoid the repetitivecomputation of each subfield overlap which may lead to calculation errors in final result. Inthe aspect of algorithm implementation, OpenMP technology and object-oriented analysismethod are adopted, and the calculation part of fluid is abstracted into a base class, in which physical parameters of fluid and calculation functions of algorithm are included. Theobject-oriented method and the thought of partition parallel computation are combined, andeach sub field is regarded as an independent object whose data can be exchanged and statecan be changed. The OpenMP technology is used to create multiple threads, and each thread isresponsible for the operation of an independent fluid object to complete all the calculationtasks of the subfield. Then, according to the features of the software, dynamic linklibrary technology is applied to implement the modular design of CFD calculation part, andthe conversion of the software operation platform from32bit to64bit is fulfilled. To verifythe correctness and reliability of the developed module, comparison tests are operatedbetween the original CFD simulation software and the commercial software FLUENT.In this paper, the research and development of parallel calculation module of CFDsimulation is systematically completed. After understanding the structure of the existing DEMsimulation software in the project group, the parallel calculation module of CFD simulation isembedded into the DEM simulation software, thus the DEM-CFD coupling simulationsoftware is developed, and some improvements of this coupling software are realized. Thetwo new functions are: coupling calculation of fluid and non-spherical particles andcoupling calculation of fluid and non-spherical particles with multiple attributes. At the sametime, based on the original discrete format of the software, a new unconditionallystable discrete format is added, compared with the two-level upwind format and the QUICKformat, this hybrid format has higher computation efficiency and is unconditionally stable, soit can always produce solutions more real physically. Because Gauss-Seidel iterative methodis used to solve fluid control equations, and the Gauss-Seidel iterative method often takes along time to solve such equations, in order to speed up the process of solution, theRossoneri node strategy is adopted to improve the solution, which transforms the serialsolution to a multithreading parallel solution. By using such approach, the calculation speed issuccessfully increased and the simulation time is effectively reduced.In conclusion, through the use of domain decomposition method and the application ofmultithreading parallel technology as well as DLL, the traditional SIMPLER algorithm isimproved to be a parallel algorithm. Testing result indicates that this simulation result showesa similar tendency to the one gotten by using CFD software with the serial SIMPLER algorith,what’s more the simulation speed is abviously increased and the calculation time isreduced. Finally, through the improvements of the coupled DEM-CFD simulation software,the scope of software application is expanded, and a better user experience is achieved.