code: def Ra(L: float, Ts: float, Tf: float, alpha: float, beta: float, nu: float
         ) -> float:
      
  
      return g * beta * (Ts - Tinf) * L**3.0 / (nu * alpha)
docs: Calculate the Ralleigh number.
  
      :param L: [m] heat transfer surface characteristic length.
      :param Ts: [K] heat transfer surface temperature.
      :param Tf: [K] bulk fluid temperature.
      :param alpha: [m2/s] fluid thermal diffusivity.
      :param beta: [1/K] fluid coefficient of thermal expansion.
      :param nu: [m2/s] fluid kinematic viscosity.
  
      :returns: float
  
      Ra = Gr*Pr
  
      Characteristic dimensions:
          * vertical plate: vertical length
          * pipe: diameter
          * bluff body: diameter