Chinese Journal of Ship Research
The analysis of a propeller's low-frequency vibration and sound radiation characteristics:Jellyfish mode
WU Chongjian1,2,WANG Chunxu*1,2,CHEN Zhigang1,2,DU Kun1,2,LEI Zhiyang1,2 1 China Ship Development and Design Center,Wuhan 430064,China 2 National Key Laboratory on Ship Vibration and Noise,Wuhan 430064,China found a new component of propeller noises which is induced by the excitation of the low-frequency elastic mode of the propeller. In particular,the model where all the blades of a propeller vibrate in the same phase is stimulated will result in strong acoustic radiation. We have named the mode shape in which all the blades of a propeller vibrate in the same phase as "Jellyfish Mode". Also, the low-frequency narrow-band sound radiation of the propulsion system induced by the excitation of "Jellyfish Mode" is named by the "Jellyfish Effect". To reveal its mechanism,a study of the low frequency intrinsic modal characteristics of the propeller is carried out.[Methods]the refined mesh FEM method is utilized to study the dynamic feature of the propeller. The modal characteristics of the propeller both in air and water are obtained and the influence of the structural detuning of the propeller on the modal characteristics is analyzed. Then,the cyclic symmetric structure vibration theory is cited to validate and further study the low-frequency dynamic property of the propeller.[Results] The research revealed the grouping modal characteristics of the propeller both in air and water. Furthermore,the single frequency mode and repetition frequency mode characteristics in a mode group are also brought out. The research results showed that the structural detuning of the propeller has a great effect on the repetition frequency mode while it has little effect on the single frequency mode. The refined mesh FEM numerical analysis results are consistent with the theoretical results. The study revealed the low frequency natural dynamic characteristics of the propeller and the research above can provide theoretical supports for propeller noise analysis and control,which is of great significance in theoretical and practical engineering. noise;Jellyfish mode cyclic symmetry structure single frequency mode repetition frequency mode;structural detuning
Abstract:[Objectives]We words:propeller Key [Conclusions]
0引言
随着机械噪声的持续降低[1-2],舰船推进系统噪声凸显,成为舰船主要的噪声源,并呈现出低频宽带、低频线谱突出的特点[3]。推进系统噪声控制涉及的要素较多,如螺旋桨、轴系、船体结构、艉部线型、操纵面等,噪声机理复杂,涉及多学科与多物理场耦合,噪声控制难度大。螺旋桨是舰船最典型的推进器,也是推进系统振动声辐射最主要的激励源和辐射源。螺旋桨噪声理论将噪声主要成分划分为了空化噪声、唱音、中高频噪声和低频噪声,其中低频噪声又可细分为低频宽带噪声和离散谱噪声[4-5]。在进行螺旋桨声学研究时,一般将桨叶当作刚体处理,而与桨叶弹性有关的振动声辐射主要是“唱音”,其机理是桨叶随边的局部模态与该处小尺度规则涡列结构的自激振动,该噪声频率相对较高[6]。螺旋桨低频噪声由其工作在舰船尾部非均匀、非定常伴流场中的非定常激振力产生,一方面,直接辐射声;另一方面,激励推进轴系、船体结构振动辐射声,并成为推进系统噪声的主要成因[7-8]。2011~2012在前期研究基础上,我们在 年将一种量级突出的低频窄带谱辐射噪声成因与螺旋桨进行了关联。与传统的螺旋桨低频噪声特征不同,深入的理论分析和试验验证证明该特征谱由螺旋桨同相模态产生,将该模态命名为“水母模态”,而由该模态激发引起的声辐射则称为“水母2015~2017 3效应”。 年,又连续 次在行业会上对噪声成分的特征、机理进行了阐释和补充。本文将基于精细化有限元分析,揭示螺旋桨的低频模态特征,然后引用循环对称结构动力学理论对其动力学特征进行理论归纳。
1 螺旋桨低频模态特性精细化数值分析
1.1 对象及数值模型
1为了说明螺旋桨的低频模态特征,选取图E1619所示的意大利船模水池 螺旋桨作为分析对7 0.485 m,象。该桨是一个 叶大侧斜螺旋桨,直径0.608。盘面比螺旋桨为中心对称结构。为减小数值模型对计算结果的影响,数值模型须尽量保证对称性。1/7取整个桨的 几何结构,按照全六面体精细化网格划分,然后通过圆周阵列得到完全循环对称的计算模型。为适应螺旋桨复杂扭曲的外形,提高
观察同组内模态频率和振型,发现亦呈现明5 3个显的单频和重频特征。 个模态实际上只有2 2模态频率,其中 个模态频率各自对应 个模态振型,属于重频模态;1 1个模态对应 个振型,属于单频模态。4在振型特征方面(图 ),单频模态振型可描NNNNN,即“水母模态”;2述为 组重频模态的振型NPNPN(PNPNP)和NNPPP(PPNNN),可以描述为2 2,3)组两两正交。 组重频模态的振型全部为( 2 3合,即 个桨叶振动与另外 个桨叶振动反相,不存在(1,4)组合(即PPPNP)模态特征。
1.3 水介质中螺旋桨低频模态特性
为了研究介质环境对螺旋桨低频模态特征的1所示模型为对象,基于有限元/边界元影响,以图(FEM/BEM)耦合模态分析方法[9],建立螺旋桨数值计算模型并分析其在水环境中的模态特性。
模态的聚集分组特征;二是组内单频、重频模态特
征。
2 桨叶失谐的影响分析
上节的建模分析从几何模型、单元数量和单元阶次等方面进行了精细化的分析,确保了结构
动力学意义上的理想对称性。将螺旋桨因加工误差(外形)、密度不均(质量)和微裂纹(刚度)等缺陷而无法保证完全中心对称状态的情况称为“失谐”,本节将研究螺旋桨微失谐状态下的模态特性。
1在图 所示桨模的任意叶尖设置集中质量, 1/1 000,以模拟螺旋桨的微小质取值为桨叶质量 4量失谐。空气介质中的模态频率如表 所示。对1照表 可见,在微失谐情况下,模态频率依然呈现7 1分组的特征,每 阶模态一组,第 组模态仍以单1 2 2阶桨叶第 阶模态聚集;第 组模态以单桨叶第