Strategic Control of Variable Density Jets in Crossflow

CBET-0755104卡拉戈兹这项研究考察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源系统中，包括用于温度模式因子和NOx排放控制的稀释空气射流、涡轮叶片的气膜冷却射流和高速燃烧室中的热反应燃料射流。控制与这种射流相关的轨迹、扩散、混合以及温度或密度场可能对提高能源效率具有重要意义。这些研究建立在美国国家科学基金会资助的皮S流研究的基础上，该研究探索了与等温、等密度横向急流及其活动强迫有关的剪切层不稳定性。这表明，随着密度和/或温度的变化，流场中可能存在一系列潜在的有趣现象。对于固定射流雷诺数的等温、等密度射流，射流从自由射流过渡到自由射流。到横向喷流吗？随着横流速度的增加。观测到上游剪切层不稳定的频谱特征发生了显著的变化。射流出口附近的不稳定性形成了强烈的多种模式，当射流与横流的速度比在10到3.5之间时，这些模式的强度相互超越。在较低的比例下，横向喷流剪切层不稳定性除了产生较高的谐波外，还发展出一个很强的主模，这表明它是一个非线性演化过程。这些剪切层模式的变化是在实验中看到的，其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，剪切层由对流不稳定转变为可能的绝对不稳定或自激。横向急流切变层对低层强迫的响应与这一解释一致。这种稳定性特征说明了横向喷流的原因？S能够以一种比自由喷流更好的方式与周围气体混合，但也提出了通过喷流强迫来控制喷流行为的策略。当速度比在3.5左右时，中低水平的正弦强迫会影响射流的响应，从而改善和控制射流的扩散和侵彻。然而，对于较低的速度比，需要具有规定的时间脉冲宽度的非常强的强迫才能显著影响射流。基于这些观测和众所周知的与低密度或加热自由喷流有关的不稳定性的转变，PI计划探索与变密度和非等温横向喷流有关的剪切层不稳定性，并根据与能源产生系统相关的具体应用开发喷流的战略控制程序。假设横流中加热或低密度射流将经历绝对不稳定流动发展的加速，即在比等温情况下更高的动量通量比时发生不稳定。PI假定，与横流中密度较高或较冷的射流相关的不稳定性会产生相反的影响。因此，对于冷却稀释空气射流、涡轮叶片的气膜冷却和燃烧室中的热反应射流的应用，需要根据不稳定的不同特征开发不同的控制策略。皮？S目前的国家科学基金会项目已经为当地高中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，本科生将继续在变密度横向射流研究中接受培训和就业，希望得到NSF REU的支持。将继续为当地高中生以及潜在的初中生和小学生进行外展演示和示威，以弥补加利福尼亚州削减对公立学校的资金？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.