HELP! I need to plot my values from if statement, and cant figure out how

if H < 1.5           %Pressure drop is less than 1.5 psi
            D = (Qc.^0.381)/(19.17*(H./(Cr.*L))^0.206);
                 
        elseif H >= 1.5     %Pressure drop is greater than or equal to 1.5 psi
            D = (Qc.^0.381)/(18.93*((P1a.^2-P2a.^2).*Y./Cr.*L)^0.206); 
end

xx = linspace(0,2*Qc,1000);
fplot

this is the part of the function that does the calculation and I can't figure out how to plot the D vs. xx

[Rik]

If they are the same size, you can use plot. If that is not the case, you will need to describe your variables in more detail.

[ragheed idrees]

ok here is all i have, i managed to plot D, but im also supposed to place a marker at the value of D on the plot and i cant do that.

function [D,P2] = SizingGasPipes(Q,P1,L,H,Case,Unit)
%This function computes the pipe size required to keep the pressure drop 
%within an acceptable range for a specified capacity (cubic feet per hour 
%or BTU per hour) and upstream pressure P1


%Giving error when user supplies an illegal value
if H>P1
    clc
    disp('Error: Pressure drop cannot be less than downstream pressure')
    P2=('Error');
    D=('Error');
    return
elseif Q<=0
    clc
    disp('Error: Flow rate cannot equal zero or negative');
    P2=('Error');
    D=('Error');
elseif P1<=0
    clc
    disp('Error: Downstream pressure cannot equal zero or negative');
    P2=('Error');
    D=('Error');
    return
elseif L<=0
     clc
    disp('Error: Length of pipe cannot equal zero or negative');
    P2=('Error');
    D=('Error');
    return
end

%Calculate inlet pressure
P1a = P1 +14.7;
P2a = P1a + H/27.7;
%generate 100 numbers from 0 to Q

switch Case %seperate Natural Gas from Propane values
    
    case 'NG'   %Case 1 is Natural Gas
        Cr = 0.6094;
        Y = 0.9992;
        btucfh=1040;
    case 'PP'   %case 2 is Propane
        Cr = 1.2462;
        Y = 0.991;
        btucfh=2500;
    otherwise
        disp('Gas type invalid')
end

switch Unit %Deciding the units to convert the Q accordingly
    
    case 'CFH'
    xscale=1;
       xtext = 'Capacity is in cubic feet per hour';
    case 'BTU'
       xscale= btucfh/1000;
       xtext = 'Capacity is in British Themal Units per hour';
    otherwise 
        disp('Units invalid')
end

%changing Q according to units supplied
Qc=Q/xscale;

%Calculations for the capacity of natural gas or propane piping
D = zeros(1,1000);
xx = linspace(0,2*Qc,1000);
for ii = 1:1000
    Qc(ii)=ii+1;
if H < 1.5           %Pressure drop is less than 1.5 psi
            D(ii) = (Qc(ii).^0.381)/(19.17*(H./(Cr.*L))^0.206);
 
        elseif H >= 1.5     %Pressure drop is greater than or equal to 1.5 psi
            D(ii) = (Qc(ii).^0.381)/(18.93*((P1a.^2-P2a.^2).*Y./Cr.*L)^0.206); 
end
end
plot(xx,D)
end

[ragheed idrees]

how do i use MarkerIndices here?

[Rik]

I you want to add a marker at only a single point, it is easier to use hold on and a single point with the appropriate marker at the appropriate position.

[ragheed idrees]

I see, my dilemma is that for me to plot it i need a for loop, but to get the correct asnwer (point on plot) I only get it without the for loop, how do I plot the graph and at the same time find the value without the for loop?

[Rik]

Which of the 1000 point do you want to put a marker?

Answer 1

[Raghunandan V]

Hi,

Just a small correction in the code

if H < 1.5           %Pressure drop is less than 1.5 psi
   D = (Qc.^0.381)/(19.17*(H./(Cr.*L))^0.206);
else  %Pressure drop is greater than or equal to 1.5 psi
   D = (Qc.^0.381)/(18.93*((P1a.^2-P2a.^2).*Y./Cr.*L)^0.206); 
end
xx = linspace(0,2*Qc,1000);
plot(xx, D)

This is only possible if your xx and D are of same dimention

[ragheed idrees]

thank you sir, I have done that but now i cant find the value for D in order to point it on the plot. Can you please help me with that?

[Raghunandan V]

Oh! Are all the inputs to calculate the value of D available?