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Askervein Neutral

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Scope

The benchmark is open to participants of the Wakebench project using flow models over topography in neutral conditions. The study will show the changes in the mean flow field above the Askervein hill for different incident flow angles defined by the following wind directions:

  • 210º, perpendicular to the long axis of the hill (classical reference),
  • 130º, parallel to the long axis, and
  • 90º, in the wake of upstream hills.

Objectives

Test model fine-tuning strategies that will be applied in complex terrain sites. Evaluate turbulence models in a test site with reasonably well defined boundary conditions. Evaluate the ability of the models to predict wind direction variations close to the ground induced by local topography.

Data Accessibility

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

Input data

The conditions for simulating the Askervein flow field in neutral conditions are:

  • Digitized map covering an area of 15x19 km based on 1:25000 maps (elevation lines every 10 m). Higher resolution digitized map of 2.5x2.5 km of the Askervein hill at 1:5000 (lines every 2 m)
  • Roughness map of the 15x19 km based on 1:25000 maps. Roughness levels: 0.0002 m (water bodies), 0.4 m (build-up area) and 0.03 m (background roughness)
  • Coordinates of met masts along lines A, AA and B and at RS, CP and HT

Inlet conditions shall be based on M-O profiles fitted to RS data.

Validation data

The validation dataset is based on ensemble mean values of:

  • Fractional-Speedup-Ratio (FSR) and normalized added turbulent kinetic energy (TKE*) with respect to the reference inlet position, at 10 m above ground level along mast lines A, AA and B
  • FSR and TKE* vertical profiles at the reference (RS), hilltop (HT) and centre point (CP) positions

Velocity and TKE values will be normalized with respect to the RS position. The validation dataset includes measurements during the following runs (Taylor and Teunissen, 1987):

  • 210º: MF03-B (URS = 10 m/s, WDRS = 210º, Ri = 0.0116, 3 hours), MF03-C (URS = 10 m/s, WDRS = 210º, Ri = -0.0017, 1.5 hours), MF03-D (URS = 8.9 m/s, WDRS = 210º, Ri = -0.011, 3 hours), TU03_A (URS = 9.8 m/s, WDRS = 210º, Ri = -0.0038, 1 hour), TU03_B (URS = 8.9 m/s, WDRS = 210º, Ri = -0.0074, 3 hours).
  • 130º: MF30-A (URS = 12 m/s, WDRS = 130º, Ri = 0.0084, 3 hours), MF30-B (URS = 12.5 m/s, WDRS = 135º, Ri = 0.0103, 7 hours), TU30-A (URS = 7.8 m/s, WDRS = 135º, Ri = 0.0005, 2 hours), TU30-B (URS = 13 m/s, WDRS = 130º, Ri = 0.0051, 2 hours)
  • 90º: MF28-A (URS = 6.8 m/s, WDRS = 90º, Ri = 0.0078, 2 hours), MF28-B (URS = 6.5 m/s, WDRS = 95º, Ri = 0.0109, 2 hours), MF28-C (URS = 7.2 m/s, WDRS = 100º, Ri = 0.0133, 2 hours) and MF28-D (URS = 6.0 m/s, WDRS = 105º, Ri = 0.0167, 14 hours)

where MF runs corresponds to mean flow measurements, TU runs corresponds to turbulence runs and TK to TALA kite runs. Hence, the 90º case does not have turbulence data and will be used to assess the sensitivity of the mean flow to the wind direction variability in hill-hill wake conditions.

Model runs

The following simulation runs are requested, corresponding to the different wind directions of the measurements:

  • Run 1: 210º, fine-tuning
  • Run 2: 130º, blind
  • Run 3: 90º, blind
  • Run 4: 95º, blind
  • Run 5: 100º, blind
  • Run 6: 105º, blind

all in neutral conditions. The computational grid should include the hills behind Askervein. A grid dependency study should be conducted in order to assess the model sensitivity to the selected grid design. This study should be described in the evaluation and only the outputs from final runs should be provided.

The origin of the coordinate system should be placed at the HT position with X aligned with the incoming wind direction, Z pointing up and Y perpendicular to the XZ plane in a right-handed system.

Output data

The simulated validation profiles consist on horizontal profiles along lines A, AA and B at 10 m height above ground level and vertical profiles at RS, HT and CP position, of velocity components (U,V,W), turbulence kinetic energy (tke) and dissipation rate (tdr). The profiles should traverse the simulated domain from boundary to boundary. Hence, the required outputs are, in this order: X(m), Y(m), Z(m), U(m/s), V(m/s), W(m/s), tke(m2/s2), tdr(m2/s3). 

Use the file naming and format convention described in the Windbench user's guide with profID = prof#, where # = [A,AA,B,RS,HT,CP], i.e. 6 output files per user and model run.

Remarks

The benchmark is divided in two steps:

  • Run 1, with validation data provided together with the inputs. This simulation shall be used to fine-tune the model in order to match the validation dataset as close as possible. In order to evaluate the added value of model fine-tuning it is important that you describe how this is performed. Please report on the deviations with respect to default settings if validation data were not available a priori (blind conditions).
  • Runs 2 to 6: Based on the model parameterization of the first run, provide simulations for the other wind directions in blind conditions. The validation data will be provided as soon as the simulation results are submitted.

There are no guidelines on the definition of the computational mesh so please describe how you integrate grid dependency in the evaluation process. Bear in mind that grid sensitivity will be direction dependent.

Terms andConditions

Not applicable.