Streeter Pheps Equations

I need to do a program in Matlab with two first-order differential equations with the Streeter Phelps model. Please help me.

BOD: dL/dt=-(k1+ks)L+B

DO: d(D)/dt=k1L-k2D-P

Answer 1

[Matt Kindig]

What have you done so far? To get you started, look at the documentation for ode45 (and the other ODE solvers). The documentation gives really clear examples of how to set up your equations into the form required by the solvers. For first-order systems like this, it is almost trivial.

doc ode45

[Carlos Gomes]

I need to put these two equations in Matlab by creating a Script and a Function. At the end I have to obtain a graph of these two equations. So I want you to explain me that :/

Answer 2

[Walter Roberson]

I have renamed your "D" to "Df" in order to prevent confusion with differentiation.

The Maple syntax would be

dsolve( [diff(L(t), t) = -(k1+ks)*L(t)+B, diff(Df(t), t) = k1*L(t)-k2*Df(t)-P ])

with result

Df(t) = (-k1 * k2 * C2 * (k1+ks) * exp(-t * (-k2+k1+ks)) + ((-P * ks + (B-P) * k1) * exp(k2*t) + C1 * k2 * (k1+ks)) * (-k2+k1+ks)) * exp(-k2*t) / ((k1+ks) * k2 * (-k2+k1+ks))

L(t) = B / (k1+ks) + exp(-(k1+ks) * t) * C2

In the above, C1 and C2 are constants of integration

[Carlos Gomes]

I need to put these two equations in Matlab by creating a Script and a Function. At the end I have to obtain a graph of these two equations. So I want you to explain me that :/

[Walter Roberson]

http://www.mathworks.com/matlabcentral/answers/8026-best-way-s-to-master-matlab