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 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 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 Editorial - physics of droplets(2024) Planchette, Carole; Lamanna, Grazia; Pan, Kuo-LongItem Open Access Ground-based autonomous passive-optical staring sensor for orbital object detection and position measurement(2022) Wagner, Paul-Philipp; Dekorsy, Thomas (Prof. Dr. rer. nat.)Active spacecraft operations heavily rely on a space surveillance network, which continuously scans, measures, and predicts space debris particle trajectories to avoid collision risks. Recent activities in large satellite constellations in Low Earth Orbit (LEO) will accumulate more than ten thousand satellites, doubling the number of active spacecraft. Due to the high density of debris, LEO is of highest interest for space surveillance. Beside RADAR sensors, space debris laser ranging can accurately measure the distance to resident space objects allowing highly precise orbit predictions. Due to their small field-of-view (FOV), laser ranging stations rely on an a-priori orbit information, which is usually obtained by a separate sensor network. A passive-optical sensor with a larger FOV represents a complementary tool to deliver the necessary initial orbit determination. Such a sensor is much more cost effective and easier to operate than RADAR sensors and therefore of high interest for a space situational awareness (SSA) network. The development of a passive-optical sensor is described in this thesis, which operates autonomously to detect unknown orbital objects in LEO. This thesis is structured in three parts. Initially a theoretical model is presented to estimate performance of a passive-optical sensor. It shows the influences of system properties and observing conditions on the detection threshold. Furthermore, deterministic simulations using ESA’s PROOF software are performed, which provide a more detailed analysis of the detection rates and detection efficiencies during different observation conditions and system parameters. In the main part, the system setup is explained including the software development, which plays a major role for the automation of the system. An image processing technique is implemented, which is able to reliably identify objects in LEO even when disturbances are present in the source images, such as high, transparent, or smaller clouds. Astrometric calibration is used to transform the coordinates of measured objects into equatorial coordinates and a standardized data export allows for sharing the data with existing tools or databases. A weatherproofed housing protects the camera and lens. A weather station is used to trigger image acquisition. The passive-optical sensor is deployed for continuous observations and the data is automatically uploaded to a webserver. Data analysis shows the performance figures of the system. Observation campaigns under three different line-of-sight (LOS) directions are performed and the detection rate, efficiency, threshold, and uncertainties are analyzed and compared to simulations. A line-of-sight to the North under 45° elevation is found to deliver the highest detection rates, while the observed Along-Track error is about ten times larger than the Cross-Track error. Finally, the determined parameters of the passive-optical system are used to derive requirements to detect space debris as small as 10 cm in diameter.Item Open Access Characterisation of the transient mixing behaviour of evaporating near-critical droplets(2023) Steinhausen, Christoph; Gerber, Valerie; Stierle, Rolf; Preusche, Andreas; Dreizler, Andreas; Gross, Joachim; Weigand, Bernhard; Lamanna, GraziaWith technical progress, combustion pressures have been increased over the years, frequently exceeding the critical pressure of the injected fluids. For conditions beyond the critical point of the injected fluids, the fundamental physics of mixing and evaporation processes is not yet fully understood. In particular, quantitative data for validation of numerical simulations and analytical models remain sparse. In previous works, transient speed of sound studies applying laser-induced thermal acoustics (LITA) have been conducted to investigate the mixing behaviour in the wake of an evaporating droplet injected into a supercritical atmosphere. LITA is a seedless, non-intrusive measurement technique capable of direct speed of sound measurements within these mixing processes. The used setup employs a high-repetition-rate excitation laser source and, therefore, allows the acquisition of time-resolved speed of sound data. For the visualisation of the evaporation process, measurements are accompanied by direct, high-speed shadowgraphy. In the present work, the measured speed of sound data are evaluated by applying an advection-controlled mixing assumption to estimate both the local mole fraction and mixing temperature. For this purpose, planar spontaneous Raman scattering results measured under the same operating conditions are evaluated using an advection-controlled mixing assumption with the perturbed-chain statistical associating fluid theory (PC-SAFT) equation of state. Successively, the resulting concentration–temperature field is used for the estimation of local mixture parameters from the detected speed of sound data. Moreover, models using the PC-SAFT equation of state and the NIST database for the computation of the speed of sound are compared. The investigations indicate a classical two-phase evaporation process with evaporative cooling of the droplet. The subsequent mixing of fluid vapour and ambient gas also remains subcritical in the direct vicinity of the droplet.Item Open Access Characterization of effective diffusion within viscoelastic fluids with elastic instabilities(2022) Hietsch, Valerie; Ligrani, Phil; Su, MengyingWe considered effective diffusion, characterized by magnitudes of effective diffusion coefficients, in order to quantify mass transport due to the onset and development of elastic instabilities. Effective diffusion coefficient magnitudes were determined using different analytic approaches, as they were applied to tracked visualizations of fluorescein dye front variations, as circumferential advection was imposed upon a flow environment produced using a rotating Couette flow arrangement. Effective diffusion coefficient results were provided for a range of flow shear rates, which were produced using different Couette flow rotation speeds and two different flow environment fluid depths. To visualize the flow behavior within the rotating Couette flow environment, minute amounts of fluorescein dye were injected into the center of the flow container using a syringe pump. This dye was then redistributed within the flow by radial diffusion only when no disk rotation was used, and by radial diffusion and by circumferential advection when disk rotation was present. Associated effective diffusion coefficient values, for the latter arrangement, were compared to coefficients values with no disk rotation, which were due to molecular diffusion alone, in order to quantify enhancements due to elastic instabilities. Experiments were conducted using viscoelastic fluids, which were based on a 65% sucrose solution, with different polymer concentrations ranging from 0 ppm to 300 ppm. Associated Reynolds numbers based on the fluid depth and radially averaged maximum flow velocity ranged from 0.00 to 0.5. The resulting effective diffusion coefficient values for different flow shear rates and polymer concentrations quantified the onset of elastic instabilities, as well as significant and dramatic changes to local mass transport magnitudes, which are associated with the further development of elastic instabilities.Item Open Access Thin-disk multipass amplifier for power scaling of ultrafast lasers(2024) Dominik, John; Dekorsy, Thomas (Prof. Dr.)
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