HomeUpSearchMail
NEW

Simuleren van wind bij Nägeli artikel

Simuleren van wind bij Nägeli artikel by Victor Reijs is licensed under CC BY-NC-SA 4.0

Introductie

• Informatie over de molen omgeving
  
• Eigenschappen van obstakels
• Maken 3D model in SketchUp
  
• Editing 3D model in SIMSCALE
• Configureren van de CFD
• Uitvoering van de CFD
• Conclusies
• Referenties
• Acknowledgements

Sorry for the two langauges gebruikt in deze webpagina!

Vette paarse tekst heeft nog extra aandacht nodig.

Informatie over de omgeving

• The ABL wind speed is 9.03m/sec@10m (which gives 5m/sec at 2.2m) and z_0m of 0.03m. z_0m is determined from the table on page 222 of Nägeli [1953].
  Remark: z_0m =0.01 is determined from the table on page 222 of Nägeli [1953]. This though does not provide a significant change.
  
• CFD wordt gedaan voor wind uit Zuid (180deg).
• In all graphs the magnitude of the wind speed is provided.
  

Eigenschappen van obstakel

• One box (fence) of 25m (Nägeli's fence) long, 40cm thick ad 2.2m heigh
  
• Nägeli fence, has an optical porosity of around 15-20% and 45-55% [Nägeli, 1953, Bild 2].
• 3D model of Nägeli's fence in SketchUp:
  
  

• Include at the back (100m from fence) a small (dummy) box (which is needed for being able to simulate things in CFD)
  
• Sluit af als klaar: 'Save' en 'Download' -> 'STL'
• Effort: 0.5 hours.

Editing 3D-model in SIMSCALE

We are making use of SIMSCALE. Make sure you make a Community Plan account. This Community Plan allows only for 10 simulations (but one can do many more; but with more manual work). Anyway make sure you have a good CAD-model and that you train/educate yourself around SIMSCALE.
The following steps can be done:

• Het simulatie project staat hier.
  
• Import het STL model
• Edit this (not yet rotated) model by: Edit a copy
  
• Make a Flow Volume -> External waarbij de poreuze objecten (bomen: 'Excluded parts') niet deelnemen aan het 'External flow volume'.
  
  Zie voor richtlijnen mbt vrije afstanden rondom het model hier [Franke, 2007]:
  
  • Xmin, Xmax: minimaal 5*hoogte/grootte hoogste obstakel [Franke, 2007, page 17] extra aan linker en rechter kant van 3D-model. In dit geval ~5*~2.2m (fence): ~ 10m
  • Ymin: Distance in front of model (in Y-direction) minimaal 8*hoogte/grootte hoogste obstakel [Franke, 2007, page 18] extra aan voorkant van 3D-model. In dit geval ~8*~2.2m (fence): ~ 20m
  • Ymax: Distance at back of model (in Y-direction) minimaal 15*hoogte/grootte hoogste obstakel [Franke, 2007, page 18] extra aan achterkant van 3D-model. In dit geval ~15*~2.2m (fence): ~ 40m
  • Zmin: Ground (in Z-direction) minimaal om invloed van modelerings afrondingen te voorkomen:  ~ 0.1m
  • Zmax: Height (in Z-direction) minimaal 6*hoogte/grootte hoogste obstakel [Franke, 2007, page 17]. In dit geval ~6*~2.2m (fence): ~ 15m
  • By the way, the default values in SIMSCALE were always somewhat larger than the above, so its defaults are also ok. Except for Zmin; I would put that always at 0.1m.
  • In this simulation a much larger area around the fence model has been taken.
    
• Delete the dummy box, maar niet de fence
  
• Save
• Effort: 0.5 hours.

Configureren van de CFD

Take the following steps (if not default, it is explicitly mentioned in below steps):

• Goto SIMULATIONS +
• Incompressible -> Turbulence-model -> Realizable k-epsilon [Franke, 2007, page 14]
  
  
• Materials -> Air -> Apply
  Assigned Volumes -> Flow region
  
  
• Boundary conditions
• Velocity Inlet
  Assigned Faces -> the wind side
  (U) Velocity -> U_y -> ABL Formula (6.4m/sec at 10m and z_0m=0.01m, the wind direction is 180deg [S])
  (0.41*9.03/log((10+0.03)/0.03))/0.41*log((z+0.03)/0.03)
  Turbulence -> Fixed value
  (k) Turb. kinetic energy -> ABL derived Formula
  (((0.41*9.03)/(log((10+0.03)/(0.03)))^2/(0.09)^0.5)
  (ε) Dissipation rate -> ABL derived Formula
  (0.41*9.03/log((10+0.03)/0.03))^3/(0.41)/(z+0.03)
  
  Save
  
  
• Pressure outlet
  Assigned Faces -> opposite inlet side
  Save
  
  
• Wall
  Assigned Faces -> two sides and top
  (U) Velocity -> Slip
  Save
  
  
• Custom (ground)
  Assigned Faces -> bottom side
  Wall roughness -> On
  Roughness height -> 1m
  Roughness constant -> 0.5
  Save
  
  
• Mesh
  25m fence: Fineness = 7.5 (1.5Mcells)
  
  
• Mesh -> Refinements -> Inflate boundary layer
  
• Advanced concepts -> Porous media -> Darcy-Forchheimer Medium
  Forchheimer coefficent f=12.8 (Dichte Wand) or Forchheimer coefficent f=4.8 (Lockere Wand).
  
• Mesh -> Volume custom sizing
  At least 4 cells in the thickness of 40cm
  
  
• Effort: 2 hours (excluding the time needed to evaluate the Direction in perforated plate).

Uitvoering van de CFD

• Simulation Runs +
• Varying the mesh size:
  

  Fineness	Size [Mcells]	Mesh [CPUh]	Run [CPUh]
  5 default	0.5	0.8	0.8
  7.5	1.5	0.3	4.0
  9.2	10.5	1.8	26

  Fineness of 7.5 has been used in below analysis.
  
• The Forchheimer coefficent f of the modelled fence was tried to be matched with Nägeli's fence at 17.5% (Dichte Wand) The black curves (17.5% optical porosity 25m fence) are from Nägeli [Bild 4, 1953] and the colored curves (z_0m=0.03m, f=12.8, 25m fence, Fineness=7.5) are computed from CFD:
  
  There is quite some difference between Nägeli and CFD.
  
• The Forchheimer coefficent f of the modelled fence was tried to be matched with Nägeli's fence at 50% (Lochere Wand). The black curves (50% optical porosity, 25m fence) are from Nägeli [Bild 4, 1953] and the colored curves (z_0m=0.03m, f=4.8, 25m fence, Fineness=7.5) are computed from CFD:
  
  There is good mapping between Nägeli and CFD.
• Beljaars [1979, page 42] compared this with Townsend's model (by changing its C_d [to 1] for best matching)::
  
• Effort: 4 hours.
  

Conclusions

• Most default values of SIMSCALE look ok, except:
• Use Realizable k-epsilon
• Zmin should be always 0.1m
• The higher the Fineness the better, somewhere between 7.5 and 9 (instead of default of 5), or better: use your own mesh configuration.
• Thoughs about the meshing:
  
• All mesh metrics stay within their defined min and max range; at least for Fineness tested between 5 and 9.2 and this 3D-model.
  All rsiduals are smaller than 10^-3.
  
• The Dichte Wand simulation does not match well. The Lochere Wand is better simulated.
  Remark: What causes this difference? I thought perhaps it is difference between magnitude or y-direction of wind speed, but these two don't have much difference.
  
• An overview of outstanding issues/questions/etc is here.
• Now we would be able to look how Beljaars's method matches CFD. This will be worked out here.
  

References

Beljaars, A.C.M.: Windbelemmering rond windmolens. In: (1979).
Franke, Jörg  et al. COST Action 732: Best practice guideline for the CFD simulation of flows in the urban environment. Brussels, COST Office 2007.
Nägeli, Werner, Untersuchungen über die Windverhältnisse im Bereich von Schilfrohrwänden, Mitt. Schweiz. Anst. Forstl. Versuchw., 29, 213–266,1953

Acknowledgements

Disclaimer and Copyright
HomeUpSearchMail