friction Subroutine

  subroutine friction(reynolds, darcy_friction)
    !! Calculate Darcy friction factor, using Haaland equation
    !! author: M Kovari, CCFE, Culham Science Centre
    !! reynolds : input real : Reynolds number
    !! darcy_friction : output real : Darcy friction factor
    !! Darcy friction factor, using Haaland equation, an approximation to the
    !! implicit Colebrook–White equationGnielinski correlation.
    !! https://en.wikipedia.org/wiki/Darcy_friction_factor_formulae#Haaland_equation
    !
    ! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

    use fwbs_variables, only: roughness, afw

    implicit none

    real(dp), intent(in) :: reynolds
    real(dp), intent(out) :: darcy_friction

    ! Local variables !
    ! !!!!!!!!!!!!!!!!!!

    real(dp) :: bracket

    ! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

    ! Bracketed term in Haaland equation
    bracket = (roughness/afw/3.7)**1.11d0 + 6.9/reynolds

    ! Calculate Darcy friction factor
    darcy_friction = (1.8d0 * log10(bracket))**(-2)

  end subroutine friction