06 Fakultät Luft- und Raumfahrttechnik und Geodäsie
Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/7
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Item Open Access Passively mode-locked Tm-lasers for all-fiber high-energy nonlinear chirped pulse amplification(2023) Graf, Florian; Dekorsy, Thomas (Prof. Dr. rer. nat.)Item Open Access Particle image velocimetry measurements in accelerated, transonic wake flows(2022) Richter, Judith; Alexopoulos, Charalampos; Weigand, BernhardThis paper reports on particle image velocimetry (PIV) measurements in compressible accelerated wake flows generated by two different central injector types, which are mounted in a convergent-divergent nozzle. The injectors differ by the extent of their trailing edge located either in the subsonic (injector A) or supersonic flow region (injector B). In addition, the undisturbed nozzle flow without injector is studied as a reference case. The PIV results reveal typical wake flow structures expected in subsonic (injector A) and supersonic (injector B) wake flows. They further show that the Reynolds stresses Rexxand Reyysignificantly decay in all three cases due to the strong acceleration throughout the nozzle. Interestingly, in the case of injector A, the flow stays non-isotropic with Reyy>Rexxalso far downstream in the supersonic flow region. These measurements were motivated by the lack of velocity data needed to validate numerical simulations. That is why this paper additionally contains results from (unsteady) Reynolds-averaged Navier-Stokes ((U)RANS) simulations of the two wake flows investigated experimentally. The URANS simulation of the injector A case is able to accurately predict the entire flow field and periodic fluctuations at the wake centerline. However, in the case of injector B, the RANS simulation underestimates the far wake centerline velocity by about 4%.Item Open Access The effect of patterned micro-structure on the apparent contact angle and three-dimensional contact line(2021) Foltyn, Patrick; Restle, Ferdinand; Wissmann, Markus; Hengsbach, Stefan; Weigand, BernhardThe measurement of the apparent contact angle on structured surfaces is much more difficult to obtain than on smooth surfaces because the pinning of liquid to the roughness has a tremendous influence on the three phase contact line. The results presented here clearly show an apparent contact angle variation along the three phase contact line. Accordingly, not only one value for the apparent contact angle can be provided, but a contact angle distribution or an interval has to be given to characterize the wetting behavior. For measuring the apparent contact angle distribution on regularly structured surfaces, namely micrometric pillars and grooves, an experimental approach is presented and the results are provided. A short introduction into the manufacturing process of such structured surfaces, which is a combination of Direct LASER Writing (DLW) lithography, electroforming and hot embossing shows the high quality standard of the used surfaces.Item Open Access An analytical study on the mechanism of grouping of droplets(2022) Vaikuntanathan, Visakh; Ibach, Matthias; Arad, Alumah; Chu, Xu; Katoshevski, David; Greenberg, Jerrold Barry; Weigand, BernhardThe condition for the formation of droplet groups in liquid sprays is poorly understood. This study looks at a simplified model system consisting of two iso-propanol droplets of equal diameter, Dd0, in tandem, separated initially by a center-to-center distance, a20, and moving in the direction of gravity with an initial velocity, Vd0>Vt, where Vt is the terminal velocity of an isolated droplet from Stokes flow analysis. A theoretical analysis based on Stokes flow around this double-droplet system is presented, including an inertial correction factor in terms of drag coefficient to account for large Reynolds numbers (≫1). From this analysis, it is observed that the drag force experienced by the leading droplet is higher than that experienced by the trailing droplet. The temporal evolutions of the velocity, Vd(t), of the droplets, as well as their separation distance, a2(t), are presented, and the time to at which the droplets come in contact with each other and their approach velocity at this time, ΔVd0, are calculated. The effects of the droplet diameter, Dd0, the initial droplet velocity, Vd0, and the initial separation, a20 on to and ΔVd0 are reported. The agreement between the theoretical predictions and experimental data in the literature is good.Item Open Access Investigation of macroscopic nearcritical fluid phenomena by applying laser-induced thermal acoustics(2023) Steinhausen, Christoph; Weigand, Bernhard (Prof. Dr.-Ing. habil.)The political and social aspiration to reduce greenhouse gases together with increasing energy demands are driving the development of new sustainable energy solutions. To achieve long term sustainability both innovative energy sources and improvements in efficiency are essential. Higher process efficiencies have been achieved by raising combustion pressures, reaching values that now exceed the critical pressures of the injected fuels. However, for an efficient and stable combustion a profound understanding of the processes prior to the combustion, such as fluid injection, disintegration and subsequent evaporation is essential. Unfortunately, the fundamental changes in fluid behaviour at near- to supercritical conditions leading to the observed fluid phenomena are not yet fully understood. Besides fluid injection, supercritical fluids themselves have been identified as an innovative path for an efficient energy conversion and heat transfer processes. The Brayton cycle using supercritical carbon dioxide as operating fluid, supercritical water as coolant and process fluid in nuclear reactors, or the application of supercritical methane as new 'green' fuel in rocket propulsion are just a few examples. Laser-induced thermal acoustics (LITA) has been identified as a promising diagnostic tool in near- to supercritical fluid research. The latter is based on the capability to acquire speed of sound data as well as to resolve thermal and acoustic attenuation in both pure fluids and complex mixing processes, such as evaporation and jet disintegration. Moreover, based on the non-linear pressure dependencies, LITA becomes increasingly more effective in high-pressure environments. By analysing the frequency domain of recorded signals, speed of sound data can be directly determined without any equation of state or additional modelling approaches. Furthermore, acoustic damping rates and thermal diffusivities can be acquired by an analytical expression for the temporal-domain of the signals. By combining both evaluations with suitable analytic expressions for the thermodynamic properties, mixing states, such as local mixing temperatures and concentrations, can be determined. Moreover, since in complex fluids at near- to supercritical conditions acoustic damping is related to both sound dispersion and volume viscosity, important insights into the physics of supercritical fluids are provided. Concordantly, the purpose of this thesis is to apply LITA in the investigation of macroscopic fluid phenomena at nearcritical to supercritical fluid conditions. This includes the following major research objectives. First, the significance of volume viscosities in complex fluids at dense gas conditions as well as the dependency of acoustic damping on mixing states are assessed. Second, the feasibility of time-resolved LITA measurements under complex flow conditions is evaluated. To achieve these objectives an experimental test facility has been designed, which enables investigations at high pressure and high temperature conditions in both pure fluids and complex mixing processes. Moreover, the laser-induced thermal acoustics setup of the ITLR has been optimised for high pressure investigations. Also a new high-speed LITA system with an adjustable measurement volume has been developed. Furthermore, a new post-processing methodology capable of analysing both the frequency and time-domain of the signal has been developed and validated. With the developed system and routines investigations in carbon dioxide, nitrogen, and binary mixtures at gas and gas-like states have been conducted to assess acoustic attenuation and volumes viscosities. Additionally, a jet mixing process has been studied to characterise the LITA arrangement and to evaluate the dependency of acoustic damping on mixing concentration. At last, to assess the feasibility of transient LITA measurements in turbulent, physically complex flow conditions and to further characterise the evaporation process, time-resolved LITA measurements have been performed in the wake of a free falling droplet evaporating in a supercritical atmosphere.Item Open Access Modellierung des Wärmeübergangs komplexer Prallstrahlfelder an Turbinengehäusen(2021) Schweikert, Julia; Weigand, Bernhard (Prof. Dr.-Ing. habil.)Die Auslegung effizienter Flugtriebwerke in allen Betriebspunkten ist eine der zentralen Aufgaben der Luftfahrttechnik. Ein wesentlicher Bestandteil liegt dabei in einer optimalen Kühlung thermisch hochbelasteter Triebwerkskomponenten über den gesamten Zyklus einer Flugmission. Für eine optimale Auslegung der eingesetzten Kühlmechanismen müssen die Wärmeübergangscharakteristika möglichst exakt bekannt sein. Unter diesem Aspekt befasst sich die vorliegende Arbeit mit dem Wärmeübergangsverhalten an Turbinengehäusen. Die Kühlung erfolgt in der realen Anwendung über den Einsatz komplexer Prallstrahlfelder, welche derzeit noch Lücken hinsichtlich der theoretischen Beschreibung aufweisen. Im Rahmen dieser Arbeit wurde ein Versuchsstand zur experimentellen Untersuchung der komplexen Prallstrahlfelder aufgebaut. Die Bestimmung des Wärmeübergangsverhaltens erfolgt dabei auf Basis der Infrarot-Thermografie. Neben einer experimentellen Betrachtung dienen numerische Simulationen der Untersuchung der Wärmeübergangs- und Strömungscharakteristika. Eine erfolgreiche Validierung des verwendeten numerischen Setups erfolgt anhand von Referenzdaten aus der Literatur. Das experimentelle Setup wird an einem vereinfachten Prallstrahlmodell verifiziert und den numerischen Verläufen gegenübergestellt. In einer zusätzlichen numerischen Studie wird der Einfluss der thermischen Randbedingung auf die Berechnung der Nusseltzahl an der Prallplatte quantifiziert. Mit den Daten dieser Studie wird eine Methode vorgestellt, den zum Teil deutlichen Einfluss der thermischen Randbedingung zu berücksichtigen. Neben der Entwicklung geeigneter Modelle zur Beschreibung des Wärmeübergangs in realen Anwendungen, bei welchen die thermischen Gegebenheiten nicht gänzlich bekannt sind, sind die Erkenntnisse essentiell für die Betrachtung des Wärmeübergangs an dem in dieser Arbeit betrachteten Modell eines komplexen Prallstrahlfeldes.Item Open Access Influence of Weber number on crown morphology during an oblique droplet impact on a thin wall film(2023) Stober, Jonathan Lukas; Santini, Maurizio; Schulte, KathrinSpray impacts can be found in several technical applications and consist of many single droplets, which impact under different trajectories on wetted walls. This study investigates the asymmetric crown morphology resulting from an oblique impact (𝛼=60°) of a single droplet on a horizontal and quiescent wall film of the same liquid. A droplet generator with an accelerated needle releases the droplets (𝐷=1.5 mm) in a controlled trajectory on a thin film (ℎ𝑓/𝐷=0.2). The impact process is recorded from two perspectives with two synchronized high-speed cameras. Varying the Weber number within the splashing regime reveals distinct crown morphologies, which are described in detail. For 𝑊𝑒< 500, a single central finger develops at the front of the crown, with subsequent detachments of secondary droplets. At higher 𝑊𝑒 (>500), a collision of the crown with the wall film shortly after impact introduces disturbances into the rim, leading to two fingers in the middle of the front crown. A further increase in 𝑊𝑒 (>600) intensifies the crown-film interaction, resulting in an early ejection of tiny droplets and a complete breakup of the front rim. The influence of 𝑊𝑒 on the crown morphology during an oblique impact is also compared to the normal impact (90°). This study paves the way for a classification of impact regimes and a comprehensive picture of the oblique impact process, which deserve more investigation.Item Open Access Air entrapment and bubble formation during droplet impact onto a single cubic pillar(2021) Ren, Weibo; Foltyn, Patrick; Geppert, Anne; Weigand, BernhardWe study the vertical impact of a droplet onto a cubic pillar of comparable size placed on a flat surface, by means of numerical simulations and experiments. Strikingly, during the impact a large volume of air is trapped around the pillar side faces. Impingement upon different positions of the pillar top surface strongly influences the size and the position of the entrapped air. By comparing the droplet morphological changes during the impact from both computations and experiments, we show that the direct numerical simulations, based on the Volume of Fluid method, provide additional and new insight into the droplet dynamics. We elucidate, with the computational results, the three-dimensional air entrapment process as well as the evolution of the entrapped air into bubbles.Item Open Access Editorial - physics of droplets(2024) Planchette, Carole; Lamanna, Grazia; Pan, Kuo-LongItem Open Access Experimental investigation of unsteady convective heat transfer under airflow velocity and temperature variations(2021) Brack, Stefan; Poser, Rico; Wolfersdorf, Jens vonAn experimental approach to study unsteady local heat transfer characteristics due to airflow velocity and/or airflow temperature variations is presented. It uses controlled electrical heaters and rotating vanes to independently vary the flow and thermal boundary conditions. Time-resolved surface temperatures are measured using an in situ calibrated infrared thermography camera. Those surface temperatures are analyzed by modeling the transient conjugate heat transfer process in the wall to obtain locally resolved surface heat flux distributions. The applicability is illustrated for a flow and heat transfer behind a tetrahedral vortex generator on a flat plate.
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