Unsteady perturbation generator

With the aim at simulating potential interaction in turbomachines, the "quasi steady” shock wave was put into oscillations using a rotating elliptical cam placed at x=625mm in the reference system of the bump. A DC motor was used to rotate the cam up to 15,000RPMs in order to generate pressure per­turbations up to 500Hz. The rotating speed was monitored using an optical encoder located directly on the shaft of the motor. Rotating speed fhctuations and time drift were measured under ±0.024% in the worst case. Furthermore, a TTL pulse generated by the motor was used as a reference signal during unsteady pressure measurements and Schlieren visualizations in order to cor­relate both measuring techniques.

1.2 Measuring techniques

Steady state pressure measurements were performed using a 208-channels ’low speed’ data acquisition system. The scanners used feature a pressure range of ±100kPa relative to atmosphere with an accuracy of ±0.042% full scale. Taking into account the digital barometer, the overall accuracy for steady state pressure measurements is about ±43.5Pa. The sampling frequency and sampling time were respectively set to 10Hz and 200s in order to ’capture’ the lowest frequencies.

Additionally, a 32-channels high frequency data acquisition and storage sys­tem was used for unsteady pressure measurements. Accounting for the res­onance frequency of the capillarity pipes between the bump surface and the transducer, the sampling frequency was set to 8kHz with a low pass filter at 4kHz to avoid bias effects. Each channel was connected to a fast response Kulite transducer and individually programmed to fully use the 16bit AD con­version. A static calibration of all fast response transducers was performed prior and after the measurements in order to reduce the systematic error related to the drift of the sensitivity and offset coefficients. Furthermore, a dynamic calibration was performed on all pressure taps in order to estimate the damp­ing and time delay of propagating pressure waves through the capillarity tubes. The unsteady pressure measurements were thereafter corrected to account for the above estimated damping and phase-lag.

Finally, a conventional Schlieren system connected to a high speed CCD camera was used to monitor the shock motion throughout the whole test section height up to 8kHz. A special feature of the camera allows the display of the TTL signal position directly onto the pictures for referencing purpose during later post treatment. The sampling frequency and shutter speed of the camera were optimally set up depending on the perturbation frequency in order to ob­tain approximately 20 pictures per unsteady cycle (up to 500Hz). The spatial accuracy based on the camera resolution and optical system was estimated to be around ±0.33mm. However, it should be reminded that the processed im­age is an integration of density gradients throughout the channel’s width.