ASM-TurbOPark

This model is based on the work of Stefan Emeis. It was converted for use in Openwind with the help of Richard Foreman and Beatriz Canadillas.

The thrust coefficients of the turbines are convoluted using a skewed Gaussian kernel. The resulting surface is then used to apply a drag on the lower atmospheric boundary layer.

Figure 1: ASM TurbOPark settings

• Look for upwind background roughness from layers - check this option to override the default roughness length using GIS layers

• Background Roughness Length - this is the default background roughness length which is used in the absence of roughness length data coverage or when the above option is not checked.

• Check Roughness Map Out to - the distance from the turbines that the roughness length is sampled.

• Monin-Obukhov Length - this value can be used to set the default Monin-Obukhov length (MOL). The value zero is reserved to mean neutral. In general, this value is left at zero because MOL is time-varying and so should be provided in the met mast time-series data and activated in the Time Series Energy Capture settings.

• Skew Downwind Gaussian Kernel - this makes the stream-wise convolution of each turbine thrust coefficient asymmetrical with higher values moving the peak of the Gaussian function less downstream and flattening the upwind nose of the Ct surface.

• Position Ct maximum at turbine - this takes the skewed Ct surface and moves it upwind so that the maximum Ct is at the turbine hub. Whilst this tends to look more intuitively correct, it results in too much of a slowdown 2.5 rotor diameters in front of the turbine, even with high levels of skew.

• Turbine Spacing Downwind Sigma - the value of sigma or scale parameter used in the Gaussian kernel when convoluting downwind. Specified in rotor diameters.

• Turbine Spacing Crosswind Sigma - the value of sigma or scale parameter used in Gaussian kernel when convoluting crosswind. Specified in rotor diameters.

• Number of cells per max rotor - by default this is set to a value of 1.0 which is suitable for most purposes and allows the wake model to run fast. However, it means that turbines are represented in the ASM part of the model to be located at the closest grid point on a grid that has a resolution equal to the largest rotor diameter currently under consideration (i.e. extant within the energy capture run underway). Increasing this value increases the fidelity of the model but results in longer run times.

• Dimensionless Constant C - please see the literature by Emeis. Should be left as is.

• Minimum dZ - this is effectively a scale parameter which determines the ASM wake effect within the wind-farm.

• Waked wind speed is upstream by X Rotor Diameters - turbine power curves can be thought of as applying to the wind speed 2.5 rotor diameters upstream. This can be used to moderate the slowdown in front of the turbine.

• Model recovery of boundary layer slow down - this should only be unchecked when investigating the shape and size of the effect of the Ct surface. For energy capture it should always be checked on.

  • Wake Recovery dZ - This is another scale parameter; this time for the recovery of the slow down caused by the convoluted Ct surface. This should generally be around 1.0 which is the current default.

• Combine Area Effect with Wake Model - method by which TurbOPark wind speed deficits and ASM deficits are combined.

  • Maximum - the resulting wind speed deficit is the maximum of the wind speed deficits (ASM and TurbOPark).
  • RSS - the resulting wind speed deficit is the root-summed-square of the two wind speed deficits.
  • Product - the resulting wind speed is the product of the wind speed fraction due to ASM (1-deficit) multiplied by the wind speed fraction due to TurbOPark.

• TurbOPark Parameters - check this box to enable the TurbOPark portion of the ASM TurbOPark array loss model.

• Initialise ASM Ct and TI from a pre-run of this model - runs the TurbOPark model first in order to initialise the thrust coefficient and ambient turbulence intensity values used in the ASM model.

Figure 1: Single turbine ASM effect with 3RD downwind and crosswind Gaussian convolution. Red area is 8m/s. Purple is 7.51m/s. Wind from the West. Circle is 2.5RD radius. Neutral stability.

Figure 2: Single turbine ASM effect with 9RD downwind and 3RD crosswind Gaussian convolution. Red area is 8m/s. Purple is 7.83m/s. Wind from the West. Circle is 2.5RD radius.

Figure 3: As in figure 2 but with a skew of 2.0. Purple is 7.75m/s.

Figure 4: As in figure 2 but with a skew of 5.0. Purple is 7.7m/s

Figure 5: As in 4 but with the convoluted Ct Surface shifted West so that the peak coincides with the turbine hub location.

Figure 6: As in figure 4 but with wake modelling and recovery enabled.