Fractal forcing of axisymmetric turbulent jets; both fully developed and impulsively forced

为了满足欧洲航空研究咨询委员会（ACARE）的雄心勃勃的目标，即到2050年将二氧化碳、氮氧化物和噪音排放量减少90%，航空业必须采用大胆的新流量解决方案。一种这样的流动解决方案是分形强迫湍流射流。分形是一种由相同的几何形状组成的物体，这些几何形状逐渐变小，因此无论人们选择以何种长度尺度来观察它，都显得相似。先前对分形产生的湍流的研究主要集中在均匀各向同性湍流上，并且表明与规则网格产生的湍流相比，湍流强度有所增加。湍流强度的增加增加了混合，从而可以提高燃烧效率。与常规网格相比，在分形网格后的流动中，压降和随后的压力恢复也有所改善。因此，分形强迫射流在燃气涡轮发动机的组成部分射流火焰燃烧和推进喷嘴中具有潜在的应用前景。类似的几何形状也被证明可以减少现代航空发动机的喷气声特征。人们已经观察到，这种分形产生的湍流不像普遍接受的理查森-柯尔莫哥洛夫现象学那样衰减，这使它具有很大的科学意义。与均匀各向同性湍流不同，射流是一种自由剪切流，其中存在平均剪切。这种平均切变提供了一种机制，通过这种机制，能量可以从平均流转移到湍流。射流中的分形力也直接作用于剪切层，而不是网格中的分形力作用于流体。本研究还将研究在细长分形边界上湍流的发展，即所谓的“分形步枪”情况。这种“分形步枪”也将被修改为包括螺旋分形图案，这将引入漩涡射流，这是已知的稳定燃烧应用中的射流火焰。这些类型的分形强迫流的流动物理尚不清楚，这是采用这种有前途的设备进入工业应用的先决条件。因此，本研究试图使用最先进的激光诊断技术来观察分形强迫湍流射流速度场中的这些流动物理现象。这样就有可能观察到分形强迫射流产生的湍流是否以与分形网格产生的湍流相同的非平衡方式衰减。它还将决定由分形几何形状或允许沿着分形边界发展的“冲动”流与这种行为转换的力的轴向长度尺度之间是否存在根本区别。

In order to meet The Advisory Council for Aeronautics Research in Europe (ACARE) 's ambitious targets to reduce carbon dioxide, NOx and noise emissions by up to 90% in 2050 bold new flow solutions must be embraced by the aviation industry. One such flow solution is a fractal forced turbulent jet. A fractal is an object that is composed of identical geometrical shapes that are progressively smaller, and therefore appears to be similar regardless of the length scale at which one chooses to view it. Previous research on fractal generated turbulence has focused on homogeneous isotropic turbulence, and has shown an increase in turbulence intensity in comparison to turbulence generated by regular grids. This increase in turbulence intensity increases mixing, which can subsequently improve the efficiency of combustion. The pressure drop, and subsequent pressure recovery, has also been shown to be improved in the flow behind a fractal grid as opposed to a regular grid. A fractal forced jet thus has potential applications in jet flame combustion and propulsion nozzles, which are both integral components of a gas turbine engine. Similar geometry has also been shown to reduce the jet acoustic signature in modern aero-engines. It has been observed that this fractal generated turbulence does not decay in the same manner as the universally accepted Richardson-Kolmogorov phenomenology, making it of great scientific interest. Unlike homogeneous isotropic turbulence, a jet is a free shear flow, in which there is a mean shear. This mean shear provides a mechanism by which energy can be transferred from the mean flow into turbulence. The fractal forcing in the jet is also applied directly to the shear layer, as opposed to a grid in which this forcing is applied to the bulk of the flow. This study will also examine the development of a turbulent flow over an elongated fractal boundary, the so-called "fractal rifle" case. This "fractal rifle" will also be modified to include a helical fractal pattern which will introduce swirl to the jet, which is known to stabilise jet flames in combustion applications. The flow physics of these types of fractal forced flows are not understood, which is a prerequisite for the adoption of such a promising device into industrial applications. This research thus seeks to use state of the art laser diagnostic techniques to observe these flow physics in the velocity field of a fractal forced turbulent jet. It will thus be possible to observe whether the turbulence generated by a fractal forced jet decays in the same non-equilibrium manner as that generated by a fractal grid. It will also determine whether there is a fundamental difference between a flow that is "impulsively" forced by a fractal geometry or allowed too develop along a fractal boundary and the axial length scale of the forcing at which this behaviour switches over.