Verification Techniques for BVP - Nc State Universityrsmith/MA797V_S10/Lecture5.pdf · Verification...
Transcript of Verification Techniques for BVP - Nc State Universityrsmith/MA797V_S10/Lecture5.pdf · Verification...
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Verification Techniques for BVP
Note: There are fewer commercial codes for BVP than IVP which put additionalresponsibility on the scientist to ensure the validity of numerical solutions.
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Numerical Methods for BVP
Recall:
Grid:
Standard Numerical Techniques:
• Finite difference
• Galerkin (finite element)
• Shooting
Verification Strategies:
• Method of manufactured solutions
• Compare multiple methods
• Compare multiple stepsizes
• Method of nearby problems
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Verification Techniques for BVP -- Manufactured Solutions
Example: Suppose we want codes for problems of the form
Notes:• This provides analytic solution to test convergence rates for numerical methods
• If possible, avoid coefficients of 1
• Smoothness of chosen solution should be commensurate with that of finalproblem
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Verification Techniques for BVP -- Multiple Methods
Problem: Suppose we do not have an analytic solution
Example:
Strategies:
• Compare multiple methods
• Compare multiple stepsizes
• Method of nearby problems
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Verification Techniques for BVP -- Multiple Methods
Example:
Finite Difference: See Lecture 3
System:
Matrix System:
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Verification Techniques for BVP -- Multiple Methods
Example:
Galerkin: See Lecture 3
Matrix System:
Approximate Solution:
Integrals: Gaussian quadrature; e.g., 2 pt
Issues: Must maintain accuracy whenapproximating integrals
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Verification Techniques for BVP -- Checks
Check:
•Symmetry
•Boundary conditions
•Qualitative behavior
Previous Example: Is this finite difference solution correct?
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BVP Verification -- Finite Difference with Two Stepsizes
Previous Example: N = 10 and N = 100
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BVP Verification -- Finite Difference versus Finite Element
Previous Example: Finite difference and finite element with N=10
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BVP Verification -- Nearby Problem
Previous Problem: Fit with the polynomial
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BVP Verification -- Nearby ProblemPrevious Example:
Pseudo-Problem: 2nd-order polynomial
where
Two Systems:
Note: Use alternative method (e.g., symbolic)when comparing RHS
2nd-Order Polynomial: N = 10
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BVP Verification -- Nearby ProblemPrevious Example:
8th-Order Polynomial:
8th-Order Polynomial: N = 10
Notes:
• Polynomialobtained withpolyfit.m may notsatisfy BC
• It is easy todifferentiate resultsfrom polyfit.m
• Be careful of highorder polynomials ---piecewise is safer!!
Errors: 8th-order polynomials
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Spline versus Polynomial FitsExample: Consider the HIV model from Adams et al., 2005
Spline Fit 10th Order Polynomial Fit