Instability Mechanisms for Low Density and Reactive Transverse Jets

1133015 karagozian这个项目涉及到低密度非活性和活性（燃烧）射流注入横向流的研究，这个主题适用于提高效率和减少排放的一系列能源和推进系统。控制轨迹、扩散、混合、反应完成以及与此类射流相关的温度或密度场的能力是这些研究的总体目标。目前的方法主要是实验性的，包括光学诊断作为一种询问和分析流场不稳定性的手段，作为一种开发控制方法的手段。这项工作将有助于提高我们为固定式发电厂、吸气式推进系统和火箭推进系统设计和开发高效未来发动机的能力。本研究的智力优势在于详细探讨了低密度非活性和活性横向射流对流到绝对剪切层不稳定转变的机制。对于横流中的等密度射流和低密度射流，有显著证据证实剪切层变得全局不稳定或自激；这些证据包括不沿射流剪切层变化的强单音扰动的起始，抑制次谐波起始，仅在非常大的强迫振幅和/或接近基频的频率上锁定应用频率，以及霍普夫分岔的证据。我们能够利用这些不稳定性的知识在横向射流中应用策略强迫，即相对弱的正弦射流激励可以增强对流不稳定的射流，但在特定的时间脉冲宽度下需要强大的方波强迫来控制绝对不稳定的横向射流。然而，这种转变的具体机制，在不同激励条件下改变混合状态的量化，以及化学反应存在下横向射流的行为都是重要的未知数，具有重要的实际意义。这些实验研究采用激光诊断，包括粒子图像测速（PIV）与平面激光诱导荧光（PLIF）同步成像，以定量三维速度和涡度场以及横向射流中的标量输运过程。在这些探索的基础上，将通过丙酮PLIF量化与非反应流（等密度和低密度射流）相关的混合，作为确定流动和激励条件以优化射流交叉流混合的一种手段。闭环控制方法正在被开发，用于优化射流行为和对扰动条件的响应，使用非侵入式传感，其测量可能与全局流动特性有关。对于横流中的反应射流，OH*化学发光成像将用于确定火焰特性、不稳定性和流动特性。这些基础探索对于预测和控制一系列工程系统中的这种不稳定性的能力至关重要。拟议研究的更广泛影响远远超出了横向射流的实际应用和通常的“研究生水平的研究和教育”。我们在这些影响方面的记录包括本科研究培训，以及为当地高中生和其他大学中代表性不足的学生提供拓展和实验室经验。在目前的项目中，本科研究人员将继续在我们的实验室接受培训和雇用，并将继续为当地的高中和潜在的初中和小学学生提供外展演讲和演示，特别关注公立学校的学生。将为当地高中生提供在夏季获得实验室经验的机会。加州政府削减了对公立学校的资助——MESA（数学工程科学成就）项目使得像加州大学洛杉矶分校这样的大学将拓展项目扩展到公立初中和高中，作为鼓励未来工程师和科学家发展的一种手段变得更加重要。

1133015KaragozianThis project involves the study of low density non-reactive and reactive (burning) jets injected into crossflow, a topic applicable to improved efficiency and reduced emissions for a range of energy and propulsion systems. The ability to control the trajectory, spread, mixing, reaction completion, and temperature or density field associated with such jets is the overall goal of these studies. The present approach is primarily experimental, involving optical diagnostics as a means of interrogating and analyzing the instabilities in the flowfield as a means of developing methods for their control. This work will help to advance our ability to design and develop highly efficient future engines for stationary power plants, airbreathing propulsion systems, and rocket propulsion systems.The intellectual merits of this research involve detailed explorations of the mechanisms underlying convective to absolute shear layer instability transition for low density non-reactive and reactive transverse jets. For both equidensity and low density jets in crossflow, there is significant evidence to confirm that the shear layer becomes globally unstable or self-excited; such evidence includes the initiation of strong single-tone disturbances that do not vary along the jet's shear layer, the inhibition of subharmonic initiation, lock-in to applied frequencies only at very large forcing amplitudes and/or at frequencies close to the fundamental, and evidence of a Hopf bifurcation. We are able to take advantage of the knowledge of these instabilities in applying strategic forcing to the transverse jet, whereby relatively weak sinusoidal jet excitation can enhance the convectively unstable jet, but strong square wave forcing at specific temporal pulse widths is required to control the absolutely unstable transverse jet. Yet the specific mechanisms for this transition, a quantification of altered mixing states during different excitation conditions, and the behavior of transverse jets in the presence of a chemical reaction are all significant unknowns, with substantial practical relevance.These experimental studies employ laser diagnostics, including particle image velocimetry (PIV) simultaneous to planar laser-induced fluorescence (PLIF) imaging of seeded acetone, in order to quantify 3D velocity and vorticity fields as well as scalar transport processes in transverse jets. Building on these explorations will be quantification of mixing associated with the non-reactive flow (equidensity as well as low density jets) via acetone PLIF, as a means of determining flow and excitation conditions to optimize jet-crossflow mixing. Closed loop control methods are being developed for the optimization of jet behavior and response to upset conditions, using non-intrusive sensing whose measurements may be related to global flow characteristics. With respect to the reactive jet in crossflow, OH* chemiluminescence imaging will be used to determine flame characteristics, instabilities, and flow characteristics. These fundamental explorations will be critical to the ability to predict and control such instabilities in a range of engineering systems. The broader impacts of the proposed studies go well beyond the practical applications of the transverse jet and the usual "graduate level research and education". Our track record in such impacts has included undergraduate research training as well as outreach to and laboratory experiences for local high school students and underrepresented students at other universities. In the present project, undergraduate researchers will continue to be trained and employed in our laboratory, and outreach presentations and demonstrations will continue to be provided for local high school and potentially middle school and elementary school students, with special focus on students at public schools. Opportunities for local high school students to gain laboratory experiences in the summer will be provided. Cutbacks in funding by the state of California to the public schools- MESA (Mathematics Engineering Science Achievement) programs has made it even more important for universities such as UCLA to extend outreach programs to public middle schools and high schools as a means of encouraging the development of future engineers and scientists.