Experimental Investigation of the Periodic Unsteady Transonic Flow Field Around a Compressor Blade by Means of Particle Image Velocimetry (PIV)

[摘要] In the present article, a study of the time-dependent transonicflow field around an isolated airfoil in a Laval nozzleis presented. In order to obtain instantaneous flow field informationall over the flow around the blade, the ParticleImage Velocimetry (PIV) method has been employed allowingone to obtain velocity data in an illuminated light sheetwith a high data acquisition rate and high spatial resolution.From these instantaneous PIV measurements the mean velocityfield and turbulence quantities of the flow easily canbe obtained by statistical means.A single compressor blade with a NACA 3506 profile anda chord length of 78 mm is centrally mounted in the testsection. A vibration system driven by a hydraulic motor isexternally attached to the blade. The rotational motion ofthe motor is translated to a plunging blade vibration perpendicularto the chord via an eccentric shaft and connectingarm. The vibration frequency can be varied between 0 and100 Hz, and the average amplitude is approximately 0.4 mm.The entire blade-motor assembly is mounted to the tunnelsidewall via a circular disc to allow easy adjustment of theinlet flow incidence angle. The exit pressure of the nozzle canbe varied by a rotating flat plate (15 mm high, 39 mm wide,and 2 mm thick) that is mounted approximately 5 times thechord length downstream of the blade mid-chord. The rotational frequency of this assembly can be varied between 0 and100 Hz, producing upstream-running waves at the doubledfrequency (i.e., 0 to 200 Hz). The above excitation systems canbe precisely synchronized and the phase lag between themcan be freely varied. This allows for unsteady measurementsto be conducted in the presence of only the downstream perturbation,only the blade vibration, or a combination of thetwo for different phase angles.For the first test series the existing PIV measurement systemat Turbomachinery Institute (LTT) has been applied tomeasure the transonic steady flow field around the compressorblade. In another test series, measurements of the time-dependentperiodic flow field were conducted by means ofPIV. This measurement series consisted of three subtasks:the measurement of the unsteady flow field with only backpressure variation, with only blade vibration, and finallywith a combination of both back pressure variation andblade vibration. These results quantify not only the unsteadymotion of the normal shock for each case on the suction sideof the blade, but also give a detailed insight into the instantaneoustransonic flow field around the compressor blade.Furthermore, the results of the set of unsteady measurementswith both back pressure variation and blade vibration(3) is compared with the combination of the results of the setsof unsteady measurements of only back pressure variation(1) and only blade vibration (2) to validate the superpositionprinciple.Finally, phase averaging of the instantaneous flowquantitiesproduced a large database for statistical treatment (e.g.,turbulence) and thus the ability to compare the averaged resultswith those from traditional measurement techniques.

[学科分类] 力学，机械学