[shenying0710]

Hello, my friends! I met with difficulties in calculating a diffuser pipe flow.
It seems so simple a problem, but CFX can't get convergency result!
The attachment is the model picture and the CCL file.
Could you give me some valuable advices on how to get a convergency result for this question?
When the inlet velocity is less than 100m/s(approximately Mach Number 0.3), It easily gets convergency result, but when inlet velocity is more than 100m/s(approximately Mach Number 0.3), It can't get convergency result no matter how I adjust the calculation parameters.
model.jpg
mesh.jpg

# State file created: 2013/03/24 21:10:03
# CFX-14.0 build 2011.10.10-23.01
FLOW: Flow Analysis 1
SOLUTION UNITS:
Angle Units = [rad]
Length Units = [m]
Mass Units = [kg]
Solid Angle Units = [sr]
Temperature Units = [K]
Time Units = [s]
END
ANALYSIS TYPE:
Option = Steady State
EXTERNAL SOLVER COUPLING:
Option = None
END
END
DOMAIN: Default Domain
Coord Frame = Coord 0
Domain Type = Fluid
Location = SOLID
BOUNDARY: inlet
Boundary Type = INLET
Location = INLET
BOUNDARY CONDITIONS:
FLOW REGIME:
Option = Subsonic
END
HEAT TRANSFER:
Option = Static Temperature
Static Temperature = 300 [K]
END
MASS AND MOMENTUM:
Normal Speed = 200 [m s^-1]
Option = Normal Speed
END
TURBULENCE:
Option = Medium Intensity and Eddy Viscosity Ratio
END
END
END
BOUNDARY: outlet
Boundary Type = OUTLET
Location = OUTLET
BOUNDARY CONDITIONS:
FLOW REGIME:
Option = Subsonic
END
MASS AND MOMENTUM:
Option = Average Static Pressure
Pressure Profile Blend = 0.05
Relative Pressure = 0 [atm]
END
PRESSURE AVERAGING:
Option = Average Over Whole Outlet
END
END
END
BOUNDARY: wall
Boundary Type = WALL
Location = WALL
BOUNDARY CONDITIONS:
HEAT TRANSFER:
Option = Adiabatic
END
MASS AND MOMENTUM:
Option = No Slip Wall
END
WALL ROUGHNESS:
Option = Smooth Wall
END
END
END
DOMAIN MODELS:
BUOYANCY MODEL:
Option = Non Buoyant
END
DOMAIN MOTION:
Option = Stationary
END
MESH DEFORMATION:
Option = None
END
REFERENCE PRESSURE:
Reference Pressure = 1 [atm]
END
END
FLUID DEFINITION: Fluid 1
Material = Air Ideal Gas
Option = Material Library
MORPHOLOGY:
Option = Continuous Fluid
END
END
FLUID MODELS:
COMBUSTION MODEL:
Option = None
END
HEAT TRANSFER MODEL:
Include Viscous Work Term = On
Option = Total Energy
END
THERMAL RADIATION MODEL:
Option = None
END
TURBULENCE MODEL:
Option = SST
END
TURBULENT WALL FUNCTIONS:
High Speed Model = On
Option = Automatic
END
END
END
OUTPUT CONTROL:
BACKUP RESULTS: Backup Results 1
File Compression Level = Default
Option = Standard
OUTPUT FREQUENCY:
Iteration Interval = 40
Option = Iteration Interval
END
END
MONITOR OBJECTS:
MONITOR BALANCES:
Option = Full
END
MONITOR FORCES:
Option = Full
END
MONITOR PARTICLES:
Option = Full
END
MONITOR POINT: Monitor Point 1
Cartesian Coordinates = 0.2 [m], 0 [m], 0 [m]
Option = Cartesian Coordinates
Output Variables List = Velocity
END
MONITOR POINT: Monitor Point 2
Cartesian Coordinates = 0.3 [m], 0.01 [m], 0.02 [m]
Option = Cartesian Coordinates
Output Variables List = Temperature
END
MONITOR RESIDUALS:
Option = Full
END
MONITOR TOTALS:
Option = Full
END
END
RESULTS:
File Compression Level = Default
Option = Standard
END
END
SOLVER CONTROL:
Turbulence Numerics = First Order
ADVECTION SCHEME:
Option = High Resolution
END
COMPRESSIBILITY CONTROL:
High Speed Numerics = On
Total Pressure Option = Automatic
END
CONVERGENCE CONTROL:
Length Scale Option = Conservative
Maximum Number of Iterations = 500
Minimum Number of Iterations = 1
Timescale Control = Auto Timescale
Timescale Factor = 1.0
END
CONVERGENCE CRITERIA:
Residual Target = 0.000001
Residual Type = RMS
END
DYNAMIC MODEL CONTROL:
Global Dynamic Model Control = On
END
END
END
COMMAND FILE:
Version = 14.0
END

[shenying0710]

================================================== ====================
OUTER LOOP ITERATION = 83 CPU SECONDS = 1.079E+03
----------------------------------------------------------------------
| Equation | Rate | RMS Res | Max Res | Linear Solution |
+----------------------+------+---------+---------+------------------+
| U-Mom | 3.83 | 3.6E-02 | 1.1E+00 | 2.0E-02 OK|
| V-Mom | 2.74 | 3.6E-03 | 9.8E-02 | 3.2E-01 ok|
| W-Mom | 2.80 | 3.6E-03 | 8.0E-02 | 3.2E-01 ok|
| P-Mass | 1.92 | 9.1E-04 | 1.9E-02 | 9.7 5.1E-02 OK|
+----------------------+------+---------+---------+------------------+
+--------------------------------------------------------------------+
| ****** Notice ****** |
| A wall has been placed at portion(s) of an OUTLET |
| boundary condition (at 100.0% of the faces, 100.0% of the area) |
| to prevent fluid from flowing into the domain. |
| The boundary condition name is: outlet. |
| The fluid name is: Fluid 1. |
| If this situation persists, consider switching |
| to an Opening type boundary condition instead. |
+--------------------------------------------------------------------+
| H-Energy | 1.05 | 6.2E-03 | 1.8E-01 | 6.3 4.2E-03 OK|
+----------------------+------+---------+---------+------------------+
| K-TurbKE | 0.93 | 1.9E-02 | 3.3E-01 | 6.3 1.0E-03 OK|
| O-TurbFreq | 1.17 | 1.5E-02 | 4.4E-01 | 12.9 2.4E-07 OK|
+----------------------+------+---------+---------+------------------+
+--------------------------------------------------------------------+
| Notice: The maximum Mach number is 2.946E+00. |
+--------------------------------------------------------------------+
================================================== ====================
OUTER LOOP ITERATION = 84 CPU SECONDS = 1.092E+03
----------------------------------------------------------------------
| Equation | Rate | RMS Res | Max Res | Linear Solution |
+----------------------+------+---------+---------+------------------+
| U-Mom | 0.57 | 2.0E-02 | 2.3E-01 | 3.8E-02 OK|
| V-Mom | 0.33 | 1.2E-03 | 3.1E-02 | 7.5E-01 ok|
| W-Mom | 0.32 | 1.2E-03 | 2.8E-02 | 8.3E-01 ok|
| P-Mass | 1.85 | 1.7E-03 | 3.3E-02 | 9.7 4.3E-02 OK|
+----------------------+------+---------+---------+------------------+
+--------------------------------------------------------------------+
| ****** Notice ****** |
| A wall has been placed at portion(s) of an OUTLET |
| boundary condition (at 100.0% of the faces, 100.0% of the area) |
| to prevent fluid from flowing into the domain. |
| The boundary condition name is: outlet. |
| The fluid name is: Fluid 1. |
| If this situation persists, consider switching |
| to an Opening type boundary condition instead. |
+--------------------------------------------------------------------+
| H-Energy | 0.79 | 4.9E-03 | 2.1E-01 | 6.2 1.6E-02 OK|
+----------------------+------+---------+---------+------------------+
| K-TurbKE | 0.75 | 1.4E-02 | 2.4E-01 | 6.2 1.1E-02 OK|
| O-TurbFreq | 1.26 | 1.9E-02 | 5.3E-01 | 12.8 1.0E-04 OK|
+----------------------+------+---------+---------+------------------+
+--------------------------------------------------------------------+
| Notice: The maximum Mach number is 3.830E+00. |
+--------------------------------------------------------------------+
================================================== ====================
OUTER LOOP ITERATION = 85 CPU SECONDS = 1.105E+03
----------------------------------------------------------------------
| Equation | Rate | RMS Res | Max Res | Linear Solution |
+----------------------+------+---------+---------+------------------+
| U-Mom | 0.85 | 1.7E-02 | 2.3E-01 | 4.8E-01 ok|
| V-Mom | 0.61 | 7.4E-04 | 1.8E-02 | 9.1E+00 F |
| W-Mom | 0.59 | 6.9E-04 | 1.8E-02 | 1.0E+01 F |
| P-Mass | 0.98 | 1.6E-03 | 3.7E-02 | 9.7 3.2E-01 ok|
+----------------------+------+---------+---------+------------------+
+--------------------------------------------------------------------+
| ****** Notice ****** |
| A wall has been placed at portion(s) of an OUTLET |
| boundary condition (at 100.0% of the faces, 100.0% of the area) |
| to prevent fluid from flowing into the domain. |
| The boundary condition name is: outlet. |
| The fluid name is: Fluid 1. |
| If this situation persists, consider switching |
| to an Opening type boundary condition instead. |
+--------------------------------------------------------------------+
| H-Energy | 0.93 | 4.5E-03 | 1.3E-01 | 6.2 1.0E-02 OK|
+----------------------+------+---------+---------+------------------+
| K-TurbKE | 1.42 | 2.0E-02 | 2.7E-01 | 6.2 2.0E-03 OK|
| O-TurbFreq | 1.37 | 2.6E-02 | 6.8E-01 | 12.7 3.2E-06 OK|
+----------------------+------+---------+---------+------------------+
+--------------------------------------------------------------------+
| Notice: The maximum Mach number is 7.261E+00. |
+--------------------------------------------------------------------+
================================================== ====================
| Timescale Information |
----------------------------------------------------------------------
| Equation | Type | Timescale |
+----------------------+------------------------+--------------------+
| U-Mom | Auto Timescale | 2.50981E-04 |
| V-Mom | Auto Timescale | 2.50981E-04 |
| W-Mom | Auto Timescale | 2.50981E-04 |
| P-Mass | Auto Timescale | 2.50981E-04 |
+----------------------+------------------------+--------------------+
| H-Energy | Auto Timescale | 2.50981E-04 |
+----------------------+------------------------+--------------------+
| K-TurbKE | Auto Timescale | 2.50981E-04 |
| O-TurbFreq | Auto Timescale | 2.50981E-04 |
+----------------------+------------------------+--------------------+
================================================== ====================
OUTER LOOP ITERATION = 86 CPU SECONDS = 1.117E+03
----------------------------------------------------------------------
| Equation | Rate | RMS Res | Max Res | Linear Solution |
+----------------------+------+---------+---------+------------------+
| U-Mom | 0.18 | 3.1E-03 | 4.6E-02 | 1.1E+00 F |
| V-Mom | 0.72 | 5.3E-04 | 5.5E-02 | 9.1E-01 ok|
| W-Mom | 0.78 | 5.4E-04 | 4.8E-02 | 1.0E+00 F |
| P-Mass | 0.03 | 5.7E-05 | 9.4E-04 | 9.7 9.3E-01 ok|
+----------------------+------+---------+---------+------------------+
+--------------------------------------------------------------------+
| ****** Notice ****** |
| A wall has been placed at portion(s) of an OUTLET |
| boundary condition (at 100.0% of the faces, 100.0% of the area) |
| to prevent fluid from flowing into the domain. |
| The boundary condition name is: outlet. |
| The fluid name is: Fluid 1. |
| If this situation persists, consider switching |
| to an Opening type boundary condition instead. |
+--------------------------------------------------------------------+
| H-Energy | 0.81 | 3.7E-03 | 1.0E-01 | 6.2 3.0E-04 OK|
+----------------------+------+---------+---------+------------------+
| K-TurbKE | 1.17 | 2.3E-02 | 3.1E-01 | 6.2 1.5E-04 OK|
| O-TurbFreq | 1.29 | 3.3E-02 | 7.9E-01 | 12.7 2.2E-07 OK|
+----------------------+------+---------+---------+------------------+
+--------------------------------------------------------------------+
| Notice: The maximum Mach number is 7.189E+01. |
+--------------------------------------------------------------------+
================================================== ====================
OUTER LOOP ITERATION = 87 CPU SECONDS = 1.130E+03
----------------------------------------------------------------------
| Equation | Rate | RMS Res | Max Res | Linear Solution |
+----------------------+------+---------+---------+------------------+
| U-Mom | 0.03 | 9.7E-05 | 5.7E-03 | 2.8E-01 ok|
| V-Mom | 0.13 | 7.0E-05 | 4.6E-03 | 5.5E-01 ok|
| W-Mom | 0.13 | 7.0E-05 | 4.7E-03 | 6.0E-01 ok|
| P-Mass | 0.00 | 3.3E-08 | 1.5E-06 | 9.7 2.1E+01 F |
+----------------------+------+---------+---------+------------------+
+--------------------------------------------------------------------+
| ****** Notice ****** |
| A wall has been placed at portion(s) of an OUTLET |
| boundary condition (at 100.0% of the faces, 100.0% of the area) |
| to prevent fluid from flowing into the domain. |
| The boundary condition name is: outlet. |
| The fluid name is: Fluid 1. |
| If this situation persists, consider switching |
| to an Opening type boundary condition instead. |
+--------------------------------------------------------------------+
| H-Energy | 1.27 | 4.7E-03 | 4.1E-01 | 6.1 1.7E-05 OK|
+----------------------+------+---------+---------+------------------+
| K-TurbKE | 0.33 | 7.7E-03 | 2.9E-01 | 6.1 2.7E-04 OK|
| O-TurbFreq | 1.40 | 4.7E-02 | 1.0E+00 | 12.6 4.5E-06 OK|
+----------------------+------+---------+---------+------------------+
+--------------------------------------------------------------------+
| Notice: The maximum Mach number is 3.953E+03. |
+--------------------------------------------------------------------+
================================================== ====================
OUTER LOOP ITERATION = 88 CPU SECONDS = 1.142E+03
----------------------------------------------------------------------
| Equation | Rate | RMS Res | Max Res | Linear Solution |
+----------------------+------+---------+---------+------------------+
| U-Mom | 3.04 | 2.9E-04 | 1.6E-02 | 9.5E-03 OK|
| V-Mom | 3.11 | 2.2E-04 | 1.4E-02 | 4.3E-03 OK|
| W-Mom | 2.91 | 2.0E-04 | 1.4E-02 | 5.3E-03 OK|
| P-Mass | 1.32 | 4.4E-08 | 5.3E-06 | 9.7 6.3E-01 ok|
+----------------------+------+---------+---------+------------------+
+--------------------------------------------------------------------+
| ****** Notice ****** |
| A wall has been placed at portion(s) of an OUTLET |
| boundary condition (at 100.0% of the faces, 100.0% of the area) |
| to prevent fluid from flowing into the domain. |
| The boundary condition name is: outlet. |
| The fluid name is: Fluid 1. |
| If this situation persists, consider switching |
| to an Opening type boundary condition instead. |
+--------------------------------------------------------------------+
| H-Energy | 0.62 | 2.9E-03 | 4.9E-01 | 6.0 5.0E-05 OK|
+----------------------+------+---------+---------+------------------+
| K-TurbKE | 0.91 | 7.0E-03 | 2.1E-01 | 6.0 3.0E-03 OK|
| O-TurbFreq | 0.19 | 9.1E-03 | 9.8E-01 | 12.7 1.8E-06 OK|
+----------------------+------+---------+---------+------------------+
Parallel run: Received message from slave
-----------------------------------------
Slave partition : 3
Slave routine : get_TWFTFC
Master location : End of Continuity Loop
Message label : 009100015
Message follows below - :
+--------------------------------------------------------------------+
| ****** Notice ****** |
| The non-dimensional near wall temperature (T+) has been clipped |
| for calculation of Wall Heat Transfer Coefficient. |
| |
| Boundary Condition : wall |
| T+ clip value = 1.0000E-10 |
| |
| If this situation persists and you are using the High Speed Model, |
| consider enabling Mach number based blending between low speed and |
| high speed wall functions. You can do so by specifying a Mach |
| number threshold as follows: |
| |
| EXPERT PARAMETERS: |
| highspeed wf mach threshold = 0.1 # default=0.0 (off) |
| END |
+--------------------------------------------------------------------+
+--------------------------------------------------------------------+
| Notice: The maximum Mach number is 1.254E+05. |
+--------------------------------------------------------------------+
================================================== ====================
OUTER LOOP ITERATION = 89 CPU SECONDS = 1.155E+03
----------------------------------------------------------------------
| Equation | Rate | RMS Res | Max Res | Linear Solution |
+----------------------+------+---------+---------+------------------+
| U-Mom | 0.00 | 6.4E-08 | 7.8E-06 | 1.8E-02 OK|
| V-Mom | 0.00 | 4.9E-07 | 7.3E-05 | 3.1E-03 OK|
| W-Mom | 0.00 | 4.0E-07 | 7.3E-05 | 4.4E-03 OK|
| P-Mass | 0.12 | 5.2E-09 | 1.9E-06 | 9.7 6.3E+00 F |
+----------------------+------+---------+---------+------------------+
+--------------------------------------------------------------------+
| ****** Notice ****** |
| A wall has been placed at portion(s) of an OUTLET |
| boundary condition (at 100.0% of the faces, 100.0% of the area) |
| to prevent fluid from flowing into the domain. |
| The boundary condition name is: outlet. |
| The fluid name is: Fluid 1. |
| If this situation persists, consider switching |
| to an Opening type boundary condition instead. |
+--------------------------------------------------------------------+
+--------------------------------------------------------------------+
| ****** Notice ****** |
| The non-dimensional near wall temperature (T+) has been clipped |
| for calculation of Wall Heat Transfer Coefficient. |
| |
| Boundary Condition : wall |
| T+ clip value = 1.0000E-10 |
| |
| If this situation persists and you are using the High Speed Model, |
| consider enabling Mach number based blending between low speed and |
| high speed wall functions. You can do so by specifying a Mach |
| number threshold as follows: |
| |
| EXPERT PARAMETERS: |
| highspeed wf mach threshold = 0.1 # default=0.0 (off) |
| END |
+--------------------------------------------------------------------+
Parallel run: Received message from slave
-----------------------------------------
Slave partition : 2
Slave routine : get_TWFTFC
Master location : RCVBUF,MSGTAG=1032
Message label : 009100015
Message follows below - :
+--------------------------------------------------------------------+
| ****** Notice ****** |
| The non-dimensional near wall temperature (T+) has been clipped |
| for calculation of Wall Heat Transfer Coefficient. |
| |
| Boundary Condition : wall |
| T+ clip value = 1.0000E-10 |
| |
| If this situation persists and you are using the High Speed Model, |
| consider enabling Mach number based blending between low speed and |
| high speed wall functions. You can do so by specifying a Mach |
| number threshold as follows: |
| |
| EXPERT PARAMETERS: |
| highspeed wf mach threshold = 0.1 # default=0.0 (off) |
| END |
+--------------------------------------------------------------------+
Parallel run: Received message from slave
-----------------------------------------
Slave partition : 3
Slave routine : ErrAction
Master location : RCVBUF,MSGTAG=1032
Message label : 001100279
Message follows below - :

+--------------------------------------------------------------------+
| ERROR #001100279 has occurred in subroutine ErrAction. |
| Message: |
| Floating point exception: Overflow |
| |
| |
| |
| |
| |
+--------------------------------------------------------------------+

[shenying0710]

But when I calculate this problem with FLUENT by import the CFX def file, FLUENT can give good convergency result when the inlet velocity is 200m/s, while still can't get convergency result when the inlet velocity is 300m/s.

[ghorrocks]

I see you have viscous heating on, I would turn that off unless you need it.

Other than that, the FAQ describes the steps to take: http://www.cfd-online.com/Wiki/Ansys...gence_criteria

[jthiakz]

Yes, flow is sub sonic and compressible.
At glance I don’t see any problem with BC. But not clear about initialization.
Would recommend you initialize the domain only with U-velocity of 200 m/s and fix the time step based on the residence time/10 (diffuser length , inlet velocity).

[ghorrocks]

Viscous heating has nothing to do with subsonic or compressible flow. You only turn it on if the fricition/dissipation in the flow is generating heat and you care about it. This is not important in 99% of flows, so it should be turned off except if you are the 1% of flows where it is important.

Do not try to estimate residence time to get the time step. Maybe use it as a starting point, but adjust it from there based on how the simulation is going. If convergence is difficult then make it smaller, if converging easily then make it bigger. The time step you end up with is going to be quite different to anything you started with.

Initialising with 200m/s is a starting point. If that works that is good. If that causes problems I would do a simulation at 100 m/s or 150 m/s and use that as an initial condition.

Finally - this simulation looks axisymmetric. So why model it as 3D? Why not model it as a 2D axisymmetric wedge (http://www.cfd-online.com/Wiki/Ansys...tion_in_CFX.3F)

[shenying0710]

Thanks, Glenn and Jthiakz.
I tried initialization, small time steps, and turning off Viscous heating, but none of them works.
At last , I changed the inlet boundary condition to Total pressure inlet, just estimate a total pressure which can generate a inlet velociy near 200m/s， then it converges so quickly!
Then I suddenly remember that in FLUENT velocity-inlet is only suitable for incompressible flow. Maybe so is in CFX? just as I said before in this thread "When the inlet velocity is less than 100m/s(approximately Mach Number 0.3), It easily gets convergency result, but when inlet velocity is more than 100m/s(approximately Mach Number 0.3), It can't get convergency result no matter how I adjust the calculation parameters…"

By the way, this simulation is indeed axisymmetric, so I will try a 2D axisymmetric wedge . Thank you for your good advice, Glenn. In fact, I often found that in those convergency result, there are asymmetry
problems, for instance, the iso-line of wall temperature are not axisymmetric. That's really confusing.

[ghorrocks]

When you model it 2D axisymmetric you should be able to refine the mesh to a much higer degree, and achieve a much better mesh quality. This will assist convergence and accuracy.