This document is downloaded from DR‑NTU (https://dr.ntu.edu.sg) Nanyang Technological University, Singapore. Two‑stream mixing flow with streamwise vorticity Mao, Rong Hai 2005 Mao, R. H. (2005). Two‑stream mixing flow with streamwise vorticity. Doctoral thesis, Nanyang Technological University, Singapore. https://hdl.handle.net/10356/6136 https://doi.org/10.32657/10356/6136 Nanyang Technological University Downloaded on 11 Oct 2021 14:25:42 SGT ATTENTION: The Singapore Copyright Act applies to the use of this document. Nanyang Technological University Library TWO-STREAM MIXING FLOW WITH STREAMWISE VORTICITY SCHOOL OF MECHANICAL AND PRODUCTION ENGINEERING NMANGTECHNQLOGICAL UNIVERSITY SINGAPORE ATTENTION: The Singapore Copyright Act applies to the use of this document. Nanyang Technological University Library Two-stream Mixing Flow with Streamwise Vorticity Mao Ronghai M.Sc. (CAS) School of Mechanical and Production Engineering A thesis submitted to the Nanyang Technological University in fulfillment of the requirement for the degree of Doctor of Philosophy in Mechanical and Aerospace Engineering ATTENTION: The Singapore Copyright Act applies to the use of this document. Nanyang Technological University Library Abstract ABSTRACT The aerodynamic characteristics of the single-lobe forced mixer have been extensively examined in the present dissertation. Nine single-lobe models with different parameters and geometry configurations were designed and fabricated. Penetration angles, lobe wavelengths, trailing edge configurations and modifications vary from case to case. Flat plate (to simulate plane free shear layer) and convoluted plate (to generate normal K-H vortex without streamwise vortex) have also been tested as baselines for comparison. Two velocity ratios, i.e., r =1: 1 and 0.4: 1 and Reynolds numbers (based on the wavelength of the basic single-lobe forced mixer) ranged from 9,200 to 40,000 have been chosen and tested. Hot-wire anemometer (HWA) and laser Doppler anemometer (LDA) were employed to evaluate the two different types of vortices, namely Kelvin-Helmholtz (K-H in abbreviation) and streamwise vortices respectively, due to their different characteristics. The K-H vortices are shed periodically from the trailing edge of the model, while the streamwise vortices are spatially stable in the near wake. To characterize the periodical K-H vortices, fast Fourier transform (FFT) was performed on the velocity signals to extract its frequency information. Consecutively the other parameters of the K-H vortices, such as the mean wavelength and non-dimensional frequencies including Roshko number and Strouhal number could be examined at different flow conditions. It was found that there were two series of K-H vortices shed after the trailing edge of the forced mixer, with both fi-equencies in proportion to the mean velocity of the on- coming streams. Both of these two frequencies are higher than that of the flat plate case. The Strouhal number increased with Reynolds number, but its increasing ratio decreased ATTENTION: The Singapore Copyright Act applies to the use of this document. Nanyang Technological University Library Abstract gradually. As a result, it will reach certain stable peak value at Reynolds number above 5x10~. The streamwise vortices, as another contributor to the enhanced mixing performance, have been measured using a laser Doppler anemometer. Detailed 3-dimensional velocity fields at different cross-sections in the near wake after the trailing edge have been measured. It was found that at the same lobe wavelength, the initial mean streamwise vorticity was proportional to tan(ε),where ε is the penetration angle. The model with lobe wavelength double the lobe height generated higher streamwise vorticity than the other models at different wavelengths. Semi-circular and rectangular models generated similar streamwise vorticity strength, and both are higher than the triangular model. The scalloping modification was beneficial to the generation of streamwise vorticity by forming additional streamwise vortices, whereas the scarfing modification on the present single-lobe forced mixer would suppress the generation of the streamwise vorticity. The interaction of the streamwise vortices with the K-H vortices, namely 'pinched- off effect, contributes to the mixing enhancement by stretching the K-H vortex tube and increasing its vorticity. On the other hand, the evolution of the streamwise vorticity with downstream distance is mainly dominated by the viscous dissipation and the deformation effects due to the K-H vorticity, except in some regions immediately after the mixer trailing edge where the stretching effects due to the streamwise mean flow acceleration taken place. Both of these two types of vortices are contributive to the mixing enhancement. Key Words: lobed forced mixer, Kelvin-Helmholtz vortex, hot-wire anemometer, streamwise vortex, laser Doppler anemometer, interactions, mixing. ATTENTION: The Singapore Copyright Act applies to the use of this document. Nanyang Technological University Library Acknowledgements ACKNOWLEDGMENTS First of all, I would like to express my sincere gratitude to my supervisor, Associate Professor Yu Ching Man, Simon, for his patient guidance and valuable suggestions throughout this project. In particular, his role in broadening my skills in research and English writing is gratefully acknowledged. I am indebted to Associate Professor Chua Leok Poh and Assistant Professor Zhou Tongming, for their assistance in hot-wires fabrication and operation. I owe all success to my parents, my wife Ma Xinran, my colleagues and fiiends in NTU. Their encouragements and supports instilled me to do my best. Among them I would especially thank Dr. Dong Yufei and Dr. Wang Xikun, for their unreserved help over the past three years. Last but not least, the graduate scholarship from the school of Mechanical and Aerospace Engineering, Nanyang Technological University is gratefully acknowledged. ATTENTION: The Singapore Copyright Act applies to the use of this document. Nanyang Technological University Library Table of Contents TABLE OF CONTENTS ABSTRACT ................................................................................................................ i ... ACKNOWLEDGMENTS ........................................................................................ 111 TABLE OF CONTENTS .......................................................................................... iv ... NOMENCLATURE ............................................................................................... Vlll LIST OF FIGURES .................................................................................................. xi LIST OF TABLES ................................................................................................. xvii 1.1 K-H Vortices and Streamwise Vortices ....................................................... 1 1-1 -1 Active Control (Perturbation Method) ................................................. 2 1.1.2 Passive Control (Geometry Method) ................................................... 3 1.2 Lobed Forced Mixer Flow ............................................................................. 4 1-3 Mixing and Affecting Factors ........................................................................ 5 1.3.1 Mixing and Mixedness .......................................................................... 5 1.3-2 Factors Affecting Mixing ...................................................................... 7 1-4 objective ..................................................................................................... 11 1.4.1 Hot-wire Anemometer on K-H Vortices ............................................. 11 1.4.2 Laser Doppler Anemometer on Streamwise Vortices... ...................... 12 1.5 Layout of the Thesis ..................................................................................... 14 2.2.1 Mixing Mechanism of the Lobed Forced Mixer ................................. 28 2 .2.1. 1 Enhanced Mixing Interfacial Areas ........................................... 28 2.2.1.2 Two Kinds of Vortices after the Trailing Edge.. ........................ 29 2.2.2 Previous Works on Lobed Forced Mixer ............................................ 30 3.1 Wind Tunnel ................................................................................................ 40 3.2 Test Models ..................................................................................................43 3.2.1 Flat Splitter Plate ........................................................+..........+.............43 3.2.2 Basic Semi-circular Single-lobe Forced Mixer (model a) ................. 43 3.2.3 Semi-circular Single-lobe Forced Mixers With Different Heights (models b and c) ................................................................................. 44 3.2.4 Convoluted Plate (model d) ............................................................... 44 ATTENTION: The Singapore Copyright Act applies to the use of this document. Nanyang Technological University Library Table of Contents 3.2.5 Semi-circular Single-lobe Forced Mixers With Different Wavelengths (models e and f) ................................................................................. 45 3.2.6 Rectangular and Triangular Single-lobe Forced Mixers (models and h) ................................................................................. 45 3.2.7 Scalloped and Scarfed Single-lobe Forced Mixers (models i and j) .. 45 3 -3 Measurement