Numerical Methods in Heat, Mass, and Momentum Transfer
Draft Notes ME 608
Numerical Methods in Heat, Mass, and Momentum Transfer
Instructor: Jayathi Y. Murthy School of Mechanical Engineering
Purdue University
Spring 2002
c 1998 J.Y. Murthy and S.R. Mathur. All Rights Reserved
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Contents
1 Mathematical Modeling
9
1.1 Conservation Equations . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.1.1 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
1.1.2 Conservation Form . . . . . . . . . . . . . . . . . . . . . . . 12
1.2 Governing Equations . . . . . . . . . . . . . . . . . . . . . . . . . . 12
1.2.1 The Energy Equation . . . . . . . . . . . . . . . . . . . . . . 12
1.2.2 The Momentum Equation . . . . . . . . . . . . . . . . . . . 13
1.2.3 The Species Equation . . . . . . . . . . . . . . . . . . . . . . 13
1.3 The General Scalar Transport Equation . . . . . . . . . . . . . . . . . 13
1.4 Mathematical Classification of Partial Differential Equations . . . . . 14
1.4.1 Elliptic Partial Differential Equations . . . . . . . . . . . . . 14
1.4.2 Parabolic Partial Differential Equations . . . . . . . . . . . . 16
1.4.3 Hyperbolic Partial Differential Equations . . . . . . . . . . . 17
1.4.4 Behavior of the Scalar Transport Equation . . . . . . . . . . . 17
1.5 Closure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2 Numerical Methods
21
2.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
2.2 Mesh Terminology and Types . . . . . . . . . . . . . . . . . . . . . . 23
2.2.1 Regular and Body-fitted Meshes . . . . . . . . . . . . . . . . 23
2.2.2 Structured, Block Structured, and Unstructured Meshes . . . . 23
2.2.3 Conformal and Non-Conformal Meshes . . . . . . . . . . . . 25
2.2.4 Cell Shapes . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2.2.5 Node-Based and Cell-Based Schemes . . . . . . . . . . . . . 28
2.3 Discretization Methods . . . . . . . . . . . . . . . . . . . . . . . . . 29
2.3.1 Finite Difference Methods . . . . . . . . . . . . . . . . . . . 29
2.3.2 Finite Element Methods . . . . . . . . . . . . . . . . . . . . 30
2.3.3 Finite Volume Method . . . . . . . . . . . . . . . . . . . . . 31
2.4 Solution of Discretization Equations . . . . . . . . . . . . . . . . . . 33
2.4.1 Direct Methods . . . . . . . . . . . . . . . . . . . . . . . . . 33
2.4.2 Iterative Methods . . . . . . . . . . . . . . . . . . . . . . . . 34
2.5 Accuracy, Consistency, Stability and Convergence . . . . . . . . . . . 35
2.5.1 Accuracy . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
2.5.2 Consistency . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
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2.5.3 Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 2.5.4 Convergence . . . . . . . . . . . . . . . . . . . . . . . . . . 36 2.6 Closure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
3 The Diffusion Equation: A First Look
37
3.1 Two-Dimensional Diffusion in Rectangular Domain . . . . . . . . . . 37
3.1.1 Discretization . . . . . . . . . . . . . . . . . . . . . . . . . . 39
3.1.2 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
3.2 Boundary Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . 41
3.2.1 Dirichlet Boundary Condition . . . . . . . . . . . . . . . . . 42
3.2.2 Neumann Boundary Condition . . . . . . . . . . . . . . . . . 43
3.2.3 Mixed Boundary Condition . . . . . . . . . . . . . . . . . . 44
3.3 Unsteady Conduction . . . . . . . . . . . . . . . . . . . . . . . . . . 45
3.3.1 The Explicit Scheme . . . . . . . . . . . . . . . . . . . . . . 47
3.3.2 The Fully-Implicit Scheme . . . . . . . . . . . . . . . . . . . 48
3.3.3 The Crank-Nicholson Scheme . . . . . . . . . . . . . . . . . 50
3.4 Diffusion in Polar Geometries . . . . . . . . . . . . . . . . . . . . . 50
3.5 Diffusion in Axisymmetric Geometries . . . . . . . . . . . . . . . . . 53
3.6 Finishing Touches . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
3.6.1 Interpolation of . . . . . . . . . . . . . . . . . . . . . . . . 55
3.6.2 Source Linearization and Treatment of Non-Linearity . . . . . 57
3.6.3 Under-Relaxation . . . . . . . . . . . . . . . . . . . . . . . . 57
3.7 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
3.8 Truncation Error . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
3.8.1 Spatial Truncation Error . . . . . . . . . . . . . . . . . . . . 59
3.8.2 Temporal Truncation Error . . . . . . . . . . . . . . . . . . . 61
3.9 Stability Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
3.10 Closure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
4 The Diffusion Equation: A Closer Look
67
4.1 Diffusion on Orthogonal Meshes . . . . . . . . . . . . . . . . . . . . 67
4.2 Non-Orthogonal Meshes . . . . . . . . . . . . . . . . . . . . . . . . 71
4.2.1 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
4.2.2 Secondary Gradient Calculation . . . . . . . . . . . . . . . . 74
4.2.3 Discrete Equation for Non-Orthogonal Meshes . . . . . . . . 75
4.3 Boundary Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . 76
4.4 Gradient Calculation . . . . . . . . . . . . . . . . . . . . . . . . . . 79
4.4.1 Structured Meshes . . . . . . . . . . . . . . . . . . . . . . . 79
4.4.2 Unstructured Meshes . . . . . . . . . . . . . . . . . . . . . . 81
4.5 Influence of Secondary Gradients on Coefficients . . . . . . . . . . . 86
4.6 Implementation Issues . . . . . . . . . . . . . . . . . . . . . . . . . 87
4.6.1 Data Structures . . . . . . . . . . . . . . . . . . . . . . . . . 87
4.6.2 Overall Solution Loop . . . . . . . . . . . . . . . . . . . . . 88
4.7 Closure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
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5 Convection
91
5.1 Two-Dimensional Convection and Diffusion in A Rectangular Domain 91
5.1.1 Central Differencing . . . . . . . . . . . . . . . . . . . . . . 93
5.1.2 Upwind Differencing . . . . . . . . . . . . . . . . . . . . . . 95
5.2 Convection-Diffusion on Non-Orthogonal Meshes . . . . . . . . . . . 96
5.2.1 Central Difference Approximation . . . . . . . . . . . . . . . 97
5.2.2 Upwind Differencing Approximation . . . . . . . . . . . . . 97
5.3 Accuracy of Upwind and Central Difference Schemes . . . . . . . . . 98
5.3.1 An Illustrative Example . . . . . . . . . . . . . . . . . . . . 98
5.3.2 False Diffusion and Dispersion . . . . . . . . . . . . . . . . . 99
5.4 First-Order Schemes Using Exact Solutions . . . . . . . . . . . . . . 102
5.4.1 Exponential Scheme . . . . . . . . . . . . . . . . . . . . . . 102
5.4.2 Hybrid Scheme . . . . . . . . . . . . . . . . . . . . . . . . . 104
5.4.3 Power Law Scheme . . . . . . . . . . . . . . . . . . . . . . . 105
5.5 Unsteady Convection . . . . . . . . . . . . . . . . . . . . . . . . . . 105
5.5.1 1D Finite Volume Discretization . . . . . . . . . . . . . . . . 107
5.5.2 Central Difference Scheme . . . . . . . . . . . . . . . . . . . 107
5.5.3 First Order Upwind Scheme . . . . . . . . . . . . . . . . . . 108
5.5.4 Error Analysis . . . . . . . . . . . . . . . . . . . . . . . . . 109
5.5.5 Lax-Wendroff Scheme . . . . . . . . . . . . . . . . . . . . . 110
5.6 Higher-Order Schemes . . . . . . . . . . . . . . . . . . . . . . . . . 112
5.6.1 Second-Order Upwind Schemes . . . . . . . . . . . . . . . . 112
5.6.2 Third-Order Upwind Schemes . . . . . . . . . . . . . . . . . 113
5.6.3 Implementation Issues . . . . . . . . . . . . . . . . . . . . . 114
5.7 Higher-Order Schemes for Unstructured Meshes . . . . . . . . . . . . 114
5.8 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
5.9 Boundary Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . 116
5.9.1 Inflow Boundaries . . . . . . . . . . . . . . . . . . . . . . . 116
5.9.2 Outflow Boundaries . . . . . . . . . . . . . . . . . . . . . . 117
5.9.3 Geometric Boundaries . . . . . . . . . . . . . . . . . . . . . 118
5.10 Closure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
6 Fluid Flow: A First Look
121
6.1 Discretization of the Momentum Equation . . . . . . . . . . . . . . . 121
6.2 Discretization of the Continuity Equation . . . . . . . . . . . . . . . 123
6.3 The Staggered Grid . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
6.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
6.5 Solution Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
6.6 The SIMPLE Algorithm . . . . . . . . . . . . . . . . . . . . . . . . 129
6.6.1 The Pressure Correction Equation . . . . . . . . . . . . . . . 131
6.6.2 Overall Algorithm . . . . . . . . . . . . . . . . . . . . . . . 132
6.6.3 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
6.6.4 Boundary Conditions . . . . . . . . . . . . . . . . . . . . . . 133
6.6.5 Pressure Level and Incompressibility . . . . . . . . . . . . . 134
6.7 The SIMPLER Algorithm . . . . . . . . . . . . . . . . . . . . . . . . 135
6.7.1 Overall Algorithm . . . . . . . . . . . . . . . . . . . . . . . 136
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