Plot for different values

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)

Answer 1

[Walter Roberson]

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.

[Luccas S.]

Didn't know the repmat function, cool.

I don't know if you tried it, but it still gives me the same graph as before.

[Walter Roberson]

Right, I was not suggesting to you that you use repmat, I was saying that your code was the same as if you used repmat.

Suppose you have

Q = 1:3

Q = 1×3

1 2 3

HfTOTAL = 6

HfTOTAL = 6

K = HfTOTAL./Q

K = 1×3

6 3 2

So a constant divided by the different Q values

Hm = K.*Q

Hm = 1×3

6 6 6

And then you multiply by the different Q values.. the result is going to be the same as the constant repeated to the same length as Q. (A/B)*B gives back A (copied according to the size of B)

You are always going to get a straight line as long as you have

K = HfTOTAL./Q.^1.852
Hm = Hg + K.*Q.^1.852

where you are multiplying by the exact same thing you divided by.

[Luccas S.]

Hmmm, got it.

I think it would be better to try using a for or while. From what you told me it won't work out this way right?

[Walter Roberson]

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.]

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