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avi@lpsc April 6, 2012 14:16

Different flow patterns in CFX and Fluent
Hi Guys
I have some issues interpreting my results from two similar analysis, one in Fluent and other in CFX. Its a general compressible flow case of CD nozzle with supersonic diffuser attached to the exit of nozzle.
I will highlight the settings here:
Fluent: 2D axi case with density modeled as Ideal gas, Density based solver, 4 species properties given as polynomials, Pressure Inlet at Nozzle 33.5bar- Mass flow rate boundary imposed :8.65kg.s-Subsonic, Pressure outlet at Diffuser section attached to Nozzle exit 180millibar. Case Initialized with Pressure Inlet with lower value of axial velocity, Boundary layer in nozzle with Y plus approximately 10.Turbulence:SST K-omega, For this condition a converged solution shows the Nozzle mach Contour as - Full flow i.e no separation inside nozzle,No shock inside. (Maximum Mach Nbr: 8),

2. A similar case was ran in CFX with similar settings, Density: Ideal gas, Solver details : Default settings,Species Properties similar to Fluent,SST K-omega, Mass flow rate bc. A converged solution shows separated flow in nozzle and no full flow similar to Fluent results.

Grid Independent and Turbulence model study has already been conducted and all resulted in same flow pattern in Fluent i.e Nozzle full flow.

Similar GI study has also been done in CFX for with adapted grid in nozzle which shows separated flow with in nozzle.

So In a sense I am getting different results from two with similar simulations.I only uses density based solver of fluent for compressible flow problems. whether CFX solver is capable for this type of high speed flows. If it is,then what could be the possible reason for this differing results.

1D cal. shows flow will separate inside the nozzle, but point here is that even I performed both simulations in 3D, same grid, property settings same. I am getting full flow in Fluent and separated flow in CFX. What would be the possible reason for it , whether PBCS solvers are able to handle shock formation and separation better than density based solver of Fluent. Major facility change will be done based on this analysis, so I need a solution for this different results.

banty April 6, 2012 15:25


I would use pressure inlet boundary condition in place of mass flow rate BC and impose target mass flow rate with pressure outlet BC. and when u are using density based solver then in place of weak enforcement of pressure specification method (default with density based solver), specify direct pressure specification method.

avi@lpsc April 7, 2012 10:39

whether this will change the flow pattern? I have cases where I have used PressIn BC instead of mass flow and similar results for both. So what would be the reason for different results from CFX and Fluent?

banty April 7, 2012 16:14

mass flow rate boundary condition would not cause much difference but pressure outlet with default setting (weak enforcement of pressure specification method in which fluent interpolate the pressure at boundary) does not enforce the exact pressure (180 millibar in your case). This become more important when u have subsonic flow at diffuser outlet because in case of supersonic exit values are interpolated from interior.

avi@lpsc April 8, 2012 06:12

Thanks banty ..I will try this out also.


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