HomeUpSearchMail
NEW

Simple wokflow to determine a windmill's biotope

Simple workflow to determine a windmill's biotope by Victor Reijs is licensed under CC BY-NC-SA 4.0

Introduction

• Making a CAD-model (Computer Aided Design)
  
• Importing and editing the CAD-model in a CFD-program (Computational Fluid Dynamics)
  
• Run simulations in the CFD-program
• The results of simulation
  
• Evaluating the results
• Conclusions

I use on this web page De Zwiepse Molen as example. There is also a workflow for the Impington Mill.

Making a CAD-model

The program to be used for making a CAD-model will need to be able to export the CAD-model in a recognisable format for the CFD-program.
One could try a windtunnel, but that would not be easy to accomodate to the wind mill nevironment. It though looks that CFD's results are comparbale to windtunnel results (to relatively low speeds: say between 2 and 7BFT); see for instance here and here.

I am using the CFD cloud service SIMSCALE which can import STF files, and such files can be provided by SketchUp CAD-program.
Both SketchUp and SIMSCALE are profession programs/services. Pro[fessional] versions cost also considerable amounts of money (several hunderds of Euros).
Anyway we are making a CAD model with SketchUp. SketchUp Free version is well equiped for the windmill environment. In this program one can easily make 3D forms, such as: boxes, roofed boxes, cylinders, capped cones, trees, etc, etc.

Houses that are around a windmill, can be simulated by a box (with its height: the wall height plus half the roof height); or otherwise a more precise model (box with hipped roof, etc.). A windmill can be a simple standing cylinder. Don't go into too much details as that will not have much influence on the CFD results.
Furthermore, buildings below the ground/bailey height are certainly not in need of precise dimensions.

The folowing steps for making the CAD-model can be seen as a guide line (Zwiepse Molen has been taken as an example):

• Define in SketchUp the unit of the dimensions (CFD uses m[etres], so do the same in SketchUp): Model Info (i) → Length Units → Meter; Model Info (i) → Display precissio → 0.00 m; and Model Info (i) → Length Snapping → OFF
  
• One can add a piece map from of Google Map (so when get an idea where buildings are), use: ≡ → Add Location → Select Region → Import
• Having imported this, one can now trace the contours of the building-plans (Line-tool). Make sure you have zoomed in enough as that is easier to trace. Make also sure that your buildings all go down to a height of 0m. One can also disable Lineair inferences (OFF by using the Alt-button).
  There is no real need to be very accurate (say within 0.25m).
• For the heights of the objects one can use in The Netherlands: AHN (Actueel Hoogtebestand Nederland). It also provides the map references in RD (RijksDriehoeksmeting). For UK there is the Defra Data Service (which uses British National Grid and see also OS Maps).
  
• After tracing the contours of the building plans, one can increase their height by extruding them (Push-Pull tool). At the right bottom of the window (Measurements) one can see the pull height.
  
• For simplicity one simple use a box for a roofed building: with its height: height of walls + half of height of roof.
  There is no real need to be very accurate (say within 0.25m).
  Remark: Some other people recommend to use the ridge height of the roof. Need to investigate what is better.
  
• The mill is made by using a cicle (Circle-tool), pull up (Pull tool) to form a cylinder: height of top is 18m, and windshaft height is around 16m.
• Perhaps a mill should be simulated with a capped cone, but it is easier for the velocity/energy analysis if a cylinder is used (and that will also not provide signifant velocity differences).
  
• If a Google Map was used; make sure you push (Push-Pull tool) the undersides of the buildings slightly below a 0m.
  Remark: Make sure an object does not end very close to 0m; otherwise 'mesh' problems can happen during the simulation.
  
• A leafed tree (height 15.5m and distance from centre of mill 22m) was included by importing it (blobbed object) from here: ≡ → Import → Device → Gotten tree.skp
  This tree is at a nearer location where early 2023 a smaller tree stood. So this is not the present/past situation.
  The blobbed object was copied from this site.
  
• In the CAD-model only a single tree has been incorporated. In the case of Zwiepse Molen many (unwanted) rows of trees are windbreakers, so one should include such.
  Remark: Can a row of trees be simplified for a simulation? If you have ideas; let me know
• Make sure that no 3D object is touching/overlaying another object. That migth cause problems in the simulation.
  
• Give the CAD-model a name and save.
  Make sure that you check and recheck things as good as possible, before downloading if for importing it into the CFD program.
  
• And when the CAD-molen looks good, download it as STL: ≡ → Download → STL

Here is an example ZwiepseMolen.skp  (made in some 30 minutes):

Importing and editing the CAD-model in a CFD-program

So we are making use of SIMSCALE. Make sure you make a Community Plan account. This Community Plan allows only for 10 simulations, so make sure you have a good CAD-model and that you train/educate yourself around SIMSCALE.
The following steps can be done:

• Import the STL model
• Edit this model by: Edit in CAD mode
• Delete the terrain (a Sheet in SIMSCALE)
  
• Make a External Flow Volume en delete all the solids
• Save Copy
• Goto SIMULATIONS +
  

Run simulations in the CFD-program

The following steps can be done:

• Important: In the Community Plan of SIMSCALE one only has a budget for 10 simulation; so make sure you train/educate yourself beforehand!!!
  
• Goto SIMULATIONS +
  
• Incompressible → Create Simulation
• Materials → Air → Apply
• Defined the Boundary conditions: Velocity inlet (using ABL profile: a West wind v=6.44m/sec @10m [4.61 Bft]; z₀=0.5m [6.4Bft]) (West side), Pressure outlet (East side), Non-slip Wall (for the ground), Slip Wall (for the remaining sides).
  
• Run the simulation (Simulation Runs +)
• When simulation ended (took some 25min), view the results (Solution Fields).
• As much as possible default SIMSCALE options have been used (like k-omega SST and residuals < 0.01).

Here is a SIMSCALE screengrab when the simulation has finished (KTE output at windshaft height):

The results of simulation

The following can be see:

• Goto SIMULATIONS → Simulations Runs → Run 1 → Solution Fields (a West wind blows from left (West) to right (East) at 6.44m/sec @ 10m [4.61Bft])
  
• Velocity Y (summer): Looking from above (there is a thin horizontal line through centre tree and mill body; and a thin vertical line at at plane of the sails):
  

  3m height	Mid-tree (10m) height	windshaft (16m) height

• All the buildings in this example (between 1.8 and 4.5m high) were lower then the bailey (4.9m) of the windmill, so no real influence on the wind pattern at mid-tree of windshaft height.
• Velocity Y (summer): Looking towards the North; through centre the tree and windmill (there is a thin horizontal line at the mid-tree height; and a thin vertical line at at plane of the sails):
  
• One can recognise the cavity areas (dark blue) behind the tree and the mill body.
• This cavity (turbulence) will catch the lower part of the plane of the sails (so it will increase stress on the sail beams).
  
• Velocity Y (summer): Looking towards the West; at plane of the sails (there is a thin horizontal line at the windshaft height; and a thin vertical line at at plane of the sails):
  
• One can recognise that the cavity (horizontal dark blue patch) of the tree extend into the plane of the sails. Beside this cavity area due to the tree; the upwind recirculation area (vertical dark blue patch) due to the mill body (in front of the mill) can be seen.
  
• Turbulent Kinetic Energy (KTE) (summer): Looking towards the North; through centre the tree and windmill (there is a thin horizontal line at the windshaft height; and a thin vertical line at at plane of the sails). Here is the combination of the ABL's inherent turbulence and the objects' extra turbulence:
  
• One can recognise the turbulence in the cavity boundary area (green/blueish area) behind the tree and less cavity turbulence behind the mill body.
• This turbulence will catch the middle part of the plane of the sails (so it will increase stress on the sail beams).
• Remark: Would be nice if SIMSCALE includes Turbulence Intensity as output, so it can report on a normalised turbulence.
  
• An analysis of how to model a leafed or leafless deciduous tree can be seen here. If one looks at leafed tree the earlier used blobbed tree model is ok. If one wants to use Darcy-Forchheimer coefficients for porosity the below points can help:
  
• Configuring SIMSCALE's Darcy-Forchheimer coefficients (SIMULATIONS →  Advanced concepts →  Porous media + → Darcy-Forchheimer medium) and see also here:
  d_x=d_y=d_z=0
  f_x=f_y=f_z=C_m or C_d
  <depending of definition of C_m or C_d>
  
• Comparing a mid-tree in summer (leaved tree: blobbed tree) and winter (leafless [stacked-cylinder] tree: f=0.2, this should be closer to f=0.09):
  
  As expected in winter the windvelocity is less reduced.
  
  
• This simulation in SIMSCALE Community Plan gives an idea of the influences of obstacles and works well for tradtional windmill biotope.

Evaluating the results

Conclusions

Referenties

Acknowledgements

Disclaimer and Copyright
HomeUpSearchMail