Experiments on Three-Dimensional Disturbance Interactions Leading to Boundary-Layer Transition

建议编号：CTS-0245253 PROPOSAL型：调查员发起PRINPIPAL调查员：Edward B.WHITEINSTUTION：CASE WESTERSITY UNIVERSITYEXPERIMENTS关于导致边界层传输的三维扰动相互作用边界层从层流到湍流的转变是一个复杂的过程，可以通过取决于流体速度、表面几何形状、环境扰动和其他因素的许多情景来进行。尽管过渡的性质复杂，但过去对过渡主题的研究往往侧重于单一的干扰-增长机制，而不是所有其他相互竞争的机制。对于Blasius边界层，被称为Tollmien-Schlichting(T-S)波的沿流向传播的不变扰动的增长就是这样一种已被广泛研究的机制。一种新发现的被称为瞬时增长的机制也受到了极大的关注，这种机制倾向于平稳的、跨距变化的扰动。虽然在孤立地理解这些机制方面取得了很大进展，但在预测或控制可能存在多种竞争机制的真实系统中的转变开始方面进展甚微。为了解决涉及竞争扰动的现实过渡情景中缺乏有用数据的问题，将进行一项关于T-S波与Blasius边界层中的瞬变扰动相互作用的风洞调查。拟议中的实验将提供一个可控的和系统的研究T-S波、瞬变扰动和二次不稳定如何竞争和相互作用来实现跃迁。输入干扰将在较宽的参数范围内变化，并将被设计为充分利用适合于从噪声背景中提取极低幅度干扰信号的信号分析技术。研究结果将为过渡预测和控制工作提供必要的数据。除了该项目的内在科学价值外，拟议的工作还将有助于改进重要的技术系统，并促进研究在凯斯西储大学教学中的作用。过渡会影响广泛的车辆和工业系统的性能和效率，因为层流边界层会产生较低的车辆阻力，而湍流边界层通常是完全和有效的流体混合所必需的。如果设计者能够预测和控制层流变得湍流的条件，那将是非常有益的。准确的预测将缩短设计周期，减少对过渡敏感的系统典型的过度设计，而流量控制将允许从根本上更有效的系统设计。更直接的是，该项目将通过支持一个对本科生开放的风洞实验室的强有力的实验计划，并通过将在风洞中进行的工作纳入实验方法和流体力学教学，来加强研究在教学中的作用。

PROPOSAL NO.: CTS-0245253PROPOSAL TYPE: INVESTIGATOR INITIATEDPRINCIPAL INVESTIGATOR: EDWARD B. WHITEINSTITUTION: CASE WESTERN RESERVE UNIVERSITYEXPERIMENTS ON THREE-DIMENSIONAL DISTURBANCE INTERACTIONS LEADING TO BOUNDARY LAYER TRANSITIONThe laminar-to-turbulent transition of boundary layers is a complicated process that can proceed via a number of scenarios depending on fluid velocity, surface geometry, environmental disturbances, and other factors. In spite of its complicated nature, past research on the subject of transition has tended to focus on single disturbance-growth mechanisms in isolation from all other competing mechanisms. For Blasius boundary layers, the growth of streamwise-traveling, spanwise-invariant disturbances known as Tollmien-Schlichting (T-S) waves is one such mechanism that has been extensively studied. A newly discovered mechanism known as transient growth, which favors stationary, spanwise-varying disturbances, has also received significant attention. Although much progress has been made in understanding these mechanisms in isolation, little progress has been made in predicting or controlling transition onset in real systems in which several competing mechanisms can exist. To address the lack of useful data on realistic transition scenarios involving competing disturbances, a wind-tunnel investigation of the interactions of T-S waves and transient disturbances in a Blasius boundary layer will be undertaken. The proposed experiments will provide a controlled and systematic investigation of how T-S waves, transient disturbances, and the secondary instabilities of both compete and interact to bring about transition. The input disturbances will be varied over wide parameter ranges and will be designed to take full advantage of signal-analysis techniques suitable for extracting very low amplitude disturbance signals from a noisy background. The results will provide data necessary for transition prediction and control efforts. Beyond the intrinsic scientific value of the project, the proposed work will also contribute to the improvement of important technological systems and to the promotion of the role of research in teaching at Case Western Reserve University. Transition affects the performance and efficiency of a broad range of vehicles and industrial systems because laminar boundary layers result in low vehicle drag, whereas turbulent boundary layers are often desired for complete and efficient fluid mixing. It would be of great benefit if designers were capable of predicting and controlling the conditions under which laminar flows become turbulent. Accurate prediction would shorten design cycles and reduce the over design typical of systems sensitive to transition, and flow control would permit the design of radically more efficient systems. More immediately, this project will enhance the role of research in teaching by supporting a vigorous experimental program in a wind-tunnel laboratory that is accessible to undergraduates and by incorporating the work performed in the tunnel into experimental-methods and fluid mechanics instruction.