Hey everybody

Im working on a 2D simulation (one element thick) in CFX (ansys 10), and I have run my simulation of flove round a bridge profile with a K-omega model - but know that the SST model simulates seperation better - but is it possible to use this model in CFX on my 2D model? I havent been able to make i work.

I have symmetry conditions on my boundaries.

Thanks i advance

Jesper

You can use SST in 2d.

Tanks for that notice Joe

- but I cant make it work for my 2D model - but i has a nice convergence when i use both K-omega and K-epsilon models. Any input to why i cant make the SST model work then?

Tanks in adwance

Jesper

Hi Jesper,

SST model works well as any other model in 2D, I never had problems with this. What means "I cant make it work"? Stability, convergence problems or something else?

Fusion

As Fusion said, it makes no sense to say that other models work and SST doesn't. If they work, so should SST. Check how fine your mesh is, maybe refine it especially at the boundaries. Can't think of anything else.

keep your yplus very low (around 1) and sst will also work. If your yplus is high the implementation of sst in CFX can give you uncorrect answers. If your yplus is already low, then I do not know what could be the issue. It should be working well.

tanks everybody for your responses!

"cant make it work" means that i terminates with an error befor the itterations even starts. My Yplus value is 1.6 and should be suficient - rigth? I agree with you - it just dosent make sense.

But thanks anyway.

Jesper

I have now changede my advection scheme to Upwind, used and extremly small time step and refined my mesh so tha yPlus is around 1. Now the SST-models starts, but terminates after only 15 itterations due to "fatal owerflow in linear solver". I am out of ideas...anybody?

Thanks in advance

The problem isnt the turbulence model.

I know that the problem isnt the model itself, but i have no idea about what the problem then might be?

Some possibilities: - Your mesh have some elements with too little volume (near 0); - You have some elements (hexahedrons I guess) with too high aspect ratio (say >100); - You are running transient calculation without checking the Courant number for the timestep; - You are running transient calculation with a really bad initialization; - something else...

good luck, Fusion

http://www.cfd-online.com/Wiki/Ansys...ible_answer.3F

Great!

dear Jesper, I wish to underline that after 13 Messages I didn't understand yet what is your problem exactly. Try to give more detailed description.

Fusion

well when the solver says fatal overflow in linear solver, it many times means division by zero. So when we look at the sst, there are few terms those can blow up if the omega goes to zero. (cross diffusion term being the one). When you say other turbulence models are working, do you mean the standard k-omega model too. Because the cross diffusion term is in std k-w model also. So if the problem coming becomes of that, that might also blow away. A good remedy to this would be to start with good initial guess. That you can get by exporting results from k-epsilon model. And read them in k-mega model as inital guess. We all can guess only, so this is one of them.

Hey everybody

I am very sorry if i have not formulated my problem clearly enough, and confusede the reader unnessesary. Sorry. But thank you very much for your responses anyway, and for making i clear to me, that the question is not directly enough.

Lets me try to make it better (if not know, then in the future.)I am a beginner here and in CFX and are very greatful for your help in this forum!

Here goes...

I am working on a bridge profile in 2D (one element thick), and whant to determin the lift and drag coefficients for this stationary problem. My problem is scaled 1:2. I have tried to run both the K-omega and K-epsilon and both of the runs very well with a good convergens rate - and therefor my question about the SST model (and BSL). I have limited myself to make it work in steadystat first.

Here is the indputs to the model: (if this helps making i clar, what i have done in my model - and if you have the time)

Thanks in adwance - Jesper

Setting up CFX-5 Solver run ...

+---------------------------------------------------------- |CFX Command Language for Run

+----------------------------------------------------------

LIBRARY:

MATERIAL: Air Ideal Gas

Material Description = Air Ideal Gas (constant Cp)

Material Group = Air Data, Calorically Perfect Ideal Gases

Option = Pure Substance

Thermodynamic State = Gas

PROPERTIES:

Option = General Material

ABSORPTION COEFFICIENT:

Absorption Coefficient = 0.01 [m^-1]

Option = Value

END

DYNAMIC VISCOSITY:

Dynamic Viscosity = 1.831E-05 [kg m^-1 s^-1]

Option = Value

END

EQUATION OF STATE:

Molar Mass = 28.96 [kg kmol^-1]

Option = Ideal Gas

END

REFRACTIVE INDEX:

Option = Value

Refractive Index = 1.0 [m m^-1]

END

SCATTERING COEFFICIENT:

Option = Value

Scattering Coefficient = 0.0 [m^-1]

END

SPECIFIC HEAT CAPACITY:

Option = Value

Reference Pressure = 1 [atm]

Reference Specific Enthalpy = 0. [J/kg]

Reference Specific Entropy = 0. [J/kg/K]

Reference Temperature = 25 [C]

Specific Heat Capacity = 1.0044E+03 [J kg^-1 K^-1]

Specific Heat Type = Constant Pressure

END

THERMAL CONDUCTIVITY:

Option = Value

Thermal Conductivity = 2.61E-2 [W m^-1 K^-1]

END

END

END END EXECUTION CONTROL:

PARALLEL HOST LIBRARY:

HOST DEFINITION: jesper

Installation Root = C:\Programmer\ANSYS Inc\CFX\CFX-%v

Host Architecture String = intel_pentium_winnt5.1

END

END

PARTITIONER STEP CONTROL:

Multidomain Option = Independent Partitioning

Runtime Priority = Standard

MEMORY CONTROL:

Memory Allocation Factor = 1.0

END

PARTITIONING TYPE:

MeTiS Type = k-way

Option = MeTiS

Partition Size Rule = Automatic

END

END

RUN DEFINITION:

Definition File = C:/9.semester/New Folder 1/test_3.def

Interpolate Initial Values = Off

Run Mode = Full

END

SOLVER STEP CONTROL:

Runtime Priority = Standard

EXECUTABLE SELECTION:

Double Precision = Off

END

MEMORY CONTROL:

Memory Allocation Factor = 1.0

END

PARALLEL ENVIRONMENT:

Number of Processes = 1

Start Method = Serial

END

END END FLOW:

DOMAIN: halfscale

Coord Frame = Coord 0

Domain Type = Fluid

Fluids List = Air Ideal Gas

Location = Assembly 2

BOUNDARY: Inlet1

Boundary Type = SYMMETRY

Location = INL.1 2

END

BOUNDARY: Inlet2

Boundary Type = INLET

Location = INL.2 2

BOUNDARY CONDITIONS:

FLOW REGIME:

Option = Subsonic

END

MASS AND MOMENTUM:

Option = Cartesian Velocity Components

U = 46 [m s^-1]

V = 0 [m s^-1]

W = 0 [m s^-1]

END

TURBULENCE:

Option = Medium Intensity and Eddy Viscosity Ratio

END

END

END

BOUNDARY: Inlet3

Boundary Type = SYMMETRY

Location = INL.3 2

END

BOUNDARY: Outlet

Boundary Type = OUTLET

Location = OUT 2

BOUNDARY CONDITIONS:

FLOW REGIME:

Option = Subsonic

END

MASS AND MOMENTUM:

Option = Static Pressure

Relative Pressure = 0 [Pa]

END

END

END

BOUNDARY: Wall1

Boundary Type = SYMMETRY

Location = WALL.1 2

END

BOUNDARY: Wall2

Boundary Type = SYMMETRY

Location = WALL.2 2

END

BOUNDARY: Body

Boundary Type = WALL

Location = HALF SCALE 1 1 2

BOUNDARY CONDITIONS:

WALL INFLUENCE ON FLOW:

Option = No Slip

END

END

END

DOMAIN MODELS:

BUOYANCY MODEL:

Option = Non Buoyant

END

DOMAIN MOTION:

Option = Stationary

END

REFERENCE PRESSURE:

Reference Pressure = 1 [atm]

END

END

FLUID MODELS:

COMBUSTION MODEL:

Option = None

END

HEAT TRANSFER MODEL:

Fluid Temperature = 288 [K]

Option = Isothermal

END

THERMAL RADIATION MODEL:

Option = None

END

TURBULENCE MODEL:

Option = SST

END

TURBULENT WALL FUNCTIONS:

Option = Automatic

END

END

END

OUTPUT CONTROL:

RESULTS:

File Compression Level = Default

Option = Standard

END

END

SIMULATION TYPE:

Option = Steady State

END

SOLUTION UNITS:

Angle Units = [rad]

Length Units = [m]

Mass Units = [kg]

Solid Angle Units = [sr]

Temperature Units = [K]

Time Units = [s]

END

SOLVER CONTROL:

ADVECTION SCHEME:

Option = High Resolution

END

BODY FORCES:

Body Force Averaging Type = Volume-Weighted

END

CONVERGENCE CONTROL:

Maximum Number of Iterations = 200

Physical Timescale = 0.005 [s]

Timescale Control = Physical Timescale

END

CONVERGENCE CRITERIA:

Conservation Target = 0.01

Residual Target = 1.E-4

Residual Type = RMS

END

DYNAMIC MODEL CONTROL:

Global Dynamic Model Control = Yes

END

EQUATION CLASS: continuity

ADVECTION SCHEME:

Option = High Resolution

END

CONVERGENCE CONTROL:

Physical Timescale = 0.005 [s]

Timescale Control = Physical Timescale

END

END

EQUATION CLASS: momentum

ADVECTION SCHEME:

Option = High Resolution

END

CONVERGENCE CONTROL:

Physical Timescale = 0.005 [s]

Timescale Control = Physical Timescale

END

END

EQUATION CLASS: ke

ADVECTION SCHEME:

Option = Upwind

END

END

EQUATION CLASS: tef

ADVECTION SCHEME:

Option = Upwind

END

END

INTERPOLATION SCHEME:

Pressure Interpolation Type = Linear-Linear

Shape Function Option = Parametric

Velocity Interpolation Type = Trilinear

END

PRESSURE LEVEL INFORMATION:

Compressible Transient Option = Shift Pressure

Option = Automatic

Pressure Level = 0 [atm]

END

END END COMMAND FILE:

Version = 10.0

Results Version = 10.0 END

+--------------------------------------------------------------------+ | | | Solver | | | +--------------------------------------------------------------------+

+--------------------------------------------------------------------+ | | | ANSYS CFX Solver 10.0 | | | | Version 2005.07.11-10.24 Mon Jul 11 10:26:04 GMTDT 2005 | | | | Executable Attributes | | | | single-32bit-optimised-supfort-noprof-nospag-lcomp | | | | Copyright 1996-2005 ANSYS Europe Ltd. | +--------------------------------------------------------------------+

+--------------------------------------------------------------------+ | Job Information | +--------------------------------------------------------------------+

Run mode: serial run

Host computer: JESPER Job started: Thu Dec 21 10:50:56 2006

+--------------------------------------------------------------------+ | Memory Allocated for Run (Actual usage may be less) | +--------------------------------------------------------------------+

Data Type Kwords Words/Node Words/Elem Kbytes Bytes/Node

Real 78083.6 299.75 600.74 305014.0 1199.00 Integer 18687.4 71.74 143.77 72997.5 286.95 Character 2327.7 8.94 17.91 2273.1 8.94 Logical 40.0 0.15 0.31 156.2 0.61 Double 1208.0 4.64 9.29 9437.5 37.10

+--------------------------------------------------------------------+ | Total Number of Nodes, Elements, and Faces | +--------------------------------------------------------------------+

Domain Name : halfscale

Total Number of Nodes = 260496

Total Number of Elements = 129980

Total Number of Hexahedrons = 129980

Total Number of Faces = 260496

+--------------------------------------------------------------------+ | Reference Pressure Information | +--------------------------------------------------------------------+

Domain Group: halfscale

The pressure level is set by the following boundary condition:

Domain Name : halfscale

Boundary Patch Name : Outlet

However, pressure level information has also been set.

The specified information will be ignored.

+--------------------------------------------------------------------+ | Average Scale Information | +--------------------------------------------------------------------+

Domain Name : halfscale

Global Length = 1.5665E+01

Minimum Extent = 1.0000E-02

Maximum Extent = 6.2000E+02

Density = 1.2254E+00

Dynamic Viscosity = 1.8310E-05

Velocity = 4.6000E+01

Advection Time = 3.4053E-01

Reynolds Number = 4.8225E+07

Speed of Sound = 3.4027E+02

Mach Number = 1.3519E-01

+--------------------------------------------------------------------+ | Checking for Isolated Fluid Regions | +--------------------------------------------------------------------+

No isolated fluid regions were found.

+--------------------------------------------------------------------+ | The Equations Solved in This Calculation | +--------------------------------------------------------------------+

Subsystem : Wall Scale

Wallscale

Subsystem : Momentum and Mass

U-Mom

V-Mom

W-Mom

P-Mass

Subsystem : TurbKE and TurbFreq

K-TurbKE

O-TurbFreq

CFD Solver started: Thu Dec 21 10:51:49 2006

+--------------------------------------------------------------------+ | Convergence History | +--------------------------------------------------------------------+

================================================== ==================== | Timescale Information | ---------------------------------------------------------------------- | Equation | Type | Timescale | +----------------------+------------------------+--------------------+ | U-Mom | Physical Timescale | 5.00000E-03 | | V-Mom | Physical Timescale | 5.00000E-03 | | W-Mom | Physical Timescale | 5.00000E-03 | | P-Mass | Physical Timescale | 5.00000E-03 | +----------------------+------------------------+--------------------+ | K-TurbKE | Physical Timescale | 5.00000E-03 | | O-TurbFreq | Physical Timescale | 5.00000E-03 | +----------------------+------------------------+--------------------+

================================================== ==================== OUTER LOOP ITERATION = 1 CPU SECONDS = 1.492E+01 ---------------------------------------------------------------------- | Equation | Rate | RMS Res | Max Res | Linear Solution | +----------------------+------+---------+---------+------------------+ | Wallscale | 0.00 | 1.3E-07 | 2.0E-07 | 7.3 1.2E+01 F | +----------------------+------+---------+---------+------------------+ | U-Mom | 0.00 | 1.7E-03 | 5.3E-02 | 4.0E-02 OK| | V-Mom | 0.00 | 2.9E-04 | 1.2E-02 | 3.7E-01 ok| | W-Mom | 0.00 | 0.0E+00 | 0.0E+00 | 0.0E+00 OK| | P-Mass | 0.00 | 5.7E-03 | 2.6E-01 | 12.2 3.3E-02 OK| +----------------------+------+---------+---------+------------------+ | K-TurbKE | 0.00 | 1.3E-01 | 1.9E-01 | 5.8 1.8E-08 OK| | O-TurbFreq | 0.00 | 1.1E-02 | 1.0E+00 | 10.5 2.2E-08 OK| +----------------------+------+---------+---------+------------------+

Could be your BCs. Could be a transient problem you are shoehorning into SS. You have a couple of odd parameter selections Could be near wall mesh.

Need pic of the mesh + BC locations including closeup of the boundary layer region.

Hey Joe. I have don a transient simulation, and the result is the same.

For some reason i cant post pictures of the mesh here - but if you could giv me an email adr. I would like to send you the pic of the mesh.

Thanks

Jesper