Initial Patching of Particles in DDPM or DEM in ANSYS Fluent

When simulating particulate flows using the dense discrete phase model (DDPM) or discrete element method (DEM) in ANSYS Fluent software, you might want to initialize the case with a certain region filled with particles. Examples of this include an initial static bed in a bubbling fluidized bed simulation and a partially filled rotary drum in a powder mixing simulation. Although it is possible to use standard DPM injection options to continuously inject particles until the desired quantity is reached, this approach is computationally expensive and impractical when the amount of particulate mass to be injected is large. Additionally surface injection from “interior” cell facets is not recommended as this option does not provide good control over injection and may lead to numerical instability.

So what are the options currently available?

One option is to first generate an injection file using, for example, a FORTRAN™ or Python® program and then use that injection file to inject particles. A second option is to directly inject particles at the first time step using a user-defined function (UDF).  We do have a Python file as well as a UDF that can be shared with interested users.

Whichever option you use, it is important to consider following factors before patching the particles.

Number of computational particles or parcels to inject: This directly affects the computational cost. Moreover in unsteady simulations if the particles are being continuously injected at each time step through other locations, the number of tracked particles will rapidly increase.  A workaround is to inject fewer but stronger parcels as explained in a Knowledge Resource 1017 on the ANSYS Customer Portal.

Parcel size compared to cell size in a mesh: For flows involving interaction with the fluid phase, parcel size must be sufficiently smaller than the cell size in a mesh. This is a requirement of the volume fraction calculation method and convergence difficulties will arise if this condition is not met. However for DEM simulations with no interaction with fluid phase, this requirement can be ignored.

Regular or staggered arrangement of particles: A staggered arrangement of particles is preferable over a regular arrangement as the staggered arrangement avoids the possibility of most particles crossing cells at the same time, leading to a large value of source terms for the fluid phase.

Constant or varying number of particles per parcel: Varying the number of particles per parcel is useful for meshes with a large variation in cell volume (i.e. the ratio of maximum to minimum cell volume). This option avoids the jump in volume fraction that may occur when the constant option is used.

7 thoughts on “Initial Patching of Particles in DDPM or DEM in ANSYS Fluent

  1. Thank a lots for this interested tips. Could you please shared UDF for initial Patching to me at werachat@cadit.com.sg.
    Moreover, I have following question for DDPM. If initial state as no particle inside fluid domain (Euler domain), after DDPM injected, my question is that Volume Fraction (VF) of secondary phase will increasing automatically due to percentage of particles in domain or not?
    How FLUENT calculated VF on secondary phase in DDPM?

    Thanks for your advances.

    • Werachat,
      To be able to provide UDF, I will need your complete name, company email address and ANSYS Contact number for creating a technical support request.

      I will send you email to get the above information.

      Volume fraction of particles will increase as particles get injected. Depending on under relaxation value specified for Discrete Phase Sources, you will need to perform sufficient number of iterations to allow volume fraction to get fully updated.

      For more information, see following section in FLUENT 13.0 Theory Guide.
      // Theory Guide :: 0 // 16. Discrete Phase // 16.12. One-Way and Two-Way Coupling // 16.12.5. Under-Relaxation of the Interphase Exchange Terms

      Volume fraction of particles is calculated based on volume of particles in a given cell and then it will be interpolated to neighboring cells using an interpolation function (beta feature in forthcoming R14 ).

      Regards,
      Shailesh

  2. Dear
    Shailesh Ozarkar

    I Enjoyed your webinar Ask the Expert –Gas-solid Flow Modeling with ANSYS CFD 13.0-20110720, mainly the two case studies that you showed related with the chalenge problem of NETL https://mfix.netl.doe.gov/challenge/index.php .

    I am having problems importing the stl file to DM. I wonder to know if you could provide me your cas file so that I can make some modelling work with it. Moreover, I Am mainly interested on the reactor puls 2 cylones cas file or the truncated geometry cas file.

    I am also interested to know more about the UDF that you discuss in your blog post for DPM initial patching http://blog.ansys.com/2011/10/13/initial-patching-of-particles-using-ddpm-or-dem-in-ansys-fluent/ .

    Thanks in advance,

    Ricardo Santos,
    Faculdade de Engenharia da Universidade do Porto

  3. Dear Shailesh,
    I am actually interested in modelling a fluidized bed. The bed is of sand particles fluidized using air. The bed also contains biomass particles with a fixed volume fraction (initially). I want to treat biomass particles as discrete phase, while other two as continuous phases. For this, I want to use DDPM in Fluent. Can you provide the code and any other useful input.

    Regards,
    Abhishek

  4. Hello Shailesh, I am interested in the UDF. Can you please send me an email, so that I can provide you my company details/Ansys contact number? Thank you

  5. Hello, Shailesh! Thank you for your useful notes! I’m from Russian ANSYS Channel Partner Company – CADFEM-CIS, and I’m interested in multiphase modelling aspects. Is that UDF, which you recommend is DEFINE_DPM_INJECTION_INIT? If so could you please send me your example code. (email removed by moderator, we will respond privately to your request)
    Thanks in advance and Best Regards!

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