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May 16, 2016, 15:57 
Bad convergence for flow separation in Tjunction

#1 
New Member
Bing
Join Date: Apr 2016
Posts: 5
Rep Power: 3 
Hi everybody,
I'm simulating a chimney pipe with double exits in a building to obtain the resistance pressure drops versus velocities under constant temperature. I'm using the SST turbulence model and 15 layers of prism is grown in the mesh. Velocity inlet and opening outlet are given as the BCs. CFX solver failed to converge and the monitor variable pressure drops oscillate. I have plotted a isosurface of velocity residual~1.0E3 and remesh this region with refined grid, but the convergence problem still there. I also change to a physical timescale from 1s~5s (fluid residence time is about 15s) and it doesn't help, too. The only convergence is obtained by using upwind advection scheme, but this scheme is of poor accuray. Can anybody help me? Detailed setup in CFXPre: 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: Domain 1 Coord Frame = Coord 0 Domain Type = Fluid Location = EXT BOUNDARY: inlet Boundary Type = INLET Location = INLET BOUNDARY CONDITIONS: FLOW REGIME: Option = Subsonic END MASS AND MOMENTUM: Normal Speed = 2.4 [m s^1] Option = Normal Speed END TURBULENCE: Option = Medium Intensity and Eddy Viscosity Ratio END END END BOUNDARY: outlet Boundary Type = OPENING Location = OUTLET1,OUTLET2 BOUNDARY CONDITIONS: FLOW REGIME: Option = Subsonic END MASS AND MOMENTUM: Option = Entrainment Relative Pressure = 0 [Pa] PRESSURE OPTION: Option = Opening Pressure END END TURBULENCE: Option = Zero Gradient END END END BOUNDARY: wall Boundary Type = WALL Location = WALL BOUNDARY CONDITIONS: MASS AND MOMENTUM: Option = No Slip Wall END WALL ROUGHNESS: Option = Rough Wall Sand Grain Roughness Height = 0.046 [mm] 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: Fluid Temperature = 39.4 [C] Option = Isothermal END THERMAL RADIATION MODEL: Option = None END TURBULENCE MODEL: Option = SST END TURBULENT WALL FUNCTIONS: Option = Automatic END END END OUTPUT CONTROL: BACKUP RESULTS: Backup Results 1 File Compression Level = Default Option = Standard Output Equation Residuals = All OUTPUT FREQUENCY: Iteration Interval = 500 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 Coord Frame = Coord 0 Expression Value = dp1 Option = Expression END MONITOR POINT: Monitor Point 2 Coord Frame = Coord 0 Expression Value = dp2 Option = Expression END MONITOR RESIDUALS: Option = Full END MONITOR TOTALS: Option = Full END END RESULTS: File Compression Level = Default Option = Standard Output Equation Residuals = All END END SOLVER CONTROL: Turbulence Numerics = High Resolution ADVECTION SCHEME: Option = High Resolution END CONVERGENCE CONTROL: Length Scale Option = Conservative Maximum Number of Iterations = 1000 Minimum Number of Iterations = 1 Timescale Control = Auto Timescale Timescale Factor = 1.0 END CONVERGENCE CRITERIA: Residual Target = 1e06 Residual Type = RMS END DYNAMIC MODEL CONTROL: Global Dynamic Model Control = On END END END 

May 17, 2016, 00:39 

#2 
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 13,632
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Two things are a giveaway for what the problem is:
1) bluff edges which will cause gross separations 2) residuals which converge for a while but then flatline with a periodic pattern. This means you are getting transient flow behaviour. You cannot model this steady state, you will need to do a transient simulation. 

May 17, 2016, 00:59 

#3  
New Member
Bing
Join Date: Apr 2016
Posts: 5
Rep Power: 3 
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Tags 
cfx, convergence problem, flow separation, tjunction pipe 
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