The measurement of near wall flows using pneumatic wedge probes
The three hole, wedge-type pneumatic pressure probe represents a robust traverse probe design which is widely used for total and static pressure and yaw angle measurements in turbomachinery. However, unsteady flows are incorrectly averaged due to pneumatic meaning errors in the pressure pipes. Wedge probes also fail to measure the correct static pressure when operating in close proximity to a wall through which the probe is inserted. Thirdly, the aerodynamic calibration obtained for a wedge-type probe in a closed wind tunnel differs appreciably from that obtained in an open jet. If not corrected, these errors will corrupt any calculation of turbomachinery blade row performance. In this investigation, the second and third effects described above have been addressed. A factorial experiment was completed in which the influence of seven variables on the wall proximity effect was quantified. Flow visualisation studies were performed to understand the responsible flow mechanisms. Two regions of re-circulating flow were identified in the probe wake, the structure of which depended on the probe immersion. Similar re-circulatory flows were resolved from three-dimensional computational fluid dynamics (CFD) calculations of the flow over a wedge probe. A link between the probe wake re-circulations and flow over the wedge faces was established. Based on this understanding of the flow structures, a model was developed from which the wall proximity effect could be predicted for a given set of conditions. Wedge probe calibrations were completed in a closed wind tunnel and in two open jets. Discrepancies in the static pressure coefficient and yaw angle sensitivity results were found. These were partially explained in terms of modifications to the probe wake structure which occurred when the probes were calibrated in the open jet facilities. Procedures for correcting the wall proximity effect and for avoiding the facility dependence of wedge probe calibrations were developed from this understanding of the flow mechanisms involved. Based on the findings of this investigation, a novel wedge probe was designed to minimise the wall proximity effect. This probe demonstrated a reduction in the wall proximity effect, from 20% dynamic head with current designs, to 3% dynamic head at flows typical of high speed turbomachinery.