Mechanisms of Transient Growth and Turbulence Evolution in a Columnar Vortex

建议没有。柱状涡的瞬态生长和湍流演化机制本项目将通过直接数值模拟（DNS）研究环境湍流与单个大尺度相干涡（理想化为孤立柱状涡）相互作用的大尺度/细尺度涡耦合的基本问题。具有大尺度涡（相干结构，CS）特征的湍流在工程应用中经常遇到，例如涡轮、尾涡、射流、尾迹和边界层。虽然现在人们已经认识到，涡流会产生重要的工程效应，如阻力、混合、传热、燃烧和流动噪声，但湍流反过来又会通过在涡核中诱导振幅较大的波，引发环境流动中的新的不稳定性，并增强涡流衰减，从而强烈影响涡流本身的演变。CS演化的建模和预测——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。提出的方法的一个中心方面是应用新的频谱DNS技术，正确地处理适当的流域边界条件。研究表明，这种涡旋产生的强烈的、定向的次级细尺度细丝可以在涡旋柱本身的核心上诱发振幅较大的波。瞬态增长分析可以提供一种系统的方法来确定最有利于旋涡不稳定和过渡的特定“最佳”模式。这种模式——通过DNS来实现——可能导致“旁路”转换和持续的湍流。现有湍流模型未能捕捉到这些影响，这似乎是预测和观测到的涡旋衰减率存在巨大差异的原因——涡旋衰减率在飞机尾随涡旋等应用中具有重要的实际意义。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。涡旋/湍流耦合的基本问题与广泛的应用相关：从减轻飞机尾流危害到改进燃气轮机设计，再到减少车辆/飞机阻力。通过控制尾涡和边界层的瞬态增长，还可以显著节省燃料。这项研究的结果将有助于减少繁忙机场的飞机起飞间隔，提高机场利用率，并避免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，通过使用飞机尾流问题和本研究项目的发现，为他们提供研究经验，并在休斯顿学校和科学博览会上推广工程。

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.