Gauss velocity deficit fails with 0 m/s inflow

Simple gauss fails at 0 m/s

If you run the Gauss velocity deficit model without secondary_steering, yaw_added_recovery, enable_transverse_velocities, the model returns NaN when you provide an inflow of 0 m/s.

How to reproduce

I took the gch.yaml in the inputs folder, and changed:

  # Can be "true" or "false".
  enable_secondary_steering: false

  ###
  # Can be "true" or "false".
  enable_yaw_added_recovery: false

  ###
  # Can be "true" or "false".
  enable_transverse_velocities: false

Then, a simple script:

fmodel = FlorisModel("inputs/gch.yaml")
fmodel.set(layout_x=[0, 500.0], layout_y=[0.0, 0.0],wind_directions=np.array([270.0]), wind_speeds=[0])
fmodel.run()
turbine_powers = fmodel.get_turbine_powers() / 1000.0

Relevant output

turbine_powers returns nan for each of the two turbines, rather than expected 0.

The problem is coming from :
sigma_z0 = rotor_diameter_i * 0.5 * np.sqrt(uR / (u_initial + u0))

which returns nan when u_initial and u0 are 0.

Floris version

FLORIS v4.2.2 (actually discovered the issue in older version, but then separately installed 4.2.2 and saw it still exists).

Start by searching the FLORIS source for the sigma_z0 calculation shown in the issue, then reproduce the case using inputs/gch.yaml with the three steering and transverse-velocity options disabled and a 0 m/s inflow. Done means the two-turbine example returns zero power values rather than NaN.