An experimental study is described to investigate the negative effects of the tip clearance gap on the aerodynamic and acoustic performance of axial turbomachines. In addition to the increased broadband levels reported in the literature when the tip clearance is enlarged, significant level increases were observed within narrow frequency bands below the blade passing frequency. Measurements of the pressure and velocity fluctuations in the vicinity of the blade tips reveal that the tip clearance noise is associated with a rotating flow instability at the blade tip, which in turn is only present under reversed flow conditions in the tip clearance gap. A turbulence generator inserted into the tip clearance gap is found to be effective in eliminating the tip clearance noise and in improving the aerodynamic performance.
With the objective of reducing the broadband noise from the interaction of highly turbulent flow and airfoil leading edge, sinusoidal leading edge serrations were investigated as an effective passive treatment. An extensive aeroacoustic study was performed in order to determine the main influences and interdependencies of factors, such as the Reynolds number, turbulence intensity, serration amplitude and wavelength as well as the angle of attack on the noise reduction capability. A statistical-empirical model was developed to predict the overall sound pressure level and noise reduction of a NACA65(12)-10 airfoil with and without leading edge serrations in the range of chord-based Reynolds numbers of 2.5•10 5 ≤ Re ≤ 6•10 5. The observed main influencing factors on the noise radiation were quantified in a systematic order for the first time. Moreover, significant interdependencies of the turbulence intensity and the serration wavelength, as well as the serration wavelength and the angle of attack were observed, validated and quantified. The statistical-empirical model was validated against an external set of experimental data, which is shown to be accurate and reliable.
Leading edge serrations are identified as an effective passive treatment for reducing fan broadband noise due to high turbulent inflow conditions. This paper aims to investigate the isolated effect of serrated applications in a rotating frame, covering the aerodynamic and aeroacoustic performance. With this purpose, a serration design, previously analyzed in the rigid domain, is transferred to the rotating frame, following a successive approach in form of a continuous increase of the fan blade number. This is considered as a feasible way to isolate the serration effects and to provide information on fan blade interaction and possible masking effects. Comparing blades with straight and serrated leading edges by analyzing the spectral noise reduction and the overall level result in deep insights in the underlying noise reduction mechanisms. Furthermore, analysis of phase differences by means of the wall pressure fluctuations leads to the identification of rotating flow phenomena, nonsynchronized with the rotor speed. The results obtained indicate an efficient noise reduction by the serrations in the vicinity of the design point. By use of the presented successive approach, noise reduction phenomena observed with the full rotor could be identified to be of either aeroacoustic or aerodynamic nature. A reduced noise is observed for the full rotor case, showing a reduction of blade interaction effects. At reducing flow coefficients, an improved stall margin of the serrated rotor is identified that also affects the aeroacoustic signature.
Drilling during temporal bone surgery may result in temporary or permanent noise-induced hearing loss or tinnitus. This has practical implications for both the patient and the surgeon. Different surgical drill devices, routinely used in temporal bone surgery, are examined referring to their emitted sound levels and sound transport. Two surgical drills were used on a brass tubing and a steel wire to simulate sound generation during temporal bone surgery. Overview measurements were performed on human cadaver in a medical laboratory. A set-up in a silent chamber was chosen to exclude external sound sources. The noise emissions and the vibration generated by a silver diamond bur and a cutting drill (Rose bur) were registered when used on a brass tubing and a solid steel wire with sound level meter and a non-contact laser vibrometer. The highest sound rate generated by the diamond burr did not exceed 63 dB(A) when used on a solid steel wire, whereas the cutting burr emitted 76 dB(A). Both drills produced lower sound levels on the brass tubing. Again the cutting burr topped the diamond burr with 68 dB(A) against 56 dB(A). The sound emission did not exceed 76 dB(A) outside a radius 4 cm around the drill location. In conclusion, sound emission generated by different surgical burs routinely used in temporal bone surgery is lower than expected. Still, within a small radius around those burs high sound pressure levels may be induced into surrounding structures such as ossicles, labyrinth, and cochlear. Still damage is feasible when using surgical drills for a longer time period close to sensitive structures.
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