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Risø Wake Lidar Single Wake

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Scope

The benchmark is open to participants of WakeBench who want to validate the near-wake models in horizontally homogemeous terrain using lidar cross-sectional scans from 1 to 5 rotor diameter downstream.

Objectives

Determination of mean wake velocity ratio at hub height as function of downstream position for a Nordtank 500 kW stall regulated turbine in flat and horizontally homogeneous terrain at different atmospheric stabilities and wind conditions.

Data Accessibility

The benchmark is offered to participants of the IEA Task 31 Wakebench.

Input data

The necessary input parameters related to the turbine, the terrain and the ambient flow are:

  • Terrain roughness length z0=  0.07 m (from site survey, subjected to changes)
  • Location: 55° 41'04 N, 12° 05'48 E.
  • Turbine characteristics (rpm, hub height, rotor tilt): see attached technical description of the Nordtank 500kW turbine by Kurt S. Hansen.
  • Inlet profiles: see "model runs" for run-specific inlet condition at hub height (mean horizontal wind speed at hub height, reference turbulence intensity and Obukhov length, shear exponent).
  • Additional inlet informations are also provided for each test case: ensemble average (and standard deviation) of inflow conditions at three different heights, mean turbulent kinetic energy at hub height, mean electrical power produced and mean thrust coefficient for rotor model validation and potential temperature profile at two different heights..

Validation data

The validation data consists of the mean wake velocity ratio defined as (Uwake,H/U0). The wake profiles are taken at hub height  and at 1, 2, 3, 4 and 5 diameters downstream. The exact output coordinates corresponding to the lidar measurement location are provided in the section "Output data".

A plot showing the validation data for three of the models runs (see section " Model runs") is available below:

Model runs

The simulations to be conducted as part of this benchmark are the following:

  • Run 1a: Neutral (abs(Lref) > 500), Lref = 1678 m (363), Uref = 7.03 m/s (1.13), Iref = 15 %, αref = 0.19,  (low velocity). [3.2 h]
  • Run 1b: Neutral (abs(Lref) > 500), Lref = 1326 m (804), Uref = 8.92 m/s (1.44), Iref = 13 %, αref = 0.18,  (intermediate velocity). [4.7 h]
  • Run 1c: Neutral (abs(Lref) > 500), Lref = 2441 m (964), Uref = 9.86 m/s (1.81), Iref = 13 %, αref= 0.18,  (high velocity). [5.5 h]
  • Run 2: Unstable (-200 < Lref < -50), Lref = -85 m (19), Uref = 6.82 m/s (1.16), Iref = 16 %, αref= 0.16,  [3.3 h]
  • Run 3: Stable (10 < Lref < 50), Lref = 29 m (11), Uref = 6.76 m/s (0.72), Iref = 9 %, αref = 0.27,  [5.5 h]

The standard deviation within each test case bin is indicated in parentheses.

The mean Obukhov length Lref is derived from the 3D sonic anemometer located on the nearby met mast at 16.5 m a.g.l.

Iref refers to as the total 3D turbulence intensity at 34.5 m measured by the 3D sonic anemometer. 

αref refers to as the shear exponent obtained from a logarithmic fitting of the inlet profiles measured by three 3D sonic anemometers located on the nearby met mast at 16.5, 34.5 and 52.m respectively.

The number of hours used for the bin averaging is indicated.

The complete inlet conditions of each run are stored in the files RisøWakeLidar_inlet#.txt where # refers to the run index (1a,1b,1c,2,3). In these files is listed: No.10min(-),<U_16.5(m/s)>,<U_ref(m/s)>,<U_52.5(m/s)>,σ_u_16.5(m/s),σ_u_ref(m/s),σ_u_52.5(m/s),α_ref(-),TKE_ref(m2/s2),I_ref(-),L_ref(m),CT(-),Pow.(kW),Ө_0_16.5(degC),Ө_52.5/Ө_0(-),z_0(m).

Output data

For direct comparison to validation data (output1.dat), please provide the streamwise velocity component (U) normalized with the free stream velocity U0 at the location {X,Y,Z} corresponding to the lidar measurements coordinates as listed in the file RisøWakeLidar_sensor.txt. The x-axis corresponds to the main flow direction (with origin the rotor center), the y-axis is the lateral direction (with origin the rotor center) and the z-axis the vertical direction pointing upwards (with origin the ground level).  If the model uses a rotor model, please provide additional power and thrust coefficient calculation for direct comparison.

For model intercomparison (output2.dat), additional data are required: the 3 velocity components (U,V,W), turbulent kinetic energy (tke) and eddy viscosity (nu_t) for the lateral and vertical wake profile at hub height, together with the inlet profiles generated by your model.

Use the file naming and format convention described in the Windbench user's guide.