def u_calc(D, speed):
    """Calculate the impeller tip speed.

    Parameters
    ----------
    D : pint.Quantity, float
        Impeller diameter (m).
    speed : pint.Quantity, float
        Impeller speed (rad/s).

    Returns
    -------
    u_calc : pint.Quantity
        Impeller tip speed (m/s).
    """
    u = speed * D / 2
    return u.to("m/s")