Fluids

Fluids

Ensure mass conserved at interface between nozzle exit and atmosphere

    • Daidalos
      Subscriber

      Hi, I am investigating a 2D case of the flow ejection of a nozzle into atmospheric air conditions. Attached is an image of my geometry with the boundary conditions imposed. However, I would like to ensure mass flow is conserved at the interface between the nozzle exit and the atmosphere as circled in the image below. Currently in fluent, the interface is treated as an interior. Is there a way to set the mass flow at that interface to be equal to the inlet mass flow. 


      Operating pressure is atmospheric pressure. the lower boundary which i cut off from the image is a wall. 


       


       Thanks in advance.


    • Keyur Kanade
      Ansys Employee

      As ANSYS Staff, we can not download attachments. Please insert images using insert image functionality. 


      If you want to define specific mass flow rate at the marked location, why dont you remove upper part from geometry and model from that location itself. 


      Having velocity at the inlet and mass flow rate at the location you marked would be in conflict. 


      Regards,


      Keyur


       


      If this helps, please mark this post as 'Is Solution' to help others.


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    • Daidalos
      Subscriber

      Hi Keyur,


       


      thanks for your advice, the image attached is the same as the one depicted. had forgot to remove the attachment. 


      As for the boundary conditions, yeap that is kind of what i am stuck with since it does not make sense to define another BC at that region for the solver. However, I think need the upper contracting geometry as from what I understand the nozzle geometry helps define the flow geometry at the exit. I had previously tried using your method and the flow volume fraction and velocity flow was quite different to what i observe. 


       


      I am actually simulating a high pressure water jet where I expect a velocity core region which I am able to obtain with he nozzle contraction. When I tried to simulate without the upper geometry I dint observe the water potential core region and the flow was kind of a straight flow. Do you think I need a velocity UDF for this? Maybe I am thinking this wrongly would appreciate if you could advice me.


       


      Thanks and kind regards

    • Rob
      Ansys Employee

      Why would the mass flow at that position be different to the inlet? 

    • Daidalos
      Subscriber

      I think it is cause the nozzle region (inlet till the circled position) is an internal flow case, while the bottom domain is an external flow. however, since the circled position is an interior, while my outlet is the pressure outlet at the far right, the mass flow becomes only conserved at the inlet and the pressure outlet at the far right. I think I am setting up my problem correctly besides the circled region since I am unsure how to ensure mass flow is conserved between the interior of the nozzle and the exit of the nozzle when I have an additional external flow domain at the bottom. Appreciate any advice given

    • Rob
      Ansys Employee

      Mass is conserved within each cell, otherwise how would the model know how to balance over the outer boundaries?  By all means put a monitor on that interior surface but you can't fix the flow like that. 

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