HomeUpSearchMail
NEW

Workflow to determine Impington Mill's biotope

Workflow to determine Impington Mill's biotope by Victor Reijs is licensed under CC BY-NC-SA 4.0

Introduction

Making a CAD-model with SketchUp

Buildings that are around a windmill, say with a radius of 250m, 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/capped cone. 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. As a guideline include anything that is higher than the DHM-rule minus 2m.

The following steps for making the CAD-model can be seen as a guide line:

• 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 precission → 0.00 m; and Model Info (i) → Length Snapping → OFF
  
• One can add a piece map (500 by 500m with the mill in the middle) 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 Linear inferences (OFF by using the Alt-button).
  There is no real need to be very accurate (say within 0.25m).
• 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 UK there is the Defra Data Service which provides a DSM (Digital Surface Model) for 2019 and 2022 (which uses British National Grid and see also OS Maps).
• The GeoTIFF (DSM) file has been changed into a csv file by using a small program in R. This csv file has been imported in Excel for presentation and easy viewing.
  
• Here is the 2022 DSM (Digital Surface Model: green lowest; red highest) for an area of 500*500m with Impington Mill in the middle (top is North; bottom is South; right is East; and left is West):
  
• Using the DHM-rule on this 2022 DSM, one gets the following picture (top is North; bottom is South; right is East; and left is West):
  
  The colouring gives a height referenced from the DHM-rule: green from -2 to 0m; orange from 0 to 4m; brown from 4 to 8m; red from 8 to 12m; black: above 12m
  
• 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 circle (Circle-tool), pull up (Pull tool) to form a cylinder (or capped cone): height of cap is 16.2m, and windshaft height is around 15m.
• Make sure an object does not end very close to 0m; otherwise 'mesh' problems can happen during the simulation.
• Beside all buildings within a radius of 100m from the mill; the buildings that have at least a green colouring (so which are higher than DHM-rule minus 2m) are included in the CAD-model.
  
• Here is a bird eye's view of the resulting CAD-model (without trees) (top is North; bottom is South; right is East; and left is West):
  

Simulation without trees using SIMSCALE

• General guidelines how to use CFD in an urban environment can be found in this guideline [Franke, 2007].
• As much as possible default SIMSCALE options have been used.
• Open space on: the left side (25H with H=5m); right side (10H with H=10m); inflow side (7H with H=10m) and outflow side (10H with H=10m).
  
• Turbulence model: k-omega SST and residuals < 0.01
• This model (without trees) has been simulated in SIMSCALE (nl) using an ABL for an Eastern wind of 7.1m/sec (4.6Bft) @ 10m above ground level mill and an z₀ = 1m.
  

  Bird eye view of the environment of the mill (so one sees the z-plane, which is at the height of the lowest part of the sails). (top is North; bottom is South; right is East; and left is West)

  There are three planes: a vertical (x) plane just in front of the mill; a vertical (y) plane through the middle of the mill; a horizontal (z)  plane at the height of the lowest part of the sail.

  There are three planes: a vertical (x) plane just in front of the mill; a vertical (y) plane through the middle of the mill; a horizontal (z)  plane at the height of windshaft.

  
  

CAD-model including the trees

• Including trees in SketchUp Free is not easy if using Google Map, as one can't see the trees in Street map. A better way is to Import the DSM height Image, which includes buildings and trees.
  
• The trees are including by using the stacked-cylinder object.
• Starting with a standard tree (15.3m high and 15.3m diameter) in SketchUp Free.
  Scale this tree by using:
  Scale → Uniform Scale About Opposite Point → enter xyz scaling
  Scale → Blue Scale About Opposite Point → enter z scaling
  
• At first, only trees that will affect the winds between South and West are included:

• 
• De CAD model changes for every wind sector (steps of 15deg) in SIMSCALE CAD mode:
• Rotated with a certain value (Transform →→ Rotate Z-axis → Rotation angle → <value> → right click → Assign All → Apply)
• Flow volume → input X min, X max, Y min, Y max, Z min, Z max → Excluded parts → Select trees → Apply
• Flow region → Hide → Select Trees → Invert selection → Delete → Apply → Flow region Unhide → Save as Copy
  

Simulation with trees using SIMSCALE

• General guidelines how to use CFD in an urban environment can be found in this guideline [Franke, 2007].
• As much as possible default SIMSCALE options have been used.
• Open space on: the left side (25H with H=5m); right side (10H with H=10m); inflow side (7H with H=10m) and outflow side (10H with H=10m).
  
• Turbulence model: at start k-omega SST and at the end Realized k-epsilon and residuals < 0.01
• In SIMSCALE we use a Forchheimer coefficient of f=0.45 [1/m] for leafed/summer trees (using the Darcy-Forchheimer medium).
• Simulations were done for wind directions from 180 to 270deg in steps of 15deg using the project: DSM2022 Impington.
  
• Input ABL wind speed is 7.1m/sec @ 10m and z₀=0.5. A lower z₀ has been used (compared to earlier this page) as the wind speed at the mill is compared to wind speed at the Mildenhall (which is in quite open terrain).
  
• When measuring the calculated speed at 75% of the sail span (75% of 9m) in a vertical sail-plane in front of the mill body (2.7m from the centre of the mill body) with wind shaft at 15m height, the following wind speeds (relative to Mildenhall ABL wind speeds: u_(ABL)) were calculated:
• 
• When repeating the measurement of the calculated wind velocities a standard deviation of some 0.2m/sec is found.
  
• When a sail is at its lowest point (bottom, around 8.5m), the lowest wind speeds were calculated (due to upwind effect of the mill body).
  
• The wind speeds at the highest point (top, around 21.5m) are for most wind direction close to the Mildenhall ABL wind speed for such heights.
  
• The wind speeds at the low locations (over₂, bottom, over₃) is effected by buildings and trees in the neighbourhood.
• For Impington Mill: Trees (up to 19m) have the most effect compared to buildings (up to 8m).
• The influence of the terrain (at 15m height) within100m upwind is as follows for each azimuth:
  

  180	195	210	225	240	255	270
  open	nearby tree	open	far trees	open	open	open

  
  
• For wind directions of 195 and 225deg, the wind speed at all locations is reduced; due to respectively a nearby high tree (around 15.5 high at a distance of some 20m) and a line of four high trees (between 12 and 15m high at a distance between 20 and 200m).
• A few possible locations were also simulated (as these could be indicators of rotational energy or positions of sensors).
• The locations are:
• 0.4m above the mill cap: yellow (here Impington Mill has a anemometer)
  
• avg around the circumference of 75% of sail length (8 averaged): green (an indicator of the mill's rotational energy)
  The dotted green line is a trendline through the calculated values.
  
• avg of right and left locations of 75% of sail length (2 averaged): blue (a simpler indicator of the mill's rotational energy)
• in front of wind shaft: black (an alternative location for the anemometer)
• Impington: red (measured by Temple [2024])
  Remark: Is Impington's speed at same height as the Mildenhall speed?
  
• 
• The wind speed above the mill cap (yellow) is high compared to ABL (due to turbulence?).
• It is expected that the average wind speed over the circumference (green) is the best proxy for the rotational energy available to the mill. Within the azimuth of 180 to 270deg (the general wind direction is around 240deg), the wind speed at Impington Mill is between 60 and 100% of Mildenhall's.
• The average wind speed over the circumference (green) is slightly higher than the average wind speed over only left and right position (blue: at wind shaft height).
• The wind speed in front of the wind shaft (black) is low (due to upwind effects of mill cap and/or obstructions).
• In general, all the locations have similar calculated behaviour (like due to the high trees).
  
• .
  

Sensitivity analysis around number/placement of trees and building height

Scenario	1	2	3	4	5	6	7	8	9	10	11	12	13
Roof height	midway	midway	midway	ridge	ridge	ridge	ridge	ridge	ridge	ridge	ridge	ridge	ridge
Trees, placement	14sc, hand	21sc, DSM2019	25sc, DSM2022	25sc, DSM2022	25sc, DSM2022	25sc, DSM2022	25sc, DSM2022	25sc, DSM2022	25sc, DSM2022	25sc, DSM2022	25sc, DSM2022	25c, DSM2022	25e, DSM2022
Turbulence model	k-omega SST	k-omega SST	k-omega SST	k-omega SST	k-omega SST	k-omega SST	realized k-epsilon	realized k-epsilon	realized k-epsilon initialised	realized k-epsilon initialised, boundary layer	realized k-epsilon initialised, boundary layer	realized k-epsilon initialised, boundary layer	realized k-epsilon initialised, boundary layer
flow region height [m]	40	40	40	40	40	40	40	100	100	100	100	100	100
Ground roughness height [m]	small	small	small	small	0.5	10	10	1	1	1	10	10	10
View

Relative speed	Relatieve energy

1. The trees (modelled as stacked cylinders; sc) were positioned by hand, say within 10m of their actual position (the ground plane from Google Map does not incude trees).
   
2. Now DSM2019 was included as ground plane and thus trees are positioned more accurately. Some trees were missing, perhaps because it was a DSM from 2019.
   
3. The DSM2020 was included as ground plane and the height of the trees was adjusted (first echo, so they were on average some 2m higher than in scenario 2).
   
4. The height of buildings is up to ridge roof height; on average some 30% higher then when using the average roof height.
   
5. The ground layer has a roughness height of 0.5m.
6. The ground layer has a roughness height of 10m (=20*z₀  [Dong, 2015, page 11004]; Rasaders).
7. The turbulence model realized k-epsilon, which should be better for turbulence in this mill environment [pers. comm. fvgool, 2024].
   
8. The flow region height is 100m. In the earlier scenarios this flow region height was too small, as it should be at least 6*H (and H=16m in this case). Ground Roughness heights of 3, 5 and 10m did not work (Gauge pressure field started diverging). Adding the necessary boundary layer at ground level (and perhaps general Initialisation) looks to solve this issue (scenario 11).
   
9. Several parameters were initialised (gauge pressure, velocity, k, epsilon, and potential flow initialisation enabled).
10. The boundary layer in the mesh was included for the ground level (this is essential for a good mesh)
11. Increased Roughness height of ground level to 10m
12. Changed stacked cylinder (cs) trees to one cylinder (c) trees, plus the buildings were sometimes merged to improve the meshing.
13. Changed one cylinder (c) trees to one ellipsoid (e) trees.
    The ellpsoid trees do not yet calculate in SIMSCALE, but it is expected that the results will be close to scenario 11.
    Remark: A question has been asked in SIMSCALE Froum about this.
14. Depending on the tree height, change the f. This will be calculated when scenario 13 is working.
    Remark: at this moment all trees have the same f, this could be further improved by making it depending on the height.

• The 8 averaged wind speed (green) is calculated by using every 45 degrees of the wing rotation surface; while the 2 averaged (blue) is only done for the left and right position of the sails (this is the 'recommended' wind speed position). There is some 14% difference between the two. And if we do not measure at left and right but 1m above left and right; both averages are close. So, it might be good to put anemometer(s) at 1m above the left and/or right wing position.
  So it is proposed to change the definition of this 'recommended' position.
• Increasing the height of the buildings, deceased the wind velocity at shaft height (scenario 3 and 4).
  This is expected as the higher buildings will provide more blockage. It is not yet known if one should use ridge height or average roof height.
  Remark: investigate what height of a roofed building is most representative.
  
• A round roughness height of 10m (=20*z₀  [Dong, 2015, page 11004]; Rasaders), this increased the (magnitude of the) wind speed (scenarios 4, 5 and 6 & scenarios 9 and 10).
  Roughness height increases the turbulence and this will also increase the velocity.
  
• Increasing the height of the flow region to 100m, this decreased the wind velocity at shaft height (scenario 7 and 8)
  This is expected as the lower flow region will return the wind downwards and this increasing the wind at shaft height. One needs to align with normal CFD modelling rules: height flow region > 6H_max (Franke, 2007, section 5.1.1). In our case H_max = H_mill = 16m.
  So important to have the height of the flow region at least 6H_max (Franke, 2007, section 5.1.1).
• The scenarios the velocities (@ 2 averaged) are relatively insensitivity to: number of tree, tree form, the height of trees/buildings; ground roughness, turbulence model and mesh.
  
• With height flow region of 40m (scenarios 1 to 7) have a standard deviation of ~6%
  
• With height flow region of 100m (scenarios 8 to 12) have a standard deviation of ~3%

IMHO the thirteenth scenario is closest to reality. The wind speed at 15m (@ 2 averaged) is some 55% of the ABL speed at 15m.

Anemometer measurements at Impington Mill and Mildenhall

Average windspeeds (m/sec) around Impington

Looking at the average windspeed, the airport locations of Cambridge, Mildenhall and Lakenheath all could be ok for Impington Mill.

Simulated speeds

Below is an overview of the simulated speeds (wind direction of 225deg) at several locations for scenarios 8 to 12:

Using Temple's measurements

The anemometer was at around 17m and behind the sails and in front of middle of mill cap (ano old loc)
The measured averaged wind speed (m/sec) of Impington Mill (black: Anemometer) and meteorological station Mildenhall (grey: Open Fields) are provided for summer time (May to September) in below figure (Temple, 2024, @ 21:36). All speeds in below graph are referenced to ???m.

Using smartmolen data

Evaluation

At this moment the difference between measurements and CDF are very large. This needs more study! This will be done in the coming weeks.
Aspects to look at are:

1. How are the anemometer measurements at Impington Mill and Mildenhall calibrated?
   It looks, that regardless of the wind direction, that the windspeed over Impington is on average reduced to around 40% compared to Mildenhall. How is the measured wind speed converted to wind speed at wind shaft level?
   
2. Is the CAD-model correct?
   Are all trees incorporated at the correct height. The first return DSM 2022 is used from the Defra Data Service
   
3. What happens with the wind speed for relatively tree free wind direction?
   In the wind direction where not many trees exist (270deg), the calculated wind speed is quite close (98%) to ABL. In the anemometer measurements it is regardless of wind directions on average around 40%.
   
4. Is the porosity of the trees properly included in the CFD?
   To reduce the wind speed in the CFD to the levels of the anemometer measurements; the porosity of the trees should be reduced considerable. Would that still be realistic trees? The derivation of f (Darcy-Forchheimer's measure of porosity) is not yet 100% understood; the simulated empirical way has been used for now.
   
5. Even if the CDF is not 100% correct in absolute sense, it can be utilised in a relative sense for comparing different scenarios. Most simulations (if they are in general correctly setup, aka dependencies are properly configured) have this property.
   

Points 1 and 2 might be important to check (keeping in mind points 3 and 4).

When the above points are understood, the wind rose sector between 270 and 360deg will also be simulated.

References

Dong, Zhibao et al.: Aerodynamic roughness of fixed sandy beds. In: Journal of Geophysical Research: Solid Earth 106  (2015), issue February, pp. 11000-11011.
Franke, Jörg  et al. COST Action 732: Best practice guideline for the CFD simulation of flows in the urban environment. Brussels, COST Office 2007.
Temple, Stephen: Wind report. In: Mill news (2020), issue January, pp. 6-10.
Temple, Stephen: A modern molenbiotope.  https://www.youtube.com/watch?v=rNxU6xD3Iuc, 2024.

Acknowledgements

Disclaimer and Copyright
HomeUpSearchMail