Fluids

Fluids

How do I know whether the convection is laminar or turbulent?

    • Karthik R
      Administrator
      I have a static heater warming up a room how do I know whether the convection is laminar or turbulent?
    • prajput
      Ansys Employee
      I am assuming that your case is purely natural convection. In that case the Rayleigh Number would probably be your answer. However, the Rayleigh Number would inform you whether the boundary layer that forms on the heater would be laminar or turbulent. Typically transition happens when Ra = 10^9.
    • Karthik R
      Administrator
      Oh, I see. I found that the Rayleigh number is Ra = Gr*Pr. What are Gr and Pr?

    • prajput
      Ansys Employee

      The Grashof number is the ratio of the buoyancy forces to the viscous forces:

      Gr = [g*beta*(T_s - T_infinity)*L^3]/nu^2

      where

      g is the gravitational acceleration

      beta is the thermal expansion coefficient

      T_s is the surface temperature

      T_infinity is the free-stream temperature

      L is the characteristic length

      nu is the kinematic viscosity


      and the Prandtl number is the ratio of the momentum diffusivity to the thermal diffusivity:

      Pr = nu/ alpha

      where

      nu is the kinematic viscosity

      alpha is the thermal diffusivity

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