I've had occasion to look seriously into the issue of turbulence downwind of a windfarm on several occasions. There's strong circumstantial evidence that they create substantial turbulence downwind, but little hard data.
There's also a lot of research that goes into turbulence around ship / rig structures for helo operations. Most of this involves complex CFD analysis: Qinetiq at Portsdown, near Portsmouth, England have a particularly impressive setup that does a lot of research, usually concentrating upon military helicopter / small ship combinations.
As for plumes of noncombustible gas, I've not seen anything (and confess that my instinct is to believe that the safe distance will actually be quite short due to the rapid diffusion rates. But, this should be reasonably straightforward to model in CFD, confirm this with a few static measurements, then apply a comfortable safety factor. Surely attempting to flight test this would be a bit silly? - since it'll largely consist of flying an aircraft close to the ground / structure and seeing if/when the engine fails due to oxygen starvation - not a trial I'd personally like to be responsible for!
If you are in need of somebody, I may have some colleagues who are up to this sort of flow modelling if you'd like me to ask around.
G