Efficiency losses resulting from base pressure deficit and energy separation over the speed range

Intrinsic to many axial flow turbomachines, with either compressing or expanding flow regimes, is consideration of the effects of compressibility. Both subsonic and supersonic speed ranges are considered in this investigation. Subsonic surface base pressures, and wake energy separation are principal manifestations of the same phenomenon and are a direct result of periodic von Kármán vortex shedding. This is the principal cause of both wake energy separation and the related subsonic base static pressure deficit. At high subsonic speeds the wake flow was supposedly isothermal. Instead a 17oC temperature difference between the wake outer edge and its centre line was observed. This time-averaged temperature separation was a manifestation of the energy separation (Eckert-Weise) effect. At supersonic speeds the trailing edge base pressure and the energy separation in the downstream wake, exhibit different characteristics from the subsonic behaviour and should be treated differently. Shock waves from a blade trailing edge may impinge on the adjacent suction surface adversely affecting the downstream boundary layer. Supersonic flows usually involve the impingement of shock and expansion waves and this can be a steady flow effect. However other modes in the wake are possible. These include von Kármán vortex shedding from the confluence region of the wake. This is not the only form of shedding and at times anomalous (or “exotic”) vortex shedding has an important role to play.