pdepe help!! incorrect answer

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Hüseyin Cagdas Yatkin 2019-12-29

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https://ww2.mathworks.cn/matlabcentral/answers/498351-pdepe-help-incorrect-answer

评论： Hüseyin Cagdas Yatkin 2019-12-30

Hi guys,

Script is working, however I don't get the correct result for the XvsC graph, what is the problam, how can I solve that, is there any other way to solve this?

clc
clear
%%  defining the mash points
x = linspace(0,15000,10); %defining distance from 0 to 15000m which is divaded in to 10
t = linspace(0, 30, 30); %defining time from 0 to 30th day which is divided in to 30 
m = 0;
sol = pdepe(m, @pde,@pdeic,@pdebc,x,t);
%% ploting for the x(m) vs C(30, x)(mg/L) for t = 30 day
sol(30,:);
plot(x, sol(30,:))
xlabel('Distance (m)')
ylabel('Concentration (mg/L)')
title('x vs C for t = 30 day')
%% ploting for the t(day) vs C(t, 5)(mg/L) for x = 7500 m
sol(:, 5);
plot(t, sol(:, 5))
xlabel('Time (days)')
ylabel('Concentration (mg/L)')
title('t vs C for x = 50 m')
%% main function that represent our governing equation 
function [c, f, s] = pde(x, t, C, dCdx)
c = 1;
f = -49680*C;
s = -(0.24+0.45/5)*C;
end
%% defining the initial condition
function u0 = pdeic(x)
u0 = 25;
end
%% defining the boundry condition
function [pl,ql,pr,qr] = pdebc(xl,ul,xr,ur,t)
pl = ul - 13.75;
ql = 0;
pr = ur - 0.05;
qr = 0;
end

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Bill Greene 2019-12-29

The problem you have modeled makes little sense to me. You have a flow velocity of 49680 m/day and you are looking for a solution after 30 days yet you have modelled a domain of just 15000 m. What were you expecting the solution to look like in this domain after 30 days?

Hüseyin Cagdas Yatkin 2019-12-30

You gat a point yeah, I never thought in that way. Since the boundry conditions are hypothetical, I will rearrange the conditions and try again. Thanks a lot.

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