Wave driven wind
simulations with CFD
DeepWind'2013, 24-25 January, Trondheim, Norway
Siri Kalvig1, Richard Kverneland2 and Eirik Manger3
1StormGeo, Norway
2 University of Stavanger, Norway 3Acona Flow technology, Norway
• Motivation
• Wave-wind interactions
• Method
• Results
• Conclusions & comments
Introduction
Industrial PhD of Stormgeo and UiS and PhD is part of NORCOWE.
How does a ”non-flat” sea affect the wind fields?
Motivation
A typical offshore wind picture….
• Will wave induced wind at an offshore wind site result
in different wind shear and more turbulence than
expected ?
• And if so, how will this affect the turbines?
Wind sea and swell influences the atmosphere different!
Wind sea - waves generated by local wind
Swell - long period waves generated by distant storms
Most common is a mixture of wind sea and swell, and this makes the picture even more complicated.
• Field experiments and numerical simulations show that during swell conditions the wind profile will no longer exhibit a logarithmic shape and the surface drag relies on the sea state (i.e. Smedman et al. 2003 &
2009, Semedo et al. 2009).
• There is a gap between “best knowledge” (science) and “best practice” (codes, standards) and there is a need for improved guidance on the impact atmospheric stability and wave-wind interaction in the MABL can have on the offshore wind industry (Kalvig et al 2013, Wiley Wind Energy, in press)
• Swell can result in both higher and smaller effective surface drag and it is likely that swell can create different wind shear and turbulence
characteristics so that a wind turbine site will be exposed to other external environmental condition than it was designed for.
• Sullivan et al. (2008) developed a large-eddy simulation (LES) with a two-dimensional sinusoidal wave and identified flow responses for
three cases; wind opposing swell, wind following swell and wind over a swell surface with no movement.
• The flow responses in the different cases where very different and ‘fingerprints’ of the surface wave extended high up in the MBL.
Aim at develop a wave-wind simulation set up with open source CFD and with more computational effective methods.
Need to simulate wave movements!
From: Grand Valley State University, http://faculty.gvsu.edu/videticp/waves.htm
Need a new boundary condition that take into account the sinusoidal movement of the “ground”.
Solution:
Transient OpenFOAM simulation with
pimpleDyMFoam. New boundary condition
implemented with mesh transformations.
Need to simulate wave movements!
From: Grand Valley State University, http://faculty.gvsu.edu/videticp/waves.htm
Need a new boundary condition that take into account the sinusoidal movement of the “ground”.
Solution:
Transient OpenFOAM simulation with
pimpleDyMFoam. New boundary condition
implemented with mesh transformations.
• The open source CFD toolbox OpenFOAM is used for both mesh generation and CFD computations.
• Wave speeds (c), wave amplitude (a), wave length (L) are input parameters to the model.
• To start with a relatively small domain with length of 250 m and a height of 50 m was established. Various sensitivity analyses were performed where different wind velocities and sea states where studied in detail (Kverneland, master theses UiS 2012).
• Temperature and the Coriolis effect are not taking into account and only uniform wind is studied. The calculations use a Reynolds
averaging Navier-Stokes (RANS) approachs and since the wave moves it is necessary with a transient (time varying) simulation. The turbulence closure model used is the standard k-epsilon model.
Method
• NORCOWE & NOWITECH organized a wind turbine blind test in 2011-2012, BT1 & BT2.
• BT1: Eight independent modelling groups submitted 11 sets of simulations. No
obvious “winner” and large spread of results (Krogstad et.al. 2011).
Method
Currently working with the Actuator disk and actuator line method.
Aiming at coupling the wave set up with a turbine wake model.
wind following waves
wind opposing waves
In general:
The wind speed profile and the turbulent kinetic energy pattern far above the waves will be different depending on the wave state and wave direction.
wind
waves
wind
waves
waves waves
Results
Wind aligned with waves: Vertical profile (at x=210 m) of mean values of the horizontal and vertical component of the wind flow for six cases with different inlet velocity (openFOAM f”ieldAverage” is used for mean values).
waves waves
Results
Wind opposing waves: Vertical profile (at x=210 m) of mean values of the horizontal and vertical component of the wind flow for six cases with different inlet velocity.
waves waves
Results
Mean turbulent kinetic energy for wind aligned with the waves and wind opposing the wave.
waves waves
Results
Various wave states opposed with wave propagation. Vertical profile of horizontal wind speed and mean horizontal wind speed .
waves waves
Results
“Instant” velocity profiles over the wave surface. Lines for every 5 m in the interval of 145-200 m (over one whole wave length). Wind aligned and wind opposed the wave propagation result in very different response in the wind field.
Uniform wind of 5 m/s at the inlet. Wave with; c=8 m/s, a=3 m, L=40 m
waves waves
Results
“Instant” turbulence profiles over the wave surface. Lines for every 5 m in the interval of 145-200 m (over one whole wave length). Wind aligned and wind opposed the wave propagation result in very different response in the wind field.
Uniform wind of 5 m/s at the inlet. Wave with; c=8 m/s, a=3 m, L=40 m
waves waves
Results
Comparison with Sullivan et al. 2008:
A n openFOAM URANS set-up with a wave with a=1.6 m, L=100 m and c= 12.5 m/s on a domain of 1200 x 100 m is being compared with Sullivan et al’s LES simulations.
Preliminary results are
promising and it looks like we are able to capture the same dynamics as Sullivan et al. But current simulations is to
coarse and more refined simulations are needed.
Contours of the horizontal wind field for the situation of aligned (top) and opposed with wave propagation (middle), and stationary waves (bottom) . The non-dimensional field shown is mean Ux / Ug.
Summary
Wave wind simulations with openFOAM is on going PhD work at University of Stavanger /StormGeo/Norcowe.
A cost efficient CFD method for flow over wave simulations, based on RANS turbulence closure is developed.
The response in the boundary layer over the wave are very different for cases where the wind is aligned with the wave propagation and wind opposing the wave.
Case of U=5 m/s and c=10 m/s wave: A low level speed up is created in the lowest
meters for wind aligned with a fast moving wave. The profiles over the wave do not exhibit a logarithmic profile (or power law profile). Turbulent kinetic energy is slightly higher for wind opposing the wave than wind aligned with the wave.
Preliminary result shows pattern that compares well to Sullivan et al. (2008). More detailed studies need to be performed.
Next step: Test the significance and the implications of wave-wind interaction on the offshore wind turbine loads and wakes. Wave movement code and turbine modelling code need to be coupled.
References
Kalvig S, Gundmestad O-T, Winther N. A literature review on implications of wave-influenced wind and atmospheric stability for offshore wind energy. Wind Energy 2013, In press.
Kverneland Richard, “CFD – Simulations of wave-wind interaction”, Master theses, University of Stavanger 2012.
Semedo A, Saetra Ø, Rutgersson A, Kahma KK, Pettersson H. Wave-induced wind in the marine boundary layer.
Journal of the Atmospheric Sciences 2009; 66 : 2256-2271
Smedman A, Larsén X, Högström U, Kahma K, Petterson H. Effect of sea state on momentum exchange over the sea during neutral conditions. Journal of Geophysical Research 2003; 108, NO.C11, 3367. DOI: 10.1029/2002JC001526 Smedman A, Högström U, Sahlee E, Drennan WM, Kahma KK, Pettersson H, Zhang F. Observational study of marine atmospheric boundary layer characteristics during swell. Journal of the Atmospheric Sciences 2009; 66(9) : 2747-2763. DOI: 10.1175/2009JAS2952.1
Sullivan PP, Edson JB, Hristov T, McWilliams JC. Large-eddy simulations and observations of atmospheric marine boundary layers above nonequilibrium surface waves. Journal of the Atmospheric Sciences 2008; 65(4) : 1225-1245
Vincent, C. L., P. Pinson, et al. (2011). "Wind fluctuations over the North Sea." International Journal of Climatology 31(11): 1584-1595.
Acknowledgements;
Eirik Manger, Acona Flow Technology OpenCFD, academic support agreement