Strategic Control of Variable Density Jets in Crossflow

本文研究了变密度横流或横向射流近场剪切层不稳定性的基本性质和控制方法。横向射流广泛应用于能量系统中，包括用于温度模式因子和NOx排放控制的稀释空气射流喷射、用于涡轮机叶片的薄膜冷却射流以及高速燃烧器中的热反应燃料射流。控制与这种射流相关的轨迹、扩散、混合和温度或密度场，对提高能源效率具有重要意义。这些研究建立在PI？目前NSF资助的研究探索与等温，等密度横向射流及其主动强迫相关的剪切层不稳定性。这些表明，一系列潜在的有趣的现象可能存在于密度和/或温度变化的流场。对于等温，等密度射流在一个固定的射流雷诺数，射流过渡？免费飞机？到？横向喷流随着横流速度的增加。然后观察到上游剪切层不稳定性的谱特征的显著变化。射流出口附近的不稳定性发展强大的多个模式，在射流与横流的速度比在10和3.5之间的强度超过彼此。对于较低的比率，横向射流剪切层的不稳定性开发了强大的主导模式，除了高次谐波，这表明非线性演化。在这些剪切层模式的改变被认为是在实验中，他们的性格描绘和支持的线性稳定性分析和三维数值模拟。稳定性特征表明，在临界速度比，剪切层的变化从对流不稳定可能是绝对不稳定或自激。横向急流剪切层对低空强迫的响应与这种解释是一致的。这种稳定性的特点表明横向射流的原因？的能力与周围的气体混合在一个上级的方式，自由射流，但也建议通过射流强迫控制射流行为的策略。低到中等水平的正弦强迫可以影响速度比大于约3.5的射流响应，因此可以改善和控制射流扩展和穿透。然而，对于较低的速度比，非常强的强迫与规定的时间脉冲宽度，需要显着影响射流。根据这些观察和众所周知的过渡与低密度或加热自由射流的不稳定性，PI计划探索与变密度和非等温横向射流的剪切层不稳定性，并制定程序的战略控制的基础上的具体应用程序的喷气发电系统。假设横流中的加热或低密度射流将加速绝对不稳定流动的发展，即不稳定性将发生在比等温情况更高的动量通量比处。PI假设，对于与横流中较高密度或较冷射流相关的不稳定性，将发生相反的影响。因此，对于涉及冷却稀释空气射流、涡轮机叶片的薄膜冷却和燃烧室中的热反应射流的应用，需要根据不稳定性的不同特性开发不同的控制策略。私家侦探？美国国家科学基金会目前的项目已经为当地高中学生和其他大学代表性不足的学生提供了重要的外展和实验室经验。在这个项目中，本科生将继续接受培训，并希望在NSF REU的支持下从事变密度横向射流研究。将继续为当地高中和可能的初中和小学学生进行外展演讲和示威，以弥补加州对公立学校资金的削减？MESA（数学工程科学成就）计划。

CBET-0755104KaragozianThis study examines the fundamental nature and control approaches of near-field shear layer instabilities for the variable density crossflow or transverse jets. Transverse jets are widely applicable in energy systems, including dilution air jet injection for temperature pattern factor and NOx emissions control, film cooling jets for turbine blades, and hot reactive fuel jets in high speed combustors. Controlling the trajectory, spread, mixing, and temperature or density field associated with such jets could have important implications for improved energy efficiency. These studies build on the PI?s current NSF-funded research exploring shear layer instabilities associated with isothermal, isodensity transverse jets and their active forcing. These suggest that a range of potentially interesting phenomena may be present in the flow field with density and/or temperature variation. For the isothermal, isodensity jet at a fixed jet Reynolds number, the jet transitions from the ?free jet? to the ?transverse jet? as the crossflow velocity increases. Remarkable alterations in the spectral character of the upstream shear layer instability are then observed. The instability near the jet exit develops strong multiple modes that overtake one another in strength for jet-to-crossflow velocity ratios between 10 and 3.5. For lower ratios, the transverse jet shear layer instability develops a strong dominant mode in addition to higher harmonics, suggesting a nonlinear evolution. The alterations in these shear layer modes are seen in experiments, and their character is delineated in and supported by linear stability analyses and 3D numerical simulations. The stability characteristics suggest that at critical velocity ratios, the shear layer changes from being convectively unstable to possibly being absolutely unstable or self-excited. Response of the transverse jet shear layer to low level forcing is consistent with this interpretation. Such stability characteristics suggest reasons for the transverse jet?s ability to mix with surrounding gases in a superior manner to that of the free jet, but also suggest strategies for controlling jet behavior via jet forcing. Low to moderate level sinusoidal forcing can affect jet response for velocity ratios above approximately 3.5, so jet spread and penetration can be improved and controlled. However, for lower velocity ratios, very strong forcing with a prescribed temporal pulse width is required to significantly affect the jet. Based on these observations and the well known transition in instabilities associated with low density or heated free jets, the PIs plan to explore the shear layer instabilities associated with variable density and nonisothermal transverse jets, and to develop procedures for the strategic control of the jet based on specific applications relevant to energy generation systems. It is hypothesized that heated or low density jets in crossflow will experience an acceleration in the development of absolutely unstable flow, that is, the instability will occur at higher momentum flux ratios than in the isothermal case. The PIs postulate that the opposite effect would occur for the instabilities associated with the higher density or cooler jet in crossflow. Thus, for applications involving cooling dilution air jets, film cooling for turbine blades, and hot reactive jets in combustors, different control strategies based on the differing character of the instabilities will need to be developed. The PI?s current NSF project has had significant outreach to and laboratory experiences for local high school students and underrepresented students at other universities. In this project, undergraduates will continue to be trained and employed in the variable density transverse jet studies with hoped for NSF REU support. Outreach presentations and demonstrations will continue for local high school and potentially middle school and elementary school students to compensate for cutbacks in funding by the state of California to the public schools? MESA (Mathematics Engineering Science Achievement) programs.