Trying to find an x value from a certain y value on my graph

I used it again for the concentration plot, it worked the first time I did it with Ca, however when I tried to use it cb, cs and cd I got some errors, could this be due to the one-on-one mapping you mentioned?

plot(t,[ca,cb,cs,cd])

idx = knnsearch(0.9378,ca) ;

t(idx)

iwant = interp1(ca,t,0.9378);% ca = 1.1587

idx = knnsearch(0.9378,cb) ;

t(idx)

iwant = interp1(cb,t,0.9378);

idx = knnsearch(0.9378,cs) ;

t(idx)

iwant = interp1(cs,t,0.9378);

idx = knnsearch(0.9378,cd) ;

t(idx)

iwant = interp1(cd,t,0.9378);

The errors I got were:

Error using matlab.internal.math.interp1

Sample points must be unique.

Error in interp1 (line 188)

VqLite = matlab.internal.math.interp1(X,V,method,method,Xqcol);

Error in Twob (line 21)

iwant = interp1(cb,t,0.9378);

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Answer 2

Star Strider 2022-1-17

0
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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1630375-trying-to-find-an-x-value-from-a-certain-y-value-on-my-graph#answer_875990

在 MATLAB Online 中打开

I will re-post my original Answer because I believe it addresses the problem (although without the data, it is difficult to determine that with certainty) —

t = linspace(0, 10);

y = sin(2*pi*t/3) + t/10;

yval = 1;

idx = find(diff(sign(y-yval))); % Approximate Index Values For 'yval'

for k = 1:numel(idx)

idxrng = [max([1 idx(k)-2]) : min([numel(t) idx(k)+2])]; % Index Range For Interpolation

tval(k) = interp1(y(idxrng), t(idxrng), yval); % Interpolate

end

tval

tval = 1×7

0.5893 0.9546 3.3487 4.2029 6.1877 7.3731 9.0459

figure

plot(t,y)

hold on

plot(tval, ones(size(tval))*yval, '+r', 'MarkerSize',7.5)

hold off

grid

.

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Tom Goodland 2022-1-17

I didn't want to miss anything out so this is the whole script that makes all the graphs:

function main

tspan = [0 200]; %just as an example

initial_conditions = [4.3 5.1 3 0 4.4 422]; %its the values of the variables you are solving for at t=0

%there must be as many inputs to the initial conditions as variables solved

%Solve your ODE function (I picked ode45 in this case,could be another):

[t,y] = ode15s(@(t,y)fun(t,y(1),y(2),y(3),y(4),y(5),y(6)), tspan, initial_conditions);

%output t is your time range, and y an array with all your variables where:

ca = y(:,1);

cb = y(:,2);

cs = y(:,3);

cd = y(:,4);

P = y(:,5);

T_2 = y(:,6);

figure(1)

plot(t,[ca,cb,cs,cd])

figure(2)

plot(t,P)

figure(3)

plot(t,T_2)

end

function out = fun(tspan,ca,cb,cs,cd,P,T_2)

%Get your inputs and assign them to variables:

%(keep the same order as in your initial conditions!)

%Define your relationships:

k_0_1 = 4E14;

k_0_2 = 1E84;

Ea1 = 128000;

Ea2 = 800000;

R_gas = 8.314;

V0 = 4000;% L

VH = 5000;

Cv1 = 3360; % mol / h * atm

Cv2 = 53600; % mol / h * atm

Hr1 = -45400; % J / mol

Hr2 = -3.2E5;

Ta = 373;

UA = 0;

NCp = 1.26E7; % J / K

%choose your values of k1 & k2:

k1 = k_0_1*exp(-Ea1/(R_gas*T_2));

k2 = k_0_2*exp(-Ea2/(R_gas*T_2));

%Reaction rates:

r1 = k1*ca*cb;

r2 = k2*cs;

%Fd

Fd = (-0.5*r1-3*r2)*V0;

%

if Fd < 11.4 % mol / h

Fvent = Fd;

else

if P < 28.2 % atm

Fvent = (P - 1)*Cv1;

else

Fvent = (P - 1)*(Cv1 + Cv2);

end

%Define your ODEs:

dca = -r1;

dcb = -r1;

dcs = -r2;

dcd = r1/2 + 3*r2;

dp = (Fd - Fvent)*((R_gas*T_2)/VH);

dT = (V0*(r1*-Hr1 + r2*-Hr2) - UA*(T_2 - Ta))/NCp;

%Assign your ourput:

out = [dca; dcb; dcs; dcd;dp;dT];

end

Thanks for the help

Star Strider 2022-1-17

编辑：Star Strider 2022-1-17

在 MATLAB Online 中打开

What do you want to get, and from what plot or plots? (I added the legend, axis labels, and figure number titles to make them easier to refer to.)

main

Warning: Failure at t=3.426260e+00. Unable to meet integration tolerances without reducing the step size below the smallest value allowed (7.105427e-15) at time t.

T_2 = 455K at 0.92355 At t = 0.92355: ca = 2.01035 cb = 2.81035 cs = 3.00000 cd = 1.14482

function main

tspan = [0 200]; %just as an example

initial_conditions = [4.3 5.1 3 0 4.4 422]; %its the values of the variables you are solving for at t=0

%there must be as many inputs to the initial conditions as variables solved

%Solve your ODE function (I picked ode45 in this case,could be another):

[t,y] = ode15s(@(t,y)fun(t,y(1),y(2),y(3),y(4),y(5),y(6)), tspan, initial_conditions);

%output t is your time range, and y an array with all your variables where:

ca = y(:,1);

cb = y(:,2);

cs = y(:,3);

cd = y(:,4);

P = y(:,5);

T_2 = y(:,6);

figure(1)

plot(t,[ca,cb,cs,cd])

xlabel('t')

ylabel('Concentrations (?)')

title('Figure 1')

legend('ca','cb','cs','cd', 'Location','best')

figure(2)

plot(t,P)

xlabel('t')

ylabel('P')

title('Figure 2')

figure(3)

plot(t,T_2)

xlabel('t')

ylabel('T_2')

title('Figure 3')

T455K_t = interp1(T_2, t, 455);

ca_cb_cs_cd = interp1(t, [ca,cb,cs,cd], T455K_t);

fprintf('\n\nT_2 = 455K at %.5f\n\nAt t = %.5f:\n\tca\t = %.5f\n\tcb\t = %.5f\n\tcs\t = %.5f\n\tcd\t = %.5f\n',T455K_t,T455K_t,ca_cb_cs_cd)

end

function out = fun(tspan,ca,cb,cs,cd,P,T_2)

%Get your inputs and assign them to variables:

%(keep the same order as in your initial conditions!)

%Define your relationships:

k_0_1 = 4E14;

k_0_2 = 1E84;

Ea1 = 128000;

Ea2 = 800000;

R_gas = 8.314;

V0 = 4000;% L

VH = 5000;

Cv1 = 3360; % mol / h * atm

Cv2 = 53600; % mol / h * atm

Hr1 = -45400; % J / mol

Hr2 = -3.2E5;

Ta = 373;

UA = 0;

NCp = 1.26E7; % J / K

%choose your values of k1 & k2:

k1 = k_0_1*exp(-Ea1/(R_gas*T_2));

k2 = k_0_2*exp(-Ea2/(R_gas*T_2));

%Reaction rates:

r1 = k1*ca*cb;

r2 = k2*cs;

%Fd

Fd = (-0.5*r1-3*r2)*V0;

%

if Fd < 11.4 % mol / h

Fvent = Fd;

else

if P < 28.2 % atm

Fvent = (P - 1)*Cv1;

else

Fvent = (P - 1)*(Cv1 + Cv2);

end

%Define your ODEs:

dca = -r1;

dcb = -r1;

dcs = -r2;

dcd = r1/2 + 3*r2;

dp = (Fd - Fvent)*((R_gas*T_2)/VH);

dT = (V0*(r1*-Hr1 + r2*-Hr2) - UA*(T_2 - Ta))/NCp;

%Assign your ourput:

out = [dca; dcb; dcs; dcd;dp;dT];

end

EDIT — (17 Jan 2022 at 14:54)

Added these lines at the end of ‘main’ —

T455K_t = interp1(T_2, t, 455);

ca_cb_cs_cd = interp1(t, [ca,cb,cs,cd], T455K_t);

fprintf('\n\nT_2 = 455K at %.5f\n\nAt t = %.5f:\n\tca\t = %.5f\n\tcb\t = %.5f\n\tcs\t = %.5f\n\tcd\t = %.5f\n',T455K_t,T455K_t,ca_cb_cs_cd)

.

Tom Goodland 2022-1-17

Thorsten I was looking at the ode event page on matlab last night and got quite confused, could you help define this event when T = 455 K please?

Torsten 2022-1-17

https://de.mathworks.com/matlabcentral/answers/1628960-help-with-odes-and-two-different-reactions?s_tid=srchtitle

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