Compatibility and Accuracy of Mesh Generation in HyperMesh and CFD Simulation with Acusolve for Torque Converter

Transcription

1 Compatibility and Accuracy of Mesh Genen in HyperMesh and CFD Simulation with Acusolve for Converter Kathiresan M CFD Engineer Valeo India Private Limited Block - A, 4th Floor, TECCI Park, No. 176 Rajiv Gandhi Salai, Sozhanganallur, Chennai , India Umamageswari A CAE Specialist Valeo India Private Limited Block - A, 4th Floor, TECCI Park, No. 176 Rajiv Gandhi Salai, Sozhanganallur, Chennai , India Subramanian J Senior Engineer Valeo India Private Limited Block - A, 4th Floor, TECCI Park, No. 176 Rajiv Gandhi Salai, Sozhanganallur, Chennai , India Abbreviations: Finite Volume Method (FVM), Finite Element Method (FEM), Computational Fluid Dynamics (CFD), Moving Reference Frame (MRF), Converter (TC) Keywords: Converter, Impeller, Turbine, Lockup, Stator Abstract CFD is the analysis of the systems involving fluid flow, heat transfer and associated phenomena such as chemical reactions by means of computer -based simulation. The solution to a flow problem (velocity, pressure, temperature etc) is defined at nodes inside each cell. The accuracy of a CFD solution is governed by the number of cells in the grid. In general, the larger the number of cells at critical areas, better the solution accuracy. Both the accuracy of a solution and its cost in terms of necessary computer hardware and calculation time are dependent on the fineness of the grid. At present grid genen is still up to the skills of the CFD user to a design that is a suitable compromise between desired accuracy and solution cost. Over 50% of the time spent in industry on a CFD project is devoted to grid genen. Grid design is the main tasks at the input stage and subsequently the user needs to obtain a successful simulation result. This paper is mainly concentrated on grid genen on complex model based on our requirements. Nowadays lots of commercial soft wares are available for grid genen. Among these, the selection of mesh tool plays an important role to get optimal mesh for the simulation. For this work, HyperMesh has taken for grid genen. Based on our requirements such as elements quality, number of cells, mesh genen time, effort to design grid, accuracy of result are considered in this paper. This experimental analysis is performed on torque converter. The generated mesh from HyperMesh meshing tool is simulated by using FVM solver. Then mesh is generated in AcuConsole preprocessor and solution is done with AcuSolve (FEM).Then the simulation results are compared with test results. By this work it will be helpful to select suitable meshing platform for our product torque converter for CFD simulation. So that HyperMesh helps to reduce the time spent on a CFD project for grid genen. Introduction converter is mounted between the engine and the transmission system. It consists of main three parts pump, turbine and stator that transfer the power to the transmission system from the engine. Pump is connected to engine shaft which is driven by engine and imparts the energy to fluid. Turbine is connected to transmission system through gear box. It intakes the energy from fluid and transfer power to wheels. Stator is key part in the torque converter which diverts the oil flow from turbine to pump without affecting the pump rotation. This gives high stall torque which is required when vehicle is started to move. The important characteristic of torque converter is the ability to multiply torque when there is substantial difference between input and output speed. It also serves as automatic clutch to transmit power and avoiding the engine vibn transfer to transmission system that results in smoothened output power and driving comfort. Figure 1: Converter Simulation Driven Innovation 1

2 Process Methodology Converter cad model generated in CATIA Case 1 Case 2 Meshing in HyperMesh Meshing in AcuConsole Solution by FVM Solver Solution with Altair AcuSolve (FEM) Validation of results Validation of results Checking the feasibility for automation of TC Hydraulic Performance Simulation Figure 2: Process Methodology Formulae Used converter consists of 30 blades of stator, impeller and turbine. So 12 degree rotational periodic model is taken for the analysis 12 deg. (1) 12deg. (2) Where, Multiplication factor=30... (3). (4) (5) Quality criteria used for Meshing converter contains four fluid regions such as impeller, turbine, lockup and stator. This fluid model is meshed with tetrahedral elements. As per the quality requirement, maximum element size is assigned as 1 mm. Skewness for the surface mesh is kept less than 0.7 and sqewness for the volume mesh is maintained less than 0.9. The most important part in torque converter is Stator, because it directs the flow from turbine to impeller. So it is necessary to capture all the features in stator with refined mesh. For that purpose, Proximity and Curvature size function is applied to Stator fluid. As it is a periodic model, same type of mesh is generated on both periodic faces. Simulation Driven Innovation 2

3 Case1: Meshing in HyperMesh Impeller blades Turbine blades Stator blade-curvature and proximity In the first case, Surface mesh and Volume mesh is generated in HyperMesh. Skewness for Surface mesh is less than 0.7 and Skewness for Volume mesh is less than 0.9 Challenges Meshing cannot be fully automated by using Batch Mesher Figure 3: Meshing in Hyper Mesh Impeller Stator Turbine Lockup Simulation Driven Innovation 3

4 Results & Discussions Solution with FVM solver: The HyperMesh fluid model is solved using FVM solver. Steady state solver with incompressible turbulent flow settings is selected. Coupled algorithm for pressure-velocity coupling is used. As it is turbo machinery simulation, Moving Reference Frame (MRF) approach is applied for pump and turbine regions. The MRF approach implies that there is no relative mesh motion of the rotating and stationary parts. By using right hand thumb rule, rotation direction for lockup, impeller and turbine is defined. Well established Realizable K- Turbulence model (2 eqn) is selected for capturing turbulence and oil properties are assigned. Impeller rotates at engine speed and turbine speed is assigned based on the speed. To improve the calculation stability, initially calculation is performed with first order upwind scheme then it is switched to second order upwind scheme Spee d Results Comparison results HYPERMESH Difference(%) By comparing the results, there is maximum 6.29% in and 3.33% in deviation between HyperMesh results and results. HyperMesh is satisfying the quality criteria that we are following and well aligned with our process. Table 1: Comparison of results and HyperMesh results Comparison of Comparison of Ratio HyperMesh 2.00 HyperMesh Ratio Speed Ratio Speed Ratio Figure 4: Comparison of HyperMesh results with results Simulation Driven Innovation 4

5 Case 2: Meshing in AcuConsole Figure 5: Meshing in AcuConsole Periodic Boundary faces In the Second case, mesh is generated in AcuConsole. The fluid model is meshed with tetrahedral elements. As per the quality requirement, maximum element size is assigned as 1 mm. In periodic boundary condition each element is paired with other opposite element. The visualization of periodic elements is easily understandable Challenges To create periodic mesh, coordinate values are needed. But finding coordinate values in AcuConsole is difficult There is no geometry cleanup and mesh editing features Simulation Driven Innovation 5

6 Results & Discussions Solution with Altair AcuSolve (FEM) solver: Altair AcuSolve is an FEM solver used for this TC hydraulic performance simulation. Moving Reference Frame (MRF) and Spallart Allmaras (one eqn) Turbulence model is used. And other boundary conditions are same for both cases. The result obtained from AcuSolve is second order. It produces the faster convergence results Speed results Results Comparison Altair AcuSolve Difference(%) By comparing the results, there is maximum % in and % in deviation between AcuSolve results and results. Table 2: Comparison of and Altair AcuSolve results Comparison of Comparison of 250 FEM-AcuSolve 2.0 FEM-AcuSolve Speed Speed Figure 6: Comparison of and Altair AcuSolve results Simulation Driven Innovation 6

7 Benefits Summary HyperMesh is the promising software for our TC hydraulic performance simulation. It is reducing the preprocessing hours considerably when CFD model is bigger than 12 deg. In other end, AcuSolve has inbuilt preprocessor and has single GUI for meshing and solving. It avoids mesh export from preprocessor and import to solver time. And it eliminates clean-up and mesh quality improving time. Challenges Initially we faced periodic definition issue for our fluid model in AcuSolve. For post processing the contours and vectors, we need to use HyperView separately. If it is inbuilt in AcuSolve, then it will be more convenient. AcuSolve help documentation is not in detail about features and improvement is needed. Future Plans We are planning to validate further HyperMesh for our TC simulation meshing automation. Also we are planning to validate AcuSolve for other periodic angles like 36 deg, 120 deg and full model simulation to understand the results variations & correlation with test measurement. Conclusions We are looking for complete automation for TC hydraulic performance simulation. So we are validating AcuSolve competency for our process. It shows that AcuSolve can be confidently used to compare two or more designs for identifying better design quickly. However, difference between AcuSolve and test measurement is slightly larger than our current process software. We hope it will be improved by appropriate solver settings and in future release versions. ACKNOWLEDGEMENTS The authors would like to thank Altair Engineering, India for providing technical support in Altair AcuSolve. The authors would also like to thank Mr.Sriram, R&D Director and Bagath Singh R, engineering Manager, Power Train Transmissions, VIPL, Chennai for their constant encouragement REFERRENCES [1] Versteeq H.K. and W. Malalasekara An Introduction to Computational Fluid Dynamics, Longman Group Ltd, [2] Ubaldi M., Zunino P., Barigozzi G. and Cattanei A., "An Experimental Investigation of Stator Induced Unsteadiness on Centrifugal Impeller Outflow", Journal of Turbo machinery, vol.118, 41-54, [3] Ramamurthi, V., Finite Element Method in Machine Design, Narosa Publishing House, January 2009, ISBN: [4] Combès, J.F., Bert, P.F. and Kueny, J.L., "Numerical Investigation of the Rotor-Stator Interaction in a Centrifugal Pump Using a Finite Element Method", Proceedings of the 1997 ASME Fluids Engineering Division Summer Meeting, FEDSM , Simulation Driven Innovation 7