00_Basic_Problem.cpp

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//@HEADER
// *****************************************************************************
//          Tempus: Time Integration and Sensitivity Analysis Package
//
// Copyright 2017 NTESS and the Tempus contributors.
// SPDX-License-Identifier: BSD-3-Clause
// *****************************************************************************
//@HEADER
 
#include <iomanip>
#include <iostream>
#include <stdlib.h>
#include <math.h>
#include "Teuchos_StandardCatchMacros.hpp"
 
using namespace std;
 
int main(int argc, char *argv[])
{
  bool verbose = true;
  bool success = false;
  try {
    // Solution and its time-derivative.
    double x_n[2];      // at time index n
    double xDot_n[2];   // at time index n
 
    // Initial Conditions
    int n = 0;
    double time = 0.0;
    double epsilon = 1.0e-1;
    bool passed = true;   // ICs are considered passed.
    x_n   [0] = 2.0;
    x_n   [1] = 0.0;
    xDot_n[0] = 0.0;
    xDot_n[1] = -2.0/epsilon;
 
    // Timestep size
    double finalTime = 2.0;
    int nTimeSteps = 2000;
    const double constDT = finalTime/nTimeSteps;
 
    // Advance the solution to the next timestep.
    cout << n << "  " << time << "  " << x_n[0] << "  " << x_n[1] << endl;
    while (passed && time < finalTime && n < nTimeSteps) {
 
      // Initialize next time step
      double x_np1[2];    // at time index n+1
      x_np1[0] = x_n[0];
      x_np1[1] = x_n[1];
 
      // Set the timestep and time.
      double dt = constDT;
      time = (n+1)*dt;
 
      // Righthand side evaluation and time-derivative at n.
      xDot_n[0] = x_n[1];
      xDot_n[1] = ((1.0 - x_n[0]*x_n[0])*x_n[1] - x_n[0])/epsilon;
 
      // Take the timestep - Forward Euler
      x_np1[0] = x_n[0] + dt*xDot_n[0];
      x_np1[1] = x_n[1] + dt*xDot_n[1];
 
      // Test if solution has passed.
      if ( std::isnan(x_n[0]) || std::isnan(x_n[1]) ) {
        passed = false;
      } else {
        // Promote to next step (n <- n+1).
        x_n[0] = x_np1[0];
        x_n[1] = x_np1[1];
        n++;
      }
 
      // Output
      if ( n%100 == 0 )
        cout << n << "  " << time << "  " << x_n[0] << "  " << x_n[1] << endl;
 
    }
 
    // Test for regression.
    double x_regress[2];      // Regression results for nTimeSteps = 2000
    x_regress[0] = -1.59496108218721311;
    x_regress[1] =  0.96359412806611255;
    double x_L2norm_error = 0.0;
    double x_L2norm_regress = 0.0;
    for (int i=0; i < 2; i++) {
      x_L2norm_error += (x_n[i]-x_regress[i])*(x_n[i]-x_regress[i]);
      x_L2norm_regress += x_regress[1]*x_regress[1];
    }
    x_L2norm_error   = sqrt(x_L2norm_error  );
    x_L2norm_regress = sqrt(x_L2norm_regress);
    cout << "Relative L2 Norm of the error (regression) = "
         << x_L2norm_error/x_L2norm_regress << endl;
    if ( x_L2norm_error > 1.0e-08*x_L2norm_regress) {
      passed = false;
      cout << "FAILED regression constraint!" << endl;
    }
 
    if (passed) success = true;
  }
  TEUCHOS_STANDARD_CATCH_STATEMENTS(verbose, std::cerr, success);
 
  if(success)
    cout << "\nEnd Result: Test Passed!" << std::endl;
 
  return ( success ? EXIT_SUCCESS : EXIT_FAILURE );
}