Energy separation and base pressure in the wake of a circular cylinder

Turbomachinery flows are inherently unsteady. While this is necessary for the machine's operation, it also creates flow inefficiencies. In turbines that have blades with thick trailing edges an important source of unsteady flow is the shedding of vortex streets into the blade wake. The vortex streets cause a separation of the wake flow into areas of high and low energy. This energy separation is non-isentropic and is therefore a source of loss. Vortex shedding also causes the base pressure to fluctuate around a low level. A simplified wake, created by a circular cylinder in compressible cross flow, has been used to model the energy separation and base pressure fluctuation. Time-resolved total temperature and pressure measurements were made six cylinder diameters downstream of the cylinder along with the time-resolved surface pressure. These measurements have then been used to investigate the creation of entropy, and hence loss, in the wake flow and the effect vortex shedding has on the base pressure. The results showed that entropy creation is associated with vortex shedding and that vortex shedding has a very significant effect on the surface pressure over a large proportion of the cylinder circumference.