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Efficient ground noise prediction method based on secondary noise radiation model
WANG Fei , SHI Yongjie , XU Guohua
National Key Laboratory of Science and Technology on Rotorcraft Aeromechanics, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China
Received: 2015-11-17; Accepted: 2016-02-29; Published online: 2016-04-13
Foundation item: National Natural Science Foundation of China (11302103)
Corresponding author. Tel.:025-84892117,E-mail:shiyongjie@nuaa.edu.cn
Abstract: An efficient ground noise prediction method for the analysis of helicopter noise in flight based on the secondary noise radiation is developed. The method to predict the aerodynamic force and aeroacoustic characteristics based on the free-wake/FW-H, the noise propagation model that can take the effects of atmosphere absorption, ground absorption and reflection on noise radiation characteristics into account, as well as ground noise computation model are included in this method. In order to improve computational efficiency, the secondary sound radiation model built on the hypothesis of compact source is introduced between the noise source computation and noise radiation. To achieve the real time simulation of ground noise, the noise radiation model database is built according to the idea of parametric modeling. The efficiency and effectiveness are illustrated through the comparison of computed results and compute time with the direct computing method for AH-1 rotor. Furthermore, the effects of atmosphere and ground on noise radiation as well as ground noise characteristics are analyzed with this method.
Key words: noise     rotor     secondary noise radiation     atmosphere absorption     ground absorption and reflection

1 计算方法

 图 1 2种地面噪声预测方法对比 Fig. 1 Comparison between two ground noiseprediction methods
1.1 旋翼气动/噪声预测模型

 (1)
 图 2 旋翼自由尾迹模型示意图 Fig. 2 Schematic of rotor free-wake model

 (2)

1.2 二级声辐射模型

 图 3 二级声辐射模型示意图 Fig. 3 Schematic of secondary noise radiation model
1.3 噪声传播模型

 (3)
 (4)

 (5)
 (6)
 图 4 观测点噪声接收示意图 Fig. 4 Schematic of noise receiving at observation point

1.4 声场计算流程

 图 5 快速地面声场计算流程图 Fig. 5 Flowchart of quick ground noise computing
2 结果讨论与分析 2.1 计算细节

 图 6 旋翼空间位置及地面观测点分布示意图 Fig. 6 Schematic of rotor spatial position anddistribution of ground observation point
2.2 方法验证

 图 7 旋翼桨尖涡尾迹几何形状 Fig. 7 Wake geometry of rotor blade tip vortex

 图 8 2种计算方法噪声声压级计算结果对比 Fig. 8 Comparison of computed noise SPLresults of two different methods

 图 9 地面观测点声压时间历程 Fig. 9 Sound pressure time history of groundobservation points

 计算方法 声辐射点数目 观测点数目 计算用时 二级声源法 1 10 万 305 s 直接计算法 101 304 10 万 3.2 h

2.3 大气吸声影响分析

 图 10 大气吸声对噪声传播特性影响 Fig. 10 Effects of atmosphere absorption on noiseradiation characteristics
 图 11 大气吸声对噪声声压时间历程的影响 Fig. 11 Effects of atmosphere absorption on noise soundpressure time history
2.4 地面效应影响分析

 图 12 不同地面类型对噪声传播特性影响 Fig. 12 Effects of different grounds on noiseradiation characteristics

 编号 地面类型 空隙率 流阻/(kg·s-1·m-3) 地面1 间隙填以碎石的旧土路 0.400 7 500 地面2 裸露并被雨水填实的土地 0.350 17 100 地面3 被尘封并很少使用的沥青路面 0.250 120 000

 图 13 观测点噪声声压时间历程 Fig. 13 Noise sound pressure time history of observation points
 图 14 不同高度观测点接收到的声压时间历程 Fig. 14 Accepted sound pressure time history ofobservation point at different heights
 图 15 不同高度观测点总声波频域特性 Fig. 15 Frequency-domain characteristics of total acousticwave of observation point at different heights
3 结 论

1) 在旋翼噪声预测及噪声传播之间，结合点声源假设和旋翼噪声辐射的方向性特点建立了二级声辐射球，从而将随桨叶旋转的多个运动声源转换为声辐射球面上单一静态辐射点，显著提高了计算效率。当地面观测点数量为10万时，传统计算方法耗时约为3.2 h，本文所建方法仅为305 s。

2) 为实时预测直升机运动过程中旋翼噪声对地面声场影响，文中提出了“特征参数声辐射球库”。在地面声场预测中，可根据旋翼所处状态的特征参数，直接选取相应的二级声辐射球进行地面噪声辐射计算。

3) 大气、地面吸声效应对旋翼噪声具有较强的衰减作用，但几乎不会改变旋翼噪声传播的方向性；地面流阻对地面吸声效应具有较大的影响。

4) 对于距地面具有一定的高度的观测点，其接收到的噪声有直达声波与反射声波；地面反射对观测点时、频域噪声均具有较大的影响，且会发生“声共振”现象，此时低频噪声将会增大，对地面人员产生较大的影响。

5) 应当指出，本文所建快速地面声场预测方法对二级声辐射球半径具有一定的要求，当噪声传播距离较近时，应采用直接计算法进行地面声场预测。

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#### 文章信息

WANG Fei, SHI Yongjie, XU Guohua

Efficient ground noise prediction method based on secondary noise radiation model

Journal of Beijing University of Aeronautics and Astronsutics, 2016, 42(11): 2445-2453
http://dx.doi.org/10.13700/j.bh.1001-5965.2015.0756