A better understanding of unsteady flow phenomena encountered in rotor-stator interactions is a key to further improvements in turbomachinery. Besides CFD methods yielding 3D flow field predictions, time-resolving measurement techniques are necessary to determine the instantaneous flow quantities of interest. Fast-response aerodynamic probes are a promising alternative to other time-resolving measurement techniques such as hot-wire anemometry or laser anemometry. This contribution gives an overview of the fast-response probe measurement technique, with the emphasis on the total system and its components, the development methods, the operation of such systems and the data processing requirements. A thorough optimization of all system components (such as sensor selection and packaging, probe tip construction, probe aerodynamics and data analysis) is the key of successful development. After description of the technique, examples of applications are given to illustrate its potential. Some remarks will refer to recent experiences gained by the development and application of the ETH FRAP ® system.
The goal of this work was the investigation of the unsteady flow field existing at the exit of the impeller of a centrifugal compressor and the critical comparison of velocity measurements with two systems based on totally different measurement principles. The paper focuses on the measurement concepts, the accuracy and the error quantification.
A one-sensor fast-response probe (FRAP® system) and a commercially available laser Doppler velocimetry (LDV) system were used to perform wall-to-wall traverses and to measure the two main time-resolved velocity components. The test rig was run at the best point of the operating line, at a tip Mach number of 0.75.
The mean levels and fluctuations of the velocity vector components were compared. Over the middle, 70 per cent of the channel width, the FRAP® and LDV measurements typically agree within 2–5 per cent of the momentary local flow velocity in terms of both the radial and the circumferential velocity components. The time-mean distributions agree within typically 0–4 per cent.
Attention was focused on the near-hub and near-shroud layers where both measurements are affected by wall proximity errors. Largest discrepancies (of 10 per cent) in the time-mean velocity occurred at the shroud where velocity component values could differ by as much as 20 per cent momentarily. These discrepancies should be viewed in relation to the peak-to-peak amplitude of the ensemble-averaged velocity fluctuations which was of the order of 25 per cent in the present compressor.
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