Multi variable simultaneous differential equations with nested function

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Isaac Bt
Isaac Bt 2016-4-26
编辑： RB 2017-3-20
I would like to solve a set of mass transfer differential equations which are not only over time but over time and reactor length. I know I have to do a nested function but not quite sure how.
ds/dt=(d^2 s)/dx^2 - ds/dx- s
dq/dt=-∝.s
the 4 dimensionless variables are s (solvent concentration), x (reactor length) and t (time) and q (solute concentration). alpha is a constant.
The complexity is that while dq and ds are over time, there is a 2nd order d2s/dx2 term as well as ds/dx term in equation one, i.e. s is a factor of time AND distance inside reactor.
ANY help would be much much appreciated.

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John BG
John BG 2016-4-26
编辑：John BG 2016-4-26
Hi Isaac
Modifying the initial example of the function pdepe please not your s is here u:
function pdepe_exercise_01
m = 0;
x = linspace(-10,10,200);
t = linspace(0,2,50);
sol = pdepe(m,@pdex1pde,@pdex1ic,@pdex1bc,x,t); % Extract first solution component as u
u = sol(:,:,1);
figure(1);surf(x,t,u) % surface plot
title('Numerical solution computed with 20 mesh points.')
xlabel('Distance x')
ylabel('Time t')
6
figure(2) % A solution profile
plot(x,u(end,:))
title('Solution at t = 2')
xlabel('Distance x')
ylabel('u(x,2)')
% --------------------------------------------------------------
function [c,f,s] = pdex1pde(x,t,u,DuDx)
c = 1;
f = DuDx;
s = -DuDx-u;
% --------------------------------------------------------------
function u0 = pdex1ic(x)
u0 = sin(pi*x);
% --------------------------------------------------------------
function [pl,ql,pr,qr] = pdex1bc(xl,ul,xr,ur,t)
pl = ul;
ql = 0;
pr = pi * exp(-t);
qr = 1;
setting c=pi^2
Since you neither defined initial or boundary conditions i left the same as in the example.
If you find this answer of any help solving your question,
please click on the thumbs-up vote link,
thanks in advance
John
jgb2012@sky.com
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John BG
John BG 2016-4-27
编辑：John BG 2016-4-27
from MATLAB help PDEPE stands for partial differential Equations with parabolic / elliptical solutions. The kind that solutions have to do with
exp(j* ..)
for instance Electromagnetic fields trapped in a cavity, you have the waves varying with time, and [x y z]
ODE stands for ordinary differential equation: the equations have the same
f(DN_y Dn-1_y , .. , y''', y'', y', y, g(t))
basic intro: http://www.math.vanderbilt.edu/~daphne/courses/math226/MatlabODEs.pdf
ODE systems would be
f1(DN_y1 Dn-1_y1 , .. , y1''', y1'', y1', y1, g1(t))
f(DN_y2 Dn-1_y2 , .. , y2''', y2'', y2', y2, g2(t))
f(DN_y3 Dn-1_y3 , .. , y3''', y3'', y3', y3, g3(t))
as complex as ODE system may be their initial conditions and boundary conditions are relatively simple compare to those needed in PDE.
Earlier on I did not mention your second equation because that is and ODE: you solve the first one pdepe, and the you simply get q time integrating s.
If you need to build an ODE PDE knowledge base mathworks website, MATLAB help and MIT Openware are great, among many possible references
Regards
John
jgb2012@sky.com
RB
RB 2017-3-19
编辑：RB 2017-3-20
Hello! I am trying to solve the following system of ODEs I have the terminal values but the equation is involving 2*2 matrices. Solve the following system of ODE's
d/dt Y(t)=-(H(t))^{'}Y(t) d/dt V(t)=-Y(t)^{'}Q^{'}QY(t)
with the terminal conditions Y(T)=I_{2} and V(T)=0. Y, V, H and Q are 2*2 matrices and ' stands for transpose.
Also H(t)=H-Q^{'}Q*W(T-t) where W(T-t)=W(tau) is determined by the matrix Ricatti equation
dW/dtau = W(tau)H+H^{'}W(tau)-2W(tau)Q^{'}QW(tau)+I_{2} such that at tau=0 W(0)=0. I am badly stuck up. I am unable to use ode45 since I want matrix result. It would be great if you can give a suggestion. \[ H= \begin{bmatrix} -0.5 & 0.4 \\ 0.007 & -0.008 \end{bmatrix} \] \[ Q= \begin{bmatrix} 0.06 & -0.0006 \\ -0.06 & 0.006 \end{bmatrix} \] Thanks. <http://quicklatex.com/cache3/ef/ql_28653ec4bfcd28208120ea76b9009bef_l3.png>I have tried everything in MATLAB but I can not get a solution. Any kind of help would be great.
Actually I am solving a system of matrix odes where dx/dt, dy/dt, dz/dt are all 2 by 2 matrices and I need to get X,Y,Z which are again 2 by 2 matrices. My code is function [dXdt, dYdt, dVdt] = mwish7(t, X, Y, V, A, B, R) X = reshape(X, size(A)); Y = reshape(Y, size(A)); V = reshape(V, size(A)); dXdt = -A.'*X - X*A + 2*X*(B.'*B)*X - R; dYdt = -(A-(B.'*B)*X).'*Y; dVdt = -Y.'*(B.'*B)*Y; dXdt = dXdt(:); dYdt = dYdt(:); dVdt = dVdt(:); end and the ode45 program I use is A=[-0.5, 0.4; 0.007, -0.008]; B=[0.06 -0.0006; -0.06, 0.006]; R = [1 0; 0 1]; X0 = [0, 0; 0, 0]; Y0 = [1 0; 0 1]; V0 = [0, 0; 0, 0]; [T X Y V] = ode45(@(t,X,Y,V)mwish7(t, X, Y, V, A, B, R), [15 0], X0, Y0, V0) I get the error: NOT ENOUGH INPUT ARGUMENTS.
Thanks.
Regards, RB

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