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Why particle size should be smaller than mesh size to use DEM? |
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January 6, 2021, 07:21 |
Why particle size should be smaller than mesh size to use DEM?
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#1 |
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Mandeep Shetty
Join Date: Apr 2016
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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? ii) For this simulation can I consider the interia of the thermocol chunks to be negligible (and so flowing freely with water) as its density is less? iii) Are all Euler-Lagrangian solver CFD-DEM solvers, since I see that Euler_Lagrangian and CFD-DEM are used interchangeably? |
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January 6, 2021, 07:29 |
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#2 |
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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) |
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January 6, 2021, 14:39 |
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#3 | |||||
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Mandeep Shetty
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January 6, 2021, 17:56 |
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#4 | |
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q
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The backbone of the particle tracking methods in Lagrangian coordinate system is that they usually treat particles like points of assumed properties (D, T, v etc.) and they are not “resolved”. Since the parameters need to be coupled by the interpolation particle sizes need to be smaller than the computational cell for the interpolation to be meaningful. |
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January 7, 2021, 05:54 |
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#5 | ||||
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Hello granzer, let me try to be more specific:
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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). Quote:
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). Quote:
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. |
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January 7, 2021, 12:32 |
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#6 | |||
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Mandeep Shetty
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https://blogs.3ds.com/simulia/particle-methods-in-abaqus-dem-vs-sph/ |
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January 7, 2021, 13:05 |
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#7 |
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Filippo Maria Denaro
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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?
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January 7, 2021, 16:35 |
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#8 | |
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Mandeep Shetty
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So I yes it's the coupling that causes the size restriction. If we use Lagrangian-Lagrangian model like say SPH-DEM then I don't think there is a matter of size begin restricted by mesh size(not sure about this though). |
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January 7, 2021, 16:48 |
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#9 |
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Filippo Maria Denaro
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I have no experience to say more... I found this recent paper
https://onlinelibrary.wiley.com/doi/...002/cjce.23773 and it seems here that DEM is mainly the solution of the lagrangian equation for a passive tracer, that is one-way mode. |
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July 13, 2023, 16:41 |
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#10 |
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Thanks for the discussion above, which unfortunately seems very difficult for me to understand in my current stage. ..
What I just want to know is: Does StarCCM support what is called immerse boundary or fictitious domain? Because I must make the particle size larger than mesh size. |
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