UNSTEADY PRESSURE MEASUREMENT WITH CORRECTION ON TUBING DISTORTION

Yang, D. B. Sims-Williams, and L. He

School of Engineering, University of Durham, Durham, DH1 3LE, U. K.

Abstract A method of correcting distortion in measured unsteady pressures using a tubing system and off-board pressure transducers is described. This technique involves the frequency domain correction using the known tubing transfer function and not only corrects the amplitude distortion, but also eliminates the phase shift. The technique is demonstrated for surface pressures in a turbomachinery blade flitter case, and for wake measurements for a vortex shedding case.

1. Introduction

In recent years, computational methods for predicting unsteady few through turbomachines have been fully developed. For the validation of these codes, systematic, accurate, and detailed unsteady pressure experimental data are needed. Most previous measurements are confined to the use of miniature high-response pressure transducers buried in the blade surface (largely on 2D sections) of linear oscillating cascades (Buffum 1993, Carta 1978 and Fleeter, 1977), annular cascades (Bolcs and Korbacher, 1993, Fransson 1990) and ro­tating machines (Manwaring 1997, Frey 2001, Minkiewicz 1998). Due to the transducer size limitation and airfoil contour preservation as well as expensive cost, only a limited number of unsteady signals can be obtained. Unsteady (static and stagnation) pressure field patterns are not obtained; these could be used to improve understanding of the few, to identify modeling limitations, and to aid future development for both aeromechanic and aerothermal (e. g. unsteady loss) applications. With embedded transducers, the movement of the blade subjects the transducer to an acceleration, for which an extensive calibra­tion and correction is required. Various installation configurations have been designed to isolate the miniature pressure transducers from the airfoil strain and centrifugal loads to improve the durability. Improved transducer charac­teristics are desired to diminish temperature sensitivity. In order to provide the required spatial resolution of the unsteady few measurements at blade sur-

521

K. C. Hall et al. (eds.),

Unsteady Aerodynamics, Aeroacoustics and Aeroelasticity of Turbomachines, 521-529. © 2006 Springer. Printed in the Netherlands.

faces, various optical measurement techniques (pressure sensitive paints – PSP, doppler sensors, micromachined fabry-perot pressure sensors and so on) were developed. However, every method requires a complicated optical technique and expensive equipment. These issues can be avoided by using off-board pressure transducers. The blade can be instrumented by detailed static pres­sure tappings, which are connected to the off-board pressure transducer by the pneumatic tubing. This approach makes economical use of pressure transduc­ers. However, the tubing system, characterized by the tubing length, its internal diameter, and the transducer internal volume, introduces a distortion of the un­steady signals. In the area of turbomachinery aeroelasticity, this distortion of the unsteady signal was generally either neglected because of low frequencies and short tubing lengths (He & Denton, 1991), or it simply was corrected for phase lag and amplitude attenuation for a certain tubing length (Bell and He, 2000). In the present work, a correction method is used which is more gen­erally applicable in that it corrects phase lag and amplitude for all frequencies using a measured transfer function for each tube.

In contrast to the low reduced frequencies for blade flitter, in the case of forced response, higher frequencies associated with higher order modes can be excited. Even for the low modes of blade flitter applications, higher fbw ve­locities at more realistic conditions require high physical frequencies to reach realistic reduced frequencies. If off-board pressure transducers are used to measure unsteady signals, these signals will be distorted by the pressure mea­surement system, and a correction must be performed. In the present paper, a tubing transfer function approach involving a frequency domain correction is described, typical transfer functions are presented, and the correction tech­nique is demonstrated for the tubing system in isolation, for surface pressures in a turbomachinery blade flutter case, and for wake measurements for a vortex shedding case.