Search

Nasal airflow obstruction correlates with several ailments, such as higher patency, increased friction at the mucosal wall or the so-called Little's area, improper air conditioning, and snoring. Nasal dilators are frequently employed, mainly due to their ease of access and use, combined with their non-permanent and non-surgical nature. Their overall efficacy, however, has not been clearly demonstrated so far, with some studies reporting conflicting outcomes, mainly because being based on subjective evaluations. This study employs Computational Fluid Dynamics simulations to analyze the flow inside a real nose, performs an objective assessment of a nasal dilator's effect in terms of airflow and air conditioning, reporting flow paths, friction levels, heat and water fluxes and detailed temperature and humidity distributions. Coincidentally, the studied nose presents a septal deviation, with one nostril being wider than the other. The tubes of the dilator used in both nostrils are identical, as with any standard commercial dilator. Consequently, the dilator widens one nostril, as intended, but results in an obstruction in the other. This allows simultaneously addressing two situations, the nominal function of the dilator, as well as an off-design case. Results indicate a 24 % increase in nasal patency in the design situation. The effect, however, is limited, as quantified by appropriate measures, such as the flow-generated friction at the nose surfaces and the temperature fluxes. Hence, the effect of such a dilator in nominal conditions is perhaps not as large as might be hoped. In the off-design situation, nasal resistance increases by 62 %, an undesirable effect, illustrating the consequences of using an inappropriate dilator., Peer Reviewed, Postprint (published version)

The intermediate wake of a circular cylinder at Reynolds number 3300 is studied by means of highly resolved large eddy simulations. The domain spans 100 cylinder diameters D downstream of the cylinder. The intermediate wake is taken here as the region between 6D and 50D downstream of the cylinder. Initially, the velocity deficit shows a marked decrease. This rate of decrease diminishes in the region 10D to 20D downstream of the cylinder centre. Thereafter, it starts to evolve following the far-wake behaviour. The wake half-width exhibits a marked increase in the region 20D to 40D. It is in this region where the von Kárman vortex street looses its coherence., J. Fröhlich and J. Ventosa-Molina acknowledge the funding by the Deutsche Forschungsgemeinschaft (DFG), through FR1593/15-1 within PAK948. I. Rodriguez and J. Ventosa-Molina acknowledge funding by AGAUR through grant 2021 SGR 01051. The authors thank Jan Wissink for providing the inflow signals. Part of the computations were performed at the Centre for Information Services and High Performance Computing (ZIH) at TU Dresden. The authors thankfully acknowledges the computer resources at Altamira and MareNostrum Supercomputers and technical support provided by Santander Supercomputacion support group at the University of Cantabria and Barcelona Supercomputing Center - Centro Nacional de Supercomputación (RES-IM-2023-1-0016, RES-IM-2023-2-0021)., Peer Reviewed, Postprint (published version)

The version of record of this article, first published in Journal of visualization, is available online at Publisher’s website: https://doi.org/10.1007/s12650-023-00930-6, Turbines are core components in jet engines for flight propulsion, power plants, and other important energy conversion processes. They are composed of successive rows of blades so that wakes of upstream blades reach subsequent blades where they perturb the flow in an unsteady manner. At the point where a wake reaches the downstream blades, the perturbation forms a so-called negative jet. In this work, we show that the negative jet partially fulfills the conditions of an anti-splat. Based on this finding, we enhance an anti-splat detection algorithm developed by the present authors in previous work and apply it to direct numerical simulation data of a turbine cascade with unsteady wakes. This provides a sound framework and suitable visualization approaches to investigate the phenomenon even in very complex conditions, as is the alteration of the boundary layer flow along the pressure side of a turbine blade. The approach allows a very clear visualization of this interaction, which was not possible to evidence with previous methods, providing new insight into the physics of this flow. The use of flow paths shows up to which point wakes affect the boundary layer along the blade. The reported physical analysis, made possible by the proposed approach, demonstrates the usefulness of the method for the application domain. The generalization to flows in compressors, pumps, and blade-tower interaction in wind engineering and other fields is possible., This work was funded by the German Federal Ministry of Education and Research within the project Competence Center for Scalable Data Services and Solutions (ScaDS) Dresden/Leipzig (BMBF 01IS14014B). JF and JVM acknowledge funding by DFG under FR1593/15-1 within PAK948., Peer Reviewed, Postprint (published version)

Das Laserschneiden ist ein in der Industrie fest etabliertes Trennverfahren für Metalle und eine Vielzahl weiterer Materialien. Trotz der weitgehenden Beherrschung des Schneidprozesses durch empirische Untersuchungen und praktisches Erfahrungswissen sind die zugrundeliegenden physikalischen Mechanismen bisher nicht vollständig verstanden. Besonders für die qualitätsbestimmende Schnittkantenrauheit und die Gratbildung ist mit den bisherigen Verbesserungsansätzen nur eine begrenzte Beeinflussung möglich. Auch wenn die fundamentale Bedeutung der Gasströmung für die Bewegung der Schmelze allgemein akzeptiert ist, existieren für den konkreten Impuls- und Wärmeaustausch zwischen Gasströmung und Schmelze bisher nur unzureichende theoretische oder numerische Ergebnisse. Weiterhin können die in experimentellen Untersuchungen dokumentierten charakteristischen Schmelzströmungen nicht mit den bereits existierenden einfachen Modellen zur Gas-Schmelze-Interaktion abgebildet oder erklärt werden. Die vorliegende Dissertation thematisiert erstmals die lokale Wechselwirkung zwischen Schneidgasströmung und der Oberfläche des Schnittspaltes bzw. der Schmelzfilmoberfläche und untersucht insbesondere die Eigenschaften der Gasgrenzschicht. Die Arbeit verfolgt einen kombinierten Ansatz aus experimentellen, theoretischen und numerischen Untersuchungen. Die Analyse typischer Schnittkanten betrachtet detailliert die charakteristische Oberflächenstruktur der Kanten und resultiert in einer für das Schneiden mit Festkörperlasern angepassten Einteilung in horizontale Strukturzonen. Der festgestellte Zusammenhang zwischen den Prozessparametern, der Spaltgeometrie und den Zonen der Schnittkante dokumentiert ebenso wie die lokalen Ansammlungen von erstarrten Schmelzresten mit spiralförmiger Oberflächenstruktur den fundamentalen Einfluss der Gasströmung auf die globale und lokale Schmelzbewegung. Die Abschätzung der Grenzschichteigenschaften anhand semi-empirischer Kennzahlgleichungen unter den spezi, Laser cutting is a firmly established cutting process in industry for metals and a variety of other materials. Despite the fact that the cutting process has been largely mastered by empirical studies and practical experience, the underlying physical mechanisms are not yet fully understood. Particularly for the quality-determining cut edge roughness and burr formation, only a limited influence is possible with the previous improvement approaches. Even if the fundamental influence of the gas flow on the movement of the melt is generally accepted, only insufficient theoretical or numerical results exist so far for the specific momentum and heat transfer between gas flow and melt. Furthermore, the characteristic melt flows documented in experimental investigations cannot be represented or explained with the already existing simple models for gas-melt interaction. The present dissertation addresses the local interaction between cutting gas flow and the surface of the kerf respectively the melt film surface, and considers in particular the properties of the gas boundary layer. The work follows a combined approach of experimental, theoretical and numerical investigations. The analysis of typical cut edges considers in detail the characteristic surface structure of the edges and results in a subdivision into horizontal structural zones adapted for cutting with solid-state lasers. The observed correlation between the process parameters, the gap geometry and the zones of the cut edge documents the fundamental influence of the gas flow on the global and local melt movement, as do the local accumulations of solidified melt residues with a spiral surface structure. The estimation of the boundary layer properties based on semi-empirical parameter equations under the specific boundary conditions of laser cutting shows for the first time that neither a laminar nor a turbulent boundary layer flow can be assumed, but that a transitional boundary layer character is present. The transf

The article describes direct numerical simulations using an Euler–Lagrange approach with an immersed-boundary method to resolve the geometry and trajectory of particles moving in a flow. The presentation focuses on own work of the authors and discusses elements of physical and numerical modeling in some detail, together with three areas of application: microfluidic transport of spherical and nonspherical particles in curved ducts, flows with bubbles at different void fraction ranging from single bubbles to dense particle clusters, some also subjected to electro-magnetic forces, and bedload sediment transport with spherical and nonspherical particles. These applications with their specific requirements for numerical modeling illustrate the versatility of the approach and provide condensed information about main findings.

In turbomachines, rotors and stators differ by the rotation of the former. Hence, half of each stage is directly influenced by rotation effects. The influence of rotation on the flow structure and its impact on the performance is studied through wall-resolving large Eddy simulations of a rotor with large relative tip gap size. The simulations are performed in a rotating frame with rotation accounted for through a Coriolis force term. In a first step, experimental results are used to provide validation. The main part of the study is the comparison of the results from two simulations, one representing the rotating configuration and one with the Coriolis force removed, without any other change. This setup allows a very clean assessment of the influence of rotation. The turbulence-resolving approach ensures that the turbulent flow features are well represented. The results show a significant impact of rotation on the secondary flow. In the tip region, the tip leakage vortex is enlarged and destabilized. Inside the tip gap, the flow is altered as well, with uniformization in the rotating case. At the blade midspan, no significant effects are observed on the suction side, while an earlier transition to turbulence is found on the pressure side. Near the hub, rotation effects are shown to reduce the corner separation significantly., Funding was provided by the Deutsche Forschungsgemeinschaft (DFG), under FR1593/15-1 and MA4922/8-1 within PAK948. The authors would like to acknowledge stimulating discussions with the partners of this project. Computations were performed at the Centre for Information Services and High Performance Computing (ZIH) at TU Dresden., Postprint (author's final draft)

In turbomachines, rotors and stators differ by the rotation of the former. Hence, half of each stage is directly influenced by rotation effects. The influence of rotation on the flow structure and its impact on the performance is studied through wall-resolving large Eddy simulations of a rotor with large relative tip gap size. The simulations are performed in a rotating frame with rotation accounted for through a Coriolis force term. In a first step, experimental results are used to provide validation. The main part of the study is the comparison of the results from two simulations, one representing the rotating configuration and one with the Coriolis force removed, without any other change. This setup allows a very clean assessment of the influence of rotation. The turbulence-resolving approach ensures that the turbulent flow features are well represented. The results show a significant impact of rotation on the secondary flow. In the tip region, the tip leakage vortex is enlarged and destabilized. Inside the tip gap, the flow is altered as well, with uniformization in the rotating case. At the blade midspan, no significant effects are observed on the suction side, while an earlier transition to turbulence is found on the pressure side. Near the hub, rotation effects are shown to reduce the corner separation significantly.