Cylindrical three-hole pressure probe calibration for large angular range

This paper presents a brand new method to calibrate a Cylindrical Three-Hole Probe (CTHP) in a “non-nulling” operating mode. This calibration method employs the definition of a new normalization factor used in the calibration coefficients of the probe. By means of this new normalization, singular points are prevented from appearing in the calibration coefficients. In addition, the angular range attainable when using this new calibration is ±70^°, while the typical angular range of traditional calibration is approximately ±30^°. To validate this study, an uncertainty analysis of the probe using this new method is carried out, resulting in significantly low uncertainties for the whole angular range of the probe. Complementarily, the influence of the angular distance between the holes of the probe on both angular range and maximum uncertainties is also performed in the paper. From this analysis, it is established that the optimal angular separation for the holes should be chosen to be between 50 and 60 degrees. Finally, to illustrate the usefulness of this procedure, a set of experimental measurements were conducted downstream of the rotor of an axial turbomachine, using the new normalization factor in a CTHP with an angular distance of 60 degrees between the holes. The results revealed that the complex, unsteady flow within the blade passages can be perfectly captured, including those regions with high variations in the flow angle, like the shear layers of the wake fluid or both casing and hub boundary layers. In summary, this newly developed calibration method enables the measurement of large variations of the flow angle, in particular, up to 140 degrees, increasing in 80 degrees the typical angular range of the traditional calibration.