>>1652147>I mean the exact type of enclosure turbofans use. Regular turbofans have a short and wide inlet then a cowl after it.The inlet on modern turbofans is often shorter and wider than older ones, thanks to advances in aerodynamics and more extensive use of transonic/supercritical flow (where small pockets of supersonic flow may occur between fan blades, but are carefully managed in order to minimize the strength of resulting shock waves), but they still generally use a subtle divergent section to slow the mach number before it reaches the fan.
The engine core then usually occupies significant volume in the vicinity of the fan nozzle (where the cowl ends). So that section of the bypass stream is highly convergent, despite minimal apparent convergence of the outside of the cowl. And since flow is subsonic within the cowl, the duct/nozzle geometry in turn has a significant impact on the flow and pressure upstream in the vicinity of the fan. A simple ring around the perimeter of the fan would produce dramatically different thrust characteristics.
