I'm new in the world of C++ and I'm having some trouble with the boost library. In my problem I want to solve a ODE-System with 5 equations. It isn't a stiff problem. As iterative method I used both integreate(rhs, x0, t0, tf, size_step, write_output) and integreate_adaptive(stepper, sys, x0, t0, tf, size_step, write_output). Both these method actually integrate the equations but giving me non-sense results changing the size of the step from 0.001 to 5 almost randomly. The equations and data are correct. What can I do to fix this problem? Here is the code:
#include <iostream>
#include <vector>
#include <boost/numeric/odeint.hpp>
#include <fstream>
#include <boost/array.hpp>
using namespace std;
using namespace boost::numeric::odeint;
//DATA
double Lin = 20000; // kg/h
double Gdry = 15000; // kg/h
double P = 760; // mmHg
double TinH2O = 50; // °C
double ToutH2O = 25; // °C
double Tinair = 20; // °C
double Z = 0.5; // relative humidity
double Cu = 0.26; // kcal/kg*K
double CpL = 1; // kcal/kg*K
double DHev = 580; // kcal/kg
double hga = 4000; // kcal/m/h/K
double hla = 30000; // kcal/m/h/K
double A = -49.705; // Pev 1st coeff mmHg vs °C
double B = 2.71; // Pev 2nd coeff mmHg vs °C
double Usair = 0.62*(A + B*Tinair) / P;
double Uair = Z*Usair;
double Kua = hga / Cu;
double L0 = 19292; // kg/h
typedef vector< double > state_type;
vector <double> pack_height;
vector <double> Umidity;
vector <double> T_liquid;
vector <double> T_gas;
vector <double> Liquid_flow;
vector <double> Gas_flow;
void rhs(const state_type& x , state_type& dxdt , const double z )
{// U Tl Tg L G
double Ti = (hla*x[1] + hga*x[2] + Kua*DHev*(x[0] - 0.62*A / P)) / (hla + hga + Kua*DHev*0.62*B / P);
double Ui = 0.62*(A + B*Ti) / P;
dxdt[0] = Kua*(Ui - x[0]) / Gdry / 100;
dxdt[1] = hla*(x[1] - Ti) / x[3] / CpL / 100;
dxdt[2] = hga*(Ti - x[2]) / Gdry / Cu / 100;
dxdt[3] = Kua*(Ui - x[0]) / 100;
dxdt[4] = Kua*(Ui - x[0]) / 100;
}
void write_output(const state_type& x, const double z)
{
pack_height.push_back(z);
Umidity.push_back(x[0]);
T_liquid.push_back(x[1]);
T_gas.push_back(x[2]);
Liquid_flow.push_back(x[3]);
Gas_flow.push_back(x[4]);
cout << z << " " << x[0] << " " << x[1] << " " << x[2] << " " << x[3] << " " << x[4] << endl;
}
int main()
{
state_type x(5);
x[0] = Uair;
x[1] = ToutH2O;
x[2] = Tinair;
x[3] = L0;
x[4] = Gdry;
double z0 = 0.0;
double zf = 5.5;
double stepsize = 0.001;
integrate( rhs , x , z0 , zf , stepsize , write_output );
return 0;
}
And this is the final results that i get from the prompt:
0 0.00183349 25 20 19292 15000
0.001 0.00183356 25 20 19292 15000
0.0055 0.0018339 25.0002 20.0001 19292 15000
0.02575 0.00183542 25.001 20.0007 19292 15000
0.116875 0.00184228 25.0046 20.003 19292.1 15000.1
0.526938 0.00187312 25.0206 20.0135 19292.6 15000.6
2.37222 0.00201203 25.0928 20.0608 19294.7 15002.7
5.5 0.00224788 25.2155 20.142 19298.2 15006.2
Only the first iteration has the right-asked stepsize.. and obiviously the solution is not the right one.. what can i do? Thank you in advance. :)
If you read the documentation, then you will find that the constant step-size routines are integrate_const and integrate_n_steps, or possibly integrate_adaptive with a non-controlled stepper.
The short call to integrate uses the standard dopri5 stepper with adaptive step size, so that the changing step size is no surprise. You could possibly use the dense output of the stepper to interpolate values at equidistant times.