Predicting and simulating wake in stable conditions

Transcription

1 Predicting and simulating wake in stable conditions Model chain evaluation and an example Mark Žagar, Gregory S. Oxley, Yavor V. Hristov Vestas Wind Systems A/S, Plant Siting and Forecasting 15 January 2014, Vindkraftnet, DONG København

2 Overview What is stability Mesoscale model ability to predict the whole spectrum of conditions Display of stability in a large wind farm High fidelity CFD simulation of a wake loss

3 What is stability Participation of gravity (the strongest force in the atmosphere) Stratification Stable conditions Air warmer than the surface, by: Cooling of the surface Advection of warm air over colder surface Offshore especially near-shore

4 What is stability Participation of gravity (the strongest force in the atmosphere) Stratification Stable conditions Air warmer than the surface, by: Cooling of the surface Advection of warm air over colder surface Offshore especially near-shore Turbulence decays Little vertical exchange Low-level jets No 2 stable conditions are alike, perhaps slightly similar

5 Measured vs. modelled stability Mast Data dx=3km Model Data Mast Data dx=3km Model Data

6 Downslope storm, hydraulic jump WRF-ARW, dx=100m Data from many anemometers in the area confirms the extent, magnitude, and sharp edge of the events.

7 Wake loss at one wind farm and stability 30% Note the pronounced peak in losses around 10m/s, associated with a peak in stability.

8 Preliminary Study (WRF-CFD-ALM Coupling) 10 turbine problematic grouping Wake interaction zone Wind direction Hub height wind speed Instantaneous power history in wake interaction zone Normalized power history for turbine C for stable vs. neutral cases

9 Precursor Simulations Microscale CFD Model - Diurnal Diurnal cycle begins from neutral conditions at T=15h (UTC, 7h LOC) Unstable convective conditions progress to stable inversion at approx. T=26h Unstable region characterized by high free stream turbulence and low shear Stable region characterized by low free stream turbulence and high shear Low-level jet can be seen to develop between m during stable hrs.

10 Turbine + Wake Simulations Microscale CFD Model - Neutral Neutral - Instantaneous wind speed at hub height Neutral - Mean wind speed at hub height (10min averaging)

11 Turbine + Wake Simulations Microscale CFD Model - Stable Stable - Instantaneous wind speed at hub height Stable - Mean wind speed at hub height (10min averaging) 18 Nov. 2013, Gregory S. Oxley (grsox@vestas.com), - Plant Siting and Forecast, Vestas Wind Systems A/S

12 The future Needs and Expectations Industry: Accelerated use of the flow models in wind energy sector Production estimates Yaw and tilt forces and moments Industry tools and methods must adapt

13 The future Needs and Expectations RCM: Atmospheric models (e.g. WRF) benefit from flow model development Develop coupling interfaces at both ends CFD models must feel the real atmosphere CFD-LES: Complex terrain wall treatment, stable flow turbulence modelling Precursor vs. synthetic turbulence generation Is massive industrial deployment of LES models feasible? Vestas HPC: Firestorm ~15K cores (50% for CFD 50% WRF) Currently we analyze roughly 2K sites/yr (12 or 36 sectors per site) with RANS 10% of those sites move to hybrid RANS/LES modelling for a few problematic sectors only The next step of CFD-LES-ALM method is expensive How to maximize the knowledge from LES?: Large flagship sites to convince customers of physics at play Tuning and validation of lower order wake models (DWM, prescribed wake models )

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