Accuracy of Steady Pneumatic Probes in Unsteady Turbomachinery Flows

This study shows that no additional measurement error due to unsteadiness was detected, when measuring in periodic turbomachinery flows at frequencies up to 5 kHz with steady, pneumatic probes. An experiment was designed, which consisted of abstracted rotors placed in the jet of a free stream wind tunnel. Five steady and unsteady probes were compared in the periodic, turbomachinery-like wakes at Mach numbers up to 0.8. The impacts of unsteadiness, probe head size and shape, and distance between probe and rotor were systematically investigated at up to 90 operating points. Within the limits imposed by unsteady pressure transducers, the experiments demonstrated the absence of a frequency-dependent effect on the measurements by comparing the time-averaged measurements of identically shaped steady and unsteady probes. Measurements with hemispherical five-hole probes of two sizes and kielhead probes at the same location deviated significantly due to different interaction with the upstream rotor. Distance variations between probe and rotor showed that each combination of probe and flow should be evaluated individually. The study concludes that pneumatic probes offer a reasonable means to measure the mean flow downstream of a rotor, accurately reproducing time-averaged values. However, careful individual evaluation of probes is essential to minimise measurement uncertainty.