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Velocity values are normal but pressure values are too big 

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April 7, 2015, 04:01 
Velocity values are normal but pressure values are too big

#1 
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Roman Voronov
Join Date: May 2010
Location: Russia
Posts: 13
Rep Power: 16 
Hi folks!
I have a problem and I need help. I try to simulate a multisection cyclone unit. And as a result I have plots of pressure and velocity. Velocity values are close to experimental values. But pressure values are too big. I tryed to simulate this case with this settings: solver: simpleFoam (is it good choice for twisted flow?). model: simulation begins with kOmegaSST and then continues with LRR or LaunderGibsonRSTM. numerical schemes: div(phi,U) are limitedLinear 0.5 or linearUpwind Gauss linear. underrelaxation factors: 0.3 for pressure and 0.7 for others. Is it possible to affect to pressure values during simulating? Or may I offer a correction factor to get correct (close to experiment) values for current mesh scale and then multiply new values on it? Or it's nonsense? A measuring point was marked on figure by red circle. Thank you! Last edited by rv82; April 7, 2015 at 05:17. 

April 8, 2015, 14:03 

#2  
Senior Member

Quote:
Of course you can modify the numerical settings on the fly, if the 'runTimeModifiable' option is turned on in 'controlDict'. Numerical schemes and underrelaxation factors seems ok. 

April 9, 2015, 23:30 

#3 
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Roman Voronov
Join Date: May 2010
Location: Russia
Posts: 13
Rep Power: 16 
Hello taxalian! Thank you for your reply!
To my mind, RSTM models are more applicable to cyclone units with complex flows then kOmegaSST, that's based on turbulent viscosity. Boundary conditions in my case are: nu 1.5e5 (air). for U: on inlets: type surfaceNormalFixedVlaue; value uniform ( 0 0 0 ); refValue uniform 12.6 // for 1st section refValue uniform 21.2 // for 3rd section on outlet: type zeroGradient; on walls: type fixedValue; value uniform (0 0 0); for p: all inlets and walls: type zeroGradient; on outlet: type fixedValue, value uniform 0; for k: on inlets: type fixedValue; value uniform 1.07; // 1st section value uniform 3.9; // 3rd section on outlet: type zeroGradient; on walls: type kqRWallFunction; value uniform 3.9; // I don't know which value will be correct for this parameter omega: on inlets: type fixedValue: value uniform 1498; // 1st section value uniform 13588 // 3rd section on outlet: type zeroGradient; on walls: type omegaWallFunction; value uniform 1498; // similarly I don't know which value will be correct For k and omega I used this material (second formula for omega). Mesh was built with snappyHexMesh. After this calculation ends, I calculate values for epsilon and R with createTurbulentFields. Then I change RASModel from kOmegaSST to e.g. LRR, change startTime in controlDict and run the calulation again. On residual plot initial value so small after computation with kOmegaSST. Then it increases to ~0.017 and then decreases again. Tell me please how can I use 'runTimeModifiable' to affect to pressure? Thank you! Last edited by rv82; April 10, 2015 at 03:12. Reason: Added residuals plot 

April 11, 2015, 06:13 

#4  
Senior Member

Quote:
For the choice of omega i will go for the first formula i.e. the square root of turbulent kinetic energy divided by the turbulent length scale. well runTimeModifiable option is available in your controlDict file and if you make any changes in your solver settings this will apply the changes to the simulation immediately and you can check the changes in your solution log file So i would also check the following:  checkMesh if check gives some failure or nonorthogonality warnings then i would go for several nonorthogonal correctors for better convergence of the pressure equation  hoping that your mesh is good enough to resolve the near wall effects in terms of having sufficient near wall resolution i.e. appropriate yPlus value 

April 13, 2015, 03:59 

#5  
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Roman Voronov
Join Date: May 2010
Location: Russia
Posts: 13
Rep Power: 16 
Hello!
That's result of yPlusRAS for my mesh: Quote:
Quote:


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