不對稱船艉參數化設計與流場模擬分析

Abstract

大型船舶的推進效率提升常著眼於螺槳跡流場中的旋向動能回收，實作上可分類為在螺槳前方預先將流體反向旋轉和在螺槳後方安裝附屬物轉換旋向動能為推力，不對稱船艉即屬於前者；本研究利用繪圖軟體Rhinoceros之外掛程式Grasshopper進行參數化船型設計，並以商用計算流體力學軟體Star-CCM+進行流場模擬，進行船艉跡流區與推進效率之分析。 本研究選用由韓國船舶及海洋工程研究所(Korea Research Institute of Ships and Ocean Engineering，簡稱KRISO)公開發表之230米貨櫃輪和附屬的五葉螺槳為研究對象。其中主要原因係因該船與螺槳已有完整的實驗數據，對於本研究擬採用的數值模擬方法，可提供極為有用的驗證資料，以便可以檢視模擬計算結果的準確性，因此首先進行原船型阻力試驗、螺槳單獨試驗與自推試驗，並和實驗結果進行比對，以驗證計算結果的可信度。接著在維持線形平順的設計要求下，建構參數化船艉扭轉變形方法，其中使用4個參數：旋轉強度、縱向分佈函數權重控制因子、剖面曲線旋轉角度分佈帶寬及剖面曲線旋轉極值位置，將任意左右對稱之船型曲面變形成不對稱船艉。此外，為評估不對稱船艉之流場效果，提出名義跡流之旋流係數。最後改變幾何變形參數建立各式不對稱船艉船型，於設計船速24節下進行阻力試驗模擬，由模擬結果發現，縱向分佈函數權重控制因子對旋流係數之影響相當有限，而剖面曲線旋轉角度分佈與旋流係數成負相關。選擇效果最佳之船型進行自推試驗模擬，分析推進效率及船艉流場，由結果發現於各船型之自推點推進效率可提升2.96%，若於相同螺槳負荷狀況下則可提升4.02%。進一步分析自推試驗螺槳後方流場及推進效率因子，發現不對稱船艉於推進效率之提升主要來自於有效跡流係數之增加，且能有效回收螺槳後方損失的旋向能量。

The propulsion efficiency of a commercial vessel is usually related to the tangential energy loss in the wake zone of the propeller. In order to elevate the efficiency practically, we can either pre-swirl the fluid in front of the propeller or transform the energy loss into thrust through the appendages behind propellers. Asymmetric stern ship is an example for the first concept. The asymmetric stern is modeled parametrically by the program named Grasshopper in this research and simulated by Star-CCM+ to analyze the wake zone behind stern and the efficiency of propeller. The prototype of this research is the container ship developed by KRISO with also its related propeller. We first simulate the resistance and self-propulsion test of this prototype, and then compare the result with the experimental data for verification. And then we construct a parametric model that can be applied to any stern. We use four parameters including rotation strength, longitudinal distribution weighting, sectional rotation distribution bandwidth and sectional rotation position, to transform arbitrary hull form to asymmetric stern while keeping the smoothness of lines. Also for analyzing the effect of asymmetric stern, we establish rotational wake coefficient. To discuss the relation between the four parameters and the axial/tangential velocity distribution in the wake zone, we calculate various types of asymmetric stern with design speed of 24 knots, and analyze the wake in propeller plane. The simulation results show that longitudinal distribution has insignificant effect on rotational wake coefficient while radial distribution has positive correlation. We choose three types which have significant effects to simulate the self-propulsion test. The results show that the propulsion efficiency can increase 2.96% at self-propulsion point and can increase 4.02% at the same propeller loading. Besides, with the analysis of wake zone in self-propulsion test and the propulsion efficiency factor, we consider that the effect of asymmetric stern on propulsion efficiency mainly comes from the difference of effective wake coefficient.