Tom L. writes:

The big question is "why does the wake turbulence descend?"

Because it's the downwash from the aircraft's wings. Aircraft stay in the air
by pushing air downward. As the wings pass through still air, they twist the
air downward as they pass. The force required to do this engenders an equal
and opposite force that raises the wings--lift, in other words.

No downwash = no lift.

Turbulence is mostly from wingtip vortices. The vortices exist because air is
twisting over to the top of the wings from the bottom. The vortices are
necessary in order to accommodate the swath of downwash behind the aircraft,
which is descending in relation to the still air on either side of the
aircraft's path.

Is the air volume inside the vortices denser than surrounding air?

Density has nothing to do with it. The air has been pushed downward by the
wings.

Probably not. So the descent is probably not due to gravitational
force.

No, it's not gravity. The air descends because the wings pushed it down.

I am no expert on fluid dynamics and have no access to texts that
answer the question (if there are any), but figure 7-3-5 in AIM is
interesting - it shows a wake sinking at several hundred fpm
immediately after an aircraft, but than stabilizing at several hunderd
feet below the flightpath, i.e. no further sink. This might indicate
that the sink is due to wing downwash.

It is.

If that is the case, than
1. Wake turbulence in steep turns will not move just downward, but
down and out, that is: opposite lift.

Yes.

2. The speed at which it moves will depend on downwash - it's speed,
intensity, strength (?) I don't know which term would be appropriate
here. Whatever it is, it might be much smaller for GA aircraft than
for large aircraft.

The product of air mass times downwash acceleration has to be the same as the
product of aircraft weight times gravity. So a larger and heavier aircraft
produces a larger downwash, albeit not necessarily a faster one.

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