Investigation of Coaxial Indeterminate-Origin Nozzles for Jet-Mixing Enhancement Purposes

当流体从喷嘴排出时产生射流，并且在许多工程应用中使用射流混合增强，例如燃料-空气燃烧室、空气动力学流动控制和喷气发动机排气冷却。两个非常重要的动机，了解和改善射流混合的行为是抑制热和声学签名的飞机喷气发动机。从军事角度来看，当热发动机排气和周围空气之间的混合增加时，隐身性会得到增强，并且更快地消散热信号。另一方面，减少飞机的声音信号，可减低噪音对社区的污染，并可在噪音水平与以前相若的情况下，以更高的飞行速度飞越民用地区。喷气机会有规律地产生涡流，改善相互混合的关键是加强涡流的产生和相互作用。一种有前景的技术涉及沿喷嘴唇缘沿着产生起伏或凹口，从而形成峰和谷。在这种情况下，喷流的原点不能精确定位在一个固定的位置，它被称为一个不确定的原点（IO）喷流。早期对不同喷管唇口设计的研究在改善喷流混合方面取得了相当大的成功，一些设计概念已被纳入隐身和民用飞机发动机排气喷管中，以分别降低它们的热和声学特征。然而，通过考虑两个IO喷射喷嘴同心/同轴布置的情况，这些改进可以显著地推进。初级内部和次级环形射流之间的额外相互作用可以提供进一步提高射流混合水平的方法。该项目将研究与单个IO射流相比，各种同轴IO射流配置如何进一步提高射流混合能力，通过观察这些配置如何影响基本的涡流物理。深入了解影响同轴IO射流如何与周围环境混合以及它们如何不同于单个IO射流的最关键的几何和流动因素。非常重要的是，这个研究课题还没有得到充分的研究之前，在合成涡动力学。因此，在同轴配置的两个IO喷流之间的相互作用是一个很好的机会，追求进一步的科学理解。另一方面，这些见解可能有助于未来的设计和优化类似的喷嘴在设计阶段，通过提供第一手的知识，不同的几何形状和流动条件将如何影响其混合水平。由于对一种应用有利的条件可能与对另一种应用有利的条件有很大不同，因此设计人员必须认识到这些基本的流动差异。为了实现这些目标，拟议的研究将通过将同轴IO射流排放到静止水箱中，以水作为内部和环形射流的工作流体，对同轴IO射流进行实验研究。水的性质使其成为理解基本涡流物理学的极好的流体介质。选择的几何配置将被研究沿着与内射流相对于环形射流的直径和速度的比率的变化。定性流动可视化和定量流动测量将用于整理所观察到的流动行为，即所得涡流动力学和涡流结构与同轴IO射流配置系统实现的射流混合水平之间的相互作用，从而可以有效地识别导致有利于改善射流混合的流动现象的几何和流动参数的任何组合。这些结果将导致显着改善我们目前的国家的最先进的科学理解的基本流动物理与这些同轴IO射流，并可能有助于实际的工程应用以及。

Jets are produced when fluids exhaust from nozzles and jet-mixing enhancements are used in many engineering applications such as fuel-air combustion chambers, aerodynamic flow-control and jet engine exhaust cooling. Two very important motivators for understanding and improving jet-mixing behaviour are suppression of heat and acoustic signatures from aircraft jet engines. From military perspectives, stealth is enhanced when mixing is increased between the hot engine exhaust and the surrounding air and dissipates the heat signature faster. On the other hand, reduction in acoustic signatures of aircrafts will lead to lower noise pollution to the community and allow higher flight speeds across civilian areas at similar noise levels as before.Jets produce flow vortices regularly and the key to improve mutual mixing is to enhance their production and interactions. One promising technique involves creating undulations or notches along the jet nozzle lip such that peaks and troughs are formed. In this case where the jet origin cannot be pinpointed at a fixed location, it is known as an indeterminate-origin (IO) jet. Earlier researches on different nozzle lip designs have shown considerable success in improving jet-mixing and some design concepts have been incorporated into stealth and civilian aircraft engine exhaust nozzles to reduce their heat and acoustic signatures respectively. However, these improvements may be advanced considerably by considering cases where two IO jet nozzles are arranged concentrically/coaxially. Additional interactions between the primary inner and secondary annular jet streams may offer ways to improve jet-mixing levels further.The project will study how various coaxial IO jet configurations may further enhance jet-mixing capabilities when compared to single IO jets, by looking at how these configurations affect the fundamental vortex flow physics. Insights into the most critical geometric and flow factors affecting how coaxial IO jets mix with their surrounding and how they differ from single IO jets will be revealed. Very importantly, this research topic has not been studied adequately before in terms of the resultant vortex dynamics. Hence, the interactions between the two IO jet streams in coaxial configurations represents an excellent opportunity to pursue further scientific understanding. On the other hand, these insights may aid future design and optimization of similar jet nozzles during the design stage by providing first-hand knowledge on how different geometric and flow conditions will influence their mixing levels. As the conditions favourable towards one application may be significantly different from those favourable towards another, it is important that designers appreciate these fundamental flow differences.To accomplish these aims, the proposed research will study coaxial IO jets experimentally using water as working fluid for both inner and annular jets by exhausting them into a quiescent water tank. The nature of water makes it an excellent fluid medium for understanding the fundamental vortex flow physics. Selected geometric configurations will be studied along with variations to the ratios of diameter and velocity of the inner jet with respect to those of the annular jet. Qualitative flow visualization and quantitative flow measurements will be used to collate the observed flow behaviour in terms of the resultant vortex dynamics and the interactions between the vortex structures to the jet-mixing levels achieved with the coaxial IO jet configurations systematically, such that any combinations of geometric and flow parameters leading to flow phenomena favourable towards improved jet-mixing can be identified effectively. These results will lead to significant improvements in our current state-of-the-art scientific understanding of the fundamental flow physics associated with these coaxial IO jets and may contribute towards actual engineering applications as well.