CFD Online Discussion Forums (https://www.cfd-online.com/Forums/)
-   CFX (https://www.cfd-online.com/Forums/cfx/)
-   -   Troubles modelling flow through a grid (https://www.cfd-online.com/Forums/cfx/18330-troubles-modelling-flow-through-grid.html)

 Hans Klaufus March 24, 2000 06:08

Troubles modelling flow through a grid

Dear All,

Currently I'm involved in simulating the flow through several grids using CFX. Since I had some troubles in the original (complex) problem, I started to research the effects of a (simple) grid placed in a two-dimensional duct with constant cross section. One boundary a inlet, opposite boundary mass flow boundary, halfway inbetween the grid covering the complete cross section.

I thought of modelling the grid using the porous medium option in CFX. But a porous medium must be modelled with at least three cells width for interpolation reasons. For simulating a grid I then need to refine the local mesh too much to be attractive (or even possible??).

And since I'm not interested in simulating the flow IN the grid (porous medium) but on the effect of it on the SURROUNDING flow, I thought of modelling the resistence of the grid using the fortran routine USRBF to define some bodyforces. I now defined the grid as a USER3D patch and skipped the definition of porosity and just set the resistence parameters. This reduced the width of the modelled grid from three cells to one cell, which comes closer (but not close enough) to reality.

From the CFX manual I found: B = Bf - (Rc + Rf|v|)v For me Bf = 0.

From the 'Chemical Engineer's Handbook' I found: Rc = 0 Rf = 3.25E+4 for a 50% porous grid.

So for a grid of 1 mm depth and an inlet velocity of 1.11 m/s I would expect a pressure drop of: dP = Rf*v*v*dx = 3.25E+4*1.11*1.11*0.001 = ~40 Pa.

Since the modelled grid has a width of 1 cell, I need to rescale the body forces in USRBF. Duct length 1.5 m No. cells in length direction: 200 Actual width of grid: 0.001m So the scale factor would be: 0.001/(1.5/200) = 0.1333

In contrast to my expectations CFX returns (after convergence) with a pressure drop of ~0.006 Pa.

Question: Can someone tell me what's wrong in my models, or in my assumptions???