The CFD case file to be provided to Rocky for a two-way coupling simulation can be either:
An Eulerian Multiphase simulation setup, where one of the phases should be defined as the particulate phase;
A setup comprising a single fluid phase, where the Multiphase model is turned off.
Some specific configurations for the model definitions and solver numerics are presented below.
As described above, the Eulerian approach can be used within the CFD model to perform the coupling between the two solvers when more than one fluid phase is needed. To do so, the Eulerian Multiphase model should be selected in order to set the coupled simulation. The number of Eulerian phases should be equal to the number of fluid phases +1, where the additional phase is the particle phase.
Note: If the simulation comprises only one fluid phase, the single-phase coupling approach offers a more efficient coupling model than the multiphase coupling. The single-phase coupling does not use the Eulerian Multiphase model; See section Single fluid phase for details.
The particle phase should be set as a secondary phase and the momentum exchange terms between the fluids and particles are calculated by Rocky. Therefore, the user does not need to set momentum transfer coefficients between the particle and fluid phases. However, in the case of more than one fluid phase, the phase interaction between fluid phases should be properly defined during this step. The material used in the particle phase must be used only for this phase. Additional setup operations related to the particle phase will be performed by Rocky.
Since Rocky is responsible for the particulate phase solution, the solid volume fraction and velocities information should come from the DEM solver. Thereby, the particulate phase velocities and volume fractions should be set equal to 0 in all boundaries.
Also, the particle volume fraction and velocities should be set to 0 during the case initialization when the Multiphase coupling approach is being used. Particle volume fractions will be updated during the coupling initialization process. The user also has the option of providing the initial flow field of the fluid to Rocky no matter whether the multi-phase or the single-phase approach is used.
Note: When setting up the multiphase solution, the secondary phase created to model the particles should have volume fraction equal to 0 for the entire domain. Rocky will initialize the secondary phase according to the initial particle state in Rocky. There is no need to care about the secondary phase velocity as it is not used in the coupling simulation.
It is also possible to initialize the coupling using a Fluent data file with an initial flow field. In this case also the volume fractions of the particle phase must be 0 throughout the domain.
If a simulation comprises only one fluid phase, the single-phase coupling detailed in section Single-phase coupling is the indicated coupling approach instead of the more general multiphase coupling. As the single-phase approach makes it unnecessary for conservation equations (section Multiphase coupling) to be solved for the solid phase, this reduces the computational load thereby speeding up the simulation. In addition to being faster, the single-phase approach tends also to be more robust when compared to the multiphase approach.
Phase Coupled SIMPLE method should be used as pressure-velocity coupling method in the multiphase coupling approach. First Order should be used as the transient scheme. Be careful to choose a reasonable time step. Time step size will be updated after coupling starts, in order to be an integer multiple of the Rocky time step.
Note: For the single-phase coupling approach, any pressure-velocity coupling method can be chosen.
By default, Rocky does not export simulation data without specific input from the user. In order to enable transient solution files, you must specify that Rocky export the simulation data as usually done in Fluent. When enabled, the transient solution files are saved in the simulation sub-folder of the Rocky project.
The only way to quantify the amount of particle volume inside a cell in Fluent is by post-processing the Rocky User-Defined Memory for particle volume fraction. Section Particle phase User-Defined Memories (UDMs) describes the particle phase UDMs that are available for post-processing.
Note: This is true even for the single phase coupling approach, as Fluent does not provide information about the porosity profile for post-processing.
In order to perform two-way coupled simulations with Fluent, Rocky automatically employs its own set of User-Defined Functions (UDFs) and reserves an amount of User-Defined Memory (UDM) for the coupling according to the procedures described by the Fluent Customization Manual. In order to not conflict with Rocky UDFs, third-party UDFs that are part of the simulation must comply with the following:
The third-party UDFs must be loaded automatically by the Fluent case.
If third-party UDMs are employed, they must be reserved and accessed by the third-party UDF according to the section Reserving UDM Variables of the Fluent Customization Manual.
If third-party UDMs are employed, the third-party UDFs must support unloading and reloading.
Table Particle phase User-Defined Memories (UDMs) details the physical quantities related to the particle phase that are available as User-Defined Memories (UDMs) in Fluent resulting from two-way coupled simulations.
Table 8.1: Rocky user-defined memories about the particle phase for post-processing in Fluent. UDMs 11 though 13 are available only in thermal simulations. The numeration of the UDMs may contain an offset if the case employs third-party UDMs according to section Third-party User-Defined Functions (UDFs).
| UDM | Particle Phase Property | Unit |
|---|---|---|
| 0 | Volume fraction | - |
| 1 | Volume fraction source |  |
| 2 | Average velocity X |  |
| 3 | Average velocity Y |  |
| 4 | Average velocity Z |  |
| 5 | Explicit momentum source X |  |
| 6 | Explicit momentum source Y |  |
| 7 | Explicit momentum source Z |  |
| 8 | Implicit momentum source X |  |
| 9 | Implicit momentum source Y |  |
| 10 | Implicit momentum source Z |  |
| 11 | Average temperature |  |
| 12 | Explicit energy source |  |
| 13 | Implicit energy source |  |