Jet engine core axial compressors (that of subsonic turbofans anyway) are designed to keep air flowing the right way at max RPM(or close to it). Below that, the airflow slows more and more, and becomes unstable. In other words, the compressor can no longer compress the large air volume flowing into it. To prevent this, variable inlet vanes are used to reduce the airflow volume into the compressor at low RPM.
My question is: why was this method chosen? When you're running the engine at low power/RPM, the closed vanes will create drag by presenting their blunt face to the air. So why not use a streamlined variable geometry inlet which can forespill the excess air with less drag? such as the following:
TOP: low RPM, a sliding annular door narrows the annular gap
BOTTOM: high RPM, the door slides back, widening the annular gap