This may not be 100% accurate in nomenclature, but it gets the point across. Laminar flow is uniform and very easy to model mathematically, hence the reference to its "clean-ness" (e.g. clean [simple yet functional] design, clean [elegant] math proofs, etc). Its opposite: turbulence. laminar:turbulent::clean:dirty. You get turbulence/wake when there's something upstream of you disturbing the flow patterns. Air has less potential energy in it once it has been disturbed (the random motion of turbulence detracts from it's ability to generate pressure differences via flow manipulation). Riding in dirty air happens when the rich man's yacht in front of you sails upwind of your own personal rich-mans-yacht. Your vessel can still sail, but will sail a little bit slower, due to there being less available energy in the wind for you to tap into. This becomes a problem w/ turbines for obvious reasons (ask yourself why they don't just put multiple turbines all on a spoke for cost-of-installation savings, or even why they always seem to choose 3-bladed turbines [a very cool/interesting mathematical optimization problem!]).