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December 19, 2006, 06:34 
SST in 2D?

#1 
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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 Komega 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 

December 19, 2006, 09:53 
Re: SST in 2D?

#2 
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You can use SST in 2d.


December 19, 2006, 11:39 
Re: SST in 2D?

#3 
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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 Komega and Kepsilon models. Any input to why i cant make the SST model work then? Tanks in adwance Jesper 

December 19, 2006, 12:05 
Re: SST in 2D?

#4 
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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 

December 19, 2006, 18:09 
Re: SST in 2D?

#5 
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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.


December 19, 2006, 20:45 
Re: SST in 2D?

#6 
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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.


December 20, 2006, 04:06 
Re: SST in 2D?

#7 
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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 

December 20, 2006, 05:50 
Re: SST in 2D?

#8 
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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 SSTmodels starts, but terminates after only 15 itterations due to "fatal owerflow in linear solver". I am out of ideas...anybody?
Thanks in advance 

December 20, 2006, 08:09 
Re: SST in 2D?

#9 
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The problem isnt the turbulence model.


December 20, 2006, 10:07 
Re: SST in 2D?

#10 
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I know that the problem isnt the model itself, but i have no idea about what the problem then might be?


December 20, 2006, 10:58 
Re: SST in 2D?

#11 
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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 

December 20, 2006, 13:30 
Re: SST in 2D?

#12 
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December 20, 2006, 13:50 
Re: SST in 2D?

#13 
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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 

December 21, 2006, 00:37 
Re: SST in 2D?

#14 
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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 komega model too. Because the cross diffusion term is in std kw 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 kepsilon model. And read them in kmega model as inital guess. We all can guess only, so this is one of them.


December 21, 2006, 05:56 
Re: SST in 2D?

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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 Komega and Kepsilon 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 CFX5 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 = 1.831E05 [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 = 2.61E2 [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 = kway 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 = VolumeWeighted 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.E4 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 = LinearLinear 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 2005.07.1110.24 Mon Jul 11 10:26:04 GMTDT 2005     Executable Attributes     single32bitoptimisedsupfortnoprofnospaglcomp     Copyright 19962005 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 = 1.0000E02 Maximum Extent = 6.2000E+02 Density = 1.2254E+00 Dynamic Viscosity = 1.8310E05 Velocity = 4.6000E+01 Advection Time = 3.4053E01 Reynolds Number = 4.8225E+07 Speed of Sound = 3.4027E+02 Mach Number = 1.3519E01 ++  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 UMom VMom WMom PMass Subsystem : TurbKE and TurbFreq KTurbKE OTurbFreq CFD Solver started: Thu Dec 21 10:51:49 2006 ++  Convergence History  ++ ================================================== ====================  Timescale Information    Equation  Type  Timescale  ++++  UMom  Physical Timescale  5.00000E03   VMom  Physical Timescale  5.00000E03   WMom  Physical Timescale  5.00000E03   PMass  Physical Timescale  5.00000E03  ++++  KTurbKE  Physical Timescale  5.00000E03   OTurbFreq  Physical Timescale  5.00000E03  ++++ ================================================== ==================== OUTER LOOP ITERATION = 1 CPU SECONDS = 1.492E+01   Equation  Rate  RMS Res  Max Res  Linear Solution  ++++++  Wallscale  0.00  1.3E07  2.0E07  7.3 1.2E+01 F  ++++++  UMom  0.00  1.7E03  5.3E02  4.0E02 OK  VMom  0.00  2.9E04  1.2E02  3.7E01 ok  WMom  0.00  0.0E+00  0.0E+00  0.0E+00 OK  PMass  0.00  5.7E03  2.6E01  12.2 3.3E02 OK ++++++  KTurbKE  0.00  1.3E01  1.9E01  5.8 1.8E08 OK  OTurbFreq  0.00  1.1E02  1.0E+00  10.5 2.2E08 OK ++++++ 

December 21, 2006, 08:50 
Re: SST in 2D?

#16 
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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. 

December 23, 2006, 09:40 
Re: SST in 2D?

#17 
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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 

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