How do i fix this error?

Answer 1

[dpb]

Paste the code as text and the complete error message; can't do anything with an image.

The statement

f=zeros(num_pipes,1);

creates a vector of three elements so when you write

for i=1:num_pipes
  ...
  f(i)=32*f*pipe_length(i)*Q(i)^2 ...

then the RHS is going to also be a 3-vector of zeros that you're trying to store into one location. Can't do that.

One presumes the second, unsubscripted f is supposed to be a friction factor based on some other correlation. Fix that to be some other scalar variable.

[Daniel]

% Constants
e = 0.26e-3; % Roughness coefficient in meters
rho = 1000; % Density of fluid in kg/m^3
mu = 1e-3; % Viscosity of fluid in Pa.s
gc = 9.81; % Acceleration due to gravity in m/s^2

% Pipe properties (You need to define your network)
pipe_length = [100, 150, 200]; % Length of each pipe in meters
pipe_diameter = [0.15, 0.2, 0.1]; % Diameter of each pipe in meters

% Initial flow rates (You need to provide initial guesses)
Q = [0.01, 0.02, 0.03]; % Initial flow rates in m^3/s

% Hardy Cross Method Parameters
max_iterations = 100; % Maximum number of iterations
tolerance = 1e-6; % Convergence tolerance

% Initialize variables
num_pipes = length(pipe_length);
H = zeros(num_pipes, 1);
iteration = 0;
converged = false;

while iteration < max_iterations
    % Calculate friction losses for each pipe
    f = zeros(num_pipes, 1);
    for i = 1:num_pipes
        A = pi * (pipe_diameter(i) / 2)^2; % Cross-sectional area of the pipe
        hydraulic_radius = A / (pi * pipe_diameter(i)); % Hydraulic radius
        Reynolds_number = rho * Q(i) / (mu * hydraulic_radius); % Reynolds number
        
        % Calculate Darcy-Weisbach friction factor using the given equation
        f(i) = (32 * f * pipe_length(i) * Q(i)^2) / (pi * gc * pipe_diameter(i)^5);
        
        % Calculate head loss in the pipe
        H(i) = 4 * f(i) * pipe_length(i) * (Q(i)^2) / (2 * gc * pipe_diameter(i));
    end
    
    % Calculate flow corrections
    delta_Q = zeros(num_pipes, 1);
    for i = 1:num_pipes
        sum_dQ = 0;
        for j = 1:num_pipes
            if i ~= j
                sum_dQ = sum_dQ + (H(i) - H(j)) / (f(i) - f(j));
            end
        end
        delta_Q(i) = -sum_dQ;
    end
    
    % Update flow rates
    Q = Q + delta_Q;
    
    % Check for convergence
    if max(abs(delta_Q)) < tolerance
        converged = true;
        break;
    end
    
    iteration = iteration + 1;
end

Unable to perform assignment because the left and right sides have a different number of elements.

if converged
    disp('Converged to a solution:');
    for i = 1:num_pipes
        fprintf('Pipe %d: Flow rate = %f m^3/s\n', i, Q(i));
    end
else
    disp('Did not converge to a solution within the maximum number of iterations.');
end

[Daniel]

Here is the full code with the error message.

[Torsten]

% Calculate Darcy-Weisbach friction factor using the given equation
        f(i) = (32 * f * pipe_length(i) * Q(i)^2) / (pi * gc * pipe_diameter(i)^5);        

This f (bold)

f(i) = (32 * f * pipe_length(i) * Q(i)^2) / (pi * gc * pipe_diameter(i)^5);

is a vector while this f(i) (bold)

f(i) = (32 * f * pipe_length(i) * Q(i)^2) / (pi * gc * pipe_diameter(i)^5);

is a scalar.

So you try to assign a vector

(32 * f * pipe_length(i) * Q(i)^2) / (pi * gc * pipe_diameter(i)^5);

to a scalar

f(i)

- consequently the error message.

@dpb suspected that f is a friction factor you forgot to specify in your code - thus that f is different from f(i). We don't know.

Shouldn't the headloss be prescribed externally if you want to compute volumetric flowrates ? I get the impression that both quantities are treated as unknowns in your code.