Research On Solid Model Conversion Method From BREP To CSG

Monte Carlo simulation tools basically use CSG(Construction Solid Geometry)method to construct geometric models, while CAD systems commonly useBREP(Boundary Representation)method for three-dimensional solid modeling.Theformeris based on that CSG is conducive to the relative position judgment betweenparticle and3D solid model in Monte Carlo simulation,the latter depends on theexcellent3D solid model description ability of BREP. In order to apply the powerful3D modeling capabilities of CAD systemsto Monte Carlo simulation, the core problemto be solvedis the BREP->CSG conversion method.This paper firstly analyzed the differences betweenMonte Carlo simulation toolsand CAD systems in geometric modeling, introduced the necessity and engineeringapplication requirement of BREP->CSG conversion method research, then discussedthe theoretical system of BREP->CSG conversion method in detail, including the basissolid model representation methods and the current domestic and internationalmainstream conversion algorithms. After summarized and refined these conversionmethods,oriented to specific engineering demands, this paper accomplishedengineering implementation of the conversion method, put forward the algorithm ofconversion from CAD neutral file based on STEP standard to MCNP geometry file.The overall algorithm is divided into five sub-modules from top to bottom asfollows:STEP file parsing module, separating half-spaces generation module, featurepoints generation module, feature points classification module,and MCNP geometryfile generation module.The algorithm firstly imports and parses the STEP file, and thenadds the essential separating half-spaces for STEP file->MCNP geometry fileconversion,followed by feature points set construction and feature points classificationoperation for determining the solid model CSG expression,and finally the CSGexpression is interpreted as MCNP formatted geometry file. The five sub-modules areindependent logically,the upper module’s output is the input of the lower module; theyare connected functionally because of internal call relationship. They cooperate toachieve the final STEP file->MCNP geometry fileconversion.Then based on the C++language and MATLAB calculation engine, this paper implemented STEP file->MCNP geometry file conversion algorithm by program development. The coreissues and key technologies of each sub-module were researched, analyzed and solved.The kernel source code is presented here.After completed theengineering development work, program testing andalgorithm verification werecarried outin the following way: using Pro/E to construct3D solid models, converting the STEP files by the conversion program, andusingSimpleGeo for MCNP geometry filesgraphical verification assisted bymanualchecking. Firstlya series of basicvolume elements were tested, then some solid modelscomprised bybasiccurved surfaceswere tested,finally the program converted the STEPfile of anaccelerator component model used in actual project and the results in linewith expectations were achieved.