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CFD wind flow modelling over the Nant y Moch site using AnsysWindmodeller software

Site• Significant forestry • Complex terrain• 9 masts (70m)

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Setup

• Central square mesh approx 8.7km • Outer circular mesh approx 26km diameter• Lopes da Costa forestry model

Grid Convergence

Grid convergence based on normalised velocity

• Convergence at approx 2.7million cell mesh• Corresponds to 53m cell resolution• Assumes 9million mesh is fully converged

• 60deg sectors

• coarse

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Scaled Velocities

Forestry coefficient

Indicates that optimum forestry coefficient for this site is between 0.02 and 0.05

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Canopy Scaling

• Density of forest reduces with height• Effect not accounted for by application of forestry coefficient• Scaling the forestry canopy heights down by a fraction and keeping the forestry

coefficient constant can - to a certain extent – maybe account for this variation• Even more detailed forestry input?

Boundary representation

• Several of the masts are placed close to forestry boundaries• Solution at these masts will be affected by the accuracy of the forestry boundary

representation.• Figure to the right shows a mesh with 800,000 cells (cell res 75m) and this does

improve with a denser mesh but even using a 9 million (cell res 33.5m) there is still a mismatch.

• This could have a substantial effect on the model results.

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Comparison with another CFD model at another SSE site –Normalised Velocity• Steep valley site but not as complex as Nant y moch• Two masts• High level comparison – differences in mesh config and application• Different size areas were mapped - Model A 18kmx16km - Windmodeller 22km Ø• Windmodeller mesh significantly denser on average (3.6million cells compared to

approx 460,000 cells in Model A) but had a coarser resolution throughout wind farm region.

• Model A simulations presented per 30deg sector averaged from 10deg simulations –Windmodeller 30deg sector

Windmodeller, K = 0.05Windmodeller, K = 0.0Model AMeasured Data

Comparison with another CFD model at another SSE site -Shear

Windmodeller, K = 0.05Model AMeasured Data

Windmodeller, K = 0.05Model AMeasured Data

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Conclusions

• Forestry modelling has a huge impact on the solution produced by the CFD as illustrated by the step change between 0.00 and 0.02

• Canopy scaling should be investigated further as a means of reducing the error in velocity.

Grid Convergence shear

Grid convergence based on shear

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Forestry Model

• Lopes da Costa• Svensson

Modelling parameters

• Ansys Windmodeller package• Central mesh approx 8.7km• Outer mesh approx 22km

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Overview

Modelling was carried out by Gary Ryan at the University of Bristol for SSE renewables

Ansys – Windmodeller

• Site• Modelling• Conclusions