Vane–Probe Interactions in Transonic Flows

In this article, a numerical investigation of the effect of a miniaturized five-hole probe downstream of a transonic low-pressure turbine vane row is presented. First, a numerical calibration of the probe was performed in uniform flow conditions, as is the case for any traditional calibration, for a wide range of Mach number, yaw angle, and pitch angle conditions. The effect of the probe on the general flow field throughout the turbine vane segments was then evaluated by performing a comparison between a setup with vanes only (no probe) and with vanes and probes. It was found that, as the probe traverses downstream the vane, the probe impact on the vane isentropic Mach number depends on the probe circumferential position. The highest impact was observed when the probe is located at the upper mid-passage (θ = 0.5), consisting of a relatively small reduction of the isentropic Mach number on the vane suction side of just 0.02. To assess the accuracy of the quantities “measured” by the probe, the probe-determined flow field was compared to the flow field of the vanes-only setup. A nonnegligible modification of the probe-determined local distributions of Mach number, yaw angle, and pitch angle is revealed with respect to the undisturbed flow. Further investigation involving stagnation point tracking showed that the artificial high circumferential variation of the yaw angle is not caused by a modification of the vane outlet flow angle, but is induced by nonuniform flow conditions downstream of the vanes. With knowledge of the above, a two-step correction is used to account for the effects of the nonuniformity of the flow, and its impact is evaluated on 2D and 3D flow regions. A significative effect of the correction was found on the probe-determined yaw angle, in which the difference from the vanes-only data was reduced to below 1 deg, except near the endwalls where larger discrepancies remain due to probe–endwall interactions. A shortfall of the correction was instead observed on the probe-determined Mach numbers. Finally, the pitch-wise averaged quantities were evaluated. It was observed that the highest differences between probe-determined and undisturbed data occur where radial gradients of total pressure are stronger and that the two-step correction had almost negligible impact on the pitch-wise averaged quantities.