Dissertation•
Développement d'une méthode de mesure de la masse volumique par diffusion Rayleigh appliquée à l'étude du bruit de jets, et contribution à l'étude du screech dans les jets supersoniques sous détendus
06 Dec 2017-
TL;DR: In this article, the authors present a set of developpements specifiques de diagnostiques optiques and leur application a l’etude aeroacoustique des jets rapides a haut nombre de Reynolds, which result in visualization par strioscopie and, de maniere preponderante dans ce manuscrit, de mesure de masse volumique par diffusion Rayleigh.
Abstract: Dans ce travail de recherche, on presente des developpements specifiques de diagnostiques optiques et leur application a l’etude aeroacoustique des jets rapides a haut nombre de Reynolds. Les resultats experimentaux presentes ici resultent de visualisation par strioscopie et, de maniere preponderante dans ce manuscrit, de mesure de masse volumique par diffusion Rayleigh. Ces methodes de caracterisation d’ecoulement, appliquees aux jets subsoniques ou supersoniques, ont ete associees a des mesures de bruit en champ lointain. La mesure par diffusion Rayleigh, qui repose sur la lumiere diffusee par les molecules constituantes du gaz, et n’est donc pas intrusive. Des difficultes apparaissent neanmoins pour exploiter les resultats lorsque le milieu diffusant contient des poussieres. Bien que l’air des ecoulements obtenu en soufflerie soit filtre, la quantite residuelle de poussieres a rendu necessaire le developpement d’une methode de nettoyage du signal en post-traitement. Le niveau des signaux obtenus par diffusion Rayleigh est tres faible, et domine par du bruit appele shot noise. Un gain significatif sur le niveau de ce bruit a ete obtenu en optimisant la chaine d’acquisition apres analyse des systemes existants. De plus une methode de traitement du signal derivee d’une methode existante a permis de calculer des spectres de masse volumique malgre le shot noise avec un seul capteur, la ou il en fallait deux auparavant. Les profils de p obtenus par cette technique ont montre qu’il existe une loi de similarite permettant de superposer les profils mesures a differentes positions axiales. Cette loi est identique pour les jets issus de trois tuyeres aux geometries differentes, et a des nombres de Mach de 0.7 et 0.9. Une loi de similarite est egalement observee pour p’rms si les profils sont mesures suffisamment loin de la tuyere. L’etude des spectres dans la couche de melange a mis en evidence un maximum faiblement marque autour d’une frequence centrale comprise entre St = 0:2 et St = 2 dans les regions mesurees, plus marque que dans les spectres de vitesse, et dont le comportement differe selon l’etat initialement laminaire ou turbulent du jet. L’evolution de la forme des spectres en fonction de la difference de masse volumique entre le jet et le milieu ambiant, ainsi qu’en fonction du nombre de Mach, a egalement ete etudiee. Une loi permettant de superposer les spectres a ete definie empiriquement sur la plage de variation des differents parametres. Des mesures simultanees entre l’acoustique en champ lointain et la masse volumique dans l’ecoulement ont ete realisees pour un jet a Mj = 0:9 et un jet a Mj = 1:32. Ces resultats ont permis l’estimation de coherences spectrales et de moyennes conditionnelles. Les resultats obtenus mettent en evidence la presence de structures liees au rayonnement acoustique dans une region situee proche de l’axe du jet en aval du cone potentiel. Pour finir, une etude a ete realisee sur le screech dans les jets supersoniques sous-detendus. Elle a permis d’identifier la position de la source de la retroaction acoustique pour les modes A1, A2, et B, ainsi que la structure du cycle de la boucle qui determine les changements de frequences observes aux sauts de modes.
Citations
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01 Jan 2014
TL;DR: In this paper, the authors reviewed some fundamental drivers of UFP emissions and dispersion, and highlighted unresolved challenges, as well as recommendations to ensure sustainable urban development whilst minimising any possible adverse health impacts.
Abstract: Ultrafine particles (UFP; diameter less than 100 nm) are ubiquitous in urban air, and an acknowledged risk to human health. Globally, the major source for urban outdoor UFP concentrations is motor traffic. Ongoing trends towards urbanisation and expansion of road traffic are anticipated to further increase population exposure to UFPs. Numerous experimental studies have characterised UFPs in individual cities, but an integrated evaluation of emissions and population exposure is still lacking. Our analysis suggest that average exposure to outdoor UFPs in Asian cities is about four-times larger than those in European cities but impacts on human health are largely unknown. This article reviews some fundamental drivers of UFP emissions and dispersion, and highlights unresolved challenges, as well as recommendations to ensure sustainable urban development whilst minimising any possible adverse health impacts.
32 citations
Journal Article•
TL;DR: In this paper, the role of upstream travelling acoustic waves in generating perturbations in the jet shear layer at the nozzle exit is directly observed for the first time, and a structure that rapidly evolves into a large-scale vortex ring forms at the trailing edge of this initial instability, first observed at approximately $$\frac{x}{d}=0.3
Abstract: The use of modern ultra-high speed cameras to acquire time-resolved schlieren image sequences of supersonic jet impingement is presented. The use of these cameras, with framerates of up to 1 million frames per second, allows for the first time-resolved visualizations of the impinging jet acoustic feedback loop. The role of upstream travelling acoustic waves in generating perturbations in the jet shear layer at the nozzle exit is also directly observed for the first time. The arrival of the acoustic wave at the nozzle lip generates a sinusoidal variation in density gradient that persists until a distance of $$\frac{x}{d}=0.3$$
. A structure that rapidly evolves into a large-scale vortex ring forms at the trailing edge of this initial instability, first observed at approximately $$\frac{x}{d}=0.25$$
.
14 citations
TL;DR: In this paper, a new spectroscopic setup to analyze Rayleigh-scattered light from a high-speed jet was created at the NASA Ames Research Center for simultaneous measurement of velocity, temperature, and density.
Abstract: A new spectroscopic setup to analyze Rayleigh-scattered light from a high-speed jet was created at the NASA Ames Research Center for simultaneous measurement of velocity, temperature, and density. The point-measurement arrangement uses a narrow-linewidth continuous-wave laser and a stabilized, air-spaced, Fabry–Perot etalon to resolve the Rayleigh–Brillouin (RB) spectrum. Light scattered from a 0.4 mm-long and 0.15 mm-diameter probe volume was collected and 10% of the light was split to measure the scattering intensity via a photo-multiplier tube, which provided a measure of the gas density and density fluctuation spectra. The rest was directly imaged through the Fabry–Perot interferometer to an EMCCD camera. A new software program was developed in the Matlab® platform to model the fringes seen in the camera image. At first, a small part of the incident laser light was analyzed to find the instrument function. When Rayleigh-scattered light was passed, a change in the fringe diameter corresponding to the Doppler shift from the air velocity and a thickening of the fringe corresponding to the thermal broadening were observed. A least-squares fit utilizing Tenti’s S6 model of the RB scattering provided velocity and temperature at the probe volume. A high-speed, clean-air jet, operated in the Mach range of 0 ≤ M ≤ 1.2 provided validation of the technique, and demonstrated applicability in shock-containing flows. The compact, transportable, efficient, and affordable setup was fairly accurate: uncertainties were < 7 m/s in velocity, < 5 K in temperature, and < 1% in density. Spectra of density fluctuations were measured over a frequency range of 50 kHz.
Principle of Rayleigh scattering technique, (a) schematic representation; (b) superimposed Fabry–Perot images from the present setup, U = 323 m/s, T = 249 K.
3 citations
TL;DR: The tested Rayleigh scattering setup is shown to perfectly satisfy the theoretical predictions, and allows us to measure the spectrum of light intensity variations with an outstanding linearity within a dynamic range proportional to the square-root of the measurement time.
Abstract: Rayleigh scattering of a laser beam can be used for time-resolved local measurements of density in flows. A key-point of the approach lies in extracting flow density statistics from the measurement of the scattered-light power. Complex hardware equipment and software procedures are involved in this process. The present paper offers a method to assess the entire chain of acquisition of a typical setup in realistic conditions, and an analysis of its dynamic performances with respect to the theoretical expectations. This is achieved by simulating the light intensity fluctuations typically observed with a Rayleigh scattering setup, by use of a known light intensity fluctuation signal generated using a light-emitting diode modulated in output power. The tested Rayleigh scattering setup is shown to perfectly satisfy the theoretical predictions, meaning that it allows us to measure the spectrum of light intensity variations with an outstanding linearity within a dynamic range proportional to the square-root of the measurement time.
1 citations
02 Aug 2021
1 citations
References
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TL;DR: In this article, the principles of Optics are discussed and a discussion of the relationship between the principles and the application of Optica Acta is presented. But this discussion is limited.
Abstract: (1961). Principles of Optics. Optica Acta: International Journal of Optics: Vol. 8, No. 2, pp. 181-182.
3,717 citations
TL;DR: In this article, a general scheme for educing coherent structures in any transitional or fully turbulent flow is presented, based on smoothed vorticity maps in convenient flow planes, which recognizes patterns of the same mode and parameter size, and then phase-aligns and ensembles them to obtain coherent structure measures.
Abstract: This is a personal statement on the present state of understanding of coherent structures, in particular their spatial details and dynamical significance. The characteristic measures of coherent structures are discussed, emphasizing coherent vorticity as the crucial property. We present here a general scheme for educing structures in any transitional or fully turbulent flow. From smoothed vorticity maps in convenient flow planes, this scheme recognizes patterns of the same mode and parameter size, and then phase-aligns and ensemble-averages them to obtain coherent structure measures. The departure of individual realizations from the ensemble average denotes incoherent turbulence. This robust scheme has been used to educe structures from velocity data using a rake of hot wires as well as direct numerical simulations and can educe structures using newer measurement techniques such as digital image processing. Our recent studies of coherent structures in several free shear flows are briefly reviewed. Detailed data in circular and elliptic jets, mixing layers, and a plane wake reveal that incoherent turbulence is produced at the ‘saddles’ and then advected to the ‘centres’ of the structures. The mechanism of production of turbulence in shear layers is the stretching of longitudinal vortices or ‘ribs’ which connect the predominantly spanwise ‘rolls’; the ribs induce spanwise contortions of rolls and cause mixing and dissipation, mostly at points where they connect with rolls. We also briefly discuss the role of coherent structures in aerodynamic noise generation and argue that the structure breakdown process, rather than vortex pairing, is the dominant mechanism of noise generation. The ‘cut-and-connect’ interaction of coherent structures is proposed as a specific mechanism of aerodynamic noise generation, and a simple analytical model of it shows that it can provide acceptable predictions of jet noise. The coherent-structures approach to turbulence, apart from explaining flow physics, has also enabled turbulence management via control of structure evolution and interactions. We also discuss some new ideas under investigation: in particular, helicity as a characteristic property of coherent structures.
1,117 citations
Book•
01 Jan 2001
TL;DR: A review of recent developments in shadowgraph and schlieren visualization can be found in this paper, where the authors present a detailed overview of the shadowgraph technique for flow visualization.
Abstract: a review of recent developments in schlieren and schlieren visualization joseph shepherd lecture # 09: flow visualization techniques: schlieren and shadowgraph, schlieren and interferometry part 01 schlieren and shadowgraph techniques osfp schlieren and shadowgraph techniques module 5: schlieren and shadowgraph lecture 26 shadowgraph and schlieren techniques schlieren techniques for the visualization of current visualization based on refractive-index affects shadowgraph and schlieren techniques towards a schlieren camera northwestern university schlieren photography principles rit scholar works introduction to shadowgraph and schlieren imaging chapter 2 laser schlieren and shadowgraph springer background oriented schlieren applied to study shock mae 123 : mechanical engineering laboratory ii -fluids recent developments in schlieren and shadowgraphy schlieren and shadowgraph techniques: visualizing optical considerationsand limitations of the schlieren method principles and techniques of schlieren imaging systems mice a simple classroom demonstration of natural convection module 5: schlieren and shadowgraph lecture 27: schlieren retroreflective shadowgraph technique for large-scale flow optical methods for visualization of ultrasound fields schlieren & shadowgraph techniques steps forward physically-based interactive schlieren flow visualization schlieren and shadowgraph techniques pdf download shadow, schlieren and color interferometry physically-based interactive flow visualization based on schlieren & shadowgraph techniques steps forward background oriented schlieren (bos) and other flow size 50,24mb doc book schlieren and shadowgraph techniques 6 shadowgraph techniques springer pradipta kumar panigrahi krishnamurthy muralidhar schlieren and shadowgraph techniques jlip application of the shadowgraph flow visualization shadowgraph, schlieren and interferometry in a 2d a fluid motion estimator for schlieren image velocimetry schlieren and shadowgraph techniques springer shadowgraph, schlieren and interferometry in a 2d quantitative fourier analysis of schlieren masks: the schlieren, shadowgraph and direct photography fkm.utm schlieren and shadowgraph techniques for °uid physics four decades of utilizing shadowgraph techniques to study color schlieren imaging with a two-path, double knife edge schlieren and shadowgraph techniques gerrymarshall free download schlieren & shadowgraph techniques book shadow-schlieren-book-1 rit people
906 citations
TL;DR: Theoretical results concerning the instability of axisymmetric jets are reviewed in this paper for inviscid parallel jet flow and various parameters affecting jet instability such as shear layer thickness, Mach number, temperature ratio, and external flow velocity are discussed.
540 citations
TL;DR: In this paper, the spectral properties of Rayleigh scattering are discussed and a review of the new advances in flow field imaging that have been achieved using the new filter approaches is presented.
Abstract: Rayleigh scattering is a powerful diagnostic tool for the study of gases and is particularly useful for aiding in the understanding of complex flow fields and combustion phenomena. Although the mechanism associated with the scattering, induced electric dipole radiation, is conceptually straightforward, the features of the scattering are complex because of the anisotropy of molecules, collective scattering from many molecules and inelastic scattering associated with rotational and vibrational transitions. These effects cause the scattered signal to be depolarized and to have spectral features that reflect the pressure, temperature and internal energy states of the gas. The very small scattering cross section makes molecular Rayleigh scattering particularly susceptible to background interference. Scattering from very small particles also falls into the Rayleigh range and may dominate the scattering from molecules if the particle density is high. This particle scattering can be used to enhance flow visualization and velocity measurements, or it may be removed by spectral filtering. New approaches to spectral filtering are now being applied to both Rayleigh molecular scattering and Rayleigh particle scattering to extract quantitative information about complex gas flow fields. This paper outlines the classical properties of Rayleigh scattering and reviews some of the new advances in flow field imaging that have been achieved using the new filter approaches.
508 citations