Within the aeronautical field there has been a considerable amount of work done on various aspects of propeller and rotor noise The applications in aircraft have included advanced design propellers and prop-fans being developed for future aircraft Although the impetus for these developments was fuel economy, the increasing demand for quiet aircraft means that the noise signature is an increasingly important element in the design Another fruitful area of noise work in the aeronautical field is that relating to helicopter rotors, and whilst there is considerable civil pressure to make helicopters quieter, there is a significant military interest in the helicopter noise signature as well.

In the marine field the interest in quiet propellers is predominantly for military applications such as anti-submarine warfare. The civil requirement is less stringent, as the main interest is in avoiding cavitation and in reducing propeller induced shipboard vibration.

Propeller noise is only one aspect of reducing the noise of a vehicle, other sources are machinery noise and flow noise over the vehicle itself This chapter will address only the propeller noise

Although there are considerable similarities between the theories applied to propellers operating in air and in water, one must be aware that the regimes covered by the appropriate theories are very different Consider for example the values of various non-dimensional parameters for the two cases, eg Mach number, Reynolds number, advance ratio, reduced frequency and the (wavenumber radius) product (ka) which is a measure of the acoustic compactness of the source Such quantities depend very much on the details of each design, but some very striking differences do occur The advance ratios of the prop-fan and the marine propeller are of a similar order, but the helicopter rotor has an advance ratio close to zero The Reynolds number of the marine unit tends to be higher than the other two, with the helicopter having the lowest value The most striking difference is in the Mach number, which is transonic for the prop-fan, in the high sub-sonic for the helicopter rotor, and less than 01 for the marine case, thus, although the reduced frequencies (s) are of order unity in all cases the product (OM) is of order unity in the airborne cases and of order 01 in the marine case This indicates that compressibility will play an important role in the flow over the blade in the airborne case but not in the marine case Similarly the factor (ka) shows significant differences between the various rotors, being greater than unity in the airborne case and much less than unity in the marine case This indicates that the source is acoustically compact in the mane case but that significant phase differences can occur in the airborne case Clearly, the major differences between the two cases occur as a result of the large difference in sound speed between au and water The other major change between air and water results from the large difference in density.

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