Fig. 3 Three Dimensional Graph of Position/SPL/Frequency in U=10 m/s
The error involved in the SPL measurements is less than 0.5 dB.
Hot wire anemometer and a linearizer type DIS-A were used to measure the mean velocity and the turbulence intensity inside the arrays tested in air. The hot wire probes could be moved to any location within the arrays. The hot wire system was calibrated and linearized by means of a calibration wind tunnel. The error involved in the hot wire measurements is less than 2 %.
Spectral analysis was carried out by a fast fourier transform spectrum analyzer type ONO SOKKI CF-360. It was sampling frequency of 5120 Hz for the frequency range 0-2 kHz. 1024 data points were sampled per spectrum. Unless otherwise stated, 64 spectra were averaged for each measurement. The frequency resolutions of the spectra were 5 Hz.
A miniature Pitot static tube located upstream of the tube bank was used with a Betz manometer t6 monitor upstream velocities (U') and the averaged gap velocities (U) were calculated on a basis of the minimum free flow area in the bank.
3. RESULTS AND DISCUSSION
3.1 Test Results
Using the described test facility tube banks of three tube spacings were placed in the test section. For each bank the air velocity through the bank was increased and the sound pressure spectrum was recorded at several velocities within the available range of the tunnel. A selection of results from these experiments are presented in the form of three dimensional plots of SPL/frequency/position as shown in Fig. 3. All these diagrams clearly show a broad band peak in the sound pressure spectrum. These characteristics are typical of vortex shedding (fs, 2fs) and are consistent with observations made by other experimenters investigating this phenomenon. These diagrams also show the acoustical resonance to be excited at a frequency (fa) which is well removed from that of vortex shedding (fs) and these frequencies bear no simple relationship to each other. Similar results were obtained for the geometries in the exhaust gas economizer, however, the sound pressure spectra from these banks did not reveal the characteristics vortex shedding peak (fs).
