豐滿船形之形狀因子探討：船模阻力試驗與計算流體力學模擬比較

INVESTAGATION OF FORM FACTOR FOR A FULL FORM SHIP : COMPARISON BETWEEN SHIP MODEL TEST AND COMPUTATIONAL FLUID DYNAMICS

本研究旨在探討形狀因子與無因次參數雷諾數及福勞德數之關係，並且以一球艏油輪交互驗證電腦計算模擬及船模實驗兩種求解阻力之方法。在船模實驗方面，選定福勞德數0.1至0.2區間執行球艏油輪模型之阻力試驗，並使用ITTC 1957平板阻力公式以及Prohaska Method推估該船之形狀因子。在計算模擬方面，本研究設定其中4個對應模型試驗的船速，利用商用計算流體力學(CFD)軟體模擬自由液面流場行為與船體所受阻力其分量。與實驗相較，計算所得之總阻力誤差小，顯示在低速時CFD結果亦可採用實驗Prohaska Method推估形狀因子，不需再透過忽略自由液面的對疊模型模擬得到。計算結果的阻力分量顯示，形狀因子主要來自壓差阻力。在有自由液面效應下，壓差阻力與雷諾數造成的黏性壓差阻力，和福勞德數造成的波浪壓差阻力皆有密切關聯。在非常低速條件下，興波阻力消失，留下黏性壓差阻力，證實Prohaska外插方法之合理性。

This study investigates the relation between form factor and two non-dimensional parameters, namely Reynolds number and Froude number. An oil tanker with bulbous bow is selected and the resistance is evaluated by ship model test and by computer simulation method. In terms of ship model test, the resistance test was conducted under Froude number from 0.1 to 0.2 for a scale model of oil tanker, to which ITTC 1957 model-ship correlation curve and Prohaska method are applied to estimate form factor. As for computation simulation, four ship speed points are set corresponding to exper-iment, and commercial computational fluid dynamics (CFD) software is applied to simulate behavior of free surface and resistance component. Compared with the experiment result, the error of total resistance by CFD is low, which implies that it is acceptable to estimate form factor by Prohaska method from CFD results under circumstances of low ship speed. Therefore, a double-body model, though reducing computational complexity, is not necessary to eliminate free surface effect. The resistance component of CFD simulation result shows that form factor originates mainly from the presence of pressure resistance. In addition, viscous resistance is related to Reynolds number, while wave making resistance is related to Froude number, and both of them highly affect pressure term. In the case of very low speed, wave making pressure resistance vanishes while viscous pressure resistance remains, which proves Prohaska extrapolation method is a proper way for form factor estimation.