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

射流是当流体从喷嘴中排出时产生的，在许多工程应用中，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，都使用了射流混合增强技术。两个非常重要的动机，以了解和改善喷气混合行为是抑制热和声学特征的飞机喷气发动机。从军事角度来看，当热发动机排气与周围空气之间的混合增加并更快地消散热特征时，隐身性就会增强。另一方面，减少飞机的声音特征，可减少对社区的噪音污染，并可在与以前噪音水平相似的情况下，以更高的速度飞越平民区。射流有规律地产生涡旋，改善相互混合的关键是增强涡旋的产生和相互作用。一种很有前途的技术涉及沿喷嘴唇产生波动或缺口，从而形成波峰和波谷。在这种情况下，射流的起源不能精确地定位在一个固定的位置，它被称为不确定起源（IO）射流。早期对不同喷嘴唇形设计的研究在改善射流混合方面取得了相当大的成功，一些设计理念已被纳入隐身和民用飞机发动机排气喷嘴中，分别用于降低其热量和声学特征。然而，考虑到两个IO喷嘴同心/同轴布置的情况，这些改进可能会大大提高。主要内部和次要环形射流之间的额外相互作用可能提供进一步提高射流混合水平的方法。该项目将通过观察不同的同轴IO射流配置如何影响基本的涡流物理特性，来研究与单一IO射流相比，不同的同轴IO射流配置如何进一步增强射流混合能力。将揭示影响同轴IO射流与周围环境混合的最关键几何和流动因素，以及它们与单个IO射流的区别。非常重要的是，这一研究课题在此之前还没有得到充分的研究。因此，两个IO射流之间的相互作用在同轴配置代表了一个极好的机会，以追求进一步的科学理解。另一方面，这些见解可以提供有关不同几何和流动条件如何影响其混合水平的第一手知识，从而有助于在设计阶段对类似喷嘴的设计和优化。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，因此设计师认识到这些基本流差异是很重要的。为了实现这些目标，拟议的研究将实验研究同轴IO射流，用水作为内部和环形射流的工作流体，通过将它们排出到静态水箱中。水的性质使它成为理解基本涡旋物理的一种极好的流体介质。将研究选定的几何结构以及相对于环形射流的直径和速度比的内部射流的变化。定性流动可视化和定量流动测量将用于根据所产生的旋涡动力学和旋涡结构与同轴IO射流配置系统实现的射流混合水平之间的相互作用来整理观察到的流动行为，以便有效识别导致有利于改善射流混合的流动现象的任何几何和流动参数组合。这些结果将导致我们目前对与这些同轴IO射流相关的基本流动物理的最先进科学理解的重大改进，并可能有助于实际的工程应用。

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.