航速对船舶横摇阻尼影响的数值模拟

Numerical simulation of effects of forward speed on ship roll damping

摘要:

为认清船舶横摇机制、探究航速对横摇阻尼的影响,模拟了DTMB5512船模的横摇自由衰减过程,并根据短期预报结果评估了不同横摇阻尼求解方法用于横摇运动分析的可靠性。采用计算流体动力学（CFD）中的重叠网格技术,考虑横摇、横荡和垂荡三自由度运动,得到了不同航速和初始横倾角下的横摇自由衰减曲线,横摇幅值与横摇周期均与试验结果吻合良好,验证了数值方法的可靠性。通过对横摇衰减曲线拟合,可分别得到线性和二次阻尼系数,并基于势流理论计算了不同海况下的横摇单幅有义值。结果表明:傅汝德数（Fn）由0.138增加到0.410,横摇衰减速度明显加快,伴随着船体周围兴波的快速上升。相同航速时,初始横倾角由10°增加到20°,线性阻尼系数无明显变化；相同初始横倾角时,高航速对应更大的线性阻尼系数。等效阻尼为线性阻尼和二次阻尼在对应横摇幅值下的组合,可以有效减弱横摇消灭曲线拟合所引入的影响。根据横摇短期预报结果,Fn=0.138和Fn=0.280航速,CFD法得到的横摇阻尼系数精度均高于经验公式法,在高海况下表现得尤为明显。等效阻尼系数更适合用于横摇运动的定量分析,随着航速的增加而增加。

Abstract:

To understand ship roll mechanisms and explore effects of forward speed on roll damping, roll free decay process of the DTMB5512 ship model was simulated. The reliability of different roll motion analysis methods was also evaluated according to short-term prediction results. By employing overset grid technology in computational fluid dynamics (CFD) and releasing roll, sway and heave degrees of freedom, roll free decay curves at various speeds and initial heel angles were obtained. The simulated roll amplitudes and periods were in good agreement with experimental results, confirming the reliability of the numerical method. Linear and quadratic roll damping coefficients were derived from fitting of roll free decay curves. Furthermore, roll single significant values were predicted at various sea states based on potential theory. The results indicate that as the Froude number (Fn) increases from 0.138 to 0.410, the roll decay rate significantly accelerates, accompanied by an enhanced wave elevation around the hull. At the same speed, with initial heel angle increasing from 10° to 20°, there shows no obvious differences in linear damping coefficients. For the same initial heel angle, higher speeds corresponds to larger linear damping coefficients. The equivalent damping is derived from combination of linear and quadratic damping at corresponding roll amplitude, which can effectively weaken the interference from roll extinction curve fitting. According to short-term roll prediction results, for speeds corresponding to Fn=0.138 and Fn=0.280, roll damping coefficients obtained from CFD method are both more accurate than that from empirical formula method, especially obvious at higher sea states. The equivalent damping coefficients are more suitable for quantitative analysis of roll motion, increasing with speed.