Generation And Application Of Unstructured Tetrahedral Mesh For Centrifugal Pumps

Mesh is proper medium for numerical simulations in CFD, whose quality directly influences numerical accuracy and may even determines whether calculating results are reasonable. Due to complex shapes and drastic variety of curvature, unstructured tetrahedral mesh generation algorithm has enjoyed great popularity in mesh generation for centrifugal pumps. However, it still faces so many challenges in generating high-quality tetrahedron. Therefore, how to accurately and efficiently generate well shaped tetrahedron consistent with computational domain is one problem that need urgently solved in numerical simulation of centrifugal pump. It is of great significance to produce high-quality mesh for numerical simulation, promote grid generation theory and method, and provide guidance for high-quality mesh generation in other types of pumps and mechanical components with complex geometrical shapes.This research is just the conquering work supported by the National Natural Science Fund "unstructured CFD mesh generation of pump based on RM-DT algorithm, No. 50825902", aiming at the existing problems during mesh generation. A set of tetrahedral mesh generation algorithms applicable to centrifugal pump is presented based on theoretical analysis, example testification, numerical simulations and experimental study. The main research contents and important conclusions obtained are as followings:1. The former research conclusions on unstructured tetrahedral mesh generation technology were systematically summarized. The main problems within the mesh quality criteria, unstructured surface and solid mesh generation as well as mesh optimization were deeply discussed. Some interfering factors that may affect inner flow numerical simulation and PIV inner flow measurements for centrifugal pumps were generalized.2. Modeling methods for trans-flux components in whole computational domain of centrifugal pump were established through analysis of their effect on the complexity of meshing, the quality of generated meshes and the efficiency of generating process. Based on this, P3D, a three-dimensional parametric modeling and assembling software, was developed by means of VC 6.0 platform and Pro/TOOLKIT. The software introduced 3D solid template driven by dimension variables. And, to accomplish design input and data transmit, the easy-to-use intuitive interface with powerful interaction and visualization was designed by adopting a dynamic link library to call MFC. Examples show that the program runs well with high efficiency.3. A new tetrahedral mesh quality criterion based on geometrical characteristics was proposed to strengthen the ability of evaluating tetrahedron. Comparative calculations with nine common criteria were conducted. And it can be identified that the new criterion shows better advantages in distinguishing poor-quality elements, the change of element shape as well as the time cost. On these bases, coupling with numerical simulation and PIV experiment in bending duct, a means for obtaining grid-independent solution was established. Simultaneously, the relationship between the quality criterion and numerical accuracy was deeply analyzed. The results show that the proposed criterion is actually responsible for causing changes of mesh quality in large error region (Ere>0.15). Furthermore, it can correctly judge out the elements which have great effect on numerical accuracy.4. To remove mapping distortion during surface mesh generation, an algorithm integrating advancing front technique (AFT) and Delaunay triangulation (DT) was firstly presented based on Riemannian metric. The algorithm used AFT to produce background grids in parametric space. During refinement, the traditional Delaunay incremental inserting kernel was replaced by inserting the center of triangle circumscribed ellipse. This algorithm took the mapping distortion into consideration and accelerated the convergence speed by introducing Riemannian metric and reasonable background mesh. Examples of centrifugal pumps with different specific speeds show that the algorithm is easy to implement with good stability, and the high-quality meshes can be obtained.5. To eliminate sliver elements in 3D Delaunay meshes, a relatively better algorithm was improved on the basis of analyzing principles of refinement methods. The algorithm optimized the rule of judging sliver elements. The harsh requirement for locations of inserted points was resolved through producing almost no small sliver elements after inserting new points. The practical applications indicate that the improved algorithm can eliminate sliver elements completely.6. To produce tetrahedral meshes with smooth boundaries, an optimization-based smoothing (OPS) was proposed. The factors that affect optimizing effect, such as objective function and method of solving optimal solution, were compared. The new algorithm was guaranteed to improve through choosing an optimal objective function. The objective function was transformed into a function term containing boundary and interior mesh quality, and a weighty coefficient was added. In this way, boundary and interior mesh quality can be controlled simultaneously. Examples indicate that the algorithm can improve both boundary and interior mesh quality, and yield optimal results with less time when the weighty coefficient is 0.7.7. A mesh quality improvement strategy combining OPS and topological optimization was firstly putted forward to overcome the existing defect that can't improve full-boundary element quality in OPS. According to the sharing form of edge (or face) and its adjacent element, the approach applied different topological optimization methods. The topological correlation of mesh after optimization rather than the element quality in connected region was considered. In this way, this kind of element was transformed into non-full-boundary element. Examples show that the approach shows better advantage in boundary and whole mesh quality than OPS.8. The validity of tetrahedral mesh generation algorithm was made clear by numerical simulation and PIV experiment in centrifugal pumps. A Comparison investigation of sixteen centrifugal pumps with different specific speeds was conducted. In order to confirm the optimal numerical methods, the effects of some factors on numerical simulation, such as computational domain, turbulence models, boundary conditions of inlet and outlet, pressure-velocity coupling algorithm and the convergence precision, were discussed. Based on this, two sets of meshes generated by some commercial software and the proposed algorithm were used in numerical simulation. And calculating results were compared with test. The results show that using the proposed algorithm, the worst element quality and average mesh quality is better under similar number of meshes. For energy characteristic, calculating errors of head and efficiency for both meshes are within 3.5%, but the proposed algorithm is of higher precision. Meanwhile, the corresponding results obtained in inner flow field are more close to the experimental values.