High velocity molecular outflows with bipolar morphology are detected in association with young stellar objects within dense interstellar clouds. Recent observations suggest that the flow could be “confined to a relatively thin, swept-up shell surrounding an evacuated wind cavity” (1). A shell structure characteristic of the wind-cloud interaction had been predicted in earlier theoretical works (see for instance (2)). More recently, models with different (not shell-shaped) geometries were presented, e.g. (3).