Implementation Issues

A block diagram of the apparatus used in measurements of TTF of a static pressure tapping and the pneumatic tubing is presented in Fig. 1.

A swept sine wave is generated which covers the range of frequencies of interest, and this is fed to an audio amplifier and loudspeaker. For the blade flitter case, the frequency range used was 0.1 Hz to 50 Hz, with a sweep period 0.75 second when logging sets of 2048 samples at 800 Hz. The loudspeaker produces pressure fluctuations with roughly the same wave forms as the input voltage. The loudspeaker is connected to a small cavity via a short rubber tube

to isolate mechanical vibrations. A reference pressure transducer is directly connected to the small cavity and used to record the pressure inside the cavity. A static pressure tapping used in the unsteady pressure measurement (0.3 mm diameter for blade flitter case) is also connected to the cavity. A length of plas­tic tube is used to connect the static pressure tapping with the other (off-board) pressure transducer as would be done for the aerodynamic measurements.

In the blade flitter case, the reference transducer (type: Sensym 113LP01d – PCB, -1-+1 mbar range) uses the ambient pressure as a reference, and the test transducer (type: Sensym 142C01D, 0-1 psi range) uses the total pressure of the setting chamber of the wind tunnel as a reference, which is the same as that in unsteady pressure measurements. The tubing system includes the trans­ducer’s internal volume, the connector, the Portex plastic tubing, and the brass tube with six static tappings – the tapping style for the blade flitter case.

The definition used to calculate the complex transfer function is:

1 M

M j=i

where M is the number of sets used to average TF(f). The Fourier coefficients A(f) and B(f) are defined above.

In order to obtain smooth transfer function desired for correcting pressure signals, M, can be greater than 20. A Hanning window function is used to reduce the effect of the finite data length, which has been found to improve the quality of the results.