Update Empirical Gaussian default deflection_rate

[misi9170]

When the Empirical Gaussian (emgauss, EmG) model was first introduced in #628, we provided a rough set of default parameters. Those were then updated in #723 as we gained more experience using the EmG model. One of the parameters that was updated was the deflection_rate parameter of the wake deflection model, which had a value of 15 originally (in #628) and was updated to a value of 30 in #723. The deflection_rate parameters describes how "quickly" (in terms of downstream distance) the wake approaches its maximum displacement due to yaw misalignment or nonzero tilt of the rotor. For details, see the EmG wake deflection model documentation, where the deflection_rate parameter is denoted $c$.

The current value of 30 for the deflection_rate is overly pessimistic about the efficacy of wake steering---while the wake deflects appropriately, it does so too "slowly", so the wake has not deflected sufficiently by 5 rotor diameters downstream to produce a net gain in a two-turbine wake steering scenario. To better reflect the results produced by the GCH model, which we take to be our best model for describing wake steering in ideal conditions in small farms, this PR reduces the deflection_rate to 22 (approximately halfway between the original value of 15 and the current value of 30).

As always with the EmG model, we advise users to tune parameters to match their observations, or at least check carefully that the default values are in good agreement with their data.

Results for a two-turbine farm of NREL 15MW turbines with 500m spacing (~4D) in aligned flow at a wind speed of 8m/s are shown below. The number reported in brackets is the percentage improvement in farm power (summed over both the upstream and downstream turbine) when the upstream turbine is yawing at 30 degrees, over the non-wake steering case (upstream turbine at 0 degrees).

GCH benchmark [3.37%]

EmG with deflection_rate of 15 (and all other parameters unchanged) [6.78%]

EmG with deflection_rate of 30 (and all other parameters unchanged, as is currently the default) [0.34%]

EmG with deflection_rate of 22 (and all other parameters unchanged, as would be the new default) [2.85%]

Thanks to @erickaloz for finding and pointing out the issue with the current default parameters!