# DEM-CFD point force coupling vs volume averaged coupling

Hi everyone, I was wondering what are some typical use cases and considerations for choosing between the two implementations of yade DEM-CFD coupling ? I've only tried the Point Force Coupling (icoFoam) for my problem due to no turbulence. However, the two papers that I looked at the solve the same problem both used the averaged volume approach. Another question is does the Point Force Couple method consider the gravity of the fluid or particles?

Thank you!

## Question information

- Language:
- English Edit question

- Status:
- Answered

- For:
- Yade Edit question

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- Last query:
- 2019-10-06

- Last reply:
- 2019-10-07

Deepak (deepak-kn1990) said : | #1 |

Hi,

In the simple point force coupling, the hydrodynamic force formulation is derived from the Stokes drag equation, this is valid when the Reynolds number at the particle scale and the Stokes number is < 1, which means the inertial fluid forces are negligible compared to the viscous drag force (doesn't matter if the flow is turbulent or not). Moreover, we consider the 'suspension' to be dilute, i.e. the volume fraction of the particles are < 1-3%

In the case of Re (at the particle scale) or if the stokes number of the particle is 1 or more, or in case of 'dense suspensions ', where the volume fraction is higher, the inertial effects have to be considered. For this case one has to use the vol. averaged approach.

Note : In both the cases, the particle size has to be smaller than the grid cell size.

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