## DYNAMIC SIMULATION OF CIVIL AIRCRAFT DITCHING BASED ON CFD METHOD

LUO Wenli,1), CHEN Haoyu

Aircraft Structure and Stress Division, Shanghai Aircraft Design and Research Institute, Shanghai 201210, China

Abstract

A numerical simulation was carried out to investigate the dynamic impact for civil aircraft ditching. The simulation used Reynolds averaged N-S equation from the computational fluid dynamics (CFD). Water-air interface and ditching process were modeled through the combination of volume of fluid (VOF) method and full-flow moving mesh. The position and attitude of aircraft were determined by solving 6-degrees-of-freedom equations. Firstly, the method was validated through typical examples, and then it was used for ditching simulation and analysis of a regional aircraft. The tail-mounted-engine and high-tail layout was focused when researching ditching characteristics. The results show that ditching movement and load characteristics can be well simulated. During water entry period, the regional aircraft is subjected to a large water impact load, while the suction force generated by rear fuselage makes the aircraft pitching up. Afterwards, the suction force decreases as speed decreases, and the aircraft gradually pitches down and approaches to a stable state.

Keywords： civil aircraft; ditching; computational fluid dynamics; water impact load

LUO Wenli, CHEN Haoyu. DYNAMIC SIMULATION OF CIVIL AIRCRAFT DITCHING BASED ON CFD METHOD. Mechanics in Engineering, 2022, 44(2): 293-302 DOI:10.6052/1000-0879-21-381

PM计算的原理一般是基于动量守恒或势流理论,忽略黏性、可压缩性、重力和气垫效应等等,多用于简单外形的物体入水问题,在精确描述民用飞机水上迫降动力学问题时存在一定难度。FEM绝大部分基于商用有限元分析软件,对于气动力的模拟存在困难,另外,计算结果依赖于流固耦合算法,常见的问题是容易引起流体渗漏,导致流体和固体载荷不统一。SPH避免了由于网格划分产生的问题,但在流体与固体的边界处理上存在较大难度,在模拟精度上存在劣势,且建模复杂度较高。实际水上迫降过程中,飞行速度必须维持在可以保持飞机平稳下降的水平,气动力的影响无法忽略。此外,由空气和水面压差导致的机身尾部的吸力也是影响迫降运动状态和全机载荷的重要因素,因此FVM具有较大的优势。

2007年,Streckwall等[13]使用基于FVM和流体体积(volume of fluid, VOF)法的COMET软件对飞机水上迫降过程进行了模拟,对比试验表明计算结果较为准确,并直观体现了飞机入水过程中的水面运动和飞机表面压强分布;2012年,Zhang等[14]模拟了一种支线客机的水上迫降过程,表明吸力对于飞机水上迫降的姿态影响较大,且空气模型是吸力产生的必要条件。同一时期,屈秋林等[15]和Guo等[16]详细地研究了尾吊高平尾、翼吊低平尾和翼身融合三种不同布局飞机的水上迫降特性,认为正常布局飞机的低平尾具有抑制飞机过度上仰的作用,从而有利于减缓过载和局部压力。2019年,吴宗成等[17]基于FVM采用滑移动网格研究了波浪对水上迫降特性的影响。由于水上迫降过程中的水平位移较大,目前基于FVM的方法大多采用动网格实现对运动过程的模拟,通过网格变形和重构模拟飞机和计算域的运动,需消耗庞大的计算资源。本文在已有FVM方法的基础上采用全流场运动网格的方法,添加约束保持水面高度不变,使流场相对水面自由运动,大大缩减了计算量。首先通过典型算例对该方法进行验证,然后模拟分析了某型飞机的水上迫降运动过程。

## 1 仿真方法

$\left.\begin{array}{l}V_{i}^{n}=V_{i}^{n-1}+\frac{F_{i}^{n-1}}{m}\left(t^{n}-t^{n-1}\right) \\\theta^{n}=\theta^{n-1}+\frac{M^{n-1}}{I}\left(t^{n}-t^{n-1}\right)\end{array}\right\}$

$\begin{array}{c}(x, y, \theta)^{n}=(x, y, \theta)^{n-1}+\left(V_{x}, V_{y}, \theta\right)^{n-1}. \\\left(t^{n}-t^{n-1}\right)\end{array}$

## 2 二维垂直入水算例

### 图1

Fig.1   Experiment diagram of the two-dimensional example

2.2.1 流场设置

### 图2

Fig.2   Mesh of the two-dimensional example

2.2.2 时间步长设置

### 图3

Fig.3   Pressure results of different time step

### 图4

Fig.4   Pressure comparison of circle example

### 图5

Fig.5   Result comparison of wedge example

### 图6

Fig.6   Water surface deformation comparison

## 3 三维水上迫降算例

### 图7

Fig.7   Model of three-dimensional example

### 图8

Fig.8   Mesh of the three-dimensional example

### 图9

Fig.9   Results comparison of the three-dimensional example

## 4 某型支线飞机水上迫降模拟

### 图10

Fig.10   Mesh of the regional aircraft

### 图11

Fig.11   Results comparison of the regional aircraft

4.3.1 运动过程

### 图12

Fig.12   Water surface position and pressure distribution during ditching

0.6 s左右俯仰角达到第一个峰值约32$^\circ$。由于该支线飞机采用T型尾翼,高置平尾,因此即使俯仰角到达峰值时平尾仍然位于水面以上,局部喷溅的水花对平尾的冲击力较小,无法起到抑制俯仰角继续上仰的作用。0.6 s以后,由于巨大的水阻力使得滑行速度迅速降低、后体吸力减弱,同时中后机身以及发动机短舱着水部位受到较大的冲击力,产生低头力矩,从而使得飞机逐渐低头。过程中飞机持续下沉,中后机身和发动机不断排开水面,冲击力出现小幅增大。

0.8 s~1.0 s之间,随着飞机的低头前机身逐步入水,在此过程中受到较大的冲击力产生抬头力矩,使得1.0 s以后加速度和俯仰角均出现第二次峰值,但由于此时速度已经大幅度降低,第二次峰值相较第一次有明显降低。此后飞机逐渐趋于稳定,1.2 s时机身底部的压力分布已经显著降低且分布较为均匀。

4.3.2 受力分析

### 图13

Fig.13   Time history of force during ditching

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