function [f] = friction(Re,ed)
   if (Re < 2.0e+3)
       f = 64/Re;
   else

   f_hD =  (1./( -1.8*log10( 6.9/Re + (ed/3.7)^(1.11)  )   ) ).^2; % Haaland
   
   cW = @(f) 1./sqrt(f) + 2*log10( 2.51/(Re*sqrt(f)) + ed/3.7 ); %Colebrook-White
   f = fzero(cW,f_hD); 
   endif
endfunction