【作者】 冯威；

【导师】 袁兆成；

【作者基本信息】 吉林大学 ， 动力机械及工程， 2004， 硕士

【摘要】 汽车排放出的有害气体是当今社会的一大公害，随着这一问题的日益解决，以及人们对生存环境的日益关心，汽车的噪声问题也成为了当前汽车研究领域的一大课题。据统计，在汽车的噪声污染中，发动机的噪声占71％。国外工程师对发动机噪声的研究，主要借助于先进的测试手段以及各种CAD\CAE工具，其分析结果比较精确，可用于实践，并且由于计算手段的先进，使问题从提出到解决的时间较短，从而满足了生产及市场的需求。而国内企业和研究所由于资金的匮乏，难以购置昂贵的仪器设备，因此利用软件进行设计优化显得尤为重要。降低内燃机辐射噪声是降低内燃机整体噪声底行之有效的方法之一。实践表明，内燃机的罩壳类零件具有壁薄和表面平而大的特点，往往是主要的表面噪声辐射源。资料表明油底壳辐射噪声占发动机整体辐射噪声的比重较大，油底壳的辐射噪声有时可占发动机总噪声的15~22%左右。因此降低油底壳的辐射噪声对于降低发动机整体噪声有着极大的意义。以往对油底壳噪声分析中，人们只分析油底壳，而把机油对其的影响单纯的看作阻尼，为了更准确的对其进行仿真，本文建立了油底壳及机油的液固耦合模型，对整体进行分析。有限元法在力学领域中的应用已相当成熟，但由于声辐射问题需要对整个外部声场划分三维网格，使得单元数量和求解工作量巨大，甚至无法求解。因此，有限元技术在声学领域里的应用仅限于内场分析和简单规则结构的外场分析。而边界元法则利用边界积分方程，使问题的维数降低了一维，并且既能求解有界区域问题，也能求解无界区域问题。将两者结合将达到很好的效果，有着广泛的应用。本文将有限元及边界元相结合，对CA6110油底壳进行液固耦合仿真，然后分析其辐射声场。并分析了各参数对其辐射声场的影响。 <WP=76>目前来讲，有限元法是求解系统动力响应最成功、最实用的方法，通过对振动结构的离散化并考虑适当的边界条件，可以很容易求解各种复杂结构在复杂激励作用下的响应。模态分析是动力学分析的基础，同时也是验证有限元模型最直接有效的方法。本文首先采用脉冲激励法，即锤击法，对油底壳进行了试验模态分析，得到了油底壳的一些固有频率和振型。然后用有限元大型分析软件Ansys建立了油底壳的有限元模型，并计算其自由模态。通过对二者模态结果相比较，发现误差相差均不超过10％。认为模型正确有效，可以作为后续分析的基础。然后进行了油底壳表面位移响应分析。首先通过试验测定发动机正常工作时油底壳受到的激励，同时测定了油底壳五个大平面中心的表面位移响应频谱。然后建立液固耦合有限元模型，施予正确的边界条件及试验测得的激励，对其进行谐响应分析。便得到了油底壳表面位移响应频谱。可以作为后续油底壳辐射声场分析的速度边界条件。再提取油底壳五大平面中心节点处的表面位移响应频谱，与试验结果相比较，吻合较好，证明了液固耦合有限元模型的正确性，以及谐响应分析方法的正确性。由前文所述，对于声场的仿真分析，边界元法显然是最好的方法。本文采用直接边界元的方法，对油底壳的外场进行仿真研究。首先建立了油底壳半消声室结构的声学边界元模型，将有限元液固耦合谐响应分析得到的油底壳表面位移响应频谱作为油底壳辐射声场分析的速度边界条件进行直接边界元计算。得到了油底壳表面声压，域点声压，以及输入、输出功率级，声辐射效率频谱等参数。找到了峰值频率点，确定了对辐射噪声贡献较大的几个频率点。将油底壳分成10个部分，对其进行声贡献分析，找到峰值频率点处声贡献较大的位置，以便将来对结构进行改进。在对CA6110柴油机正常工作时其油底壳液固耦合辐射声场分析的基础上，本文还考虑了各种参数变化对油底壳辐射噪声的影响。第一，在声贡献比较大的地方适当加筋，增加油底壳的结构刚度，再进行辐射声场分析，使得，油底壳输出声功率级明显下降，但声辐射效率略有增加。第二，对不含油油底壳，及油底壳正常工作允许范围内油量最少的油底壳的表面位移响应及辐射声场进行了分析。并将其和含正常油量的油底壳进行了比较。发现在分析频率范围内，总输入声功率级随油量的增加而增加，总输出声功率级在稍少油量情况下较大，说明油量不足时反而增加了对油底壳的激励，从而增加辐射声能。声辐射效率<WP=77>随油量的增加而减小了。第三，分析计算了发动机刚刚开始工作，机油温度取20℃时油底壳的表面位移响应及辐射声场。将其和发动机平稳运转时，机油温度取100℃时的油底壳进行比较发现在分析频率内随油温增加总输入声功率减少，总输出声功率却增加，声辐射效率增加。第四，改变油底壳材料为复合钢板(即采用三明治结构)以增加其结构阻尼。取阻尼损耗因子为0.1，0.3，0.5时分析计算液固耦合油底壳的表面位移响应及辐射声场，并将其与CA6110发动机油底壳（材料为A8钢板，阻尼损耗因子取0.005）进行了比较，发现在分析频率内随损耗因子增加总输入和输出声功率级降低，且在高频影响较大。由于从前文知输出声功率级在低频处较高，阻尼损耗因子较低时对其影响相对较明显，声辐射效率在损耗因子较大时（大于0.3）随之增加而降低，在损耗因子较小时?更多还原

【Abstract】 The exhaust of automobile has been one of the social effects of pollution. With the problem solved step by step, the noise of automobiles have been a main problem. It’s investigated that 71 percent of the automobile noise is produced by engine.Foreign engineers mainly use advanced testing apparatuses and CAD\CAE measures to study engine noise. The results are relatively accurate, and can be used in practice. Because the measures they use are advanced, the time they need is comparatively short. This meets to the requirements of production and market. As regards domestic corporations, they have less capital to purchase expensive apparatuses. So using software to optimize and design is very important.To reduce the noise of an engine, reducing its radiant noise is one of the effective methods. It has proved that the components of shell type in engine has thin wall and big plane and is the main radiant point of surface noise. Some data indicate the radiant noise of oil pan is an important part of engine radiant noise. In some conditions 15~22 percent of engine noise is come from oil pan. Above all, reducing the radiant noise of oil pan is meaningful to reducing engine noise.In most papers of oil pan radiant noise analysis, only oil pan was considered, oil was just regarded as damp and was not analyzed. For more accurate simulating the oil pan, this paper built a liquid-solid couple model of oil pan and oil, and analyzed them as a whole.FEM has matured in mechanics. But analyzing acoustic radiation need mesh the exterior acoustic field into 3D mesh, which increases the element number and solving workload and even lead the mesh cannot be solved. Therefore，FEM only applied in interior acoustic field or exterior acoustic field of simple <WP=79>and regular structure. Different from FEM, BEM uses boundary integral equation, which decreases the dimension of the problem. Therefore the method can solve the problems of both limitary and limitless field. Combine the two methods will gain a fine effect.This paper combined FEM and BEM, simulated the solid-liquid couple oil pan of CA6110 diesel engine, analyzed its radiant field, and then analyzed some parameters’ effect to its radiant field. At present, FEM is the most successful and practical method to solve the system dynamical response. Through dispersing the vibration structure and apply suitable boundary condition, FEM can easily solve all kinds of structure responses.Modal analysis is the basic of dynamics, and as well as the effective method to validate FEM model. This paper tested the experiment mode of the oil pan via the impulse prompting method, and got the oil pan’s nature frequency and mode. Then built FEM model of oil pan through Ansys, which is a FEM analysis software, and computed its free mode. Through comparing the two kinds of modal results, it was find that the errs are all not more than 10 percent, so it can be concluded that the model is right and can be used for lasting analysis.After the modal analysis, this paper analyzed the surface displacement response of oil pan. First, tested the prompting that oil pan undertake when the engine in working order, and the surface displacement response spectra on the centers of the five big planes at same time. Then, built liquid-solid couple FEM model, applied accurate boundary condition and prompting which come from the experiment mentioned above. After harmonic solution, the surface displacement spectra of the oil pan were gotten, which can be the velocity boundary condition of BEM analysis. Through observing the surface displacement response spectra of the nodes which in the centers of oil pan’s five big planes and comparing them with the trial results, it is find that they are consistent, so the model and method can be considered as right.Just as noted above, BEM is a best method to simulation an acoustic field. This paper simulation the exterior acoustic field of oil pan with direction BEM. First, built a semi-anechoic chamber BEM model of oil pan’s acoustic field, <WP=80更多还原