# Sexbierum Double Wake Neutral

**Scope**

The benchmark is open to participants of the Wakebench project using wake and, possibly, atmospheric boundary layer models. This is a realistic wake case measured at the Dutch Experimental Wind Farm at Sexbierum that has the added complexity of multiple wake interaction. It should test a wake model’s ability to reproduce the wake merging process.

This benchmark follows the single-wake case (*Sexbierum_SingleWakeNeutral*). Participants are strongly encouraged to participate in both exercises for a more complete model evaluation.

**Objectives**

Demonstrate how wake models perform and capture the wake merging process in the presence of atmospheric shear and turbulence.

**Data Accessibility**

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

**Input data**

The conditions for simulating the Sexbierum double wake are:

- Hub height mean wind speed:
*U0*= 8.0 m/s - Hub height turbulence intensity: ‹
*u’u*’›1/2/*U0*= 0.135 - Aerodynamic roughness height:
*z*0 = 0.047 m - Location: 53.22° N, 5.48° E
- Wind direction: -15° to +30°, where 0° is the direction of the line between turbines T38 and T36 as shown in Figure 3.
- Stability: This is not given by Cleijne (1992) and data is averaged over a long (months) time period, so assume neutral stability.
- Turbine model: WPS 30. See attached Cp and Ct curves and consult Bulder (1993) for specifications.
- Turbine locations.

**Figure 4**: Definition of the coordinate system taken from Cleijne (1992, 1993).

**Validation data**

The validation data consists of mean and turbulent inflow point measurements and wake profiles (profiles as a function of wind direction). The wake profiles are taken at three different heights above, at, and below hub height on mast *b*, and at hub height on masts *a* and *c*.

**Model runs**

The participant should use a domain size sufficient for his or her model to work properly and devoid of spurious boundary effects. Due to the differing levels of computational cost of the various models, the participant should choose how many wind directions to run, but at least the 0° case should be modeled.

**Output data**

1. For direct comparison to data (*output1.dat*): Provide values of the mean velocity components (*U*, *V*, and *W*), turbulent kinetic energy (*tke*), and if possible the three velocity component variances (‹*u’u’*›1/2, ‹*v’v’*›1/2, and ‹*w’w’*›1/2) and the three Reynolds shear stress components (‹*u’v’*›, ‹*u’w’*›, ‹*v’w’*›) at the sensor locations on mobile mast *b* and on the single sensor locations on masts *a* and *c*(see Figure 3). Also provide the values of mean wind speed (*U0*) and turbulence intensity (*I*) at the hub height at the location of meteorological mast 7 (see Figure 1). Provide the average power production of turbines T38, T37, and T36. Use the file naming and format convention described in the Windbench user's guide.

2. Additional data for model intercomparison (*output2.dat*): Vertical lines of mean (*U*, *V*, and *W*) and turbulent (*tke*, ‹*u’u’*›1/2, ‹*v’v’*›1/2, ‹*w’w’*›1/2, ‹*u’v’*›, ‹*u’w’*›, and ‹*v’w’*›) data that extend from the surface to 1D above to upper extent of the rotor with sufficient resolution to show the profiles at 1D, 2D, 3D, and 4D downstream of T37 on the line beteen T38-T37-T36. This is to be taken for the wind direction down this T38-T37-T36 line (this is similar to Figure 5).

**Figure 5**: Vertical lines of data extraction for output2.dat.

*3. Additional data for model intercomparison ( output3.dat): Horizontal lines of mean (U, V, and W) and turbulent (tke, ‹u’u’›1/2, ‹v’v’›1/2, ‹w’w’›1/2, ‹u’v’›, ‹u’w’›, and ‹v’w’›) data that extend from 2D upstream of T38 to 2D downstream of the T36 and passing through the center of the rotors, ¼D above the center of the rotors, and ¼D below the center of the rotors when the wind is down the T38-T37-T36 line (this is similar to Figure 6).*

**Figure 6**: Horizontal lines of data extraction for output3.dat.

**Remarks**

The velocity vector is based in a wind-direction-local coordinate system, as shown in Figure 4. Take care to rotate the output into this coordinate system for each wind direction you run.

**Terms andConditions**

Not applicable.