Different Resistance Falues do not result in different system pressures

[eric.johanson]

I am working on establishing a system pressure based on adjustable louvers.  To simulate the louvers, I am using a volumeric resistance model with a free flow %.  I figure by varying the flow thru value, I can achieve the system pressure I am targeting.  However, if I use different free flow values, the system pressure does not change.  If I apply a solid, pressure goes way up.  If I apply air, system pressure goes way down.  If I apply an % free flow value, system pressure changes to a constant value...No matter what % is applied...  From 90% to .01%.  What am I missing here?  I have setup a very simple test part, attached.

[Jon.Wilde]

Hi Eric,

Thanks for the file (although we only need a CFZ in future if it helps). I just have two points:

1. Ugrade to CFD 2015 SP2, there are some changes to the way we model resistances that you will find beneficial here
2. Your mesh is too coarse. The air needs to be finer and we need 4-5 elements (use a uniform mesh) through a 3D resistance thicknes, with one element CFD will be unable to capture the pressure drop properly
3. You need to use Advection Scheme 5 for volumetric resistances (ADV1 is OK for surface ones, see below)
4. Consider using a surface resistances
5. Run for more than 100 iterations, if you have sufficient mesh CFD should stop automatically once converged

Kind regards,

Jon

[eric.johanson]

Jon:

Thanks for the quick response.  I'm am on CFD 2014, v 14, and cannot update to 2015.  So I've taken your input to refine the mesh of the resistance part, then change the iterations to 300 and went with Advection 5.  Still no pressure delta between 90% or 5% resistance.  Listed are pre-change and post-hange results (csv attached).  Do you have any other ideas?  Seems like a fundamental capability that CFD should perform or that I'm not understanding correctly.  I'll play with surface resistances next.

Pre-Changes...

Mat    System Pressure    System Velocity    Inlet Pressure    Iterations
Alum    0.028900    245.66    0.0278026    100
Air    0.000666    17.41    0.0003625    100
90.00%    0.002025    40.56    0.0017188    100
5.00%    0.002025    40.55    0.0017182    100

Post Changes...

Alum    0.045648    229.512    0.0432426    178
Air    0.000998    19.3044    0.0003511    131
90.00%    0.009667    105.759    0.0090153    132
5.00%    0.009630    105.542    0.0089789    136

Thanks.

-Eric J

[eric.johanson · Accepted solution]

Jon:

I converted to the surface resistance method, and now the result appear to be valid.  It appears that the volumetric resistance method is not applicable for this situation.

Thanks for your help.

-eric

snapshot of results curve attached.

[Jon.Wilde]

Hi Eric,

Good to know it is working.

Either approach should be OK, although we need to be sure we have the correct solver settings for each.

Kind regards,

Jon