def Gr(L: float, Ts: float, Tf: float, beta: float, nu: float, g: float):
return g * beta * (Ts - Tf) * L**<NUM_LIT> / nu**<NUM_LIT><EOL>
Calculate the Grashof number.

:param L: [m] heat transfer surface characteristic length.
:param Ts: [K] heat transfer surface temperature.
:param Tf: [K] bulk fluid temperature.
:param beta: [1/K] fluid coefficient of thermal expansion.
:param nu: [m2/s] fluid kinematic viscosity.

:returns: float

.. math::
    \\mathrm{Gr} = \\frac{g \\beta (Ts - Tinf ) L^3}{\\nu ^2}

Characteristic dimensions:
    * vertical plate: vertical length
    * pipe: diameter
    * bluff body: diameter