|The Performance Assessment of Wave and Tidal Array Systems (PerAWaT) project, launched in October 2009 with £8m of ETI investment. The project delivered validated, commercial software tools capable of significantly reducing the levels of uncertainty associated with predicting the energy yield of major wave and tidal stream energy arrays. It also produced information that will help reduce commercial risk of future large scale wave and tidal array developments.
This study aims to build methodologies to simulate the wake of several turbines in a large basin using the Telemac2D and Telemac3D softwares, developed by EDF
In order to have confidence in the simulations of tidal turbines in a large basin with Telemac2D, these steps were followed:
With these methodologies, we have the ability to accurately simulate several turbineson a large basin with Telemac2D.
- First, we verify that the FORTRAN subroutine (DRAGFO) used to simulate a tidal turbine in Telemac2D produces the same results when launched with one processor as it does with several processors.
- Next, a simple flow than the wake downstream a tidal turbine is analyzed: this simple case is the wake of a flow around a cylinder, which is simulated for two models of turbulence in Telemac2D (k-epsilon model and constant viscosity model). The validated results from this analysis (mesh size, turbulence model ...) were used to get some guidance in our study which concerns the flow around the tidal turbines and not around cylinders.
- Third, the wake of a tidal turbine in a flume is simulated and validated in Telemac2D using laboratory measurements made by the LNHE department of EDF. The methodology for simulating one tidal turbine is defined, in particular a drag force is applied on a rectangular box in order to simulate the drag force due to the turbine.
- Once it was established that a single tidal turbine in a flume could be modeled, two varying alignments of tidal turbine arrays were simulated in Telemac2D using the same two models of turbulence.At this stage, two methodologies are defined:
- the first one for the method where a rectangular box is used for each turbine, and
- the second methodology for the global box containing allturbines method. In this second case, the methodology also explains how to calculate an equivalent drag coefficient to simulate a set of turbines in a large scale basin.
- In both methodologies, the way of choosing the mesh size (near and far from the rectangular box(es) and in the transition area ranging from a fine mesh to a larger mesh), the turbulence model, the drag coefficient, and the limitations are given.
The same approach can be done in three dimensions and the following steps were followed with Telemac3d to develop the methodology:
The simulations are compared to the EDF LNHE measurements to adjust the parameters in order to obtain realistic simulations.
- First, it is verified that the subroutine (SOURCE) used to simulate a tidal turbine in Telemac3D produces the same results when launched with one processor as it does with several processors.
- Second, a tidal turbine in a flume is simulated with Telemac3D.
Then, numerical analysis shows that the Telemac2D approach is transferable to Telemac3D. This lays the groundwork for defining the methodology to be used to simulate multiple turbines in a large maritime area with Telemac3D (particularly, the methodology for calculating a drag coefficient equivalent to a set of turbines for large-scale basin applications).
This methodology defines also the choice of the mesh size (near and far from the horizontal cylinder(s)), the turbulence model, the drag coefficient, and the limitations.
Finally, methodologies which enable the simulation of a tidal array in a coastal basin scale model have been set up for both 2D and 3D. It is built on the step-by-step process which had been used for the validation task, which increases confidence in the ability of the developed routines to assess large scale effects of tidal energy extraction at basin scale. It consists in running simulations at a local scale with individual devices, in extracting the equivalent array drag coefficient from these, and in inserting that piece of information in the large basin scale model.
Telemac2D and Telemac3D are now capable of assessing the impact ofany array layouts on large scale hydrodynamics effects, as the software user has the opportunity of combining the use of the DRAGFO and SOURCE routines with an appropriate methodology.