自由液面效應對螺槳非定常力所引起聲場之影響探討

The Acoustic Pressure Due to the Unsteady Forces of a Marine Propeller Considering Free Surface Effect

The main objective of this paper is to investigate the free surface effect on the acoustic pressure due to unsteady forces of a marine propeller operating in a non-uniform ship wake. These unsteady forces include two components: axial thrust and tangential torque force. In this paper, acoustic pressures at both far and near fields are calculated by the analytic solution of the linear wave equation in time domain and the negative Lloyd Mirror effect is used to simulate a soft free surface. From the computational results, it is found that the dominant direction of the acoustic pressure due to the propeller unsteady thrust, when considering the free surface effect, is shifted about 45 degrees from x-axis in x-y plane. The interference of the acoustic pressure due to the propeller unsteady torque with free surface effect is strongly dependent on the angular position of the field point. Its acoustic field is dominant in y-axis. When the shaft immersion of the propeller is reduced, the interference is generally more obvious. 本論文主要目的在探討自由液面效應對船用螺槳之非定常力所引起之聲場之影響，探討之螺槳是作動於非均勻船艉激流中。本文所提及之螺槳非定常力包含軸向推力及切向扭矩力。依據線性波動方程之解析解，首先進行螺槳非定常力對遠近自由場所造成之聲場計算；再者，使用映射法將實際之自由液面考慮為一平軟面，藉此考慮自由液面之存在。由計算結果可知：推力項擾動聲壓在考慮自由液面效應後，於x-y平面中，噪音強度最強之方向，由原本沿螺槳軸，x軸，轉至與x軸夾角約為45度之方向上。自由液面對扭矩力項擾動聲壓所造成之干射效果和場點所在之角度位置有極大相關，且考慮自由液面後噪音強度最主要仍會表現於y軸上。當螺槳軸吃水越淺，自由液面效應會越明顯。