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Time and place: Nov. 25, 2022 12:15 PM1:00 PM, Niels Henrik Abels hus, 9th floor

Ingeborg Gjerde (Simula Research Laboratory) presents joint work with Ridgway Scott (University of Chicago).

Abstract: Airflow around airplane wings is characterized by a wide range of flow scales, making it highly challenging to capture numerically. From a simulation viewpoint, the following questions are still being actively investigated: Why do airplanes fly? Can one reliably simulate the lift and drag of an airplane wing? In this talk, I will provide no good answers to these questions. Instead, I want to talk about some interesting results I've stumbled into tangentially, including:
- (Nonlinear) kinetic energy instability analysis, also referred to as Reynolds-Orr instability
- Slip boundary conditions and their connection to D'Alembert's paradox
- Stokes' paradox and its connection to weighted Sobolev spaces. I will show numerical results computed for flow around a cylinder, which serves as a proxy for flow around an airplane wing. In particular, I will talk about the impact of the friction boundary condition on the drag force and flow stability. Finally, I will comment on how these results might be interpreted in view of: New Theory of Flight, J. Hoffman, J. Jansson, C. Johnson (2016), Journal of Mathematical Fluid Mechanics.

Time and place: Nov. 11, 2022 12:15 PM1:00 PM, Niels Henrik Abels hus, 9th floor

In this talk, I will go through my past research before joining UiO, particularly at The University of Texas at Austin. This will include a brief introduction to the development of stable and adaptive finite element methods for challenging problems in engineering science. Second, I will focus on modeling efforts in coastal ocean hydrodynamics, including a review of the underlying physics and assumption and a review of the current state-of-the-art. I will also introduce several related to my focus of storm surge modeling and how the models are used by stakeholders beyond academia. 

Time and place: Oct. 28, 2022 12:15 PM1:00 PM, Niels Henrik Abels hus, 9th floor

When a body (such as an offshore structure and ship) exists on the surface of the ocean, it is influenced by waves. At the same time, waves are deformed by the body. This interaction is essential for considering the problems of bodies in waves. Although these are complicated systems, the theory is well-established based on linear potential flow, and this explains these phenomena very well.
In the seminar, some applications of potential theory-based analysis are shown, including the seakeeping of a ship, multi-bodies interaction, and elastic plate in waves. In addition, the progress of the study of wave-ice interaction in a marginal ice zone is presented which is a current work in UiO.

Time and place: Oct. 21, 2022 1:10 PM2:00 PM, Niels Henrik Abels hus, 9th floor

The survival of green plants depends on the efficient use of photosynthesis in the leaves, where sunlight, water, and CO2 are transformed into sugar – the raw material, which builds up even the largest trees. The dissolved sugars are transported by osmosis through the sieve tubes of the phloem, a vascular system, which runs through the veins of the leaves and on through the stem, all the way down into the roots. The sugar production sites (mesophyll) are distributed over the entire leaf, and it is important for the functionality of the leaf that they are all able to export their sugars. For conifer needles the linear venation architecture makes this challenging, and they have an extra “transfusion tissue” that bridges between production and transport. We are currently studying this complex collection of interdigitated water -and sugar-carrying cells by micro X-ray tomography on intact needles and by network modelling, to understand the pathways for water and for sugars (running in opposite directions) with huge pressure differences (say 3 MPa) across tiny length scales (say 5 microns).

Thomas Bohr is Professor of Physics at the Physics Department of the Technical University of Denmark.

Time and place: Oct. 21, 2022 12:00 PM12:45 PM, Niels Henrik Abels hus, 9th floor

Diffusion and reactions are central to understanding life. However, studies often focus on dilute systems, while the interior of living cells is crowded with macromolecules that occupy about 20 % to 40 % of the cell volume, affecting virtually all intracellular processes [1]. In this talk, I will mainly focus on diffusion, emphasising the effects significant to crowded intracellular environments, such as polydispersity of crowders [2], macromolecular shapes, interactions [3], and softness [4]. We will also briefly discuss how reactions proceed under crowding, paying particular attention to enzymatic reactions [5] and the cooperativity of divalent binding [6].

Time and place: Sep. 23, 2022 12:15 PM1:00 PM, Niels Henrik Abels hus, 9th floor

Stephen Hladky presents work in collaboration with Margery A. Barrand (both Department of Pharmacology, University of Cambridge).

Abstract: Extravascular fluxes of marker substances and some wastes are sufficiently fast that there is almost certain to be a component of flow augmenting their diffusion in the parenchyma. There have been two major proposals for how this flow is produced and where it is important. The evidence for the classical and glymphatic hypotheses will be reviewed. Extravascular, and in particular perivascular, routes for fluid movement out of the parenchyma to lymphatics may be important in the development of hydrocephalus.

Time and place: Sep. 19, 2022 3:05 PM4:00 PM, Niels Henrik Abels hus: Undervisningsrom 1119

We combine a systematic approach for deriving general a posteriori
 error estimates for convex minimization problems using convex duality relations with a recently derived generalized Marini formula. The resulting a posteriori error estimates are essentially constant-free and apply to a large class of variational problems including the p-Dirichlet problem, as well as degenerate minimization, obstacle and image de-noising problems.
For the p-Dirichlet problem, the a posteriori error bounds are equivalent to the classical residual type a posteriori error bounds and, hence, reliable and efficient.

Time and place: Sep. 9, 2022 12:15 PM1:00 PM, Niels Henrik Abels hus, 9th floor

José Carlos Nieto Borge is a marine physicist and associate professor at the Universidad de Alcalá de Henares, Spain.

Time and place: Sep. 2, 2022 12:15 PM1:00 PM, Niels Henrik Abels hus, 9th floor

We consider the linearized elasticity equations, discretized using multi-patch Isogeometric Analysis. To solve the resulting linear system, we choose the Dual-Primal Isogeometric Tearing and Interconnecting (IETI-DP) Method with a scaled Dirichlet preconditioner. We are interested in a convergence analysis. See more details below.

We will present the results of numerical experiments that demonstrate our theoretical findings.

Time and place: June 24, 2022 12:15 PM1:00 PM, Niels Henrik Abels hus, 9th floor

The simulation of multi-phase fluids has attained growing interest in the last decades. While for single-phase flow with the Navier-Stokes system the basic model is well understood, for multi-phase systems additional challenges by the necessity to track the transition zones or interfaces between different fluid components arise. 
We propose to use a phase field as a smooth indicator function to describe this situation. Using phase-field models, one introduces a small layer of mixed fluids as a so-called diffuse interface. One benefit of phase-field models is, that they can naturally deal with topology changes and can easily be extended to cope with contact line dynamics.
This model allows for discussing the optimal control problem for two-phase flow. We introduce a thermodynamically consistent phase-field model for two-phase flow including a model for contact line dynamics and introduce an energy stable numerical scheme. 
This scheme allows us to investigate the time-discrete (open loop) optimal control problem, where we investigate different control actions to steer a given distribution of phases towards the desired distribution. We derive the existence of solutions to the optimal control problem and provide first-order optimality conditions.

Hybrid format via Zoom possible on demand (contact timokoch at uio.no)

Time and place: June 17, 2022 12:15 PM1:00 PM, Niels Henrik Abels hus, 9th floor

Migration of particles and deformation and coalescence of droplets can be highly affected by the non-Newtonian properties of the carrier fluid. 
In this talk, we focus on the effects of viscoelasticity and yield stress (plasticity) on particle and droplet dynamics. Results are presented from numerical simulations of Navier-Stokes equations coupled with viscoelastic and elastoviscoplastic constitutive models. We will investigate how to yield stress and elasticity affect the final position of a particle in channel flows, and how complex fluid droplets deform, merge and jump on a superhydrophobic surface.

Hybrid format via Zoom possible on demand (contact timokoch at uio.no)

Time and place: June 10, 2022 1:00 PM1:45 PM, Niels Henrik Abels hus, 9th floor

Brain source analysis is an important tool in brain research. It is used for example during operation planning for epilepsy patients. Given EEG (electroencephalography) and MEG (magnetoencephalography) measurements the goal is to reconstruct the brain activity, i.e. the electric potential in the brain. This poses an inverse problem. It was observed in experiments, that the accuracy of the inverse problem strongly depends on the quality of the forward simulation, in particular the head model. We discuss how modern numerical methods like discontinuous Galerkin (dG) methods and cut-cell techniques can increase the robustness of the forward problem and simplify the overall workflow. Hardware-oriented design of numerical methods allows for improved speed of the inverse simulation by making use of modern hardware resources. In order to compute the forward problem efficiently, we propose an algebraic multigrid solver for cut-cell dG methods. We introduce the challenges of EEG/MEG inverse modeling and discuss how different parts of the problem can be improved using modern numerical methods.

Hybrid format via Zoom possible on demand (contact timokoch at uio.no)

The talk will start at ca. 13:00 right after the talk by Andrea Bressan.

Time and place: June 10, 2022 12:15 PM1:00 PM, Niels Henrik Abels hus, 9th floor

Abstract: Many constructions have been proposed to attain adaptivity within the IGA framework (HB-, T-, LR-splines among others).
The idea of all approaches is to break the tensor product structure of the space in order to allow a spatially varying resolution.
Here we present an approach for adaptivity in IGA aimed at solving PDEs with preconditioned Krylov methods. 
All the main ingredients of this work are already known, but their combination looks promising for the 
development of isogeometric solvers with a good balance of flexibility and computational efficiency.

Hybrid format via Zoom possible on demand (contact timokoch at uio.no)

Time and place: June 3, 2022 12:15 PM1:15 PM, Niels Henrik Abels hus, 9th floor

Abstract: Dynamic Contact Lines are moving lines at which two fluids (eg air and water) and a solid meet. They offer longstanding paradoxes in the theory of continuum mechanics. I describe some aspects of their computation: a curtain-coating numerical experiment, a withdrawing plate, and a droplet between parallel plates.  All computations are done using Volume-Of-Fluid methods on octree grids, and comparisons with phase field and molecular dynamics approaches performed in collaboration with KTH, Stockholm.

Hybrid format via Zoom possible on demand (contact timokoch at uio.no)

Time and place: May 20, 2022 12:15 PM1:15 PM, Niels Henrik Abels hus, 9th floor

Abstract: I present the electrophoretic transport phenomenon of spherical soft particles. Electrophoresis is one of the important electrokinetic techniques, which is often used to characterize, and separation of colloids.  It is commonly used as a separation technique and often used in the separation of DNA, protein molecules, serum to identify paraproteins, etc. Electrophoretic transport phenomenon is also used to understand the electric properties of several bioparticles including virus, bacteria, humic cells and macromolecules and may be used to understand the transport of cargo vessel in treatments of various diseases, e.g., cancer, inflammation, multiple myeloma, rental pathological disorders and macroglobulinemia, etc. Thus, the proper understanding of the electrophoretic transport of soft particles is important to understand the characteristics features of various bio-colloids and macromolecules, which can be viewed as soft particles. In this talk, I will present some of the existing simplified models for electrophoretic transport of soft particles. In addition, I have further extended it for the real situation, considering the effect of pH-dependent charge densities of the inner core and peripheral soft polymeric layer, effect of hydrodynamic slip length of the hydrophobic core surface, etc. 

Hybrid format via Zoom possible on demand (contact timokoch at uio.no)

Time and place: May 13, 2022 12:15 PM1:15 PM, Niels Henrik Abels hus, 9th floor

I present a simple, efficient, three dimensional, time dependent model for bone regeneration in the presence of porous scaffolds to bridge critical size bone defects. The essential processes are an interplay between the mechanical and biological environment which we model by a coupled system of PDEs and ODEs. The mechanical environment is represented by a linear elastic equation and the biological environment through reaction-diffusion equations as well as as logistic ODEs, modelling signalling molecules and cells/bone respectively. Material properties are incorporated using homogenized quantities not resolving any scaffold microstructure. This makes the model efficient in computations, thus suitable as a forward equation in optimization algorithms and opening up the possibility of patient specific scaffold design and this model is used as a PDE constraint for the optimization of polymer scaffold porosities. Our numerical findings show that our model for example recovers and quantifies clinically relevant stress shielding effects that appear in vivo due to external fixation of the scaffold at the defect site.

This talk is part of the Mechanics Lunch Seminar series. Bring-your-own-lunch and lots of questions.

Time and place: Apr. 29, 2022 12:15 PM1:15 PM, Niels Henrik Abels hus, 9th floor

Abstract: T cell activation is initiated upon T cell receptor (TCR) binding to cognate peptide-MHC complexes on the surface of antigen presenting cells (APCs). This initiates the formation of the immunological synapse – a specialized hub for bidirectional membrane transfer between T cells and APCs which is essential for processes such as downregulation of triggered TCR and T cell-mediated stimulation of B cells. Here, I will present some recent advancements in our understanding of how this is regulated.

This talk is part of the Mechanics Lunch Seminar series. Bring-your-own-lunch and lots of questions. Hybrid format via Zoom possible on demand (contact timokoch at uio.no)

Time and place: Apr. 22, 2022 12:15 PM1:15 PM, Niels Henrik Abels hus, 9th floor

Abstract: What links a baby’s first breath to adhesive debonding, enhanced oil recovery, filtration or multiphase microfluidics? These processes all involve two-phase flows in rigid or elastic confined vessels and are often prone to interfacial instabilities. The canonical viscous fingering instability, which occurs when air displaces a viscous fluid in the narrow gap between two parallel plates, offers a versatile testbed for such phenomena. In this talk, I will use both experiments and numerical simulations of depth-averaged models to explore several aspects of bubble dynamics in Hele-Shaw cells. I will first show how the onset of fingering can be suppressed when replacing the upper plate of the vessel with an elastic sheet. Interfacial flows in narrow gaps can also exhibit considerable disorder, but they are rarely investigated from a dynamical systems’ perspective. I will show how compliance can promote rich multiplicity of front propagation modes in a channel before turning to bubble propagation in a rigid channel with a depth perturbation. There I will explore how the bubble’s organised transient dynamics is orchestrated by weakly-unstable steady propagation modes, and how its long-term behaviour may be practically unpredictable.

This talk is part of the Mechanics Lunch Seminar series. Bring-your-own-lunch and lots of questions. Hybrid format via Zoom possible on demand (contact timokoch at uio.no)

Time and place: Apr. 8, 2022 12:15 PM1:15 PM, Niels Henrik Abels hus, 9th floor

Abstract: A random, labyrinthine pattern emerges during slow drainage of a granular-fluid system in two- dimensional confinement. Compacted grains are pushed ahead of the fluid-air interface, which becomes unstable due to a competition between capillary forces and the frictional stress mobilized by grain-grain contact networks. We reproduce the pattern formation process in numerical simulations and present an analytical treatment that predicts the characteristic length scale of the labyrinth structure. The pattern length scale decreases with increasing volume fraction of grains in the system and increases with the system thickness. By tilting the model, aligned finger structures, with a characteristic width, emerge. A transition from vertical to horizontal alignment of the finger structures is observed as the tilting angle and the granular density are varied. The dynamics is reproduced in simulations. We also show how the system may explain patterns observed in nature, created during the early stages of a dike formation.

This talk is part of the Mechanics Lunch Seminar series. Bring-your-own-lunch and lots of questions. Hybrid format via Zoom possible on demand (contact timokoch at uio.no)

Time and place: Mar. 18, 2022 12:15 PM1:15 PM, Niels Henrik Abels hus, 9th floor + Zoom

Abstract: Because of their huge compressibility difference with their surrounding media, air bubbles in water have a special relationship with acoustic waves: they are sub-wavelength resonators. In this presentation, I will show that this characteristic has great implications for both the surrounding fluid, because of the steady streaming effect, but also for the acoustic waves.

This talk is part of the Mechanics Lunch Seminar series. Bring-your-own-lunch and lots of questions.

Time and place: Mar. 11, 2022 12:15 PM1:15 PM, Niels Henrik Abels hus, 9th floor + Zoom

This talk is part of the Mechanics Lunch Seminar series. Bring-your-own-lunch and lots of questions.

Time and place: Feb. 18, 2022 12:15 PM1:15 PM, Niels Henrik Abels hus, 9th floor + Zoom

Abstract: This talk is about the non-integer (fractional) derivative, its mathematical formulation by Abel in 1823, and present-day applications in modeling power-law behavior. These applications are in acoustics of complex media like tissue and sediments as well as in rheology, turbulence, and dielectrics. It will build on my book “Waves with Power-Law Attenuation”, Springer, 2019.

The talk will be streamed online. Please contact "timokoch at uio.no" for the Zoom link. This talk is part of the Mechanics Lunch Seminar series. Bring-your-own-lunch and lots of questions.

Time and place: Feb. 11, 2022 12:15 PM1:15 PM, Zoom + Niels Henrik Abels hus, 9th floor

Abstract: Gravitational settling of a droplet in air onto a soft substrate is a ubiquitous event relevant to many natural processes and engineering applications. We study this phenomenon by developing a three-phase lubrication model of droplet settling onto a solid substrate coated by a thin, soft compressible layer of elastic material. By combining scaling analysis, analytical methods and numerical simulations we elucidate how the resulting droplet dynamics is affected by the soft layer. We discuss extensions to droplet settlings onto thin viscous liquid films and elastic sheets. Our results provide new insight into the coupled interactions between droplets and solids coated by a thin film of a soft material.

Talk can be followed online on Zoom as well as in person. Please contact "timokoch at uio.no" for the Zoom link. This talk is part of the Mechanics Lunch Seminar series. Bring-your-own-lunch and lots of questions.

Time and place: Feb. 4, 2022 12:15 PM1:15 PM, Zoom

Abstract: We investigate the simulation of a rising bubble and a stationary droplet interaction to gain a better understanding of the rising dynamics and the morphology changing of bubble-droplet aggregate. A detailed study is conducted on the interaction process under different-size bubbles with various combinations of spreading factors. The current simulation framework consists of the conservative phase-field Lattice Boltzmann equation (LBE) for interface tracking and the velocity-pressure LBE for hydrodynamics. We simulate the contact line dynamics to confirm the method's accuracy. We further investigate the morphology changing of two contact droplets under different combinations of spreading factors and depict the final morphologies in a diagram. The separated, partially engulfed and complete engulfed morphologies can be replicated by systematically altering the sign of the spreading factors. The rising bubble and droplet interaction is simulated based on different final morphologies by adding a body force. The results show that the aggregate with double emulsion morphology can avoid distortion and maintain a greater terminal velocity than the aggregate with partially engulfed morphology.

Talk is online on Zoom. Please contact "timokoch at uio.no" for the Zoom link. This talk is part of the Mechanics Lunch Seminar series. Bring-your-own-lunch and lots of questions.

Time and place: Jan. 28, 2022 12:15 PM1:15 PM, Niels Henrik Abels hus, 9th floor, seminar room 919

Abstract: The fungal kingdom is one of the most species-rich organismal groups, containing up to 6 million species worldwide with a large diversity of ecosystem functions. Multicellularity has evolved independently in fungi, and over time many different growth forms and structures have originated. I will present some basics on fungal growth and the formation of complex multicellular structures.

Online participation is possible too. Please contact "timokoch at uio.no" for the Zoom link. This talk is part of the Mechanics Lunch Seminar series. Bring-your-own-lunch and lots of questions.