Why same OpenModelica code behaves differently in two files? - graph

Question

I'm trying to model the trajectory of projectile in modelica, but when I model air_pressure dependency on the height I get weird result, now if I simply copy only the air_pressure's code to another file, I get pretty fine output. Why's that so?
This is the original code for projectile,
class proj
//position
Real x(start = 6371000);
Real y(start = 0);
Real z(start = 0);
Real r(start = 6371000);
Real row(start = 6371000);
Real h(start = 0);
//velocity
Real v_x(start = 2000);
Real v_y(start = 0);
Real v_z(start = 0);
Real v_mag(start = sqrt(4000000));
Real v_row(start = sqrt(4000000));
//acceleration
Real a_x();
Real a_y();
Real a_z();
Real a_mag();
//total force
Real ft_x();
Real ft_y();
Real ft_z();
Real ft_mag();
//Gravitational force
Real fg_x();
Real fg_y();
Real fg_z();
Real fg_mag();
//Drag force
Real fd_x();
Real fd_y();
Real fd_z();
Real fd_mag();
Real energy();
//Fictitious force
Real ff_x();
Real ff_y();
Real ff_z();
Real ff_mag();
//gravitation variables
Real g(start = G * earth_mass / earth_radius ^ 2);
//Drag variables
Real air_density(start = surface_density);
Real air_pressure(start = surface_pressure);
//_________________________________________________________________________
//Earth parameters
parameter Real earth_radius = 6371000;
parameter Real earth_mass = 5.972 * 10 ^ 24;
parameter Real earth_rot_vel = 7.292115 * 10 ^ (-5);
//Missile parameters
parameter Real missile_effective_area = 0.785398;
parameter Real missile_mass = 200;
parameter Real Cd = 0.02;
//Air parameters
parameter Real temp_lapse_rate = 0.0065;
parameter Real air_molar_mass = 28.97 / 1000;
parameter Real surface_temp = 303.15;
parameter Real surface_pressure = 101325;
parameter Real surface_density = 1.1644;
//constants
parameter Real pi = 3.14159265358979;
parameter Real G = 6.67384 * 10 ^ (-11);
parameter Real R = 8.3144621;
//__________________________________________________________________________
equation
//position
der(x) = if h < 0 then 0 else v_x;
der(y) = if h < 0 then 0 else v_y;
der(z) = if h < 0 then 0 else v_z;
r = sqrt(x ^ 2 + y ^ 2 + z ^ 2);
row = sqrt(x ^ 2 + y ^ 2);
h = r - earth_radius;
//velocity
der(v_x) = a_x;
der(v_y) = a_y;
der(v_z) = a_z;
v_mag = sqrt(v_x ^ 2 + v_y ^ 2 + v_z ^ 2);
v_row = sqrt(v_x ^ 2 + v_y ^ 2);
//acceleration
a_x = ft_x / missile_mass;
a_y = ft_y / missile_mass;
a_z = ft_z / missile_mass;
a_mag = ft_mag / missile_mass;
//total force
ft_x = fg_x + fd_x + ff_x;
ft_y = fg_y + fd_y + ff_y;
ft_z = fg_z + fd_z + ff_z;
ft_mag = sqrt(ft_x ^ 2 + ft_y ^ 2 + ft_z ^ 2);
//Gravitational force
fg_x = -1 * fg_mag * row / r * x / r;
fg_y = -1 * fg_mag * row / r * y / r;
fg_z = -1 * fg_mag * z / r;
fg_mag = G * earth_mass * missile_mass / r ^ 2;
//Drag force
fd_x = if v_mag > 0 then -1 * fd_mag * v_row / v_mag * v_x / v_mag else 0;
fd_y = if v_mag > 0 then -1 * fd_mag * v_row / v_mag * v_y / v_mag else 0;
fd_z = if v_mag > 0 then -1 * fd_mag * v_z / v_mag else 0;
fd_mag = 0.5 * air_density * v_mag ^ 2 * Cd * missile_effective_area;
//Fictitious force
ff_x = missile_mass * earth_rot_vel * (2 * v_y + earth_rot_vel * x);
ff_y = missile_mass * earth_rot_vel * (-2 * v_x + earth_rot_vel * y);
ff_z = 0;
ff_mag = sqrt(ff_x ^ 2 + ff_y ^ 2 + ff_z ^ 2);
energy = 0.5 * missile_mass * v_mag * v_mag - G * earth_mass * missile_mass / r;
//Gravitation variables
g = fg_mag / missile_mass;
//Drag variables
air_density = air_pressure * air_molar_mass / (R * surface_temp);
air_pressure = if h > 46600 then 0 else surface_pressure * (1 - temp_lapse_rate * (r - earth_radius) / surface_temp) ^ (g * air_molar_mass / (R * temp_lapse_rate));
end proj;
This the extracted air_pressure's code,, that works fine alone but behaves weirdly with whole projectile code,
class check
Real g;
Real h(start = 0);
Real pressure;
//Earth parameters
parameter Real earth_radius = 6371000;
parameter Real earth_mass = 5.972 * 10 ^ 24;
//Air parameters
parameter Real temp_lapse_rate = 0.0065;
parameter Real air_molar_mass = 28.97 / 1000;
parameter Real surface_temp = 303.15;
parameter Real surface_pressure = 101325;
//constants
parameter Real pi = 3.14159265358979;
parameter Real G = 6.67384 * 10 ^ (-11);
parameter Real R = 8.3144621;
equation
g = G * earth_mass / (earth_radius + h) ^ 2;
h = time;
pressure = if h > 46600 then 0 else surface_pressure * (1 - temp_lapse_rate * h / surface_temp) ^ (g * air_molar_mass / (R * temp_lapse_rate));
end check;
This is the air_pressure graph vs time from the class proj
This the air_pressure graph vs time(=height) from class check containing only air_pressure's code

Answer 1

You should check the value of h in your system. I suspect something strange is happening with h.

Answer 2

Its not clear what x refers to. From the start value, x seems to refer to the distance from the center of the earth and if this is the case then what is r? I am assuming x,y & z refer to the position of the projectile in 3D space w.r.t a point of reference on the ground (surface of the earth) and if this correct, then I would suggest defining x, y in terms of latitude and longitude and z as the height from the surface of the earth.
This should fix the ambiguity in h.