非平行圆柱流致噪声形成机理及降噪技术研究

The landing gear is a vital aircraft component used for landing and take-off. The primary noise source during take-off and landing is the landing gear. Noise continues to be an ongoing problem for existing aircraft and is a major concern for next generation designs. The noise is a problem not only for the passengers but also for the people dwelling around the airport, particularly for the children. Eventually, the children residing around airport experience hearing loss and poor academic performance. The noise from the landing gear is generally broad banded, originating due to complex, unsteady flow separation from the upstream strut and the subsequent interactions of the separated flow/vortex with the downstream strut. To understand the behavior of this complex interactive noise generation, two tandem non-parallel cylinders can be adopted as a representative tractable model of the landing gear struts. Here we are motivated to examine the detailed fluid dynamics and flow-induced noise generated from two non-parallel cylinders, where the included angle between the cylinders and the cylinder orientation will be varied. The brief objectives of the project are to (i) examine the vortex shedding frequency, shear-layer/vortex interaction, impingement process, and formation of spiral vortices in the gap and the wake; (ii) measure the radiated noise and hotwire-measured velocity simultaneously and find the correlation between them to assimilate underlying physics behind the noise; and (iii) establish a state-of-art means to suppress or reduce the noise. This project will impact upon social, academic and economic indices, providing data to aircraft industries.

起落架是飞机至关重要的组成部分，关系到飞机起飞和降落时的飞行安全。同时起落架还是飞机重要的噪声源。噪音问题不仅是当下亟需解决的难题，更关系到新一代飞机的设计与研发。噪音不仅会影响乘客的乘机体验，尤其还会对生活在机场周围的儿童产生听力上的伤害。起落架通常会产生宽频噪声，这是因为各构件之间存在着复杂的、非稳态的流动分离和流固耦合现象。为理解其复杂的交互式形成机理，考虑到起落架的具体结构，可以选用两根非平行的串列圆柱作为实验模型。受此启发，我们决定通过改变两圆柱的夹角和指向来研究起落架周围的流动特性及其噪声形成机理。研究目标如下：（1）探究涡脱频率，剪切层/旋涡之间的相互作用和相关的撞击过程，间隙和尾流中螺旋形旋涡的形成过程与圆柱夹角和指向的关系；（2）同步测量噪声和速度，揭示与噪声产生有关的机理；（3）发展一种先进的降噪技术。本项目具有重要的学术和经济价值并可为设计人员提供可靠的理论依据。

降低以起落架噪声为主的飞机机体噪声，是国产大飞机研制工作中的重点技术问题之一，也是提高其国际竞争力的关键所在。本项目以非平行串列双圆柱为模型对起落架噪声的形成机理及基于螺旋绊线的噪声抑制技术进行了系统且深入的实验研究。利用包括粒子图像测速仪（PIV）在内的多种流体测量技术和多层次的数据分析方法，对起落架模型所涉及的流体力、压力分布、噪声声压级水平、湍流形态等动力学特性进行了细致的探究。一方面研究了柱体绕流特性随柱体间距的变化规律，另一方面探究了螺旋绊线的直径、数目、安装位置和旋向对噪声抑制效果的影响，最后对相关的噪声抑制机理做出了详细阐释。本项目的主要研究成果包括：1. 随着柱体间距的减小，相关的流动形态可以分为三类，即双涡脱(co-shedding)、双稳态(Bistable)和再附着(Reattachment)流态。2. 受上游圆柱与来流方向夹角的影响，下游圆柱的绕流在不同展向位置处会出现上扬(Upward)和下扫(Downward)运动。3. 当螺旋绊线的数目为1.0，螺距为1.0，绊线与柱体直径比为0.05时，噪声抑制效果最为明显。噪声的抑制效果主要由上游圆柱的绊线所决定，且左旋向为最佳。4. 在最佳控制工况（上游圆柱上半部绊线左旋，下半部绊线右旋，下游圆柱绊线右旋）下，全频段内的最大降噪量为12分贝。5. 研究发现利用螺旋绊线实现降噪的机理如下：绊线使得柱体间隙处的回流强度和脱落漩涡强度减弱；流动与柱体之间的非定常耦合作用弱化；回流区和尾流区的脉动速度、雷诺剪切应力、湍动能大幅削弱。上述有关起落架噪声形成机理和降噪技术研发的成果，不仅为大飞机设计中起落架结构的优化提供了一定的理论依据，而且在一定程度上为新型起落架降噪技术的研发与应用提供了数据支持和技术支撑。本项目已圆满完成项目计划书中所拟定的研究内容，实现了既定的研究目标。相关的研究成果已经发表在湍流相关领域的核心期刊，并在多个国内外学术会议上进行了报导。