Getting blank plot in MATLAB

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Satish Jawalageri 2020-7-14

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评论： Satish Jawalageri 2020-7-15

Hi, could I know where I might be wrong in the following code as Im getting blank graph. The parameter vel and acc are time dependent which in turn Fh is function of time.

function [Fh] = hydro(t)
Cd = 0.6;
Ca = 0.9699;
R = 4.7;
for t=0:0.1:10
    vel = compute_wavevelocity(t)
    acc = compute_waveacceleration(t)
    fun = sym((0.5*997*Cd*2*R*abs(vel)*vel)+(Ca*997*pi*(R^2)*acc)+(pi*(R^2)*997*acc));
    Fh(t==0:0.1:10) = eval(int(fun,0,-120));
end
plot(t,Fh), xlabel('time(s)'), ylabel('Fh')
title ('Fh vs Time')
end

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Image Analyst 2020-7-14

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Do you really even need symbolics? Or could you do it numerically, something like this:

Cd = 0.6;
Ca = 0.9699;
R = 4.7;
all_t=0:0.1:10
for k = 1 : length(all_t)
    t = all_t(k);
    vel = compute_wavevelocity(t)
    acc = compute_waveacceleration(t)
    fun = (0.5*997*Cd*2*R*abs(vel)*vel)+(Ca*997*pi*(R^2)*acc)+(pi*(R^2)*997*acc);
    Fh(k) = fun;
end
plot(all_t,Fh), xlabel('time(s)'), ylabel('Fh')
title ('Fh vs Time')

Satish Jawalageri 2020-7-15

Thanks for your reply.

I will check this but here Fh is integration of fun.

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回答（1 个）

Cris LaPierre 2020-7-14

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This is because t is a scalar. When you plot, it has the value it was assigned on the last loop of the for loop: 10.

Try changing the x input in your plot command:

plot(0:0.1:10,Fh)

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Satish Jawalageri 2020-7-15

编辑：Satish Jawalageri 2020-7-15

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Could I know how to run this state space model and ode45 function and Is it correct what I have done. Here, F1 and F2 are fuction of Fh.

State space model: (Output.m)

function [sdot] = Output(t,x)
Ze = zeros(3);
I = eye(3);
M = [8070000,0,-629468070;0,8070000,112980;-629468070,112980,6.800000000000000e+10];
Ad = [8.053095218400001e+06,0,-4.831857131040000e+08;0,2.167940435676214e+05,0;-4.831857131040000e+08,0,3.865485704832000e+10];
C = [0,0,0;0,3.241885080000000e+05,0;0,0,1.301151158327999e+09];
Cm = [4.12e+04,0,-2.82e+06;0,1.19e+04,0;-2.82e6,0,3.11e+08];
MA = M+Ad;
Q = C+Cm;
Fwt = 2.793977550000000e+04;    %Wind force on turbine tower
Fg = -79086000;                 %Gravitational force
Fbuoy = 7.844814740000000e+07;  %Buoyancy force
Fp = 2.712318560000001e+06;     %Heave force
Fmx = -334731.8545;             
Fmz = -3517000;                 
t = 0:0.1:10
for i = 1:length(t)
    
    Fh = hydro(t(i))
    F1(1,i) = Fmx+27939.6+6.5*10^5+Fh(1,i);
    F2(1,i) = Fmz+Fg+Fbuoy+Fp;
    F3(1,i) = -112510430.2+3.44*10^6+266.5*10^5+(Fh(1,i)*18);
 A = [Ze, I; -inv(MA)*Q, Ze];
 B = [Ze; inv(MA)];
 F = [F1;F2;F3];
  
 Svec = [x(1);x(2);x(3);x(4);x(5);x(6)];
% 
 sdot(1,i) = A*Svec + B*F;
end
end 

ODE45:

%Initial Condition 
x(10) = 0; x(20) = 0; x(30) = 0;
x(40) = 0; x(50) = 0; x(60) = 0;
IC = [x(10);x(20);x(30);x(40);x(50);x(60)];
%time span
t0 = 0; tf = 10;
tspan = t0:0.1:tf;
[t, x] = ode45(@Output,tspan,IC);
IC = [IC;x(end)];
%x(1) = x(:,1);
%x(2) = x(:,2);
%x(3) = x(:,3);
%x(4) = x(:,4);
%x(5) = x(:,5);
%x(6) = x(:,6);
%Plot Results 
figure(1), clf 
plot(t,x(:,1)), xlabel('time(s)'), ylabel('surge(m)')
title ('Surge vs Time')