Fluids

Fluids

LPBS conditions

    • alshareef
      Subscriber

      Hello,

      I'm trying to simulate steady laminar 2D pressure driven flow between two parallel plates in the slip regime with Knudsen number (Kn = 0.05). I was able to turn on the low pressure boundary slip (LPBS) in Fluent by turning on the energy equation so the option will appear at the bottom of the models window. I also have read that in order for the model to apply slip, the Lennard-Jones length has to be adjusted based on the desired pressure, Knudsen number, and the characteristic length. This can be done easily using Eq. (7.2-17) in Fluent 12.0 Theory Guide. With these modifications, I was able to have a slip at the walls of the channel without changing the wall boundary condition. My questions are:

      1- What is the limit of low pressure in order for the LPBS to work accurately? is there a threshold of the low pressure in order for this model to work? Can I use a very small length scale with near atmospheric pressure "and adjust L-J number accordingly" to have slip?

      2- Does the operating pressure in the setup window need to be altered or modified?

      3- Can the LPBS be used with SIMPLE, SIMPLEC, PISO, COUPLED, or only with a specific one?

      If anyone has any experience with the LPBS model and in the slip flow simulation using FLUENT and are willing to answer my questions and share their experience, I will highly appreciate it.

      Thanks,

      Sultan

    • Rob
      Ansys Employee

      Check the Knudsen Number still gives continuum flow (I assume you've done that): that's the critical parameter along with size.  Also have a look in literature, University of Sheffield (Tippetts, Tesar, Allen, Low & Zimmerman) have published at various points through the 90's and 00's along with (I think) some images of the mesh. 

      As an aside, why Fluent 12? That must be about 12-13 years old by now: it's the first Fluent build that was in Workbench! 

    • alshareef
      Subscriber

      Hi rwoolhou,

      I use the characteristic length along with the Knudsen number desired to calculate the mean free path. Then I use the mean free path along with the pressure that I chose to calculate the Lennard-Jones number to be edited in the material section. This way, the conditions should produce slip at the wall, which I was successful to do. I just need to check what is the pressure range that I can operate on, so this model LPBS still valid. 

      I'm actually using ANSYS 19R2, I was just reading the LPBS formulation from an old Fluent manual :-)

      My questions in the previous post still need to be answered if you have any clue about this LPBS.

      I have read ANSYS manual along with the Theory Guide, it mentioned how to turn the model on and what is the algorithm used. However, it doesn't elaborate in any details nor there is an example on it.

      Thanks for your quick reply

      Sultan

    • Rob
      Ansys Employee

      Not sure, but the manual does mention a few millitorrs at the top of the page.  Have you checked the reference given in the manual?  Otherwise, we can't help: we're limited to "public" information as we're ANSYS employees so can't explain beyond what's in the manual. 

    • alshareef
      Subscriber

      Thanks a lot for your quick reply. I will check the reference in the manual.

      Regards,

      Sultan

    • zhangdongjie
      Subscriber
      Low pressure boundary slip only works in 3D. How can I open this button in 2D?n
    • Rob
      Ansys Employee
      If it's only available in 3d you can't open it in 2d. You could use a 2.5d model, ie 3d but only one cell thick.
    • zhangdongjie
      Subscriber
      thanks,i will try it
    • zhangdongjie
      Subscriber
      Your answer helped me a lot, but when I chose density based instead of pressure based. Low pressure boundary slip disappeared again. How can I get him out?n
    • zhangdongjie
      Subscriber
      Your answer helped me a lot, but when I chose density based instead of pressure based. Low pressure boundary slip disappeared again. How can I get him out?nn
    • Rob
      Ansys Employee
      See 7.2.3. Slip Boundary Formulation for Low-Pressure Gas Systems in the 2020R2 Theory Guide.
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