Fluids

Fluids

How to give gravity vector?

    • Parthasarathi
      Subscriber

      Dear community members,


       


      I have a query in enabling gravity vector in Fluent. Consider, 2D pipe - multiphase with water is stagnant and air bubbles are moving from the bottom. I would like to enable the gravity vector for the air bubbles. I attached two figure. In Fig. 1, air bubbles are flowing from the bottom, so I believe y=-9.81 m/s2 would be given. Is that right? What would be the gravity vector for Fig. 2. 


      Thank you. 


       

    • Keyur Kanade
      Ansys Employee

      as ansys employee, we can not download attachments. can you please insert images. 

    • Rob
      Ansys Employee

      I can't open the attachment (we're not allowed). If  +y is up  gravity will be -9.81 m/s/s in the y direction. 

    • raul.raghav
      Subscriber

      Inserted the above image for reference. Rob provided the answer for Fig. 1.


      For Fig. 1, gravity will be -9.81 [m s^-2] in the y-direction. For Fig. 2, gravity should be -9.81 in the x-direction, if the bubbles are moving strictly along the +x direction.


      Follow up question: Are you trying to simulate the movement of bubbles in vertical and horizontal pipes?




    • DrAmine
      Ansys Employee

      A good habit is to go to View and change the Camera >Up Vector to the direction one wants to have the vertical direction. In that direction, you input the negative of the absolute value of the earth gravity constant.


       


      Bear in mind: Bubbles in 2D planar are not spherical but cylinders. So force balance is not well whenever bubble dynamics are of interest. Only the axisymmetric case is good here (with the limitation the x-axis has to be symmetry axis)


       

    • Parthasarathi
      Subscriber

      Thank you for your response Admin. I will insert the pictures in future. 

    • Parthasarathi
      Subscriber

      Thank you for the detailed response Rahul. I would like to simulate bubble behavior through PBM (in a vertical pipe). 

    • Parthasarathi
      Subscriber

      Thank you Amine. 


      Yep. I would like to simulate for axisymmetry geometry. For understanding about gravity vector, I made 2D geometry. But I wonder how the spherical bubbles will be cylinders in 2D planar. I thought it would be circle in 2D planar. 

    • raul.raghav
      Subscriber
      In 2D planar, you define a depth in the z-direction which will be used to calculate/report the forces and fluxes. Therefore, your circular 2D bubbles will be treated as cylinders and not as spheres, as Amine mentioned above.
    • DrAmine
      Ansys Employee

      Raul provided a concise reply. To add: the depth can be changed under "Reference Values". 

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