I have a function and I would like for it to print out an answer if I feed it a value.

I am using the Colebrook equation, which is a function of "f" and "Re" PLEASE READ(RE IS WHERE 126,400 IS). I haven't used MATLAB in a long time. Basically if I plug in a value for f into the equation, I get a value for Re. I need to do this for f = 0:0.0001:1 (which is a 10000x1 double) so I'll get the respective values for Re with respect to f = 0:0.0001:1. How do I go about doing this in MATLAB? I understand that I need to set the equation equal to zero, but I do not know what to do after that.

Here is the equation

[Stephen23]

@Peter Phung: I don't see Re in the equation.

[Peter Phung]

Oh I'm sorry. The Re is supposed to be where 126,400 is

Answer 1

[Star Strider]

Probably the easiest way is to have the Symbolic Math Toolbox solve for ‘Re’, then do the calculations in an anonymous function created by matlabFunction:

syms f Re 
Eqn = 1/sqrt(f) == -2*log(4.2E-5/3.7 + 2.51/(Re*sqrt(f)));
Re_sol = solve(Eqn, Re);

ReFcn = matlabFunction(Re_sol)
f = linspace(0, 1, 1E+4);

figure
semilogy(f, ReFcn(f))
grid
xlabel('f')
ylabel('Re')

This produces:

ReFcn = @(f) (1.0./sqrt(f).*(2.51e2./1.0e2))./(exp(1.0./sqrt(f).*(-1.0./2.0))-1.135135135135135e-5);

You might want to use abs(ReFcn(f) in the plot, to avoid the ‘Warning: Negative data ignored’ message.