BYOM function derivatives.m (the model in ODEs)
Syntax: dX = derivatives(t,X,par,c)
- t is the time point, provided by the ODE solver
- X is a vector with the previous value of the states
- par is the parameter structure
- c is the external concentration (or scenario number)
Time t and scenario name c are handed over as single numbers by call_deri.m (you do not have to use them in this function). Output dX (as vector) provides the differentials for each state at t.
function dX = derivatives(t,X,par,c)
global glo % allow for global parameters in structure glo (handy for switches)
The state variables enter this function in the vector X. Here, we give them a more handy name.
Cw = X(1); % state 1 is the external concentration Ci = X(2); % state 2 is the internal concentration
The parameters enter this function in the structure par. The names in the structure are the same as those defined in the byom script file. The 1 between parentheses is needed as each parameter has 5 associated values.
kd = par.kd(1); % degradation rate constant, d-1 ke = par.ke(1); % elimination rate constant, d-1 Piw = par.Piw(1); % bioconcentration factor, L/kg Ct = par.Ct(1); % threshold external concentration that stops degradation, mg/L % % Optionally, include the threshold concentration as a global (see script) % Ct = glo.Ct; % threshold external concentration that stops degradation, mg/L
Calculate the derivatives
This is the actual model, specified as a system of two ODEs:
if Cw > Ct % if we are above the critical internal concentration ... dCw = -kd * Cw; % let the external concentration degrade else % otherwise ... dCw = 0; % make the change in external concentration zero end dCi = ke * (Piw*Cw-Ci); % first order bioconcentration dX = [dCw;dCi]; % collect both derivatives in one vector dX