. Noise Control by Vane Sweep

Sweep as lean is another noise reduction mechanism. Positive sweep is de­fined as the axial displacement of the vane leading edge from its baseline radial position, as shown in Figure 2. Vane sweep modifies the spanwise phase vari­ation of the incident gust as does lean. For the case of a uniform ft>w, the

ing the sweep angle will increase the wake intersections with the vane leading edge.

As for the case of lean, numerical calculations are validated by comparing them to results of Schulten, 1997 and, Envia and Nallasamy, 1999. Schulten has concluded that for high Mach number sweep is only effective for large positive sweep angles. For low positive sweep angles sweep is ineffective and there may be an increase in the noise level. It was also indicated that sweep is more effective for reducing upstream noise levels than for downstream noise levels. Envia and Nallasamy have presented experimental evidence showing sweep effectiveness for high positive angles, and numerical results which con­cluded that positive sweep is always effective and negative sweep is always ineffective. In what follows we attempt to resolve the differing conclusions.

Case Study: B = 16, V = 24. Figure 7 compares the upstream and downstream RAPC versus the sweep angle a. The upstream acoustic power level is reduced for both positive and negative sweep. On the other hand, there is a reduction in the downstream sound power for negative sweep, and an ini­tial increase for small positive sweep angles, confirming results calculated by Schulten, 1997. The downstream sound power decreases for sweep angles
larger than 12.5o. Also, the rate of sound power reduction is larger for sound propagating upstream than downstream. The stronger reduction is attributed to the difference in the axial wavelength. The upstream axial wavelength is smaller than the downstream wavelength, thus variations in vane sweep cause more phase differences. These results are in good agreement with those of Schulten for the same geometry and Mach number. In fact, Schulten predicts the angle of effective sweep to be about 12o, and that the reduction rate in the upstream sound power is much larger than the downstream sound power.

Figure 7. Relative power change for downstream and upstream power levels for different sweep angles

Numerical results indicate that sweep is effective for negative angles. Envia and Nallasamy, 1999 used a combined analytical/numerical technique which concluded that sweep was only effective for positive sweep. The differences between the present results and theirs are attributed to two factors: (i) In their model the wake decay was accounted for and thus the up-wash of the wake is different for positive and negative sweep. In our model we do not account for wake decay, and we beleive it is unlikely to introduce such significant changes. (ii) Our computations are three-dimensional which are different at high fre­quency from the strip-theory approach used by Envia and Nallasamy.

To demonstrate that the vane sweep introduces spanwise phase variations, the unsteady pressure difference along the vane span for four cases is shown in Figure 8. The first two plots (a) and (b) are for negative sweep, and the plots (c) and (d) are for positive sweep. There is significant spanwise radial loading, associated with vane sweep, compared to the case of zero lean and sweep in Figure 2. Clearly this can not be modeled using strip-theory.

3. Summary for Noise Control

Noise reduction by passive means is possible and simple to apply. It is found from this study that implementation of vane lean or sweep may reduce the sound levels for the tonal noise components. It is important to note that

Figure 8. Unsteady pressure difference across vane for different sweep angles and ю = 3 n, Mx = 0.5.

vane lean is dependent on a specific rotor/stator design, tip-hub ratio, and Mach number. Caution should be taken when specifying the lean angle, as a rule of thumb negative lean angles may be more beneficial, however, it is crucial to check the results numerically at the design phase. Vane sweep is efficient for large positive sweep angles. Negative sweep angles, even-though unconven­tional, may show promise to future sound reduction mechanisms. Both lean or sweep effectiveness depends on the number of propagating modes. For a single frequency (tonal noise), the vanes can be designed optimally to reduced the propagating sound power, however, for a frequency spectrum (broadband noise), the number of propagating modes is large, thus vane lean or sweep may not be beneficial.

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