Help storing values for Ma and Cd so I can create a plot for them.

Question

Dennis Williams 2020-7-31

0
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https://ww2.mathworks.cn/matlabcentral/answers/573481-help-storing-values-for-ma-and-cd-so-i-can-create-a-plot-for-them

回答： Sudheer Bhimireddy 2020-8-5

% initializing variables

h = 0.0001; %delta t [s]

k = 1.4; %specific heat ratio for air

R = 1716.5; %gas constant [(ft*lbf)/(slugs*R)]

g = 32.174; %acceleration due to gravity [ft/s^2]

W = 55; %weight [lbf]

m = W/g; %mass [slugs]

d = 3/12; %diameter [ft]

A = pi*1/4*d^2; %Area [ft^2]

N = 55000; % number of iterations

xf = 5280*7.1; % final distance [ft]

x = zeros(1,N); % distance [ft]

y = zeros(1,N); % height [ft]

Vx = zeros(1,N);

Vy = zeros(1,N);

V = zeros(1,N);

%Accelx = zeros(1,N);

%Accely = zeros(1,N);

mach = 7*1125; %conversion Ma units

theta(1) = 89.165031*(pi/180); % initial angle [rad] | can't assume it's constant, needed to find x_o, y_o, V_o

Vx(1) = mach*cos(theta(1)); % first step of velocity in x-dir [ft/s]

Vy(1) = mach*sin(theta(1)); % first step of velocity in y-dir {ft/s]

V(1) = sqrt((Vx(1))^2 + (Vy(1))^2);

x(1) = 0; % first step of position[ft]

y(1)= 0;

for n = 2:N

[T, p, rho] = atmos_funEE (y(n-1)); %calling function given to find rho, p, T at y

c = sqrt(k*R*T);

V = sqrt((Vx(n-1))^2 + (Vy(n-1))^2); %V = velocity [ft/s]

Ma = V/c; % Ma number

[Cd] = Ma_Cd_curvefit (Ma); %Curve fit of Ma vs. Cd

x(n) = x(n-1) + h*Vx(n-1);

Vx(n) = Vx(n-1) + h*(-1/2*1/m*rho*A*Cd*V*Vx(n-1));

y(n) = y(n-1) + h*Vy(n-1);

Vy(n) = Vy(n-1) + h*(-g-(0.5*(1/m)*rho*A*Cd*V*Vy(n-1)));

end

plot(x,Cd)

plot(x,Ma)

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Dennis Williams 2020-7-31

Whenever I plot (x,Cd) and (x,Ma), I get a blank screen without a line as I was expecting as if only a point is being plotted and and not all the points from all the iterations

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Answer 1

Sudheer Bhimireddy 2020-8-5

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/573481-help-storing-values-for-ma-and-cd-so-i-can-create-a-plot-for-them#answer_475621

在 MATLAB Online 中打开

The problem is inside the for loop.

Change this

V = sqrt((Vx(n-1))^2 + (Vy(n-1))^2); %V = velocity [ft/s]
Ma = V/c; % Ma number  

to

V(n)  = sqrt((Vx(n-1))^2 + (Vy(n-1))^2); %V = velocity [ft/s]
Ma(n) = V(n)/c; % Ma number  

and try again to plot. Similarly check the size and values of Cd. Also when you have two plots and if you want both of them to be on the same plot. use 'hold on', otherwise the latest plot() will overwrite the previous one.