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

轴对称湍流射流的分形强迫；

基本信息

批准号：
EP/L023520/1
负责人：
Oliver Buxton
金额：
$ 12.88万
依托单位：
Imperial College London
依托单位国家：
英国
项目类别：
Research Grant
财政年份：
2014
资助国家：
英国
起止时间：
2014 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=EP%2FL023520%2F1
关键词：
Fractal forcing axisymmetric turbulent jets

项目摘要

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.

为了实现欧洲航空研究咨询理事会（ACARE）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。分形是一种由逐渐变小的相同几何形状组成的物体，因此无论人们选择观看它的长度尺度如何，它看起来都是相似的。以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。湍流强度的这种增加增加了混合，这随后可以提高燃烧效率。与规则网格相比，在分形网格后面的流动中，压降和随后的压力恢复也被证明得到改善。因此，分形强制射流在射流火焰燃烧和推进喷嘴中具有潜在的应用，这两者都是燃气涡轮机发动机的整体部件。类似的几何形状也被证明可以减少现代航空发动机中的喷流声学特征。已经观察到，这种分形产生的湍流不会以与普遍接受的Richardson-Kolmogorov现象学相同的方式衰减，使其具有极大的科学兴趣。与均匀各向同性湍流不同，射流是自由剪切流，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中。射流中的分形强迫也直接应用于剪切层，而不是网格中的这种强迫应用于大部分流。本研究也将探讨在一个细长的分形边界，所谓的“分形步枪”的情况下，湍流的发展。这种“分形步枪”也将被修改为包括螺旋形分形图案，该图案将向射流引入漩涡，这已知在燃烧应用中稳定射流火焰。这些类型的分形强制流动的流动物理学不被理解，这是一个先决条件，采用这样一个有前途的设备到工业应用。因此，本研究旨在使用最先进的激光诊断技术来观察这些流动物理的分形强迫湍流射流的速度场。因此，可以观察到由分形强制射流产生的湍流是否以与由分形网格产生的湍流相同的非平衡方式衰减。它还将确定在由分形几何形状“冲动地”强迫的流动或允许沿分形边界沿着发展的流动与这种行为转换时强迫的轴向长度尺度之间是否存在根本差异。