UNDERWATER ACOUSTİCUNDERWATER ACOUSTİC

PROPELLER NOSİEPROPELLER NOSİESELİM YILMAZ SELİM YILMAZ

ÖZGÜR SUBAŞIÖZGÜR SUBAŞI

DefinitionDefinition

Self noise generated by the ship's propellers is known as propeller noise. Propeller noise is mainly caused by cavitation, which occurs when bubbles form on the low pressure side of the propeller blade and grow to full size very quickly (in about 2 microseconds), then collapse. The collapse of these bubbles gives a continuous spectrum of noise, which dominates the higher frequency range of ship's noise and is speed related.

That is, cavitation is greater at higher speeds, because the propellers produce more bubbles Cavitation is a much more significant factor in surface ships than in submarines. This is because submarine cavitation, which is relatively slight in shallow water, can be almost completely eliminated in deeper water. The deeper the submarine is, the greater the hydrostatic pressure, thus the less cavitation. In addition, improved design in submarine screws (propellers) significantly reduces cavitation.

In the design of naval or research vessels, reduction of In the design of naval or research vessels, reduction of underwater noise radiated from the vessel is of underwater noise radiated from the vessel is of primary importance for the reliable operation of primary importance for the reliable operation of onboard acoustic instruments. Considerable part of onboard acoustic instruments. Considerable part of noise generated by the ship system is the noise generated by the ship system is the underwater noise and the major sources underwater noise and the major sources contributing to this are due to the machinery, contributing to this are due to the machinery, propeller and background hull flow noise as propeller and background hull flow noise as described by Ross(1976).described by Ross(1976).

Amongst these sources the propeller noise , Amongst these sources the propeller noise , particularly for the cavitating propeller, is the particularly for the cavitating propeller, is the most harmful one for acuistic survey most harmful one for acuistic survey operations since the dominant noise levels operations since the dominant noise levels can cover a wide frequency band, as can cover a wide frequency band, as reported e.g. Sasajima et al(1986). Therefore reported e.g. Sasajima et al(1986). Therefore the design of low noise propellers for these the design of low noise propellers for these types of vessels is utmost important and types of vessels is utmost important and requires feedback from model tests in requires feedback from model tests in cavitation tunnels, so we examined the cavitation tunnels, so we examined the details of propeller noise with cavitation details of propeller noise with cavitation tunnel.tunnel.

Similar to the cavitation inception tests, Similar to the cavitation inception tests, the measurements of the noise of the the measurements of the noise of the same model propeller for varying same model propeller for varying cavitation numbers and advance cavitation numbers and advance coefficients of the propeller were taken coefficients of the propeller were taken at two different values of the dissolved at two different values of the dissolved gas content in four groups.gas content in four groups.

• • The first group was the measurements of The first group was the measurements of the noise with the propeller in the free the noise with the propeller in the free flow.flow.

• • The second group The second group was the was the measurements at measurements at varying levels of the varying levels of the free-stream free-stream turbulence by using turbulence by using the same turbulence the same turbulence generator facility.generator facility.

• • The third group was to investigate the The third group was to investigate the effect of the roughness on the noise.effect of the roughness on the noise.

• • The fourth group involved the noise The fourth group involved the noise measurements of the propeller measurements of the propeller subjected to the combined effects of subjected to the combined effects of varying levels of the free-stream varying levels of the free-stream turbulence and blade roughness. turbulence and blade roughness.

The analyses of the sources The analyses of the sources contributing to the tunnel's background contributing to the tunnel's background noise indicated that the dynamometer noise indicated that the dynamometer was the major source regardless of was the major source regardless of operational conditions. However, the operational conditions. However, the contribution of the wire meshes to the contribution of the wire meshes to the background noise level varied background noise level varied depending upon the operational depending upon the operational conditions.conditions.

The analyses of the net propeller noise for The analyses of the net propeller noise for the effect of the free-stream turbulence the effect of the free-stream turbulence and roughness displayed extremely and roughness displayed extremely complex trends which were difficult to complex trends which were difficult to interpret particularly for the low frequency interpret particularly for the low frequency region between 50 Hz and 1000 Hz.region between 50 Hz and 1000 Hz.

In the high frequency In the high frequency region, which is beyond region, which is beyond 1000 Hz, the effect of the 1000 Hz, the effect of the free-stream turbulence on free-stream turbulence on the net propeller noise the net propeller noise depended upon the depended upon the operating conditions and operating conditions and displayed a complicated displayed a complicated trend with the development trend with the development of the cavitation. However, of the cavitation. However, the levels of noise were the levels of noise were increased with increasing increased with increasing levels of the free-stream levels of the free-stream turbulence for the non-turbulence for the non-cavitating case.cavitating case.

• • In the low frequency region, similar to the In the low frequency region, similar to the effect of the free-stream turbulence it was effect of the free-stream turbulence it was impossible to observe any distinct trend for impossible to observe any distinct trend for the effect of the blade roughness. However, the effect of the blade roughness. However, in the high frequency region and at in the high frequency region and at atmospheric condition, the levels of the atmospheric condition, the levels of the propeller noise were increased with propeller noise were increased with decreasing the level of roughness while this decreasing the level of roughness while this trend disappeared and became complex trend disappeared and became complex when cavitation was present.when cavitation was present.

• • Although the effect of the free-stream Although the effect of the free-stream turbulence on the inception of cavitation has turbulence on the inception of cavitation has been found similar to that of the blade been found similar to that of the blade roughnessroughness, , it is difficult to say that these two it is difficult to say that these two mechanisms had the same effect on the mechanisms had the same effect on the noise level of the propeller. In contrast to the noise level of the propeller. In contrast to the case in the cavitation inception tests, the case in the cavitation inception tests, the blade roughness reduced the noise level of blade roughness reduced the noise level of the propeller while the increasing free-stream the propeller while the increasing free-stream turbulence increased the noise similar to the turbulence increased the noise similar to the case for the inception tests. This may be case for the inception tests. This may be because of the different behaviour of the because of the different behaviour of the bubble dynamics affected by these bubble dynamics affected by these mechanisms.mechanisms.

• • As the dissolved gas As the dissolved gas content was increased, the content was increased, the propeller noise was reduced propeller noise was reduced slightly in the high frequency slightly in the high frequency region, displaying a cushioning region, displaying a cushioning effect on the noise effect on the noise characteristics of the propeller.characteristics of the propeller.