Plot for different values

Question

Luccas S. 2021-8-15

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/898797-plot-for-different-values

编辑： Luccas S. 2021-8-15

Hi, I'm trying to vary the value of Q but the values aren't changing.

HfTOTAL = 26.8708
Hg = 30
Q = 0:50:450;
K = HfTOTAL./Q.^1.852
Hm = Hg + K.*Q.^1.852 % m 
KQ =  K.*Q.^1.852
plot(Q,Hm,'k','LineWidth',2)

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Answer 1

Walter Roberson 2021-8-15

1
链接

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https://ww2.mathworks.cn/matlabcentral/answers/898797-plot-for-different-values#answer_767532

在 MATLAB Online 中打开

Just for fun, let is assign Q.^1.852 to a variable, say Q18

HfTOTAL = 26.8708
Hg = 30
Q = 0:50:450;
Q18 = Q.^1.852;
K = HfTOTAL./Q18;

So K is something divided by Q18

Hm = Hg + K.*Q18 % m

And there you take K times Q18. But Q is something divided by Q18, so except for loss of precision due to round-off error, or inf or nan effects, then K*Q18 is going to be the same as the something that was then divided by Q18. So

Hm = Hg + repmat(HfTotal, 1, length(Q));

but that's just a repeated constant

KQ = K.*Q18

Again that is just the same as repmat(HfTotal, 1, length(Q)); so it is just a repeated constant.

plot(Q,Hm,'k','LineWidth',2)

You plot the different Q values with the repeated constant on the Y axes.

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Walter Roberson 2021-8-15

在 MATLAB Online 中打开

Using a for loop will not help.

HfTotal = 6
HfTotal = 6
for Q = 1 : 3
   K(Q) = HfTotal./Q
end
K = 6
K = 1×2
     6     3
K = 1×3
     6     3     2
for Q = 1 : 3
  Hm(Q) = K(Q) .* Q
end
Hm = 6
Hm = 1×2
     6     6
Hm = 1×3
     6     6     6

The problem is that you are multipling by the exact same thing you are dividing by.

Luccas S. 2021-8-15

编辑：Luccas S. 2021-8-15

I found my mistake, silly thing.

I should be plotting as a function of the Q m³/h in a part I hadn't posted, and I was plotting as a function of the Q m³/s. Never that it was going to work.

Sometimes it's better to go for a walk and come back hahaha