What I'm going to say is my own opinion. I don't claim it's true, that's just my understanding of wake turbulences... I'm eager to learn, so feel free to correct me if I'm wrong :)
I'm assuming wake turbulence is moving air (airflow) caused by an aircraft flying ahead, wether airflow moving down, up, sideways or spiralling - I understand the pressure difference - Without going deep in particles motion (air molecules) I think you just have to consider four things :
your position behind a the leading aircraft : right behind its fuselage, right behind one of its wing tip, beyound its wingtip trail, from the axis of the leading aircraft.
your distance behind the leading aircraft : a few feet just behind (without colliding with) or miles away ?
wake size increases as you go far behind the aircraft producing the wake turbulence, but its strength decreases accordingly.
The point 3 tells you where NOT to position your aircraft behind another. Especially at a (relative) location where the wake is still powerfull, and its size is large enough to affect your controls surfaces and wing envelope, and send your aircraft in an uncomfortable motion, brutal gain or lost of lift, differential lift, etc.
So, right behind another aircraft, you can fly without been affected (much) by the wake turbulence, because the wake size is not large enough to send your aircraft sideways, however, you still encounters slight turbulences, especially if you fly just right behind the wingtips. Worst situation is when you fly several hundreds feets behind the aircraft : large volume of strong spiralling airflow that you can't visualize. Miles aways, the airflow velocity is assumed to have dissipate enough because of its lost of strength due to expansion and air resistance.
Of course, the wake turbulence of a Cessna is not the same of an A380. Weaker and smaller for a Cessna, that dissipates pretty quickly. This leads me to introduce point four :
- The size of the plane creating the wake, and the size of the following plane. Surprisingly, you can put an A350 right behind another A350, but you can't put a Cessna right behind an A350. This, because of the three points above : The Cessna will likely be affected by the slightest amount of airflow while the A350 wont much, because of surfaces size (wings/ailerons) The wake (especially the wingtip vortex) hasen't grow enough to hit the following A350 hard enough.
However, that doesn't mean there aren't risks. Main risk in such close formation is vibration. Test pilots are trained to identify their origin and deal with.
And also, I don't have a comparable aircraft in size, so I'll take a DC10, or a B52 : I think you can't put such earlier generation of aircraft right behind an A350 (or 787) because of Wing technology (this related to the question "does the A350 has something special ?") Old aircraft have much less flexible wings, that makes them really sensible to brutal airflow. I'm not a physicist, but more flexible wings better distribute the load factors along said wing, considerably reducing vibrations and fatigue.
That's why wake turbulences are more dangerous at lower speed : the wake can grow enough to compromise the flight enveloppe of the following aircraft even at the tightest separation. That's why wake turbulence are a serious issue at lower altitudes where the moving airflow is really dense and affect surfaces greatly. Logically, wide wakes should dissipate more quickly in dense atmosphere, but the (tiny) spiralling vortex can last several minutes and is still very dangerous especially for smaller aircraft.
Fighter jets in airshow and flight formation :
a) They fly at high speed => not enough expansion to affect nearby followers
b) tandem formation requires lining up with leading aircraft => good distribution in differential lift.
c) in tandem formation, followers are usually below (rarely above) the preceding aircraft.
Finally, a Cessna control surfaces (and flaps) are more sensible than the ones that equip a 777. Never get a Cessna below and behind a 777 in a five to ten minutes timespan unless you know where the wake turbulence path has drifted and/or landed.
Side note : I'm not a pilot. I'm just interrested in how an aircraft flies. There are tons of documents out there about wake turbulences, wake vortices, stall schemas, etc. Some are contradictory, some are complementary. At a molecule level, the magic of an airflow looks like a perfect example of the theory of chaos. But the above is the summary of what I found here and there. That's no science, just opinion.