"If a conductor is carrying high alternating currents, the distribution of current is not evenly distributed throughout the cross section of the conductor.
This is due to two independent effects known as the "Skin Effect" and the "Proximity Effect".
If a conductor is comprised of a large number of concentric circular elements or strands, those strands at the center of the overall conductor will be enveloped by a greater magnetic flux than those at the outside. Thus the self-induced back e.m.f. will be greater towards the center of the conductor thereby causing the current density at the center to be less than at the conductor surface. The extra concentration at the surface is known as the skin effect and results in an increase in the effective resistance of the conductor. The magnitude of the skin effect is influenced by the frequency, the size of the conductor, the amount of current flowing and the diameter of the conductor.
The proximity effect also increases the effective resistance and is associated with the fields of two conductors which are close together."
Reference "Electrical Cables Handbook" D. MCAllister
This 880 page book is out of print but hopefully it is in your company library or in an university engineering library near your location.
As you might imagine, the mathematical analysis is quite complex because of all the possible variations so your question regarding the skin effect of a strand or strands randomly moving in and out of the center of a few carefully defined conductors might be a great topic for a Masters or Ph.D. Thesis in Electrical Engineering, Engineering Physics or Theoretical Physics.
As far as Litz wire is concerned, take a look at this web page
www.litz-wire.com/technical.html
Kindest regards,
Peter J. Stewart-Hay
Principal
Stewart-Hay Associates
www.Stewart-Hay.com 
