Modelling atmospheric stability with CFD: The importance of tall profiles

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1 ENERGY Modelling atmospheric stability with CFD: The importance of tall profiles VindKraftNet Seminar on Profiles Jean-François Corbett, Global Head of CFD Service 1 SAFER, SMARTER, GREENER

2 DNV GL CFD Service: capturing wind flow at wind farm scale In-house methodology and software tools, STAR-CCM+ as RANS calculation engine Focus on the most important controlling physics 2

3 Observed impact of atmospheric stability Measurements show that stability within the ABL can substantially influence the spatial variation of the wind resource at microscale Stable BL Measured speedups between two masts in Sweden, binned by direction and stability conditions 3

4 Stability should be accounted for in microscale flow models. But what does that mean? What kind of stability? Static CFD (RANS) Stable? (SBL, FA) Unstable? (ML) Neutral? (RL, ML) Where? Within the ABL? Above the ABL? Both Which impacts? Turbulence? Mean flow? Both Figure source: Stull, Meteorology for Scientists and Engineers

5 Acknowledgements Jim Bleeg did the bulk of the technical development work that I will be presenting today Dnyanesh Digraskar produced some of the validation results that I will show you 5

6 CFD model of the stable atmosphere 6

7 Typical vertical profiles of potential temperature in the lower troposphere Day Night Stratification here, here, and here FA impacts wind speeds down here. Stable BL ABL 7

8 Homogeneous boundary conditions Inflow boundary condition profiles are formulated based on established theory for the nocturnal boundary layer. Achieving homogeneity is no small feat! Neutral Distance from inflow boundary 8

9 CFD simulation results for an idealised case 9

10 Impact of Stability In the Free Atmosphere (SIFA), neutral boundary layer over a two-dimensional hill Wind speed normalized at top of the hill --- BL bounding streamline Flow direction Accounting for stability in the free atmosphere (SIFA) above the neutral boundary layer significantly affects the wind speed near the ground Wind speed at the top of the hill and just downstream is much higher than in the surrounding terrain To understand the trend, focus on the ABL thickness 10

11 Impact of stability in the FA and ABL, neutral and stable boundary layers over a two-dimensional hill Accounting for stable stratification within the boundary layer, as well as above, causes the solution to further deviate from traditional approaches. Flow direction Waves downstream of the hill are due to gravity waves in the uniformly stratified free atmosphere 11

12 Vertical component of wind speed above idealized 2D hill predicted by CFD with stable BL and FA The hill induces pronounced gravity waves in the uniformly stratified free atmosphere Vertical component of velocity (m/s) Ground elevation (m) 12

13 Impact of stability, flow over a three-dimensional hill With a 3D hill, the contrast between the wind speed at the top of the hill and the surrounding terrain is not quite as pronounced when accounting for SIFA. Flow direction The stable boundary layer still causes a downslope speedup, but the downstream gravity waves are less evident near the ground. 13

14 Impact of stability, flow over simple terrain (short 2D hill) Despite the simple terrain, there are material differences between the three results (>5%). Flow direction Max slope: 0.6 With SIFA the contrast between the wind speed at the top of the hill and well off the hill is much larger. The stable boundary layer increases wind speed on the downslope 14

15 CFD simulation results for real-world cases 15

16 DNV GL CFD with stability above and within the ABL has been run in 13 different countries to date 57 wind farms and 1162 mastto-mast cross-predictions 16

17 Validation summary: 57 sites, 1162 mast-to-mast predictions Accounting for stability in CFD within and above the ABL significantly reduces prediction errors relative to the industry standard. CFD RMSE is lower at 84% of the sites Mean site RMSE is 2.6% for CFD (with stability) and 4.4% for WAsP (stability neglected) 17

18 How accounting for stability affects prediction accuracy at three example sites in Europe Accounting for stable stratification in the free atmosphere (FA) consistently increases accuracy relative to traditional CFD. Accounting for stability variation in the ABL further reduces prediction errors. France site Poland site Sweden site (3 masts) (3 masts) (4 masts) 18

19 Visualising the effects of stability 19

20 Impact of stability on flow across turbine rotor Neutral flow is more or less straight Stability suppresses vertical mixing, allowing the Coriolis force to cause significant veer across the height of the rotor area 20

21 Impact of stability on flow across turbine rotor Neutral flow is more or less straight Stability suppresses vertical mixing, allowing the Coriolis force to cause significant veer across the height of the rotor area 21

22 22

23 Room for improvement 23

24 Inlet profiles are currently idealised, based on theory Many parameters are best guesses, based on typical global averages: thickness of ABL, capping inversion, and residual layer potential temperature gradients in capping inversion, and residual layer FA Stable BL Day Night ABL 24

25 Symptom: Δ Elevation bias physics need fixing On average, WAsP-BZ has a reproducible Δelevation bias, 3-4% / 100m High masts overpredict at low turbines; low masts underpredict at high turbines Current CFD model has much smaller trend with opposite sign (!) Likely cause: our initial parameter guesses are not perfect!!! OMG! LOL On average: Capping inversion a bit too hard Low mast High turbine High mast Low turbine BL a bit too thin 34 sites in US Ongoing work to make better BCs using balloon measurements, mesoscale results promising! Avg. Error: CFD 2.1%, WAsP-BZ 3.1% 25

26 Conclusion 26

27 Conclusion As demonstrated by: Measurements Numerical experiments An extensive global validation of results on commercial sites so far a substantial accuracy gain can be achieved at typical hub heights when microscale wind flow models account for the impacts of stable stratification Not only within the ABL, but also above the ABL, at heights up to a few km Both on buoyancy in the mean flow, and on turbulence Accounting for more of the controlling physics offers substantial benefit and is the way forward Clearly there is still some way to go Next step is implementing realistic inlet boundary conditions with the help of mesoscale results and balloon probe measurements 27

28 Thanks for your attention Jean-François Corbett SAFER, SMARTER, GREENER 28

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