Fluent-Intro_15.0_L10_BestPractices.pdf

1 © 2013 ANSYS, Inc. February 28, 2014 ANSYS Confidential 15.0 Release Lecture 10: Best Practice Guidelines Introduction to ANSYS Fluent 2 © 2013 ANSYS, Inc. February 28, 2014 ANSYS Confidential Lecture Theme: The accuracy of CFD results can be affected by different types of errors. By understanding the cause of each different error type, best practices can be developed to minimize them. Meshing plays a significant role in the effort to minimize errors. Learning Aims: You will learn: •Four different types of errors •Strategies for minimizing error •Issues to consider during mesh creation such as quality and cell type •Best practices for mesh creation Learning Objectives: You will understand the causes of error in the solution and how to build the mesh and perform the simulation in a manner that will minimize errors Introduction Introduction Error Types Best Practices for Meshing Summary 3 © 2013 ANSYS, Inc. February 28, 2014 ANSYS Confidential Motivation for Quality CFD-Results are used for many different stages of the design process: •Design & optimization of components and machines •Safety analyses •Virtual prototypes When undertaking a CFD model, consideration should be given to the purpose of the work: •What will the results be used for? •What level of accuracy will be needed? Introduction Error Types Best Practices for Meshing Summary 4 © 2013 ANSYS, Inc. February 28, 2014 ANSYS Confidential Different Sources of Error There are several different factors that combine to affect the overall solution accuracy. In order of magnitude: •Round-off errors –Computer is working to a certain numerical precision •Iteration errors –Difference between ‘converged’ solution and solution at iteration ‘n’ •Solution errors –Difference between converged solution on current grid and ‘exact’ solution of model equations –‘Exact’ solution  Solution on infinitely fine grid •Model errors –Difference between ‘exact’ solution of model equations and reality (data or analytic solution) Introduction Error Types Best Practices for Meshing Summary 5 © 2013 ANSYS, Inc. February 28, 2014 ANSYS Confidential Round-Off Error Inaccuracies caused by machine round-off: •High grid aspect ratios •Large differences in length scales •Large variable range Procedure: •Check above criteria •Define target variables •Calculate with: – Single-precision – Double-precision •Compare target variables Introduction Error Types Best Practices for Meshing Summary 6 © 2013 ANSYS, Inc. February 28, 2014 ANSYS Confidential Iteration Error Example: 2D Compressor Cascade Check for monotonic convergence (Residual) Introduction Error Types Best Practices for Meshing Summary Rmax=10-2 Rmax=10-3 Relative error: 0.18% Rmax=10-4 Iteration 35 Iteration 59 Iteration 132 0.01% Iteration errors: Difference between ‘converged’ solution and solution at iteration ‘n’ Isentropic Efficiency Iteration Number Convergence criterion 7 © 2013 ANSYS, Inc. February 28, 2014 ANSYS Confidential • Define target variables: – Head rise – Efficiency – Mass flow rate – … • Select convergence criterion (e.g. residual norm) • Plot target variables as a function of convergence criterion • Set convergence criterion such that value of target variable becomes “independent” of convergence criterion • Check for monotonic convergence • Check convergence of global balances Iteration Error - Best Practice Introduction Error Types Best Practices for Meshing Summary 8 © 2013 ANSYS, Inc. February 28, 2014 ANSYS Confidential Discretization Error All discrete methods have solution errors: •Finite volume methods •Finite element methods •Finite difference methods •. Difference between solution on a given grid and “exact‘ solution on an infinitely fine grid Exact solution not available  Discretization error estimation hhex eff Introduction Error Types Best Practices for Meshing Summary 9 © 2013 ANSYS, Inc. February 28, 2014 ANSYS Confidential Discretization Error Estimation Impinging jet flow with heat transfer 2-D, axisymmetric Compared Grids: •50  50  800  800 SST turbulence model Discretization schemes: •1st order Upwind •2nd order Upwind • Target quantities: – Heat transfer – Maximum Nusselt number H r D D= 26.5mm or 101.6mm Introduction Error Types Best Practices for Meshing Summary 10 © 2013 ANSYS, Inc. February 28, 2014 ANSYS Confidential Discretization Error Estimation 150 160 170 180 190 200 -3.47E-170.0050.010.0150.02 Nu_max 1/N_Cells 1st order2nd order Introduction Error Types Best Practices for Meshing Summary The plot shows •If the grid is fine enough, 1st and 2nd order solutions are the same •On coarser meshes, the 2nd order solution is closer to the final solution Practical alternatives for industrial cases are: •Compare solutions from different order schemes •Compare solutions on locally or regionally refined meshes 11 © 2013 ANSYS, Inc. February 28, 2014 ANSYS Confidential Model Errors Inadequacies of (empirical) mathematical models: •Base equations (Euler vs. RANS, steady-st。