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No description available.

Vibrations of small cylinder in a jet flow are investigated experimentally. Because of the flow field in shear layers of jet flow is very complex and filled with vortex structures, so the flow induced vibration phenomena in jet flow is different from the flow induced vibration in uniform flow. The major subject in this experiment is to discuss the major cause of small cylinder vibrations, and the flow field influenced by the cylinder vibration. About flow measurement, velocity measurement by hot-wire is applied. As for the vibration measurement, by using the principle of electromagnetic, a new measurement technology was successfully developed. This new vibration measurement can measure the vibrations in two axial, so as to describe the orbit of vibrations. To find the interrelation of flow field and cylinder vibrations, flow measurement and vibration measurement was carry on at the same time. It is shown that when the jet velocity is increased constantly, small cylinder will vibration intensely. The fixed velocity is called critical velocity. If add a perturb, the vibration will occur in advance. The dominant frequency of cylinder vibration, fr, will be the same with it's nature frequency, fn, in the critical velocity, but when the flow velocity keep on increasing, the dominant frequency, fr, will also increase. Besides, the relation of reduced velocity and mass damping was found in this case. The orbits of vibrations are all like ellipse, and the orbit is different with different reduced velocity. The vibration amplitude be changed into three sections that have different reduced velocity, and different orbit. About the flow field, the velocity profile in potential core is not influenced by vibrations of small cylinder, but the velocity fluctuations in shear layer indeed be inflected. At the fixed velocity region, the dominant frequency of flow is the same with dominant frequency of vibrations when the flow at downstream of small cylinder in shear layer. This phenomena only exist when the vibration amplitude under the fixed range.

Jet in crossflow (JICF) configurations are often used for secondary fuel injection in staged-fuel combustion systems. The high temperature, vitiated crossflow in these systems is inherently unsteady and characterized by random, turbulent fluctuations and coherent, acoustic oscillations. This thesis presents the results of an experimental investigation into the dynamics of non-reacting and reacting jets injected into unsteady, vitiated crossflow. The flow structure and flame stabilization of jets with different momentum flux and density ratios relative to the crossflow are characterized using simultaneous time-resolved stereoscopic particle image velocimetry (SPIV) synchronized with OH planar laser induced fluorescence (PLIF). A modified trajectory scaling law is developed to account for the influence of near-field heat release on the jet trajectory. The second part of this work focuses on the response of a JICF to crossflow forcing. Acoustic drivers are used to excite natural resonances of the facility, which are characterized using the two-microphone method. Spectral analysis of SPIV results shows that, while the jet response to crossflow velocity fluctuations is often negligible, the fluctuating crossflow pressure induces a significant fluctuating jet exit velocity, which leads to periodic jet flapping. The flame response to crossflow forcing is studied using flame edge tracking. An analytical model is developed that predicts the dependence of the jet injector impedance upon important JICF parameters. In the final part of this work, vortex tracking and Mie scattering flow visualization are used to investigate the effect of near-field heat release on the shear layer dynamics. A phenomenological model is developed to explain the effect of combustion on the shear layer stability of density stratified, reacting JICF. The results of this study demonstrate the important effects of near-field heat release and crossflow acoustics on the dynamics of reacting JICF.

<p>The aerodynamic excitation of the diametral acoustic modes of an axisymmetric cavity-duct system is investigated experimentally. The change experienced by the acoustic diametral modes with the increase of the mean flow Mach number is investigated numerically. The first objective of this research is to examine the ability of the axisymmetric free shear layer forming along the cavity mouth to excite the asymmetric diametral modes which do not have preferred azimuthal orientations. The dependency of the system aeroacoustic response on both the cavity length and its depth is investigated to determine the limitations imposed by the relative dimensions of the cavity on the excitation process. The azimuthal behaviour of the self-excited diametral modes is also characterized.</p> <p> An experimental set-up is designed to ensure the coincidence of the frequencies of the shear layer oscillation with the acoustic resonance frequencies. The selection of the test section dimensions is based on finite element simulation of the acoustic diametral modes for several geometries. To simulate the diametral modes at different flow Mach numbers, a finite difference code is developed based on a two-step computational aeroacoustic approach. This approach allows the simulation of the acoustic field, taking into account the convection effect of the mean flow.</p> <p>The experimental results show that the diametral modes are very liable to be self-excited when the mean flow Mach number is higher than 0.1. The level of acoustic pressure during the diametral mode resonance increases rapidly with the increase of the ratio of the cavity depth, d, to the pipe diameter, D. However, the maximum acoustic pressure during each resonance decreases with the increase of the ratio of the cavity length, L, to the pipe diameter, D. The selfexcitation of the diametral modes is sustainable with d/D as small as 1/12. Further reduction in this ratio may result in complete suppression of the resonance. For deep cavities, d/D>3/12, the first and second diametral modes are more liable to excitation than the higher order modes. This is attributed to the fact that the low order modes have relatively higher radial acoustic particle velocity amplitude at the cavity mouth compared to the higher order ones. For d/D=l/12, the higher order modes have relatively higher radial acoustic particle velocity amplitude and consequently their tendency to be self-excited increases. For long cavities, L/D>2/3, the duct longitudinal acoustic modes start to be excited and become more dominant as the cavity length is further increased. The excitation mechanism of these longitudinal modes has not been investigated in this work since sufficient details already exist in the literature.</p> <p>The azimuthal behaviour of the diametral modes is characterized for all the tested cases. For short cavities, the diametral modes are classified as spinning modes; while for long cavities, L/D> 1/2, the orientation of the mode changes randomly over time. Small imperfections in the axisymmetric geometry result in what is described as partially spinning modes. An analytical model is developed to describe quantitatively the spinning behaviour of the diametral modes. The free shear layer and the diametral modes are found to be fully coupled in the azimuthal direction. The random behaviour of the diametral modes in the case of long cavities is attributed to the increase of randomness in the turbulent shear layer </p> <p>The numerical simulations show that the diametral modes experience considerable changes with the increase of the mean flow Mach number. At the cavity mouth, both the amplitude and phase distributions of the acoustic particle velocity are altered with the increase of the Mach number. This demonstrates the importance of considering the effect of the mean flow on the acoustic power production process. Moreover, the resonance frequency of the diametral modes decreases with the increase of the Mach number.</p> / Thesis / Doctor of Philosophy (PhD)

The effect of direct, small-scale excitation on the evolution of a plane shear layer which forms at the edge of a backward facing step is investigated experimentally using high resolution particle image velocimetry and hot-wire anemometry. Actuation is effected at frequencies that are over an order of magnitude higher than the characteristic (or natural) formation frequency of the layer by a spanwise array of piezoelectrically-driven synthetic jet actuators that are placed near the edge of the step. The actuation has significant effects on the evolution of both large- and small-scale motions within the shear layer inducing an increase in small-scale dissipation and simultaneous suppression of turbulence production. While the fundamental instabilities that lead to the formation of large scale motions are typically suppressed, low-frequency amplitude-modulation of the actuation signal allows the formation of large scale motions and entrainment which, in concert with the small-scale actuation, lead to enhancement of the turbulent shear stresses throughout the shear layer. Amplitude modulation is also used to assess the effect of flow transients that are induced by step or low duty cycle actuation. The present findings suggest strategies for controlled suppression or enhancement of mixing in the near field of the shear layer.

The aerodynamic steering forces and moments on a wind tunnel model of an axisymmetric bluff body are altered by induced segmented attachment of the separated flow over an azimuthal Coanda surface. The model is suspended in the wind tunnel by eight thin wires for minimal support interference within the wake. Each wire is instrumented with a miniature strain gage sensor for direct dynamic force measurements. Control is effected by an array of synthetic jet actuators that emanate from narrow, azimuthally-segmented slots, within a backward facing step. The aerodynamic effects are characterized using hot-wire anemometry and PIV measurements. In the first set of experiments, the array of synthetic jets is distributed around the perimeter of the circular tail end which is extended into a Coanda surface. The fluidic actuation results in segmented vectoring of the separated base flow along the rear Coanda surface and induces asymmetric aerodynamic forces and moments that can effect steering during flight. Transitory modulation of the actuation waveform of multiple actuators around the tail leads to the generation of significant dynamic side forces of controlled magnitude and direction with the potential utility for flight stabilization and fast maneuvering. In a second set of experiments the array of the synthetic jets is placed upstream of a mid-body axisymmetric cavity. A single jet induces a quasi-steady, nearly-matched force couple at the upstream and downstream ends of the cavity. Furthermore, transitory activation of multiple jets can be used to control the onset and sequencing of the couple forces and therefore the resultant force and moment.

The normal injection of a liquid jet into a gaseous crossflow has many engineering applications. In this thesis, detailed numerical simulations based on the level set method are employed to understand the physical mechanism underlying the jet ``surface breakup''. The numerical observations reveal the existence of hydrodynamic instabilities on the jet periphery. The temporal growth of such azimuthal instabilities leads to the formation of interface corrugations, which are eventually sheared off of the jet surface as sheet-like structures. The sheets finally undergo disintegration into ligaments and drops during the surface breakup process. Temporal linear stability analyses are employed to understand the nature of these instabilities. To facilitate the analysis, analytical solutions for the flow fields of the jet and the crossflow are derived. We identify the ``shear instability'' as the primary destabilization mechanism in the flow. This inherently inviscid mechanism opposes the previously suggested mechanism of surface breakup (known as ``boundary layer stripping''), which is based on a viscous interpretation. The influence of the jet-to-crossflow density ratio on the flow stability are also studied. The findings show that a higher density jet leads to higher wavenumber instabilities on the jet surface and thereby subsequent smaller drops and ligaments. The stability characteristics of the most amplified modes (i.e., the wavenumber and corresponding growth rate) obtained from stability analyses and numerical simulations are in good agreement. The stability results of the jet also show that the density may have a non-monotonic stabilizing/destabilizing effect on the flow stability. To investigate such effect, the concept of wave resonance are employed to physically interpret the inviscid instability mechanism in two-phase flows with sharp interfaces and linear velocity profiles. We demonstrate that neutrally stable waves are formed due to the density jump in the flow, in addition to the well-known vorticity (Rayleigh) waves. Under certain conditions, such neutral waves are capable of resonating and generating unstable modes. The resonance of different pairs of neutral waves, therefore, results in either stabilizing or destabilizing effect of density variation. We predict similar reasoning behind the density behavior in the jet in crossflow configuration with smoothly varying velocity and density profiles.

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Large-eddy simulations of turbulent flows past a circular cylinder have been performed at sub-, critical and super-critical Re using an orthogonal curvilinear grid solver, CFDship-Iowa version 6.2. An extensive verification and validation study has been carried out. Various aspects of the flow field have been investigated. The aspect ratio of the computational domain has major effects on the results. In general, large aspect ratio produced best results for the sub-critical Re. Small dependency on both aspect ratio and grid resolution was observed for the critical Re. Small aspect ratio and conservative scheme produced best results for the super-critical Re. Overall flow features and the drag crisis phenomenon have been correctly predicted. A lot of experimental and numerical studies of flow past a circular cylinder were collected and used for the validation of the present LES study. Integral and local variables were in fairly good agreement for the sub-critical Re. Sharp behavior including drag crisis was predicted for the critical Re. Although some discrepancy including early formation of turbulent separation was observed, local flow structures including separation bubble were observed for the super-critical Re. The formation of secondary vortex near the cylinder wall and its evolution into separation bubble were observed. The spectral analysis showed that the separation bubble had the instabilities close to the shear layer frequency. The proximity of shear layer to the cylinder enhanced the mixing process of boundary layer and shear layer and led to the formation of separation bubble. A snapshot POD method was used to extract flow structures in the boundary layer, shear layer and wake. In the boundary layer, the secondary vortices and separation bubble were successfully extracted. Due to the weak TKE distribution, specific flow structures were hard to find in the shear layer. Large two-dimensional flow structures representing the Karman shedding vortices were extracted for the sub- and super-critical Re.