Section 5.25 Spiral Classifiers

Basically, the purpose of drill collars is to furnish weight on bit. However, both size and length of drill collars have an effect on bit performance, hole deviation, and drill pipe service life. Drill collars may be classified according to the shape of their cross-sections as round drill collars (conventional drill collars), square drill collars, or spiral drill collars (drill collars with spiral grooves). 

This node behaves as an unstable node since profiles originate from it. Mathematically, however, it is classified as an unstable focus due to characteristic spiraling (see Section 3.8.1). 

Winfree discovered scroll waves [60] and classified them [61]. The simplest cylindric scroll wave can be produced by a parallel shift of a plane spiral wave in the direction perpendicular to its plane. A shift in the phase of rotation between cross-sections perpendicular to the axis (filament) of the scroll creates a twisted scroll wave which is unstable in a uniform medium, and evolves back to a cylindric scroll. 

---