Ordered structures and jet noise

A series of measurements of near field pressures and turbulent velocity fluctuations were made in a jet having a Reynolds number of about 50,000 in order to investigate more quantitatively the character and behavior of the large scale structures, and to ascertain their importance to the jet noise problem. It was found that the process of interaction between vortices can be inhibited by artificially exciting the shear layers with periodic disturbances of certain frequency. The turbulent fluctuation amplitudes measured at four diameters downstream decreased considerably. Finally, it was observed that the passage frequency of the structures decreased with x in a similar manner as the frequency corresponding to the maximum intensity radiation emanating from the same value of x

Contributions of numerical simulation data bases to the physics, modeling and measurement of turbulence

The use of simulation data bases for the examination of turbulent flows is an effective research tool. Studies of the structure of turbulence have been hampered by the limited number of probes and the impossibility of measuring all desired quantities. Also, flow visualization is confined to the observation of passive markers with limited field of view and contamination caused by time-history effects. Computer flow fields are a new resource for turbulence research, providing all the instantaneous flow variables in three-dimensional space. Simulation data bases also provide much-needed information for phenomenological turbulence modeling. Three dimensional velocity and pressure fields from direct simulations can be used to compute all the terms in the transport equations for the Reynolds stresses and the dissipation rate. However, only a few, geometrically simple flows have been computed by direct numerical simulation, and the inventory of simulation does not fully address the current modeling needs in complex turbulent flows. The availability of three-dimensional flow fields also poses challenges in developing new techniques for their analysis, techniques based on experimental methods, some of which are used here for the analysis of direct-simulation data bases in studies of the mechanics of turbulent flows

Thermo-fluid-dynamics of impinging swirling jets

The superimposition of a tangential motion on a conventional round jet has been demonstrated to significantly affect the large-scale topology of the flow. Swirling flows are widely employed, in the impinging configuration, in several industrial processes which involve both non-reacting and reacting applications. In the present dissertation, the simultaneously acquired thermal and three-dimensional velocity fields of an impinging hot jet emerging from a custom swirl generator in a cold ambient are presented. The velocity and temperature fields are experimentally measured using time-resolved Tomographic PIV and high-speed Infrared thermography in a combined system. A detailed description of a custom swirl generator is provided, and the time-averaged velocity profiles of a free swirling flow are discussed in order to estimate the swirl number. The instantaneous three-dimensional dynamics in proximity of the nozzle is discussed and the main features of a free swirling jet are investigated through the application of Proper Orthogonal Decomposition technique. The time-dependent features of velocity and temperature fields of an impinging swirling jet are investigated through the description of the time sequences of the temperature fluctuations and the synchronised instantaneous vortical structures. Taking advantage of the simultaneous acquisition and of the knowledge of the relative positioning of thermal and velocity frames, two different correlation techniques are applied, and their outcomes discussed

Noninvasive Cardiac Flow Assessment Using High Speed Magnetic Resonance Fluid Motion Tracking

Cardiovascular diseases can be diagnosed by assessing abnormal flow behavior in the heart. We introduce, for the first time, a magnetic resonance imaging-based diagnostic that produces sectional flow maps of cardiac chambers, and presents cardiac analysis based on the flow information. Using steady-state free precession magnetic resonance images of blood, we demonstrate intensity contrast between asynchronous and synchronous proton spins. Turbulent blood flow in cardiac chambers contains asynchronous blood proton spins whose concentration affects the signal intensities that are registered onto the magnetic resonance images. Application of intensity flow tracking based on their non-uniform signal concentrations provides a flow field map of the blood motion. We verify this theory in a patient with an atrial septal defect whose chamber blood flow vortices vary in speed of rotation before and after septal occlusion. Based on the measurement of cardiac flow vorticity in our implementation, we establish a relationship between atrial vorticity and septal defect. The developed system has the potential to be used as a prognostic and investigative tool for assessment of cardiac abnormalities, and can be exploited in parallel to examining myocardial defects using steady-state free precession magnetic resonance images of the heart

Characterization of Flow Dynamics in a Heart Simulator by means of PIV = Charakterisierung der Strömungsdynamik in einem Herzsimulator mittels PIV

Diese Arbeit konzentriert sich auf die strömungsmechanische Charakterisierung von Regurgitationsjets, die durch insuffiziente Mitralklappen entstehen. Die Untersuchungen wurden mit Particle Image Velocimetry (PIV) Messtechnik und eigens entwickelten Post-Processing-Skripten durchgeführt. Die Mitralinsuffizienz ist die häufigste Form der Herzklappenerkrankung und bezeichnet den Rückfluss von Blut in den linken Vorhof aufgrund einer insuffizienten Mitralklappe, was zu verschiedenen gesundheitlichen Komplikationen führt. Eine genaue Diagnose der Mitralinsuffizienz ist für eine erfolgreiche Behandlung von entscheidender Bedeutung. Bei komplexen Regurgitationsjets ist die Aussagekraft herkömmlicher Methoden jedoch limitiert. Diese Studie zielt darauf ab, das Verständnis für die komplexe Strömungssituation von Regurgitationsjets zu verbessern und so zur Verbesserung der Diagnose und damit der Therapie beizutragen. Dazu wurden phasengemittelte und phasenaufgelöste 2D2C-PIV-Experimente für verschiedene Mitralklappengeometrien im Herzsimulator durchgeführt. Der Laserlichtschnitt wurde traversiert, um quasi 3D Geschwindigkeitsdaten zu erhalten. Drei generische Mitralklappengeometrien unterschiedlicher Größe wurden untersucht: eine runde Lochblende, ein spitzes Oval und eine Tropfenform. Zusätzlich wurden eine exzentrische Klappengeometrie und eine patienten-spezifische Klappe untersucht. Die Geschwindigkeitsdaten wurden bezüglich zeitlichem Strömungsverlauf und Form des Jets untersucht. Die Wirbeldynamische Untersuchung basierte auf der Wirbelstärke, sowie dem Q- und $\Gamma_1$-Kriterium. Die Ergebnisse der generischen Mitralklappen zeigten typische Merkmale von pulsierenden Jets. Unterschiede in der radialen Geschwindigkeitsverteilung konnten mit den unterschiedlichen Öffnungsformen in Verbindung gebracht werden. Die Sattel-behafteten radialen Profile der Axialgeschwindigkeit am Auslass können durch die geringe Einlasslänge und die scharfen Öffnungsränder erklärt werden. Zusätzlich zu periodisch wiederkehrenden Anfangswirbeln wurden Kelvin-Helmholtz-Instabilitäten in der Grenzschicht stromaufwärts detektiert. Die exzentrische Klappengeometrie zeigte ähnliche Beobachtungen, jedoch um den Winkel der Exzentrizität gedreht. Die patientenspezifische Mitralklappe ergab, im Vergleich zu den generischen Fällen,eine deutlich instabilere und turbulentere Strömungssituation, was auf die komplexere Öffnungsform, die höhere Reynoldszahl und die größere Flexibilität der Klappenblätter zurückzuführen ist. Form und Richtung des Jets unterlagen starken zeitlichen und räumlichen Schwankungen. Gemittelte PIV-Auswertungen ergaben ein dominantes Anfangswirbelpaar, während Einzelbildpaarauswertungen keine eindeutige Wirbelerkennung zeigten. Für zukünftige Untersuchungen wird empfohlen, das Innere des Atriums mit einer realistischeren Geometrie zu modellieren und Hochgeschwindigkeits-PIV zu verwenden

Vortex Motions in the Solar Atmosphere: Definitions, Theory, Observations, and Modelling

Vortex flows, related to solar convective turbulent dynamics at granular scales and their interplay with magnetic fields within intergranular lanes, occur abundantly on the solar surface and in the atmosphere above. Their presence is revealed in high-resolution and high-cadence solar observations from the ground and from space and with state-of-the-art magnetoconvection simulations. Vortical flows exhibit complex characteristics and dynamics, excite a wide range of different waves, and couple different layers of the solar atmosphere, which facilitates the channeling and transfer of mass, momentum and energy from the solar surface up to the low corona. Here we provide a comprehensive review of documented research and new developments in theory, observations, and modelling of vortices over the past couple of decades after their observational discovery, including recent observations in Hα, innovative detection techniques, diverse hydrostatic modelling of waves and forefront magnetohydrodynamic simulations incorporating effects of a non-ideal plasma. It is the first systematic overview of solar vortex flows at granular scales, a field with a plethora of names for phenomena that exhibit similarities and differences and often interconnect and rely on the same physics. With the advent of the 4-m Daniel K. Inouye Solar Telescope and the forthcoming European Solar Telescope, the ongoing Solar Orbiter mission, and the development of cutting-edge simulations, this review timely addresses the state-of-the-art on vortex flows and outlines both theoretical and observational future research directions