Mechanisms of Transient Growth and Turbulence Evolution in a Columnar Vortex

建议编号：CTS-0554165主要研究人员：F.HUSSAININT：HOUSTONN大学柱状涡中的瞬变增长和湍流演化这个项目将通过环境湍流与单个的大尺度相干涡旋相互作用的直接数值模拟(DNS)来关注大尺度/细尺度涡度耦合的基本问题-理想为孤立的柱状涡。以大尺度涡(相干结构)为特征的湍流在工程应用中经常遇到，如涡轮、尾涡、射流、尾迹和边界层等。虽然CS现在被公认为是重要的工程效应--如阻力、混合、热传递、燃烧和流动噪声--但湍流反过来又通过在涡核中诱导大振幅波、在环境流动中触发新的不稳定性并增强CS衰变来强烈地影响CS本身的演变。CS演变的模拟和预测--涉及大尺度和细尺度湍流之间的复杂耦合--仍然是一个艰巨的挑战。所提出的方法的一个中心方面是应用新的频谱DNS技术，该技术正确地解决了适当的流域边界条件。研究表明，这种涡旋孕育出的强烈的、方位向定向的二次细丝可以在涡柱的核心上诱导出大振幅的波。瞬变增长分析可以提供一种系统的方法来确定最有利于涡不稳定性和转换性的特定的最佳模式。这类模式--要通过域名系统来实现--可能会导致“绕过”过渡和持续的湍流。现有的湍流模型未能捕捉到这些影响，这似乎是预测的涡流衰减率和观测到的涡流衰减率之间存在巨大差异的原因--在飞机尾涡等应用中，这是一个具有基本实用意义的量。这项研究将产生广泛的教育、科学、技术影响，并最终产生非常重大的经济影响。涡流/湍流耦合的这一基本问题与广泛的应用有关：从减轻飞机尾流危险到改进燃气轮机的设计，再到减少车辆/飞机的阻力。通过控制尾涡和边界层中的瞬时增长，还将显著节省燃料。这项研究的发现将有助于最大限度地减少繁忙机场的飞机起飞间隔，提高机场利用率，并消除对昂贵的新跑道的需求。研究生和本科生将与NASA/JSC工程师合作，与高中科学教师和学生合作，向他们传授研究经验，并通过使用飞机尾迹问题和该研究项目的发现，在休斯顿学校和科学博览会上普及工程学。

PROPOSAL NO.: CTS-0554165PRINCIPAL INVESTIGATOR: F. HUSSAININSTITUTION: UNIVERSITY OF HOUSTONMECHANISMS OF TRANSIENT GROWTH AND TURBULENCE EVOLUTION IN A COLUMNAR VORTEX This project will focus on the fundamental problem of large-scale/fine scale vorticity coupling through direct numerical simulations (DNS) of ambient turbulence interacting with a single, large-scale coherent vortex - idealized as an isolated columnar vortex. Turbulent flows featuring large-scale vortices (coherent structures, CS) are commonly encountered in engineering applications, e.g. turbines, trailing vortices, jets, wakes and boundary layers. While CS are now well recognized to be responsible for important engineering effects - such as drag, mixing, heat transfer, combustion, and flow noise - turbulence, in turn, strongly affects the evolution of the CS itself by inducing large-amplitude waves in the vortex core, triggering new instabilities in the ambient flow, and enhancing CS decay. The modeling and prediction of CS evolution - involving the intricate coupling between large- and fine-scale turbulence - remains a formidable challenge. A central aspect of the proposed approach is the application of novel spectral DNS techniques that correctly address the proper flow domain boundary conditions. It has been shown that intense, azimuthally-oriented secondary, finer-scale filaments that such vortex breeds can induce large-amplitude waves on the core of the vortex column itself. Transient growth analysis can provide a systematic way to identify specific 'optimal' modes most conducive to vortex instability and transition. Such modes - to be pursued through DNS - can cause 'bypass' transition and sustained turbulence. The failure of existing turbulence models to capture these effects appears to underlie the large discrepancies in predicted and observed vortex decay rates - a quantity of fundamental practical relevance in applications such as the aircraft trailing vortices. This research will have broad educational, scientific, technological, and, ultimately, very significant economic impact. This fundamental problem of vortex/turbulence coupling is relevant to a wide spectrum of applications: ranging from the alleviation of the aircraft wake hazard to the improved design of gas turbines to reducing vehicle/aircraft drag. Significant fuel savings will also result via control of transient growth in trailing vortices and boundary layers. The findings from this research will help minimize spacing of aircraft takeoffs in busy airports, enhancing airport utilization and obviating the need for expensive new runways. Graduate and undergraduate students, in collaboration with NASA/JSC engineers, will work with high-school science teachers and students to give them hands on research experience and to popularize engineering in Houston schools and science fairs via the use of the aircraft wake problem and findings from this research program.