line method (ALM) - Chalmers
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Tidal power plant simulations using large eddy simulation (LES) and the actuator line method (ALM) Sam T Fredriksson 1 , Göran Broström 1 , Björn Bergqvist 2 , Johan Lennblad 2 , and Håkan Nilsson 3 1 Dept. Of Marine Sciences, University of Gothenburg, Sweden, 2 Minesto AB, Sweden, 3 Dept. of Mechanics and Maritime Sciences, Chalmers University of Technology, Sweden Abstract The share of the renewable energy in the global energy mix is to be increased according to the sustainable development goals of the UN. Tidal energy potentially can here play a substantial role for the electric power generation. The tidal power plant Deep Green developed by Minesto uses a novel technology with a “flying” kite that, with its attached turbine, sweeps the tidal stream with a velocity several times higher than the mean flow. Eventually these power plants will form arrays requiring knowledge of (1) the interaction between individual power plants as well as (2) how the power plants and the arrays will influence the surrounding environment. The tidally oscillating turbulent boundary layer flow is in the present study analyzed using Large Eddy Simulations (LES) utilizing two different modeling techniques (pseudo‐spectral and finite volume method). The boundary layer flow is analyzed both undisturbed and with a sweeping tidal power plant. The power plant is modeled using the Actuator Line Method (ALM). This method has been reformulated in order to be able to take arbitrary pathways of the actuator line into account. The results for the undisturbed flow simulations show, e.g., variations of the turbulence intensity depending on pre‐ or post‐tidal peak flow for equivalent volume mean flow. The results for the modeled power plant show, e.g., that the wake flow downstream of the power plant that can be related to the size of the pathway size. Model Setup The LES model is setup to resemble the conditions at a site, west of Holy Island along the north wet coast of Wales, where the first Deep Green is to be deployed. The depth is 80 m and a bottom roughness length is used to model bottom roughness following the observations of frequent boulders with dimension roughly 2x2x2 m 3 at the bottom. The model is forced using a full tidal cycle (12 h) sinusoidal varying body force, and the body force amplitude is adjusted to fit the maximum tidal peaks present at the site (1.6, 2.0, and 2.4 m/s respectively). For the Coriolis force we assume that the force perpendicular to the main flow is balanced by a pressure gradient. The volume mean flow in the LES model during a number of tidal cycles are shown below for the case with a maximum tidal peak of 2.0 m/s. The instance for the third tidal peak (between 25 and 26 h) and where the volume mean flows of 1.6 m/s are indicated with a black line and red lines, respectively. The instantiations field at time the first red line (approx. 24.5 ) is used as initial conditions for the simulation with the Deep Green. References: Fredriksson et al. (2016). http://iopscience.iop.org/article/10.1088/1757‐899X/276/1/012014 Broström et al. (2018), Some modelled characteristics of tidal turbulence in medium depth water, Ocean Science meeting, AGU, Seattle, USA. Acknowledgement: This project is financed by the Swedish Energy Agency. Velocity Field and Vortices The instantaneous velocity fields after approximately 15 trajectories have been run after the mean current of 1.6 m/s at approximately 24.5 . The Deep Green (visualized by the green isosurface of the force field) clearly affects the velocities here given at domain boundaries and at yz‐planes at ݔ= ݔ , ݔ + ܦ ݕ, ݔ +2 ܦ ݕ, ݔ + 3 ܦ ݕ, and ݔ +4 ܦ ݕwhere ݔ is the center and ܦ ݕis the width of the trajectory, respectively. The induced vortices (indicated by isosurfaces of a positive value of the second invariant of the velocity‐gradient tensor) are visible the full domain length. Velocity Deficit Comparison of mean flow velocities, at locations downstream of the Deep Green trajectory center ( ݔ , ݕ , ݖ ), show the downstream wake. a) Vertical profiles at the ݖݕ‐planes, through ݕ= ݕ . b) Horizontal profiles at the ݖݕ‐planes, through at ݖ= ݖ . U [m/s] 2.0 1.5 1.0 0.5 0 ܪݖ⁄ ݑ ത ଵ ݑ ത ଵ௭ ൗ ܦݕ௬ ⁄ ݑ ത ଵ ݑ ത ଵ௭ ൗ a) b) Turbulence Intensity The turbulence intensity ܫൌ ߪ ݍ ഥ ݖ⁄ is shown below for the case with a maximum tidal peak of 2.0 m/s. Here ߪଶ ݍൌ ݎ ݍ, ݎis the variance, ݍis the velocity fluctuation, and ഥ ݖis the horizontal mean velocity. The instance for the third tidal peak (between 25 and 26 h) and where the volume mean flows of 1.6 m/s are indicated with a black line and red lines, respectively. It is seen that the turbulence intensity varies strongly with time and is asymmetric around the tidal peak. It is further noticed that it is anisotropic in different directions. Volume mean tidal flow Dashed line before and full line after tidal peak during volume mean flow of 1.6 m/s, respectively. Above bottom ߪ ݔ ഥ ݖ⁄ Time ߪ ݍ ഥ ݖ⁄ Depth Turbulence intensity Time Volume mean velocity ݏ⁄
Transcript of line method (ALM) - Chalmers
Tidal power plant simulations using large eddy simulation (LES) and the actuator line method (ALM)Sam T Fredriksson1, Göran Broström1, Björn Bergqvist2, Johan Lennblad2, and Håkan Nilsson31Dept. Of Marine Sciences, University of Gothenburg, Sweden, 2Minesto AB, Sweden, 3Dept. of Mechanics and Maritime Sciences, Chalmers University of Technology, Sweden
AbstractThe share of the renewable energy in the globalenergy mix is to be increased according to thesustainable development goals of the UN. Tidal energypotentially can here play a substantial role for theelectric power generation. The tidal power plant DeepGreen developed by Minesto uses a novel technologywith a “flying” kite that, with its attached turbine,sweeps the tidal stream with a velocity several timeshigher than the mean flow. Eventually these powerplants will form arrays requiring knowledge of (1) theinteraction between individual power plants as well as(2) how the power plants and the arrays will influencethe surrounding environment.The tidally oscillating turbulent boundary layer flow
is in the present study analyzed using Large EddySimulations (LES) utilizing two different modelingtechniques (pseudo‐spectral and finite volumemethod). The boundary layer flow is analyzed bothundisturbed and with a sweeping tidal power plant.The power plant is modeled using the Actuator LineMethod (ALM). This method has been reformulated inorder to be able to take arbitrary pathways of theactuator line into account.The results for the undisturbed flow simulations
show, e.g., variations of the turbulence intensitydepending on pre‐ or post‐tidal peak flow forequivalent volume mean flow. The results for themodeled power plant show, e.g., that the wake flowdownstream of the power plant that can be related tothe size of the pathway size.
Model SetupThe LES model is setup to resemble the conditions at asite, west of Holy Island along the north wet coast ofWales, where the first Deep Green is to be deployed.The depth is 80 m and a bottom roughness length isused to model bottom roughness following theobservations of frequent boulders with dimensionroughly 2x2x2 m3 at the bottom. The model is forcedusing a full tidal cycle (12 h) sinusoidal varying bodyforce, and the body force amplitude is adjusted to fitthe maximum tidal peaks present at the site (1.6, 2.0,and 2.4 m/s respectively). For the Coriolis force weassume that the force perpendicular to the main flowis balanced by a pressure gradient. The volume meanflow in the LES model during a number of tidal cyclesare shown below for the case with a maximum tidalpeak of 2.0 m/s.
The instance for the third tidal peak (between 25 and26 h) and where the volume mean flows of 1.6 m/sare indicated with a black line and red lines,respectively. The instantiations field at time the firstred line (approx. 24.5 ) is used as initial conditionsfor the simulation with the Deep Green.
References: Fredriksson et al. (2016). http://iopscience.iop.org/article/10.1088/1757‐899X/276/1/012014Broström et al. (2018), Some modelled characteristics of tidal turbulence in medium depthwater, Ocean Science meeting, AGU, Seattle, USA.
Acknowledgement: This project is financed by the Swedish Energy Agency.
Velocity Field and VorticesThe instantaneous velocity fields after approximately 15 trajectories have been runafter the mean current of 1.6 m/s at approximately 24.5 . The Deep Green(visualized by the green isosurface of the force field) clearly affects the velocitieshere given at domain boundaries and at yz‐planes at = , + , + 2 , +3 , and + 4 where is the center and is the width of the trajectory,respectively. The induced vortices (indicated by isosurfaces of a positive value of thesecond invariant of the velocity‐gradient tensor) are visible the full domain length.
Velocity DeficitComparison of mean flow velocities, at locations downstream of the Deep Greentrajectory center ( , , ), show the downstream wake. a) Vertical profiles at the
‐planes, through = . b) Horizontal profiles at the ‐planes, through at = .
U [m/s]2.0
1.5
1.0
0.5
0
⁄
⁄
a) b)
Turbulence IntensityThe turbulence intensity ⁄ is shownbelow for the case with a maximum tidal peak of 2.0m/s. Here , is the variance, isthe velocity fluctuation, and is the horizontalmean velocity. The instance for the third tidal peak(between 25 and 26 h) and where the volume meanflows of 1.6 m/s are indicated with a black line and redlines, respectively. It is seen that the turbulenceintensity varies strongly with time and is asymmetricaround the tidal peak. It is further noticed that it isanisotropic in different directions.
Volume mean tidal flow
Dashed line beforeand full line aftertidal peak duringvolume mean flow of1.6 m/s, respectively.
Above bo
ttom
⁄
Time
⁄
Depth
Turbulence intensity
Time
Volume mea
n velocity
⁄
