Strategic Control of Variable Density Jets in Crossflow

本研究探讨了变密度横流或横向射流的近场剪切层不稳定性的基本性质和控制方法。横向射流在能源系统中有着广泛的应用，包括用于温度模式因子和NOx排放控制的稀释空气喷射，涡轮叶片的膜冷却射流，以及高速燃烧室的热反应燃料射流。控制与此类射流相关的轨迹、扩散、混合以及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的基础上？目前，美国国家科学基金会（nsf）资助了一项研究，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。这表明，随着密度和/或温度的变化，流场中可能会出现一系列潜在的有趣现象。对于固定雷诺数的等温等密度射流，射流从？飞机有空吗?到？横向飞机吗?随着横流速度的增加。然后观察到上游剪切层不稳定的光谱特征的显著变化。射流出口附近的不稳定性发展出强大的多模态，当射流与横流的速度比在10到3.5之间时，这些模态的强度相互超过。在较低的比例下，横向射流剪切层失稳除了高次谐波外，还发展为强主导模态，表明其非线性演化。这些剪切层模式的变化可以在实验中看到，它们的特征在线性稳定性分析和三维数值模拟中得到了描述和支持。稳定性特征表明，在临界速度比下，剪切层从对流不稳定变为绝对不稳定或自激。横向射流剪切层对低层强迫的响应与这一解释一致。这样的稳定性特征说明了横向喷射的原因。S与周围气体混合的能力优于自由射流，但也提出了通过射流强迫控制射流行为的策略。低至中等水平的正弦压力会影响速度比约为3.5以上的射流响应，因此可以改善和控制射流的扩散和穿透。然而，对于较低的速度比，需要非常强的强迫与规定的时间脉冲宽度显著影响射流。基于这些观察和众所周知的与低密度或加热自由射流相关的不稳定性转变，pi计划探索与变密度和非等温横向射流相关的剪切层不稳定性，并开发基于与发电系统相关的特定应用的射流战略控制程序。假设横流中加热或低密度射流在绝对不稳定流的发展过程中会加速，即不稳定将在比等温情况下更高的动量通量比下发生。pi假设在横流中，与高密度或较冷的射流相关的不稳定性会发生相反的效果。因此，对于涉及冷却稀释空气射流、涡轮叶片的气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征开发不同的控制策略。π吗?他目前的NSF项目为当地高中生和其他大学中代表性不足的学生提供了重要的推广和实验室经验。在本项目中，本科生将继续接受变密度横向射流研究的培训和就业，并希望得到NSF REU的支持。外展演讲和演示将继续为当地的高中和潜在的初中和小学的学生，以弥补加州政府对公立学校的资金削减？MESA（数学工程科学成就）项目。

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.