大型船舶推減係數之精算及推進效率優化之研究

Abstract

船舶推進的流場中，船殼與螺槳的互動可視為勢流現象。本研究發展一帶有環流分佈的邊界元素法求解拉普拉斯方程，以計算船殼在螺槳作動下的阻力增量。根據Lagally定律可推導推減係數的尺度效應，是由於在模型和實船的跡流場速度不一樣所造成。推減係數的修正必需與文中所提出的平衡型自推試驗流程搭配，方可在不使用船殼摩擦修正項之下得到自推點。本文第二部分建立一套船舶推進試驗的數值模擬系統，結合計算流體力學、邊界元素法、以及船殼-螺槳互動求解模組，達成迅速且精確的模擬。此系統更進一步採用升力線理論及最佳環流分佈假設，使得在船型初步設計階段即能算得其推進性能。在此模擬系統架構下進行船型推進效率優化。文中以參數化方法建立等排水量的三階貝茲斷面積曲線，在保證達成曲線的平順度之下，以模式搜尋演算法找到五種斷面積的設計型態，並透過Lackenby船型變形方法及線性組合法研究參數與水動力之關係。在模擬30個船型後，導出最優效率前緣，並在此前緣上發現影響船殼效率的關鍵因子，以供未來在修改艉部線形的參考。在搭配重新設計的螺槳後，最優秀船型-螺槳組合的傳遞馬力可下降17.3%，但以拖曳阻力上升3.2%作為代價。此一結論顯示船艉線形設計必需在阻力以及效率之間做取捨。

Based on the flow characteristics in ship propulsion, the interaction between ship hull and propeller is assumed to be inviscid in nature. A boundary element method (BEM) with circulation present is employed to solve the Laplace equation for the augmented resistance of the hull. The governing equation for wake interaction is built upon the stream surface contraction caused by the propeller’s induced velocity. According to the Lagally theorem, thrust deduction scaling originates from the scaling of the effective wake. The former is a potential-based interaction, while the latter is based on a viscous boundary layer basis. The scale correction of the thrust deduction factor works with the proposed balanced self-propulsion test procedure without using a skin friction corrector. An implementation of the self-propulsion framework integrates computational fluid dynamics method (CFD) and BEM methods but decouples the hull-propeller interaction solvers from them. This approach drastically reduces computation time, such that a propulsion simulation may be completed within minutes. The framework is validated for a moderate speed containership, and further extended by the lifting line method for faster hull performance analysis. On the hull form design aspect, challenges of high efficiency ship designs have attracted much attention due to the requirement for reduction in NOx emission. Optimizations for the propulsive efficiency are feasible by utilizing the decoupled approach of propulsion simulation, and many combinatorial options from hull forms and propellers are accomplished without suffering long simulation times in CFD. A parametric hull form transformation model, based on the cubic Bezier curve formulation to keep displacement constant, is proposed. Thirty hull forms of five sectional area curve patterns are studied and an optimal set is found. Compromises between resistance, hull efficiency, and propeller efficiency exist along the optimal frontier, on which a design principle is derived. According to this principle, a successful modification of the tanker brings a reduction in delivered horse power by 17.3% due to a 23.7% decrease in thrust deduction, but with paying a price of a 3.2% increase in its towed resistance.