UNDERWATER ACOUSTİC

PROPELLER NOSİE SELİM YILMAZ ÖZGÜR SUBAŞI

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

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

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

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

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

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

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

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

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

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

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

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

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