Search

The current revolution in the field of machine learning is leading to many interesting developments in a wide range of areas, including fluid mechanics. Fluid mechanics, and more concretely turbulence, is an ubiquitous problem in science and engineering. Being able to understand and predict the evolution of turbulent flows can have a critical impact on our possibilities to tackle a wide range of sustainability problems (including the current climate emergency) and industrial applications. Here, we review recent and emerging possibilities in the context of predictions, simulations, and control of fluid flows, focusing on wall-bounded turbulence. When it comes to flow control, we refer to the active manipulation of the fluid flow to improve the efficiency of processes such as reduced drag in vehicles, increased mixing in industrial processes, enhanced heat transfer in heat exchangers, and pollution reduction in urban environments. A number of important areas are benefiting from ML, and it is important to identify the synergies with the existing pillars of scientific discovery, i.e., theory, experiments, and simulations. Finally, I would like to encourage a balanced approach as a community in order to harness all the positive potential of these novel methods., QC 20240326

Turbulent flows over porous lattices consisting of rectangular cuboid pores are investigated using scale-resolving direct numerical simulations. Beyond a certain threshold which is primarily determined by the wall-normal Darcy permeability, Ky, near-wall turbulence transitions from its canonical regime, marked by the presence of streak-like structures, to another marked by the presence of Kelvin-Helmholtz-like (K-H-like) spanwise-coherent structures. The threshold agrees well with that previously established in studies where permeable-wall boundary conditions had been used as surrogates for a porous substrate. In the smooth-wall-like regime, none of the investigated substrates demonstrate any reduction in drag relative to a smooth-wall flow. At the permeable surface, a notable component of the flow is that which adheres to the pore geometry and undergoes modulation by the turbulent scales of motions due to the interaction mechanism described by Abderrahaman-Elena et al. (2019). Its resulting effect can be quantified in terms of an amplitude modulation (AM) using the approach of Mathis et al. (2009). This pore-coherent flow component persists throughout the porous substrate, highlighting the importance of a given substrate's microstructure in the presence of an overlying turbulent flow. This geometry-related aspect of the flow is not accounted for when continuum-based models for a porous medium or effective representations of them such as wall boundary conditions are used. The intensity of the AM effect is enhanced in the K-H-like regime and becomes strengthened with larger permeability. As a result, structured porous materials may be designed to exploit or mitigate these flow features depending upon the intended application., QC 20240327

This thesis investigates how in wall-bounded turbulent flows changes to the wall can induce changes in the flow. To this end, we investigate the use of wall boundary conditions meant to mimic the effect of textured surfaces. We also study the effect of porous walls on their overlying bulk turbulent flow and the consequences of these effects for heat transfer. Textured surfaces can alter near-wall turbulence in subtle to dramatic ways. Vanishingly small surface textures of the order of the viscous sublayer thickness cause the displacement of the near-wall turbulence-generating flow structures by either restricting or permitting transpiration from taking place close to the surface. The former leads to drag reduction and the latter to its increase. As the textures increase in size and become comparable to the turbulence scales, they alter the near-wall dynamics and cause structural changes to occur in the flow. These effects are emulated using slip and transpiration boundary conditions called the Transpiration-Resistance model (TRM). Its utility in acting as an effective model for wall roughness is assessed. It captures the effect of vanishingly small roughness well, and to a limited extent larger roughness which protrude into the buffer layer. The TRM also helps to shed light on which near-wall structures play an essential role in the near-wall cycle of turbulence. Porous walls permit the exchange of mass, momentum and energy with the overlying turbulence. Their permeable structure quickly causes the turbulence to depart from its canonical smooth-wall-like structure, and induce a Kelvin-Helmholtz-like instability which leads to the emergence of spanwise rollers. These rollers efficiently redistribute momentum and turbulent kinetic energy into the porous wall. This pronounced interaction between the wall and bulk flow regions is detrimental for drag but beneficial for the transport of heat. The potential of porous walls for enhancing heat transfer exceeds that of other, I detta arbete, undersöker vi hur turbulenta flöden påverkas av ytmodifieringar. För detta ändamål undersöker vi randvilkor som är avsedda att efterlikna effekten av ojämna ytor. Vi studerar också hur porösa ytor påverkar den överliggande turbulenta flödet. Ytstrukturer kan påverka turbulens på ett subtilt sätt. Mycket små ytojämnheter (i storleksordningen av det viskösa skiktets tjocklek) orsakar en förskjutning av de turbulensgenererande flödesstrukturerna nära väggen genom att antingen begränsa eller tillåta transpiration att äga rum nära ytan. Det förra leder till minskning av luft/vattenmotståndet och det senare till dess ökning. När ytojämnheterna ökar i storlek och blir mer synliga för turbulensen, förändrar de dynamiken nära väggen och orsakar strukturella förändringar i flödet. Dessa effekter modelleras med hjälp av glid- och transpirationsrandvilkor som kallas ”Transpiration-Resistance Model”. Användbarheten av dessa randvilkor för att fungera som en effektiv modell för ytråhet har analyserats. Det visar sig att randvilkoren är en bra modell av små ytojämnheter, och i begränsad utsträckning större ojämnheter (som penetrerar buffertskiktet). TRM ökar vår förståelse kring vilka strukturer som är viktiga i turbulenscykeln när ytor. Porösa väggar tillåter utbyte av massa, rörelsemängd och energi med den överliggande turbulensen. Deras permeabla struktur gör att turbulensen utvecklar en Kelvin-Helmholtz-liknande instabilitet som orsakar stora virvlar. Dessa virvlar omfördelar rörelsemängden och den turbulenta kinetiska energin i den porösa väggen. Denna överföring av energi bort från bulkflödet ökar motståndet men är fördelaktigt för transporten av värme. Vi visar att potentialen hos porösa väggar för förbättring av värmeöverföring är större jämfört med andra passiva väggstrukturer. Vi har även utfört en studie av ett flerflassystem vid ytor. Flerfasinteraktioner äger oftast rum på en mikroskala i porösa medier. Vi har undersökt stabiliteten av tunn vätskehinna, QC 20240327

Extracorporeal life support (ECLS) includes life-saving support in severe acute cardiac and/or pulmonary failure. In the past 20 years, centrifugal pumps have become the primary choice to deliver the required blood flow. Pumps of various designs, with different approved operating ranges, are today available to clinicians. The use of centrifugal pumps in the low flow condition has been shown to increase hemolytic and thrombogenic risks of the treatment. Further, low flow operation has been associated with retrograde flow at the pump inlet. In this study, experimental and numerical methods have been applied to investigate the operating conditions and fluid dynamical mechanisms leading to reverse flow (or backflow) at the inlet. Reverse flow was predominantly observed in pumps having a top shroud covering the impeller blades, showing a relation between pump geometry and backflow. The shroud divides the pump volume above the impeller into two regions, separating the swirling reverse flow migrating toward the upper pump volute from the main flow, reducing the dissipation of the vortical structures, and allowing the swirling reverse flow to reach further in the pump inlet. At the inlet, backflow was observed as stable recirculation areas at the side of the pump inlet., QC 20240326

The interaction between fluid dynamics and biofilm growth plays a key role in both medical and industrial applications. Biofilms, or bacteria that are embedded in a protective matrix of extracellular polymeric substances, settle on interfaces such as on implanted devices or ship hulls. These biofilms canthen cause infectious diseases or significantly increase drag. In this thesis, we investigate the influence of flow, specifically shear stress, on the development of biofilm. The first paper presents a new facility to investigate biofilm growth in laminar flow cells over extended periods of up to several weeks. Optical coherence tomography is used to obtain three-dimensional scans of the biofilm structure at regular intervals. From these time series, we derive a simple model that relates the growth of an individual microcolony to the growth of the full biofilm depending on the wall shear stress. Additionally, we show that biofilm streamers, thin, flexible filaments that extend into the bulk flow, can form on sharp biofilm structures in laminar flow, even if the substratum is a flat surface. The second contribution is a report detailing preliminary studies on biofilm experiments. We investigate the formation of biofilm in the shear layer behinda backward-facing step. The results indicate a maximum shear stress, beyond which biofilm growth is inhibited. We also provide guidelines for the design of experimental setups for the investigation of the influence of fluid dynamics on biofilm and vice-versa., Samspelet mellan fluiddynamik och biofilmtillväxt spelar en nyckelroll i både medicinska och industriella tillämpningar. Biofilmer, eller bakterier som är inbäddade i en skyddande matris av extracellulära polymera substanser, sätter sig på ytor som på implanterade enheter eller fartygsskrov. Dessa biofilmer kan sedan orsaka infektionssjukdomar eller avsevärt öka vattenmotståndet. I den här avhandlingen undersöker vi hur flöde, speciellt skjuvspänning, påverkar utvecklingen av biofilm. I den första artikeln presenteras en ny uppställning för att undersöka biofilmstillväxt i flödesceller med laminärt flöde under längre perioder på upp till flera veckor. Optisk koherenstomografi används för att få tredimensionella skanningar av biofilmstrukturen vid regelbundna intervall. Från dessa tidsserier härleder vi en enkel modell som relaterar tillväxten av en enskild mikrokoloni till tillväxten av hela biofilmen beroende på väggskjuvspänning. Dessutom visar vi att biofilm filament som sträcker sig in i bulkflödet, kan bildas på skarpa biofilmstrukturer i laminärt flöde, även om substratum är en plan yta. Det andra bidraget är en rapport som beskriver preliminära studier av biofilmsexperiment. Vi undersöker bildandet av biofilm i skjuvskiktet bakom ett bakåtvänt steg. Resultaten indikerar en maximal skjuvspänning, bortom vilken biofilmstillväxt hämmas., QC 240314

The increasing number of scientific publications in acoustics, in general, presents difficulties in conducting traditional literature surveys. This work explores the use of a generative pre-trained transformer (GPT) model to automate a literature survey of 117 articles on data-driven speech enhancement methods. The main objective is to evaluate the capabilities and limitations of the model in providing accurate responses to specific queries about the papers selected from a reference human-based survey. While we see great potential to automate literature surveys in acoustics, improvements are needed to address technical questions more clearly and accurately., QC 20240111

Rigid particles in a Stokesian fluid experience an increasingly strong lubrication resistance as particle gaps narrow. Numerically, resolving these lubrication forces comes at an intractably large cost, even for moderate system sizes. Hence, it can typically not be guaranteed that artificial particle collisions and overlaps do not occur in a dynamic simulation, independently of the choice of method to solve the Stokes equations. In this work, the potentially large set of non-overlap constraints, in terms of the Euclidean distance between boundary points on disjoint particles, are efficiently represented via a barrier energy. We solve for the minimum magnitudes of repelling contact forces and torques between any particle pair in contact to correct for overlaps by enforcing a zero barrier energy at the next time level, given a contact-free configuration at a previous instance in time. Robustness for the method is illustrated using a multiblob method to solve the mobility problem in Stokes flow, applied to suspensions of spheres, rods and boomerang shaped particles. Collision free configurations are obtained at all instances in time, and considerably larger time-steps can be taken than without the technique. The effect of the contact forces on the collective order of a set of rods in a background flow that naturally promote particle interactions is also illustrated., QC 20231205

According to the IEC 60041 standard, the pressure-time method (1D PTM) can be employed to determine the flow rate in hydraulic turbines. This method assumes a one-dimensional flow and applies to straight pipes with uniform cross-sections, with specific restrictions on the pipe length, fluid velocity, and distance between the measurement sections from any irregularities in the pipeline. However, challenges arise when applying this method in low-head hydropower plants due to the short lengths, irregularities like bends and developing flows in the intake. The present paper aims to improve the performance of the method in the presence of a bend. To this end, a test rig has been developed and measurements performed, including such geometry. The data are evaluated using the development of a newly proposed approach combining the 1D PTM based on an energy balance formulation and three-dimensional computational fluid dynamics (3D CFD) developed for axis-symmetrical accelerating flows. The updated methodology includes a correction of the experimental pressure measurements used in the 1D PTM to account for the effects of the Dean vortices present after the bend as well as the kinetic energy correction factors which deviate from known values in transient conditions. The results obtained under conditions involving the presence of bends either between or in close proximity to one show a significant improvement compared to the standard one-dimensional pressure-time method., Validerad;2024;Nivå 2;2024-02-26 (joosat);Full text license: CC-BY

The implementation of hydropower to stabilize electrical grids dictates more frequent off-design operations of these renewable energy resources. Flow instabilities under such conditions reduce the efficiency of hydro turbines. Part-load operation is particularly detrimental since the development of a rotating vortical structure termed rotating vortex rope (RVR) in the draft tube leads to periodic pressure pulsations that jeopardize turbine performance. This paper experimentally explores a novel solution involving the protrusion of flat plates into the turbine draft tube. Three flat plates equally separated by 120° were vertically installed on the draft tube wall. The plates were protruded up to 83% of the draft tube local radius under four different part-load conditions. Their impact was observed through time-resolved pressure measurements in the draft tube and vaneless space, as well as efficiency measurements. The results demonstrated successful RVR mitigation, achieving a maximum 85% reduction in pressure oscillation amplitudes. Protruding flat plates disrupted RVR periodicity and coherence, confining its orbit to the draft tube center. This approach proved particularly effective at lower part-load conditions, enhancing turbine hydraulic efficiency by increasing torque extraction. Reducing the adverse effects under part load, the proposed method appears promising in extending the operational range of hydraulic turbines., Validerad;2024;Nivå 2;2024-03-18 (hanlid);Full text license: CC BY

Packed bed reactors play a crucial role in various industrial applications. This paper utilizes the Discrete Element Method (DEM), an efficient numerical technique for simulating the behavior of packed beds of particles as discrete phases. The focus is on generating densely packed particle beds. To ensure the model accuracy, specific DEM parameters were studied, including sub-step and rolling resistance. The analysis of the packed bed model extended to a detailed exploration of void fraction distribution along radial and vertical directions, considering the impact of wall interactions. Three different samples, spanning particle sizes from 0.3 mm to 6 mm, were used. Results indicated that the number of sub-steps significantly influences void fraction precision, a key criterion for comparing simulations with experimental results. Additionally, the study found that both loosely and densely packed beds of particles could be accurately represented by incorporating appropriate values for rolling friction. This value serves as an indicator of both inter-particle friction and friction between particles and the walls. An optimal rolling friction coefficient has been thereby suggested for the precise representation for the densely packed bed of spherical char particles., Validerad;2024;Nivå 2;2024-01-17 (joosat);Full text: CC BY 4.0 License

The keyhole depth is a key measurement characteristic in the laser welding of busbar to battery tabs in battery packs for electric vehicles (EV), as it directly affects the quality of the weld. In this work, experiments are carried out with controlled and adjusted laser power and feed rate parameters to investigate the influence on the keyhole width, keyhole depth and porosities. A 3D numerical model of laser keyhole welding of an aluminum alloy (A1050) has been developed to describe the porosity formation and the keyhole depth variation. A new integration model of the recoil pressure and the rate of evaporation model is implemented which is closer to the natural phenomena as compared to the conventional methods. Additionally, major physical forces are employed including plume formation, upward vapor pressure and multiple reflection in the keyhole. The results show that keyhole depth is lower at higher feed rate, while lower feed rates result in increased keyhole depth. This study reveals that low energy densities result in an unstable keyhole with high spattering, exacerbated by increased laser power. Mitigating incomplete fusion is achieved by elevating laser energy density. The findings emphasize the critical role of keyhole depth in optimizing laser welding processes for applications like busbar-to-battery tab welding., CC BY-NC-ND 4.0 DEEDCorresponding author. E-mail address: akmee@dtu.dk (A. Meena).The authors would like to acknowledge the financial support by the European M-ERA.NET 3 call (project9468 LaserBATMAN), Innovation Fund Denmark (grant number 1139-00001), and the Swedish Governmental Agency for Innovation Systems (Vinnova grant number 2022-01257). ASSAR Innovation Arena in Skövde, Sweden is also acknowledged for the experimental activities.

The waveguide acoustic black hole (WAB) effect is a promising approach for controlling wave propagation in various applications, especially for attenuating sound waves. While the wave-focusing effect of structural acoustic black holes has found widespread applications, the classical ribbed design of waveguide acoustic black holes (WABs) acts more as a resonance absorber than a true wave-focusing device. In this study, we employ a computational design optimization approach to achieve a conceptual design of a WAB with enhanced wave-focusing properties. We investigate the influence of viscothermal boundary losses on the optimization process by formulating two distinct cases: one neglecting viscothermal losses and the other incorporating these losses using a recently developed material distribution topology optimization technique. We compare the performance of optimized designs in these two cases with that of the classical ribbed design. Simulations using linearized compressible Navier-Stokes equations are conducted to evaluate the wave-focusing performance of these different designs. The results reveal that considering viscothermal losses in the design optimization process leads to superior wave-focusing capabilities, highlighting the significance of incorporating these losses in the design approach. This study contributes to the advancement of WAB design and opens up new possibilities for its applications in various fields.

QC 20240313

The SARS-CoV-2 (COVID-19) pandemic has highlighted the crucial role of preventive measures in avoiding the spread of disease and understanding the transmission of airborne viruses in indoor spaces. This study focuses on a novel personal protective equipment consisting of a fan-peaked cap that creates a jet flow of air in front of the individual's face to reduce the concentration of airborne viruses and decrease the risk of infection transmission. Direct numerical simulation is used to analyze the effectiveness of the device under certain conditions, such as the velocity of the airflow, flow orientation, ambient conditions, and geometrical factors., QC 20240308

Wall-modeled large eddy simulation (WMLES) has been proven to be a cost-effective approach capable of resolving turbulence up to certain resolutions. Among the simplest wall models used are the equilibrium wall models, assuming the pressure gradient and convective terms balance out in the momentum equations. There is a lack of studies to assess the performance of these standard wall models in internal turbulent flows including separation regions with/without particles. Regarding this research gap, we have conducted WMLES of incompressible turbulent flows, to the authors’ knowledge, for the first time, in 90° bent pipes with and without particles using an algebraic equilibrium wall model (Spalding's function). A pipe flow simulation was conducted to confirm the simulation setup and assess the sensitivity with respect to the modeling parameters. In each case, comparisons are made with experiment or direct numerical simulation (DNS), and depending on the case, with other existing simulation methods in the literature: WMLES, standard (wall-resolving) LES, and Reynolds stress model (RSM) for Reynolds-averaged Navier-Stokes (RANS) simulations. Despite the controversy on the performance of equilibrium wall models in nonequilibrium flows, our results show acceptable accuracy of this type of wall models. Specifically in the bent pipe flow with particles, WMLES succeeded in predicting particle deposition efficiency at Stokes numbers greater than 0.5, but obtained less accurate results for smaller Stokes numbers. The WMLES errors were, however, on par with those of the standard LES employed with a tenfold higher grid cell count. Improved results would be expected if combined with auxiliary mechanisms such as stochastic models., QC 20240108

Background: : Increasingly, computational fluid dynamics (CFD) is helping explore the impact of variables like: cannula design/size/position/flow rate and patient physiology on venovenous (VV) extracorporeal membrane oxygenation (ECMO). Here we use a CFD model to determine what role cardiac output (CO) plays and to analyse return cannula dynamics. Methods: : Using a patient-averaged model of the right atrium and venae cava, we virtually inserted a 19Fr return cannula and a 25Fr drainage cannula. Running large eddy simulations, we assessed cardiac output at: 3.5–6.5 L/min and ECMO flow rate at: 2–6 L/min. We analysed recirculation fraction (Rf), time-averaged wall shear stress (TAWSS), pressure, velocity, and turbulent kinetic energy (TKE) and extracorporeal flow fraction (EFF = ECMO flow rate/CO). Results: : Increased ECMO flow rate and decreased CO (high EFF) led to increased Rf (R = 0.98, log fit). Negative pressures developed in the venae cavae at low CO and high ECMO flow (high CR). Mean return cannula TAWSS was >10 Pa for all ECMO flow rates, with majority of the flow exiting the tip (94.0–95.8 %). Conclusions: : Our results underpin the strong impact of CO on VV ECMO. A simple metric like EFF, once supported by clinical data, might help predict Rf for a patient at a given ECMO flow rate. The return cannula imparts high shear stresses on the blood, largely a result of the internal diameter., QC 20240412

Transitional and turbulent shear flows are ubiquitous, from the boundary layer developing on an aeroplane wing to the flow within the aortic arch. In this thesis, we study wall-bounded and free shear flows through direct numerical simulations. To control numerical errors and represent every flow structure, we implement the adaptive mesh refinement (AMR) technique within a spectral element method code. Using data-driven methods and causality metrics, we explore the fundamental physical mechanisms in various shear flows. The adaptive mesh refinement technique necessitates a precise evaluation of the committed error. Thus, we compare the local spectral error indicator with the dual-weighted adjoint error estimator. The former ensures a more homogeneous refinement, targeting regions with a high-velocity gradient, while the latter is goal-oriented. However, the adjoint error estimator fails in turbulent flows due to the exponential sensitivity of the adjoint linear solution to any perturbation. Alternatively, we introduce a causality-based error indicator that employs the Shannon transfer entropy, i.e. a causality metric arising from information theory, to establish causal relations between the local solution and a specified quantity of interest. Using information-theoretic causality, linear global stability analysis and modal decomposition, we investigate transitional and turbulent coherent structures. In turbulent straight pipe flows, the proper orthogonal decomposition is integrated with the Voronoi diagram to automatically discern between wall-attached and detached eddies. In spatially developing bent pipe flows, we employ the proper orthogonal decomposition to examine the swirl switching phenomenon, the origins of which continue to be a topic of debate. In the context of external flows around a cylinder, we explore two configurations: the Flettner rotor, a rotating cylinder in a wall-bounded shear flow, and the stepped cylinder, namely two cylinders of different dia, I denna avhandling studerar vi transitionella och turbulenta skjuvströmningar genom direkta numeriska simuleringar. Med hänsyn till den avgörande rollen av att kontrollera numeriska fel och representera varje skala i rummet, utvecklar, validerar och implementerar vi den adaptiva nätförfiningstekniken inom en spektralelementkod. Med hjälp av data-drivna metoder och mått för kausalitet utforskar vi de grundläggande fysikaliska mekanismerna i olika skjuvströmningar. Den adaptiva nätförfiningen kräver en noggrann beräkning av det begångna felet. Således jämför vi den lokala spektrala felindikatorn med den felestimatorn från adjunkt-ekvationen. Den förra säkerställer en mer homogen förfining, inriktad på områden med en stor hastighetsgradient, medan den senare är målinriktad. Emellertid misslyckas den adjunkta felestimatorn i turbulenta flöden på grund av den exponentiella känsligheten hos den adjunkta linjära lösningen för turbulenta störningar. Som nytt alternativ introducerar vi en kausalitets-baserad felindikator som använder Shannon-transferentropin, dvs. ett kausalitets-mått som härrör från informationsteori, för att fastställa kausala samband mellan den lokala lösningen och en specificerad kvantitet av intresse. Med hjälp av detta kausalitets-mått, linjär global stabilitetsanalys och modal dekomposition undersöker vi transitionella och turbulenta koherenta strukturer. I glatta turbulenta rörströmningar använder vi den så kallade proper orthogonal decomposition (POD) med Voronoi-diagrammet för att automatiskt skilja mellan väggnära och yttre virvlar. För strömningsfallet med ett krökt rör med 90 eller 180 grader-vinkel använder vi POD för att undersöka fenomenet swirl switching, vars ursprung fortsatt är oklart i litteraturen. I samband med den externa strömningen runt en cylinder utforskar vi två konfigurationer: Flettner-rotorn, en roterande cylinder i ett gränsskikt och den stegformade cylindern, där två cylindrar med olika diametrar är sammanfogade i ena änden., QC 240304

Based on the finite difference method, the fixed boundary method, and a staggered grid method, this paper develops a one-dimensional thermal–hydraulic analysis code to simulate the effects of heave motion and more complex oceanic motions, namely heaving coupled with inclination and heaving coupled with swaying, on the thermal–hydraulic parameters of the International Reactor Innovative and Secure (IRIS) reactor's helical coil once-through steam generator (HCOTSG). The oceanic conditions were achieved by modifying the momentum equation. The validity and accuracy of the code were obtained by comparing the results with the design parameters of HCOTSG for two types of marine nuclear reactors, the SMART reactor and the IRIS reactor, and with the calculations of some similar international prediction programs. The simulation results indicate that (1) when only heave motion is considered, increasing the amplitude and reducing the period both result in a greater magnitude and a faster rate of change in the HCOTSG parameters. (2) When the combined motion of inclining and heaving is present, the dynamic performance of the composite motion inclining towards the positive x-axis deteriorates the most severely, while its thermodynamic conditions improve slightly. In this direction, the inclining angle has little influence on the thermal–hydraulic characteristics of HCOTSG. (3) When the combined motion of heaving and swaying is present, the dynamic performance is more severely affected under swaying motion towards the negative y-axis, despite a slight improvement in thermodynamic conditions. In this direction, as the swaying angle increases, the trend of parameter variation under swaying is similar but with a greater magnitude of change. The deterioration in dynamic performance becomes more severe, although there are small improvements in thermodynamic conditions. With the swaying period increasing, the maximum and minimum values of the secondary side pressure drop show little chan, QC 20240408

This study introduces an agent-specific assessment method of traffic noise exposure in agent mobility simulations. The assessment is achieved through a combination of an energy-based noise exposure impact assessment using noise exposure cost, and the state-of-the-art traffic noise prediction tool NoiseModelling coupled with the activity-based agent mobility simulation software MATSim. The agent-specific noise exposure cost is a measure to evaluate how the noise emissions from the transport of agents relate to the noise-related impact on other agents performing stationary activities. By introducing an agent-specific level, each agent’s individual responsibility for the noise exposure may be estimated. The potential of the agent-specific noise exposure cost concept, combined with the MATSim-NoiseModelling framework, is illustrated through a case study, applying activity-based agent mobility simulations across Nantes, France. The results of the case study highlight, among other considerations, the insights that an agent-specific, activity-based noise exposure cost approach provides by visualizing the noise exposure ”footprint” resulting from an agent’s transportation activities., QC 20240513

The present study investigates unsteady wake dynamics in flow past an oscillating cylinder. A two-dimensional computational analysis is undertaken for flow past a sinusoidally oscillating cylinder. Operating parameters like Reynolds number, forcing frequency, amplitude, and wavelength are systematically varied, and their effect on wake dynamics is investigated. Lift and drag coefficients are plotted with cylinder position to characterize the flow. The flow field is analyzed using Proper Orthogonal Decomposition (POD). POD mode shapes and Power Spectral Density (PSD) plots characterize the flow response to cylinder oscillations. The effect of frequency ratio is examined in achieving synchronized vortex shedding. Previous studies focus on two extremes- the cylinder is either stationary or oscillating with perfect phase-locking. Flow past an oscillating cylinder where Karman and forcing frequencies compete for dominance is rarely reported. This paper considers such a scenario and demonstrates how POD can identify coherent structures, their significance, and associated frequencies. Further, non-symmetric vortex shedding, which is not a common observation, is captured and discussed. Large amplitude and large wavelength regimes are investigated, which remained unexplored in previous research. Coherent structures associated with those regimes are identified., QC 20240301

Thixo-elasto-viscoplastic (TEVP) fluids are very complex fluids. In addition to elasticity and viscoplasticity, they exhibit thixotropy, i.e., time-dependent rheology due to breakdown and recovery of internal structures at different length- and timescales. General and consistent methods for a priori flow prediction of TEVP fluids based on rheological characteristics are yet to be developed. We report a combined study of the rheology and flow of 18 samples of different TEVP fluids (three yogurts and three concentrations of Laponite and Carbopol, respectively, in water in both the unstirred and a stirred state). The rheology is determined both with standard protocols and with an ex situ protocol aiming at reproducing the shear history of the fluid in the flow. Micrometer resolution flow measurements in a millimeter scale rectangular duct are performed with Doppler Optical Coherence Tomography (D-OCT). As expected, the results show the existence of a plug flow region for samples with sufficiently high yield stress. At low flow rates, the plug extends almost all the way to the wall and the extent of the plug decreases not only with increased flow rate but also with increased thixotropy. The ex situ rheology protocol enables estimation of the shear rate and shear stress close to the wall, making it possible to identify two scaling laws that relates four different non-dimensional groups quantifying the key properties wall-shear stress and slip velocity. The scaling laws are suggested as an ansatz for a priori prediction of the near-wall flow of TEVP fluids based on shear flow-curves obtained with a rheometer., QC 20240222

We investigated variations in haemodynamics in response to simulated microgravity across a semi-subject-specific three-dimensional (3D) continuous arterial network connecting the heart to the eye using computational fluid dynamics (CFD) simulations. Using this model we simulated pulsatile blood flow in an upright Earth gravity case and a simulated microgravity case. Under simulated microgravity, regional time-averaged wall shear stress (TAWSS) increased and oscillatory shear index (OSI) decreased in upper body arteries, whilst the opposite was observed in the lower body. Between cases, uniform changes in TAWSS and OSI were found in the retina across diameters. This work demonstrates that 3D CFD simulations can be performed across continuously connected networks of small and large arteries. Simulated results exhibited similarities to low dimensional spaceflight simulations and measured data—specifically that blood flow and shear stress decrease towards the lower limbs and increase towards the cerebrovasculature and eyes in response to simulated microgravity, relative to an upright position in Earth gravity., QC 20240125

Large-eddy simulations of a flat-plate boundary layer, without a leading edge, subject to multiple levels of incoming free-stream turbulence are considered in the present work. Within an input-output model, where nonlinear terms of the incompressible Navier-Stokes equations are treated as an external forcing, we manage to separate inputs related to perturbations coming through the intake of the numerical domain, whose evolution represents a linear mechanism, and the volumetric nonlinear forcing due to triadic interactions. With these, we perform the full reconstruction of the statistics of the flow, as measured in the simulations, to quantify pairs of wavenumbers and frequencies more affected by either linear or nonlinear receptivity mechanisms. Inside the boundary layer, different wavenumbers at near-zero frequency reveal streaky structures. Those that are amplified predominantly via linear interactions with the incoming vorticity occur upstream and display transient growth, while those generated by the nonlinear forcing are the most energetic and appear in more downstream positions. The latter feature vortices growing proportionally to the laminar boundary layer thickness, along with a velocity profile that agrees with the optimal amplification obtained by linear transient growth theory. The numerical approach presented is general and could potentially be extended to any simulation for which receptivity to incoming perturbations needs to be assessed., QC 20240209

Small heavy particles in a fluid flow respond to the flow on a timescale proportional to their inertia or Stokes number St. Their behavior has been thought to be gradually modified as St increases. We show, on the other hand, in the steady spatially periodic laminar Taylor Green vortex flow, that particle dynamics, and their effective diffusivity, actually change in an irregular, nonmonotonic, and sometimes discontinuous manner with increasing St. At St similar to 1, we show chaotic particle motion, contrasting with earlier conclusions for heavy particles in the same flow [Wang et al., Phys. Fluids 4, 1789 (1992)]. Particles may display trapped orbits, or unbounded diffusive or ballistic dispersion, with the vortices behaving like scatterers in a soft Lorentz gas [Klages et al., Phys. Rev. Lett. 122, 064102 (2019)]. The dynamics is nonergodic., QC 20240213

The present doctoral thesis investigates the turbulent flow developing around wing sections, focusing on the impact of adverse-pressure-gradient (APG) conditions on turbulent boundary layers (TBLs) and the physics of flow separation. Both experimental and numerical methods are employed to generate high-fidelity data sets and provide an in-depth analysis of the flow. The first objective of this thesis is the development of a comprehensive database for the flow around a NACA 4412 wing profile. For this purpose, adaptive mesh refinement (AMR) is used together with the spectral-element method code Nek5000. With AMR, high-resolution Large Eddy Simulations (LES) are conducted at various Reynolds numbers (Rec = 2×105, 4×105 and 1×106) and angles of attack (AoA=5°, 8°, 11°, 14°), which were previously unattainable. The effect that strong APGs have on TBLs developing around a wing section is assessed through the collection of statistics and time series. The results demonstrate the influence of APG conditions on both the mean and variance profiles of velocity, and on the distribution and production of turbulence energy within the TBL. Additionally, the connection of APG TBLs with flow separation is explored through the development of an in-situ identification and tracking algorithm, tightly integrated into Nek5000. Our findings show that, in contrast to canonical flows, backflow events in TBLs under strong APGs extensively merge to form larger structures that grow exponentially in size, eventually leading to significant flow separation near the wing’s trailing edge. Furthermore, a wind-tunnel experimental campaign is conducted to validate and extend the numerical results. Pressure, wall-shear stress and velocity measurements were carried out in the MTL wind tunnel at KTH Royal Institute of Technology. The study also scrutinizes measurement methodologies for APG TBLs, examining uncertainties in skin-friction determination and the impact of hot-wire probe lengths on velocity va, Denna doktorsavhandling undersöker det turbulenta flödet runt vingsektioner, med fokus på effekten av negativa tryckgradienter (APG) på turbulenta gränsskikt (TBL) och fysiken bakom avlösning. Både experimentella och numeriska metoder används för att generera data och genomföra en noggrant analys av flödet. Det första målet med avhandlingen är att utveckla en omfattande databas för flödet runt en NACA 4412 vingprofil. För detta ändamål används adaptiv nätförfining (AMR) i det spektralelementbaserade programmet Nek5000. Med AMR genomförs väggupplösta large-eddy simuleringar (LES) vid olika Reynoldstal (Rec = 2×105, 4×105 och 1×106) och anfallsvinklar (AoA=5°, 8°, 11°, 14°), vilka tidigare varit ouppnåeliga. Effekten av den starka negativa tryckgradienten på TBL som utvecklas runt vingsektionen bedöms genom insamling av statistik och tidsserier. Resultaten visar på APG:s påverkan på både medel- och variansprofiler för hastighet, samt på fördelning och produktion av turbulenta energien inom TBL. Dessutom utforskas sambandet mellan APG TBL och avlösning genom utveckling av en in-situ identifierings- och spårningsalgoritm, integrerad i Nek5000. Våra resultat visar att negativa hastigheter händelser i TBL under starka APG interagerar betydligt med varandra, sammanflätar och bildar större strukturer som ökar exponentiellt i storlek, och så småningom leder till betydande avlösning nära vingens bakkant. Dessutom genomförs en experimentkampanj i vindtunnel för att validera och utöka de numeriska resultaten. Mätningar av tryck, väggskjuvspänning och hastighet utförs i MTL vindtunneln vid KTH Kungliga Tekniska Högskolan. Studien granskar också mätmetoder för APG TBL, undersöker osäkerheter i bestämningen av väggskjuvspänning samt effekterna av längden på varmtrådprober på mätprofiler för hastighetsvarians. Slutligen utförs en RANS-studie, där högupplösta data används för validering, för att bedöma optimering av flödeskontrollmetoder baserade på blåsning och sugning. Denna stud, QC 240122

In supersonic aerospace applications, aerospike nozzles have been subject of growing interest. This study sheds light on the noise components of a cold jet exhausting an aerospike nozzle. Implicit large eddy simulations (ILES) are deployed to simulate the jet at a nozzle pressure ratio (NPR)=3. For far-field acoustic computation, the Ffowcs Williams-Hawk-ings (FWH) equation is applied. A mesh sensitivity study is performed and the jet instantaneous and time-averaged flow characteristics are analyzed. The annular shock structure displays short non-attached shock-cells and longer attached shock-cells. Downstream of the aerospike, a circular shock-cell structure is formed with long shock-cells. Two-point cross-correlations of data acquired at monitoring points located along the shear layers allow to identify upstream propagating waves associated to screech. Power spectral density at monitoring points in the annular shock-cell structure allows to identify its radial oscillation modes. Furthermore, a vortex sheet model is adapted to predict the annular shock-cells length and the BBSAN central frequency. High sound pressure levels (SPL) are detected at the determined BBSAN central frequencies. Finally, high SPL are obtained at the radial oscillation frequencies for the annular shock-cell structure., QC 20240103

In an earlier study, we analyzed how audio signals obtained from three professional opera singers varied when they sang one octave wide eight-tone scales in ten different emotional colors. The results showed systematic variations in voice source and long-term-average spectrum (LTAS) parameters associated with major emotion “families”. For two of the singers, subglottal pressure (PSub) also was recorded, thus allowing analysis of an additional main physiological voice control parameter, glottal resistance (defined as the ratio between PSub and glottal flow), and related to glottal adduction. In the present study, we analyze voice source and LTAS parameters derived from the audio signal and their correlation with Psub and glottal resistance. The measured parameters showed a systematic relationship with the four emotion families observed in our previous study. They also varied systematically with values of the ten emotions along the valence, power, and arousal dimensions; valence showed a significant correlation with the ratio between acoustic voice source energy and subglottal pressure, while Power varied significantly with sound level and two measures related to the spectral dominance of the lowest spectrum partial. the fundamental., QC 20240118

This paper presents a modelling framework for incorporating wall interactions into the Maxwell-Stefan equations for ideal gases in porous media. In contrast to already existing models in literature, such as the Dusty Gas, Binary Friction and Modified Binary Friction models, the framework makes use of N−1 (with N being the number of gas species) independent mass balances for the gas species in combination with a total fluid momentum balance. This approach has the advantage that already existing numerical schemes for fluid momentum balances, such as Darcy's law or the Brinkman equations, can be used with only minor modifications. It also allows for separate initialisation of the velocity and pressure fields, something that is often beneficial when solving convection–diffusion equations numerically. The derived model is validated against both stationary and time-dependent experimental data from literature for binary Ar-He systems in porous media, using Knudsen diffusivities to define the wall friction terms., QC 20240208

Quantification of uncertainty in results is crucial in both experiments and simulations of turbulence, yet this practice is notably underutilized. This thesis project delves into statistical tools within the framework of uncertainty quantification to systematically quantify uncertainties that occur in the time varying quantities of turbulence. Two main categories of variance estimators for quantifying time averaging uncertainties in turbulent flow time series are examined in detail – the batch-means based methods and autoregressive model-based methods. The batch size is critical to estimation of uncertainty by the batch methods. We discuss reasons for biased estimates and provide guidance on the selection of batch sizes for the non-overlapping, overlapping and batch means-batch correlations estimators, to obtain consistent estimates of uncertainty when dealing with turbulence time samples. The autoregressive model (ARM)-based estimator was found to be more efficient than the batch methods, in terms of computational efficiency and sample requirements. A novel insight into the operating principle of the ARM, enabled fast quantification of uncertainty with few samples and with batch means series. The extension of univariate autoregressive processes to model entire 2D space-time fields of turbulence, through vector autoregression has been discussed and its potential as a turbulent inflow boundary condition has been illustrated. A crucial flow case that questioned the reliability of Computational Fluid Dynamics (CFD), namely flow through Food and Drug Administration benchmark nozzle device was also simulated in this doctoral thesis project, with a well-defined turbulent inflow boundary condition. Novel insights on the flow physics due to geometrical effects were obtained through statistical analysis, anisotropy invariant maps and proper orthogonal decomposition. These insights provide answers to many open questions in this domain. This work provides analyses and methods, Kvantifiering av osäkerhet i resultat är avgörande i både experiment och simuleringar av turbulens, men denna praxis är anmärkningsvärt underutnyttjad. Detta avhandlingsprojekt undersöker statistiska verktyg inom ramen för osäkerhetskvantifiering för att systematiskt kvantifiera osäkerheter som uppstår i de tidsvarierande kvantiteterna av turbulens. Två huvudkategorier av variansskattare för kvantifiering av tidsmedelvärderade osäkerheter i tidsserier av turbulenta flöden undersöktes i detalj; batch-meansbaserade metoder och autoregressiva modellbaserade metoder. Batchstorleken är avgörande för uppskattning av osäkerheter med de batchbaserade metoderna. Vi diskuterar orsakerna till avvikande skattningar och ger vägledning kring valet av batchstorlekar för de icke-överlappande, överlappande och batchmedel-batchkorrelationsskattarna, för att få konsekventa skattningar av osäkerheten vid hantering av turbulenstidssampels. Den autoregressiva modellen (ARM)-baserade estimatorn visade sig vara effektivare än batchmetoderna avseende beräkningseffektivitet och samplingskrav. En ny insikt i ARM:s funktionsprincip möjliggjorde snabb kvantifiering av osäkerheter med få stickprov och med batchmedelvärdesserier. utökningen av univariata AR-processer till att modellera hela 2D-rum-tidsfält av turbulens, genom vektorautoregression, har undersökts och dess potential som randvillkor för turbulenta inflöden har illustrerats. Ett avgörande flödesfall som utmanade tillförlitligheten av Computational Fluid Dynamics (CFD), nämligen flöde genom FDA benchmark munstycksanordning, simulerades också i denna avhandling, med ett väldefinierat turbulent inflödesrandvillkor. Nya insikter om flödesfysiken baserad på geometriska effekter erhölls genom statistisk analys, anisotropi-invarianta avbildningar och ortogonala nedbrytningstekniker. Dessa insikter ger svar på många öppna frågor inom denna domän. Detta arbete ökar simuleringarnas tillförlitlighet och utökar omfattningen av CFD till applika, QC 240202

The study focuses on developing a comprehensive design approach for a flow-through ultrasonic reactor (sonicator) to tackle challenges like low energy transfer efficiency and unstable system performance. The simulation accounts for structural vibrations, structural-fluid interactions, and pressure distributions within the cavitation zone under single-frequency excitation. Different geometrical designs of cylindrical sonicators are analyzed, with input parameters tailored to acquire higher acoustic cavitation intensity. The findings reveal a novel hexagonal ring-shaped excitation structure that reduces coupling losses, ensures uniform acoustic pressure distribution, and generates symmetric vibration mode shapes. The study emphasizes the separation of parasitic modes from the desired resonance frequency response and simulates the influence of bubbly liquid properties through complex wave numbers and harmonic responses. Experimental validation on a manufactured prototype, including mechanical and electrical impedance, sound pressure spectrum, and cavitation intensity, supports the simulated results. Ultimately, the sonicator exhibits three feasible resonance frequencies to be used pairwise at the certain temperature and input power interval for different applications., Validerad;2024;Nivå 2;2024-04-09 (sofila);Full text license: CC BY 4.0;Funder: European Union through the European Agricultural Fund for Rural Development (2016-5274); Innovativa Drycker Balsgård AB

Acoustic trapping is used in modern biophysics laboratories to study cell adhesion or aggregation, to sort particles, or to build model tissues. Here, we create an acoustic trapping setup in liquid for an undergraduate instructional laboratory that is low-cost, easy to build, and produces results in a 1-hour laboratory period. In this setup, we use a glass slide, cover slip, and double-sided tape to make the sample chamber. A piezo-electric transducer connected to a function generator serves as the acoustic source. We use this setup to measure the node spacing (millimeters) and the acoustic trap force (picoNewtons). We anticipate that the simplicity of the experimental setup, the tractability of the theoretical equations, and the richness of the research topics on the subject will lead to an undergraduate laboratory with many interesting student projects., QC 20240115

Fifth Madrid Turbulence Workshop May 29 - June 30, 2023Will be published in Journal of Physics: Conference Series (In Press)QC 20240304

Wall-bounded turbulent flows as those occurring in transportation (e.g. aviation) or industrial applications (e.g turbomachinery), are usually subjected to pressure gradients (PGs). The presence of such PGs affects greatly the development and physics of the turbulent boundary layer (TBL), making it an open research area. An important phenomena associated with the presence of strong adverse PGs (APGs) as appearing in wings, is the separation of the boundary layer, which can lead to stall., QC 20231214

Flow-accelerated corrosion and erosion (FACE) phenomena can be crucial for performance of structural elements in heavy liquid metal (HLM) cooled reactor systems. Existing experimental observations indicate that turbulent flow characteristic can affect FACE, but there is no quantitative data that can be used for model development and validation. Main recirculation pump impellers, which operate at high relative velocities and rotational flow conditions can be especially vulnerable to FACE. For comparison, the core internals operate at lower velocities and in axial flow conditions, but at higher temperatures and neutron fluence. Hence, systematic experimental data is needed to improve our knowledge on FACE phenomena. The Separate Effect Test Facility for Flow-Accelerated Corrosion and Erosion (SEFACE) is designed to obtain such experimental data including high relative velocities (up 20 ms−1) and high temperatures (400 to 550 °C) of liquid lead. This article focuses on the hydrodynamic design of SEFACE. The aim of the design is to achieve well defined flow conditions for experiments and ensure safe operation of the facility. First, we examine three design concepts (i.e., forced convection loop, rotating cylinder, and rotating disk) and motivate the choice of the rotating disk approach for SEFACE. Second, we discuss different design options, i.e., a confined rotor–stator test chamber and the unconfined rotating disk configuration. We used Reynolds-Averaged Navier Stokes (RANS) calculations to identify and solve the issues stemming from the high rotational speed. These include, for instance, lead free surface deformation, radial pressure buildup, and axial bending forces due to asymmetric test chamber. The CFD-derived torque and power predictions in rotor–stator and rotating disk systems are verified with selected empirical turbulent friction factor correlations or/and DNS calculations. We demonstrate that the developed hydrodynamic design of SEFACE solves identified iss, QC 20240108

A new Grey-Area Mitigation (GAM) method for hybrid RANS-LES is presented., QC 20231214

Direct numerical simulations of the fully developed turbulent flow through helical pipes are performed. The numerical procedure is described, and a validation of the volume force driving the flow is presented. A comparison of the turbulence statistics against literature data is also reported., QC 20231214

The wing-tip vortices are formed near the tip of finite-span, lift-generating surfaces as a result of the pressure difference between the pressure and suction sides of the wing., QC 20231214

In this paper, we investigate the problem of recovering the frequency components of a mixture of K complex sinusoids from a random subset of N equally-spaced time-domain samples. Because of the random subset, the samples are effectively non-uniform. Besides, the frequency values of each of the K complex sinusoids are assumed to vary continuously within a given range. For this problem, we propose a two-step strategy: (i) we first lift the incomplete set of uniform samples (unavailable samples are treated as missing data) into a structured matrix with missing entries, which is potentially low-rank; then (ii) we complete the matrix using a weighted nuclear minimization problem. We call the method a weighted lifted-structured (WLi) low-rank matrix recovery. Our approach can be applied to a range of matrix structures such as Hankel and double-Hankel, among others, and provides improvement over the unweighted existing schemes such as EMaC and DEMaC. We provide theoretical guarantees for the proposed method, as well as numerical simulations in both noiseless and noisy settings. Both the theoretical and the numerical results confirm the superiority of the proposed approach., QC 20231128

Complementing large-eddy simulation (LES) with wall-modelling is, perhaps, the most straight-forward way to enable high-fidelity simulations at high Reynolds numbers. At the same time, high-order methods offer the benefits of high computational efficiency and potentially faster convergence with respect to mesh refinement even outside the asymptotic regime., QC 20231214

The hydraulic turbines, especially Francis turbines, frequently run at part load (PL) conditions to meet the dynamic energy needs. The flow field at the runner exit changes significantly with a change in the operating point. At PL, flow instabilities such as the Rotating Vortex Rope (RVR) form in the draft tube of the Francis turbine. The present paper compares the features of the velocity and vorticity field of the Francis turbine draft tube at the best efficiency point (BEP) and PL operations using the Proper Orthogonal Decomposition (POD) of the 2D-PIV data. The POD analysis decomposes the flow field into coherent and incoherent structures describing the spatiotemporal behavior of the flow field. A visual representation of the coherent structures and the turbulent length scales in the flow field is extracted and analyzed for BEP and PL, respectively. The study highlights the salient features of the draft tube flow field, which differentiate the BEP and PL operation. The fast Fourier transform of the temporal coefficients confirms the presence of RVR frequency (0.29 times the runner frequency) at PL. The phase portraits of different modes elucidate the relationship between different harmonics of the RVR frequency at PL., Validerad;2024;Nivå 2;2024-04-02 (joosat);Full text license: CC BY-NC-ND 4.0

In this contribution, we present a robust and efficient computational framework capable of accurately capturing the dynamic motion and large deformation/deflection responses of highly-flexible rods interacting with an incompressible viscous flow. Within the partitioned approach, we adopt separate field solvers to compute the dynamics of the immersed structures and the evolution of the flow field over time, considering finite Reynolds numbers. We employ a geometrically exact, nonlinear Cosserat rod formulation in the context of the isogeometric analysis (IGA) technique to model the elastic responses of each rod in three dimensions (3D). The Navier–Stokes equations are resolved using a pressure projection method on a standard staggered Cartesian grid. The direct-forcing immersed boundary method is utilized for coupling the IGA-based structural solver with the finite-difference fluid solver. In order to fully exploit the accuracy of the IGA technique for FSI simulations, the proposed framework introduces a new procedure that decouples the resolution of the structural domain from the fluid grid. Uniformly distributed Lagrangian markers with density relative to the Eulerian grid are generated to communicate between Lagrangian and Eulerian grids consistently with IGA. We successfully validate the proposed computational framework against two- and three-dimensional FSI benchmarks involving flexible filaments undergoing large deflections/motions in an incompressible flow. We show that six times coarser structural mesh than the flow Eulerian grid delivers accurate results for classic benchmarks, leading to a major gain in computational efficiency. The simultaneous spatial and temporal convergence studies demonstrate the consistent performance of the proposed framework, showing that it conserves the order of the convergence, which is the same as that of the fluid solver., QC 20231031

The atmospheric convective boundary layer (CBL) consists of three basic parts: (1) the surface layer unstably stratified and dominated by small-scale turbulence of very complex nature; (2) the CBL core dominated by the energy-, momentum-, and mass-transport of semi-organized structures (large-scale circulations), with a small contribution from small-scale turbulence produced by local structural shears; and (3) turbulent entrainment layer at the upper boundary, characterized by essentially stable stratification with negative (downward) turbulent flux of potential temperature. The energy- and flux budget theory developed previously for atmospheric stably-stratified turbulence and the surface layer in atmospheric convective turbulence is extended to the CBL core using budget equations for turbulent energies and turbulent fluxes of buoyancy and momentum. For the CBL core, we determine global turbulent characteristics (averaged over the entire volume of the semi-organized structure) as well as kinetic and thermal energies of the semi-organized structures as the functions of the aspect ratio of the semi-organized structure, the scale separation parameter between the vertical size of the structures and the integral scale of turbulence and the degree of thermal anisotropy characterized the form of plumes. The obtained theoretical relationships are potentially useful in modeling applications in the atmospheric convective boundary-layer and analysis of laboratory and field experiments, direct numerical simulations, and large-eddy simulations of convective turbulence with large-scale semi-organized structures.

The thesis proposes to model the flow of gases in a room by solving the incompressible Navier-Stokes equations in three dimensions. The transportationof scalars such as temperature and concentration of CO2 is modeled by solving the advection-diffusion equation, and the effects of temperature on thevelocity is accounted for by employing the Boussinesq approximation. Theequations are solved numerically in Matlab by using the finite volume methodon a cubic domain with Cartesian coordinates. The code is verified by comparison with the well studied case of a lid-driven cavity. Different boundaryconditions are discussed and boundary conditions to model inlets and outlets along with radiators and windows are proposed. A simplified model of aroom is created and is shown to provide reasonable results for low Reynoldsnumbers (Re ≈ 200). It is also shown that the model is not efficient enoughto run simulations with higher Reynolds numbers (Re ≈ 20000) that resultfrom a realistic choice of parameters.Teknisk-naturvetenskapliga fakult

SAAB Dynamics producerar och utvecklar flertalet produkter som innehåller krutraketmotorer. Krutraketmotorers prestanda är kraftigt beroende av en effektiv upptändning och det är önskvärt att kunna prediktera detta förlopp numeriskt med god noggrannhet. Ingen etablerad simuleringsmetodik existerar i dagsläget. Detta examensarbete är ett första steg mot att undersöka hur anfyrningsförloppet kan modelleras med Computational Fluid Dynamics. Den komersiella mjukvaran Ansys Fluent har använts som simuleringsverktyg. Anfyrningsförloppet är ett transient förlopp och involverar flertalet komplicerade värmeöverföringsmekanismer. Några av de viktigaste mekanismerna är konvektiv värmeöverföring från anfyrningsgaser, värmeledning från anfyrningspartiklar och termisk strålning från både gaserna och partiklarna. Tre modeller presenteras. De modellerar en approximativ geometri från tidigare experimentella undersökningar och simulerar 2,4 millisekunder av anfyrningsförloppet. Inga kemiska reaktioner modelleras. Modell 1 simulerar konvektiv värmeöverföring från heta förbränningsgaser. Modell 2 inkluderar termisk strålning från förbränningsgaserna som modelleras med Planck-medelvärdes absorptionskoefficienter och Discrete Ordinates modellen. Totalt värmeflöde och slutgiltig temperaturfördelning längs krutranden redovisas. Modellerna predikterar liknande värmeflöden och temperaturfördelningar, med något förhöjda nivåer för modell 2. Ett experimentellt uppmätt tryck används för att validera trycknivåer i simuleringarna. Fysikalisk rimlighet av anfyrningsgasens temperatur bedöms även med den adiabatiska flamtemperaturen. Båda modeller presterar väl med avseende på dessa mått. En tredje modell som försöker modellera anfyrningspartiklar med hjälp av Discrete Phase Model presenteras även. Modellen anses inte tillförlitlig då den lider av numeriska problem för det simulerade fallet. Avslutningsvis diskuteras felkällor och fortsatt arbete för samtliga modeller., SAAB Dynamics produces and develops several products that use solid-propellant rocket engines. The performance of these rocket engines is heavily dependant upon an efficient ignition. It is desirable to be able to predict this sequence numerically with good accuracy. No established simulation methodology exists as of today however. This master thesis is a first step towards the investigation of how the initial ignition sequence can be modelled using Computational Fluid Dynamics. The commercial software Ansys Fluent has been used as a tool for simulation. The initial heat transfer during ignition is a transient sequence and involves several complex heat transfer mechanisms. Some of the most important ones include convective heat transfer from ignition gases, conduction from ignition particles as well as thermal radiation from both the gases and particles. Three models are presented. They model an approximate geometry from earlier experimental investigations and simulate 2.4 milliseconds of the initial ignition sequence. No chemical reactions are modelled. Model 1 simulates convective heat transfer from hot ignition gases. Model 2 includes thermal radiation from the ignition gases that is modelled using Planck-mean absorption coefficients and the Discrete Ordinates model. Total heat transfer rates and final temperature distributions along the surface of the propellant are reported. The models predict similar total heat transfer rates and final temperature distributions, with somewhat elevated levels for model 2. An experimentally measured pressure is used to validate the simulated pressure levels. Physical validity is also evaluated by comparing the temperature of the ignition gas to the adiabatic flame temperature. Both models perform satisfactorily with regards to these metrics. A third model that attempts to model the ignition particles using the Discrete Phase Model is also presented. The model is deemed unreliable due to numerical problems for the simulated case.

High-pressure chemistry can be used to determine the contents of blood or water samples and to discover new chemistries. However, working with chemistry at pressures of many tens, or even hundreds, of bars often requires expensive and stationary equipment, such as autoclaves or chromatographic systems like high-performance liquid chromatography (HPLC). Since the introduction of microfluidics in the '90s, researchers have attempted to develop microfluidic chips as microreactors to speed up synthesis with faster mass and heat transfer. Other researchers have made efforts to create microfluidic chip-based HPLC to reduce the cost, increase the separation quality, speed up the analysis, and even enable portable systems for on-site medical or environmental analysis. Still, fully integrated systems have not yet been realized due to a lack of fluidic control components. This thesis presents novel methods for on-chip regulation and monitoring of pressure, flow, and temperature. Papers I and II specifically suggest a method for regulating backpressure and stabilizing pressure and flows using thermally controlled restrictors. Furthermore, a collaboration was made where a pressure-regulating chip was connected to an on-chip HPLC. The purpose of this was to activate sample plugs and therefore reduce the requirement for expensive surrounding equipment and enable portability, Paper III. Paper IV explores the use of a pressurized capsule to generate high-pressure flows that are coupled to a pressure-regulating chip to stabilize and regulate the pressure. Finally, an approach for integrating pressure sensors into high-pressure tolerant microchannels has been proposed, Paper V. The work conducted has provided new insights into fluid dynamics. The regulating method employed in Paper I-IV utilizes a restrictor that alters the pressure drop as temperature changes, hence changing the viscosity of the fluid. Although this technology has been known since before, new understandings have eme

Ships carry cargo and passengers around our planet. In coastal regions, complaints about ship noise have been raised from nearby residents. Ships are also the dominant source of continuous underwater noise in the seas. In the SHIPNOISE project, we investigate levels of ship noise in coastal regions using a custom measurement station for both airborne and underwater noise from ships underway. The measurement station is developed using embedded microprocessors for low power consumption. The station is deployed at Böttö at the inlet to Gothenburg harbor, and then at Lurö in Lake Vänern. These sites are close to shipping lanes of different traffic intensity. The measured airborne noise levels indicate that there is a risk to exceed recommended indoor low-frequency noise limits for dwellings positioned up to several hundred meters from the passing ships, although the effect on public health is uncertain. The underwater noise recorded at 200-300 m range at the SHIPNOISE measurement locations during ship passages is strong enough to exceed levels for environmental impact previously demonstrated on local marine mammals, fish and possibly invertebrates. For example, harbor porpoises, herring and salmon are likely to avoid or escape the area when a loud ship passes.

This paper investigates different actuator sector model implementation alternatives and how they compare to actuator line results. The velocity sampling method, tip/smearing correction, and time step are considered. A good agreement is seen between the line and sector model in the rotor plane and the wake flow. Using the sector model, it was possible to reduce the computational time by 75 % compared to the actuator line model as it is possible to run the simulations with a larger time step without compromising the accuracy considerably. The results suggest that the proposed velocity sampling method produces the closest results to the line model with different tip speed ratios. Moreover, the vortex-based smearing correction applied to the sector model results in the lowest error values, among the considered methods, to correct the radial load distributions. Also, it is seen that reducing the time step compared to the one used for the actuator disc/sector does not provide an advantage considering the increased computational time.

Thermal-related problems in electric machinery are complex problems with different physics. Most of the problems are related to the motion of fluid due to cooling or motions of the machinery; thus, numerical methods such as computational fluid dynamics (CFD) are commonly used. However, due to the complexity of the problems, it is costly to utilize. Thus, this thesis investigated an alternative method to the most common method today, the finite volume method (FVM). The alternative method uses a lagrangian approach or a mesh-free approach, as it can also be known. The method is moving particle simulation (MPS), which is also based on the lagrangian approach of smooth particle hydrodynamics (SPH) but with modifications to improve stability, convergence, and more. Thus, the aim of the thesis is to evaluate the feasibility, accuracy, and modelling aspects of the MPS method irrelevant problems. This is done by investigating three canonical cases and comparing the results to the finite volume method (FVM) and existing literature. The canonical cases relate to the thermal issues of an electric motor, namely external cooling, the rotor-stator gap, and oil cooling, and are related to the backwards facing step, Taylor-Couette flow, and a free round single impinging jet, respectively. In all these cases, the MPS method was used and a sensitivity analysis was performed to estimate the discretization error. The FVM method was also done by the author using large eddy simulation (LES)for two of the cases, where an estimation of the discretization error was also performed. The results indicated that the MPS method did not perform well in the first two canonical cases, exhibiting low accuracy and high computational cost, thus rendering it unsuitable for internal flows. However, in the last case, the MPS method aligned with the literature and showed potential for reduced computational cost. Nevertheless, obtaining crucial thermal parameters such as the Nusselt number with a satisfacto

Integrating mock circulatory loops (MCLs) with magnetic resonance imaging (MRI) of fourth-dimensional flow (referred to as 4D flow MRI) has the potential to provide a comprehensive view of hemodynamics in the aortic region. While MCLs simulate bulk hydraulic properties of cardiovascular blood flow in specific regions of the cardiovascular system in benchtop studies, 4D flow MRI captures detailed velocity information, shear stress, and precise properties of fluid flow in a clinical context. However, this integration presents challenges because MCLs often contain large and ferromagnetic components, making them incompatible with the MRI environment. Therefore, this project focuses on designing, building, and evaluating an MRI-compatible MCL simulating aortic blood flow. This includes constructing a pump and modules to mimic the compliance and resistance of the cardiovascular system, all composed of non-ferromagnetic or -conducting materials and with appropriate size to be compatible with the MRI. Verification of the MCL was done to evaluate that target values were met in terms of pressure and flow, and its repeatability and runtime were evaluated. Finally, a 4D flow MRI scan of the loop was performed to evaluate the compatibility. The MCL achieved the target ranges for heart rate (60-180 bpm) and stroke volume (70-114 ml/beat), and a maximum cardiac output of 7.65 L/min. The MCL could be run for a minimum of one hour with a standard deviation of the mean flow of 0.0131 L/min and mean aortic pressure of 0.0737 mmHg. A general standard deviation of 0.021- 0.045 L/min for mean flow and 0.331-1.402 mean mmHg for pressure showed that the system had a high repeatability. Although an unwanted pressure fluctuation made it difficult to verify the pressure curve, tuning of resistance and compliance successfully influenced the pressure as intended. 4D flow images of the MCL were acquired, and the system did not produce any image artifacts and is considered safe for use in the M, Integrering av kardiovaskulära flödesmodeller (KFM:er) med magnetisk resonanstomografi (MRT) av fyrdimensionellt flöde (refererat till som 4D flow MRI) har potentialen att producera en omfattade helhets bild av hemodunamiken i aortaregionen. Medan KFM:er simulerar bulkhydraulika egenskaper hos kardiovaskulärt blodflöde i specifika regioner av det kardiovaskulära systemet i bänktops studier, fångar 4D flow MRI detaljerad hastighetsinformation, skjuvspänning och exakta egenskaper hos vätskeflödet i ett kliniskt sammanhang. Denna integration medför dock utmaningar eftersom att KFM:er ofta innehåller stora och ferromagnetiska komponenter, vilket gör dem inkompatibla med MRT-miljön. Därför fokuserar detta projekt på att designa, bygga och utvärdera en MRT-kompatibel KFM som simulerar blodflödet i aorta regionen. Detta inkluderar att konstruera en pump och moduler för att efterlikna det kardiovaskulära systemets följsamhet och motstånd, alla sammansatta av icke-ferromagnetiska eller ledande material och med lämplig storlek för att vara kompatibla med MRT. Verifiering av den KFM:en gjordes för att utvärdera att målvärdena uppfylldes vad gäller tryck och flöde, och dess repeterbarhet och körtid utvärderades. Slutligen utfördes en 4D fow MRI-skanning av flödesmodellen för att utvärdera kompatibiliteten. Den KFM:en uppnådde målområdena för hjärtfrekvens (60-180 slag per minut) och slagvolym (70-114 ml/slag) och en maximal hjärtminutvolym på 7,65 l/min. Den kunde köras i minst en timme med en standardavvikelse av medelflödet på 0,0131 l/min och medelaortatrycket på 0,0737 mmHg. En generell standardavvikelse på 0,021-0,045 l/min för medelflöde och 0,331-1,402 medel mmHg för tryck visade att systemet hade en hög repeterbarhet. Även om en oönskad tryckfluktuation gjorde det svårt att verifiera tryckkurvan, påverkade inställning av motstånd och följsamhet trycket som avsett. 4D-flödesbilder av den KFM:en erhölls, och systemet producerade inga bildartefakter och anses säkert för an

Continuous casting is by far the most widely used casting method for steel production all over the world. The Swedish steel industry has specialized in niche products with high quality. To obtain high-quality steel, flow control is crucial when operating the caster. Computational fluid dynamics is used in the research and development of steel production to gain knowledge and optimize flow control, with meshing being a crucial step. One challenge with meshing is finding the balance between computational cost (time and storage) and accuracy. This study investigated whether polyhedral mesh elements would be suffcient for modeling turbulent flow in continuous casting. Firstly, Ansys Fluent's polyhedral converter was used on an existing mesh containing tetrahedral and hexahedral elements. The mesh quality in terms of skewness, orthogonality, and aspect ratio was studied. The results showed that the polyhedral converter lowered the cell count and slightly improved the quality compared to the original mesh. However, more research is needed with other meshes and actual simulations to determine if it is suffcient for modeling turbulent flow in continuous casting. Following that, two new meshes were generated: one polyhedral and one poly-hexcore mesh, both using Ansys Fluent Meshing, and their mesh quality was analyzed as in the first case. These meshes were then simulated with the Large Eddy Simulation model for steel, slag, and air for 60 seconds. They were compared and evaluated using production data in the form of steel layer fluctuations and nail board measurements. The results from the polyhedral and poly-hexcore meshes showed that both meshes had good, equal quality and cell count. Even so, due to the construction of the poly-hexcore, switching between polyhedral and hexahedral elements, and the occurrence of hanging nodes, the simulation showed signs of discretization errors. This might have also contributed to the lower flow velocities at the steel layer compared to, Stränggjutning är den främsta metoden använt för stålproduktion över hela världen. Den svenska stålindustrin har specialiserat sig på nischade produkter med hög kvalitet. För att uppnå högkvalitativt stål är flödeskontroll avgörande vid drift av gjutmaskinen. Numeriska strömningsberäkningar används i forskningen och utvecklingen av stålproduktion för att skaffa kunskap och optimera flödeskontrollen, där meshing är ett avgörande steg. En utmaning med meshing är att hitta balansen mellan beräkningskostnad (tid och lagring) och noggrannhet. Denna studie undersökte om polyhedrala mesh-element skulle vara tillräckliga för att modellera turbulent flöde i stränggjutning. Först användes Ansys Fluents polyhedrala konverterare på en befintlig mesh, innehållande tetrahedrala och hexahedrala element. Meshkvalitén i termer av skevhet, ortogonalitet och aspektförhållande studerades. Resultaten visade att den polyhedrala konverteraren sänkte cellantalet och förbättrade kvaliteten något jämfört med den ursprungliga meshen. Dock behövs mer forskning med andra mesher, och faktiska simuleringar, för att avgöra om den är tillräcklig för att modellera turbulent flöde i stränggjutning. Därefter genererades två nya mesher: ett polyhedralt och en poly-hexcore, båda med hjälp av Ansys Fluent Meshing, och precis som i det första fallet, studerades deras meshkvalitet. Dessa två meshes simulerades sedan med Large Eddy Simulation-modellen för stål, slagg och luft i 60 sekunder. De jämfördes och utvärderades med hjälp av produktionsdata i form av stålnivå, fluktuationer och 'nail board'-mätningar. Resultaten från det polyhedrala och poly-hexcore meshen visade att båda mesherna hade god och likvärdiga kvalitéer samt cellantal. Trots detta, på grund av uppbyggnaden av poly-hexcore meshen, alltså växlingen mellan polyhedrala och hexahedrala element och förekomsten av hängande noder, visade simuleringen tecken på diskretiseringsfel. Detta kan också ha bidragit till de lägre flödeshastigheterna vid s