Calculate terminal velocity of animal with a body size

    Args
    ----
    sw_dens: float
        Density of seawater (g/cm^3)
    dens_gcm3: float
        Density of animal (g/cm^3)
    seal_length: float
        Length of animal (cm)
    seal_girth: float
        Girth of animal (cm)
    Cd: float
        Drag coefficient of object in fluid, unitless

    Returns
    -------
    Vt: float
        Terminal velocity of animal with given body dimensions (m/s).

    References
    ----------
    Biuw, M., 2003. Blubber and buoyancy: monitoring the body condition of
    free-ranging seals using simple dive characteristics. Journal of
    Experimental Biology 206, 3405–3423. doi:10.1242/jeb.00583

    Vogel, S., 1994. Life in Moving Fluids: The Physical Biology of Flow.
    Princeton University Press.

def diff_speed(sw_dens=1.028, dens_gcm3=1.053, seal_length=300, seal_girth=200,
        Cd=0.09):
    '''Calculate terminal velocity of animal with a body size

    Args
    ----
    sw_dens: float
        Density of seawater (g/cm^3)
    dens_gcm3: float
        Density of animal (g/cm^3)
    seal_length: float
        Length of animal (cm)
    seal_girth: float
        Girth of animal (cm)
    Cd: float
        Drag coefficient of object in fluid, unitless

    Returns
    -------
    Vt: float
        Terminal velocity of animal with given body dimensions (m/s).

    References
    ----------
    Biuw, M., 2003. Blubber and buoyancy: monitoring the body condition of
    free-ranging seals using simple dive characteristics. Journal of
    Experimental Biology 206, 3405–3423. doi:10.1242/jeb.00583

    Vogel, S., 1994. Life in Moving Fluids: The Physical Biology of Flow.
    Princeton University Press.
    '''
    import numpy

    surf, vol = surf_vol(seal_length, seal_girth)

    Fb = buoyant_force(dens_gcm3, vol, sw_dens)

    x = 2 * (Fb/(Cd * sw_dens * (surf*1000)))

    if x >= 0:
        Vt = numpy.sqrt(x)
    else:
        Vt = -numpy.sqrt(-x)

    return Vt