Originally Posted by sbaffini (Post 792571)

Not an expert in this specific field but, a general reasoning you can adopt here as well as in other cases is: if you have a grid but it doesn't capture your phenomena of interest, then a model for these phenomena requires them to happen at scales smaller than those described by the grid, ideally much smaller.

For particles, if they are the size of a mesh cell or bigger, the mesh should probably take them into account.

I'm pretty sure that DEM and Euler-Lagrange are not, in general, interchangeable. There are DEM approaches that are purely Lagrangian and don't have a grid at all (at least not in the common Eulerian sense)

Originally Posted by granzer (Post 792570)

Hello,
I am trying to simulate the flow of discreet particles in a continuous medium using the CFD-DEM coupling.
Let the continuous phase be water and particle be high-density thermocol, which has considerable size (assume chunks of themocol) but its density is smaller than water so that the chunks float (if the density of water is taken as 1000 kg/m3 let the themocol's density be 800 kg/m3)
i) I read that to use DEM the size of the particles should be less than that size of the mesh. Why is this?

Originally Posted by granzer (Post 792605)

You mean if the grid is not sufficient enough to 'resolve' the phenomena of interest ryt?.

Originally Posted by granzer (Post 792605)

If we are not able to "resolve" the phenomena we have to model for the said phenomena...but using a model for the phenomena requires the mesh size to be larger than the phenomena itself?

Originally Posted by granzer (Post 792605)

This sentence seems to answer my questions (i) and (ii). Are you suggesting that when considering particle flows, even though the particles may be bigger than the mesh, it shouldn't cause a problem for simulations?

Originally Posted by granzer (Post 792605)

(In the last sentence I am assuming you are talking about particle methods like SPH or MPM?) As you say since there are DEM that is purey Lagrangian are the terms 'DEM' and 'Lagrangian' synonymous or is it wrong to say so?

Originally Posted by sbaffini (Post 792684)

Hello granzer, let me try to be more specific:



yes



Yes, that was the general idea I was trying to promote but, as in most circumstances, there are exceptions. It really depends from the phenomena, the model and scope of the model. There are indeed models that
don't satisfy this but are known to work. Let me give you some examples:

1) Continuum model in place of molecular dynamics. We know that for the continuum hypothesis to be valid there must be enough molecules in a volume for the related averages to be meaningful. The model will still work for very small grids but the results might not be meaningful anymore.

2) Roughness models and wall functions. Wall functions are wall boundary conditions used when the grid can't resolve some features. It is only in this context that it makes sense to augment the wall function with roughness effects. If the grid can resolve the flow at the wall, by definition, it must resolve the roughness as well. So there is no roughness model outside wall functions (i.e., when the grid is not coarse enough to use them).

3) I used to work with models for micro vortex generators, which are devices with size of the order 0.1 the boundary layer thickness. The related models are just forcing terms in the equations and their intended use is for grids that, in theory, could actually describe them. Their use case is, typically, in the sense of an immersed boundary approach, you accept some error in order to more easily move them around and not having to make a mesh again. This is not a physical model, it is more a smart model that doesn't want to describe the physics but be smart and useful.

So, to be more specific, what I said is for models of physical phenomena that happen at smaller scales. Still, I know no physical model that works like, say, the micro vortex generator model I was mentioning above (that is, general scales that might or not be described by the grid).



Actually no, I meant quite the opposite. That is, if your particles are the size of, say, 10 grid cells, what is the meaning of the solution in those 10 cells which, instead, would ideally be occupied by a particle? To correctly take into account a particle of that size on your grid, you probably need the grid to describe the particle (i.e., go around it, not trough it), with boundary conditions etc.

This does not mean that there might not be models for this scenario that still work (think about my micro vortex generator example), but that is certainly not a physical model in the common sense. It is some smart model or some trick (say, immersed boundary treatment).



Again, I am not an expert but, what I know about DEM explicitly points to what I classify as Lagrangian treatment. For me, Lagrangian basically means any sort of N-body treatment, where position and velocity of N objects (particles, planets, etc.) are obtained by integrating in time their respective Newton 2nd laws.

Now, there are physical applications where these objects just interact between themselves and thus no additional field needs to be defined on an underlying Eulerian grid (yet, nearest neighbor searching would still, typically, require a sort of grid for algorithmic implementation).

To be more specific, Lagrangian is a more mathematical term that, despite what anyone can think, univocally identifies a certain treatment that we can simplify in "moving particles around without an underlying grid".

DEM is a more specific approach, but still broad enough, and what it means is basically defined by its community. Still, there are examples of applications that are DEM and don't use any underlying grid. Indeed, the presence of the underlying grid, intended in the Eulerian sense, is necessary when an interaction (backward, forward or both) between the two has to be taken into account in order to correctly describe the physics at hand.

So, clearly, in CFD, it typically happens that your approach is indeed Euler-Lagrange, because you are actually interested in the Euler part or what it does on the Lagrange part. But there are cases where the Lagrangian part is self consistent (within the approximation of the modeling assumptions, of course) and doesn't require the Euler part at all. SPH is one such example, but I think that this also applies to several applications in the realm of solid dynamics, where I don't think they use any global description for the fluid between the solid particles.

Originally Posted by FMDenaro (Post 792731)

I never worked with DEM but I wonder what the grid size is in this discussion... I assume that DEM has no requirement of a grid at all, thus the computational grid should be required by the coupling with some Eulerian approach. Is the coupling that should be the reason for discussing the proper size, not the DEM by itself, isn't it?