[Julian K.]

Hi!

I simulated flow of discrete particles past a moving 2D cylinder which captures them on contact. The particle morphology is "Particle Transport Solid". The particle diameter is 0.1 mm. The height of cells close to the cylinder's surface is approx. 0.005 mm. Therefore, close to the cylinder the particles are larger than the cell height. A reviewer of my research suggested that this is a problem? She/he said that: "When Lagrangian particle tracking is performed, the particles must always be much smaller than the size of the mesh. If this is not the case one will have to resolve all the individual particles and also account for the fluid filling factor in each grid cell."

How does CFX handle tracking of particles that are larger than mesh elements?

[ghorrocks]

CFX models all lagrangian particles as points, regardless of how big or small they are.

The reviewer is correct in that your particles will extend over many elements, and it is preferable to account for these effects. In CFX you could do this with an immersed solid model if you really wanted to.

Are you sure you need a mesh that fine next to the cylinder? What is the size of the cylinder?

[Julian K.]

So as I understand CFX calculates the particle trajectory based thinking it's a singular point and using only velocity from the cell this point is in, even though the particle is large and extends over several cells. Is that correct?

My cylinder has a diameter of 10 mm. The mesh is very fine close to it's surface as I want to resolve the boundary layer without using a wall function.

Unfortunately, all my simulations are finished and submitted my manuscript to a journal. So, it is what it is.

I compared particle capture rates of my simulations with numerical and laboratory result from literature and they agree well for a static cylinder. Unforunately, there are no results of particle capture for a moving cylinder.

Additionally, in case of a moving cylinder some particles got lost due to an integration error. I couldn't locate the source of that error, but it happened to particles close to the cylinder surface.

[ghorrocks]

Your first sentence is correct.

Do you really need the mesh to be this fine? What y+ does this give you? If you are far below 1 then you are probably too fine.

If you want to reply to this reviewer and not redo the simulations then you need to come up with a reason why modelling the particle as a point rather than its true size makes no difference to the results. Intuitively this would seem the case for your flow. You can probably make a proof (or at least an argument) along the lines of the shear in the flow is small on the particle length scale, meaning it gets exposed to the same flow conditions regardless of whether it is modelled as a point or its true size. And you would also need to discuss the relative motion of the particle to the fluid and whether the flows that induces are different between a point and the true size.

I suspect your integration error is also due to the super fine mesh.

[Julian K.]

Thanks Glenn!

I'm pretty confident that my numerical model is accurate enough to resovle the most dominant mechanisms of particle deposition on the cyilnder. Thus, I will take a look at the gradient in my flow and probably arguee that the particle experience only small shear. Hopefully that will satisfy the reviewer.