 base pressur subson speed presenc superson jet base pressur subson speed presenc superson jet paper present experiment investig superson jet base pressur bluff cylind uniform subson flow ratio jet diamet base diamet 0 1875 jet stagnat pressur slight under-expans jet increas base pressur larger jet stagnat pressur base pressur reduc simpl theori base momentum integr depend base drag jet free stream speed dimens jet base 