On the interpretation of measured profile losses in unsteady wake - turbine blade interaction studies

The interaction of wakes shed by a moving blade row with a downstream blade row causes unsteady flow. The meaning of the free-stream stagnation pressure and stagnation enthalpy in these circumstances has been examined using simple analyses, measurements, and CFD. The unsteady flow in question arises from the behavior of the wakes as so-called negative jets. The interactions of the negative jets with the downstream blades lead to fluctuations in static pressure, which in turn generate fluctuations in the stagnation pressure and stagnation enthalpy. It is shown that the fluctuations of the stagnation quantities created by unsteady effects within the blade row are far greater than those within the incoming wake. The time-mean exit profiles of the stagnation pressure and stagnation enthalpy are affected by these large fluctuations. This phenomenon of energy separation is much more significant than the distortion of the time-mean exit profiles that is caused directly by the cross-passage transport associated with the negative jet, as described by Kerrebrock and Mikolajczak. Finally, it is shown that if only time-averaged values of loss are required across a blade row, it is nevertheless sufficient to determine the time-mean exit stagnation pressure.