I was quite confused with those two methods to model a rotating impeller. When I read through the literature, the description of those two mothods confused me. MFR represents multiple frames of reference and SG is sliding grid or sliding mesh. Does any one have the experience of using the two methods in CFX? Please provide a clear and detailed explaination regarding this.

Thanks in advance!

Hi,

Sliding grids (in CFX talk a General Grid Interface or GGI) is used to join different domains together in a Multiple frame of reference (MFR) simulation. They are not different methods of doing impeller simulations.

Glenn Horrocks

Glenn, Thanks a lot! However, in the literature, it seems that they are two different methods. MFR saves a considerable computational demand compared to SG/SM. Am I right?

Hi,

As I said, MFR and sliding grids are not different methods. You are probably confused with frozen rotor versus a full transient simulation. If the frozen rotor assumption is applicable then it saves a lot of computing time.

Glenn Horrocks

MFR and sliding grids are two options for modelling rotation problems.

MRF model is a steady-state approximation. It includes the use of stationary and rotating frames. The solution proceeds with a steady transfer of information across a pre-defined interface between the two frames.

Sliding mesh is a transient approach. The two grids slide past each other in a time-dependent manner, exchanging information at the cylindrical interface.

Regards

Gab

Hi,

Do you mean transient rotor/stator (TRS) versus static GGI? TRS interfaces are used in fully transient simulations where the GGI is re-computed every timestep and the static GGI can be used in frozen rotor simulations and is only computed once. Note they are both multiple frame of reference (MFR) simulations.

Glenn Horrocks

Hi,

It's true that the MFR and sliding mesh both have multiple frame.

However, traditionally, they were recoginized as two different approaches. Search 'sliding mesh, MFR' on Google, I did not find any paper take them as the same approach.

Just list two examples:

http://www.fluent.com/solutions/articles/ja111.pdf (Fluent, page 2):

http://www.multiphase.newcastle.edu..../1999/lane.pdf (page2)

Regards!

Gab

Hi,

I think I see where the confusion has come from. What the papers you refer to call "MFR" is in CFX talk a frozen rotor simulation, and what they call "sliding grid" is in CFX talk a transient rotor stator simulation. I think we have that cleared up now!

Regards, Glenn Horrocks

Hi Glenn, These two methods were developed by Luo et al (see below). Incidentally SG came before MFR. Then and there after, MFR was invariably understood as a steady state approach and SG (or sliding mesh, as some call it) is transient.

In the CFX world, to be specific, Frozen rotor is a CFX5 language. I do not think there is I Frozen rotor in CFX4 (unless my memory is rusted) instead MFR is talked about.

1) Luo, J.Y., Gosman, D.A., Issa, R.I., Middleton, J.C, Fitzgerald, M.K. (1993). Full flow field computation of mixing in baffled stirred vessels, Transactions of the Institution of Chemical Engineers, 71A, 342-344. 1)Luo, J.Y., Issa, R.I, Gosman, D.A. (1994). Prediction of impeller induced flow in mixing vessels using MFR, ICHEME Symposium series, 136, 549-556.

regards, Eldoret

In SG, the equation of space conservation is solve alongside the momemtum equation. A domain or a block of grid is allow to deform and slide or slide past a stationary domain. The whole set of equations is solve in the same frame of reference (inertial frame).

In MRF, the grid however remains stationary throughout the calculation. But one is allow to specify different frame of reference (non-inertial frame) to different domain according to the case considered. For example, in a stirred reactor simulation, u can divide the reactor into 2 blocks, a stationary frame covering the baffle and a rotating frame (non-inertial) frame covering the impeller, u then solve the euqation implicitly but in a different frame according to the location of the cell.

SL