[atul1018]

Dear community members and seniors

I am writing in order to get some insights and guidance to understand the coupled fluid-particle interaction forces.

After going through varoius literature (main source: https://www.cambridge.org/core/journ...59D18FBBD60A59), I found out that there are mainly three approaches in CFD-DEM depending upon which forces are taken into account to calculate fluid-particle interaction force and corresponding governing equations, namely BFull (set-I), A (set-II) and B (set-III). Model A (set-II) and and model Bfull (set-I) are recommended for cfd-dem simulations, while it is recommended, not to use model B, as it may lead to incorrect results for compelx flow systems. In the different formulations, I see that pressure gradient force and bouyancy is taken together as combined force (at least in A and BFull formulations) and considered in fluid-particle interaction forces. I am not able to understand why they are combined as in my understanding bouyancy is a body force like gravity and it should not be taken as coupled forces (fluid-particle interaction forces) such as drag, lift, pressure gradient, etc.

Now when it comes to impelemntation of those formulations in cfd packages, It seems that OpenFOAM (eg solver DPMFoam) does not consider bouyancy as coupled force (which i think is correct). On the other hand, the cfdem coupled programs (eg. cfdemSolverPiso) considers gives user the option to consider select among different formulations (A, B, BFull). But again in those formulations, bouyancy is conbsidered as coupled force, combined together with pressure gradient force, why?

I also wonder, what is the source of cfd-dem implementation for openFOAM solver DPMFoam?, as to me it seems like none of the formulations (A, B, BFull) fits to the DPMFoam behavour as here bouyant force is considered as body force, not as interatcion force. On the other hand, the formulations mentioned in the paper above takes bouyancy force as fluid-particle interaction force.

Looking farward to discuss it and hopefully clear my doubts.

Best Regards
Atul

[atul1018]

Some additional thoughts.

I think OpenFOAM separates buoyancy force from fluid particle interaction forces because it provides the option to select among coupling regimes (1-way, 2-way and 4-way).

Consider a case where particle concentration is very small, therefore 1-way coupling should be enough and would accelerate the simulation many folds. In this scenario, fluid-particle interaction term in CFD equation is zero. If buoyancy is not excluded from this interaction forces, it would result incorrect results as magnitude buoyancy force might be large and of significance and definitely gonna affect the fluid motion even when the particle concentration is very small. If buoyancy is separated from interaction forces, one could have buoyant effect of particles even fluid-particle interaction force in CFD equation is turned off (one-way coupling).
On the other hand, CFD-DEM coupled codes (LIGGGHTS and OpenFOAM) doesn't give this option of selecting among coupling regimes, rather it is always two way coupling, ie all fluid-particle interaction forces (including buoyancy) is always included in CFD equation. So it doesn’t make any sense to separate the buoyancy from other interaction forces and almost all the paper based on CFD-DEM also doesn’t separate them.

After reading a lot of papers and thinking about it, this is what which make sense to me and may be the probable reason for considering the buoyancy force separately (as body forces) from other interaction forces in OpenFOAM.


Best
Atul

[Jun12123]

I have the same confusion. However, after some thought and research, I discovered the relationship between the pressure gradient force and buoyancy. Firstly, buoyancy arises due to the difference in pressure on the surface of an object, and calculating buoyancy based on the displaced volume is just a method of calculation, not the principle. The reason for the pressure difference is the pressure gradient, so the pressure exerted on the object’s surface ultimately forms a pressure difference, which is buoyancy. Secondly, imagine an object sinking at a constant speed in still water, which indicates it is in force equilibrium. Note that even in still water, there is a pressure gradient in the downward direction. However, if both buoyancy and pressure gradient force are considered simultaneously, it ultimately leads to a force imbalance.
Perhaps we can discuss this issue in detail, this is my email:2019302060190@whu.edu.cn

[TammyJSouza]

Originally Posted by Jun12123

I have the same confusion. However, after some thought and research, I discovered the relationship between the pressure gradient force and buoyancy. Firstly, buoyancy arises due to the difference in pressure on the surface of an object, and calculating buoyancy based on the displaced volume is just a method of calculation, not the principle. The reason for the pressure difference is the pressure gradient, so the pressure exerted on the object’s surface ultimately forms a pressure difference, which is buoyancy. Secondly, imagine an object sinking at a constant speed in still water, which indicates it is in force equilibrium. Note that even in still water, there is a pressure gradient in the downward direction. However, if both buoyancy and pressure gradient force are considered simultaneously, it ultimately leads to a force imbalance. poppy playtime chapter 3
Perhaps we can discuss this issue in detail, this is my email:2019302060190@whu.edu.cn

Explore whether thrust should be classified as a body force or a coupled force in the context of particle-fluid interactions in CFD simulations. This is one way thrust is treated together with pressure gradient forces in various models, especially in CFD-DEM approaches.

[Jun12123]

Thank you very much for your reply. My research direction is not CFD-DEM. I just feel that there might be some problems when seeing that the particle dynamic equation in some articles considers both pressure gradient force and buoyancy force.