Introducing viscous drag and fluid mechanics into what shapes a spiral galaxy. Astrophysicists avoid fluid mechanics like the plague, but understanding this energy disapative force is the only way to understand how spirals work. Shortcuts only lead to weird and fantastic stories, many of which are accepted in popular descriptions. Spiral flow patterns are common here on earth (some, like hurricanes are too common) and so this topic deserves an earthly description.
From Torus to Spiral Disk – Galactic Spiral Structure (Part 3)
