arXiv subjects

Dominik Krug

Publications and source records attributed to Dominik Krug.

At least 19 recordsLinked to original sources

arXiv2026

Simulations of Particle-Laden Flows with Large Dispersed-Phase Size Disparities Using Highly Scalable Parallel Adaptive Methods

The numerical simulation of multiphase flows involving dispersed components with large scale disparities, such as the collisions between millimeter-sized bubbles and micron-sized mineral particles in flotation, poses a significant computational challenge. Accurately resolving the thin boundary layers of finite-size objects while tracking massive numbers of small particles within a large turbulent domain is often prohibitively expensive on uniform grids. To address this, we present a parallel scalable computational framework that couples the lattice Boltzmann method with the immersed boundary method on a dynamically adaptive octree grid. A key algorithm is developed for the efficient parallel host-cell searching, which significantly accelerates the tracking of Lagrangian points on distributed unstructured grids. The accuracy and robustness of the code are rigorously validated against canonical benchmarks, including the flow induced by an oscillating cylinder and the sedimentation of a sphere. The framework is applied to the multiscale problem of bubble-particle collisions. In quiescent flow, the simulations accurately capture the hydrodynamic interception mechanism, reproducing the theoretical collision efficiency scaling law proportional to the square of the particle-to-bubble size ratio. Furthermore, the framework is applied to the simulation of fully resolved bubbles interacting with inertial point particles in homogeneous isotropic turbulence.

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arXiv2026

Influence of the Inhalation Route on Tracheal Flow Structures in Patient-Specific Airways using 3D PTV

The tracheal flow field shapes particle transport into the lower airways and thus influences both the spread of inhaled pathogens and the effectiveness of aerosol-based therapies. Identifying how different inhalation routes modify the flow field is therefore crucial for understanding lower-airway disease transmission and for guiding targeted drug delivery. To gain a detailed understanding of the influence of the inhalation route on the flow structures in the human trachea, the flow field in the trachea is investigated in vitro in a non-compliant, refractive-index matched silicone model of the human respiratory tract. The investigations comprise steady inhalation, and oscillatory flow to simulate calm breathing. A realistic breathing pattern is approximated by a sinusoidal waveform for two Reynolds numbers of $Re_{Tr} = [400, 1200]$, based on the bulk velocity at maximum volume flux and the hydraulic diameter of the trachea and two Womersley numbers of $Wo = [3, 4.5]$, representing the oscillation time scales. To capture the inherently three-dimensional and asymmetric nature of the flow field, 3D particle-tracking velocimetry measurements are performed using the Shake-The-Box algorithm. Using a refractive-index matched fluid consisting of water and glycerin, the complex flow structures inside the trachea are fully resolved. The PTV measurements confirm that the nasal and/or oral cavity must be considered when analyzing the flow field in the lower respiratory tract. In particular, we find that the presence of both cavities significantly alters the flow field compared to idealised, fully developed inflow conditions. However, velocity profiles in the sagittal and coronal plane in the trachea as well as contour plots of the of the normalized velocity magnitude evidence nearly identical flow structures for oral and nasal inhalation, indicating minimal influence of the inhalation route.

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arXiv2026

Coupled Level-Set Lattice Boltzmann Method on Adaptive Cartesian Grids

A novel coupled level-set lattice Boltzmann method on adaptive Cartesian grids for simulating liquid-gas multiphase flows is presented. The approach addresses the inherent challenges of accurately modeling multiphase systems characterized by sharp interfaces and large density ratios. By employing separate solution algorithms for each fluid phase which are coupled through boundary conditions at the interface the method is more accurate and more efficient. The study highlights the advantages of using lattice Boltzmann methods together with level-set techniques to track interfaces effectively while facilitating adaptive mesh refinement. Applications to various test cases, e.g., immiscible stratified flow and rising bubbles, demonstrate the method's capability to capture complex interfacial dynamics and validate its accuracy against literature data.

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arXiv2025

Comparative Analysis of the Flow in a Realistic Human Airway

Accurate simulations of the flow in the human airway are essential for advancing diagnostic methods. Many existing computational studies rely on simplified geometries or turbulence models, limiting their simulation's ability to resolve flow features such shear-layer instabilities or secondary vortices. In this study, direct numerical simulations were performed for inspiratory flow through a detailed airway model which covers the nasal mask region to the 6th bronchial bifurcation. Simulations were conducted at two physiologically relevant \textsc{Reynolds} numbers with respect to the pharyngeal diameter, i.e., at Re_p=400 (resting) and Re_p=1200 (elevated breathing). These values characterize resting and moderately elevated breathing conditions. A lattice-Boltzmann method was employed to directly simulate the flow, i.e., no turbulence model was used. The flow field was examined across four anatomical regions: 1) the nasal cavity, 2) the naso- and oropharynx, 3) the laryngopharynx and larynx, and 4) the trachea and carinal bifurcation. The total pressure loss increased from 9.76 Pa at Re_p=400 to 41.93 Pa at Re_p=1200. The nasal cavity accounted for the majority of this loss for both Reynolds numbers, though its relative contribution decreased from 81.3% at Re_p=400 to 73.4% at Re_p=1200. At Re_p=1200, secondary vortices in the nasopharyngeal bend and turbulent shear-layers in the glottis jet enhanced the local pressure losses. In contrast, the carinal bifurcation mitigated upstream unsteadiness and stabilized the flow. A key outcome is the spatial correlation between the pressure loss and the onset of flow instabilities across the four regions. This yields a novel perspective on how the flow resistance and vortex dynamics vary with geometric changes and flow rate.

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arXiv2025

A predictive model for bubble-particle collisions in turbulence

The modelling of bubble-particle collisions is crucial to improving the efficiency of industrial processes such as froth flotation. Although such systems usually have turbulent flows and the bubbles are typically much larger than the particles, there currently exist no predictive models for this case which consistently include finite-size effects in the interaction with the bubbles as well as inertial effects for the particles simultaneously. As a first step, Jiang and Krug (J. Fluid Mech., vol. 1006, 2025, A19) proposed a frozen turbulence approach which captures the collision rate between finite-size bubbles and inertial particles in homogeneous isotropic turbulence using the bubble slip velocity probability density function measured from simulations as an input. In this study, we further develop this approach into a model where the bubble-particle collision rate can be predicted a priori based on the bubble, particle, and turbulence properties. By comparing the predicted collision rate with simulations of bubbles with Stokes numbers of 2.8 and 6.3, and particles with Stokes numbers ranging from 0.01 to 2 in turbulence with a Taylor Reynolds number of 64, good agreement is found between model and simulations for Froude number $Fr \leq 0.25$. Beyond this range of bubble Stokes number, we propose a criterion for using our model and discuss the model's validity. Evaluating our model at typical flotation parameters indicates that particle inertia and settling effects are usually important. Generally, smaller bubbles, larger particles, and stronger turbulence increase the overall collision rate.

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arXiv2025

To jump or not to jump: Adhesion and viscous dissipation dictate the detachment of coalescing wall-attached bubbles

Bubble coalescence can promote bubble departure at much smaller sizes compared to buoyancy. This can critically enhance the efficiency of gas-evolving electrochemical processes, such as water electrolysis. In this study, we integrate high-speed imaging experiments and direct numerical simulations to dissect how and under which conditions bubble coalescence on surfaces leads to detachment. Our transparent electrode experiments provide new insights into contact line dynamics, demonstrating that the bubble neck generally does not contact the surface during coalescence. We reveal that whether coalescence leads to bubble departure or not is determined by the balance between surface energy, adhesion forces, and viscous dissipation. For the previously unexplored regime at low effective Ohnesorge number, a measure of viscosity that incorporates the effect of asymmetry between the coalescing bubbles, we identify a critical dimensionless adhesion energy threshold of $\approx$15% of the released surface energy, below which bubbles typically detach. We develop a global energy balance model that successfully predicts coalescence outcomes across diverse experimental conditions.

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arXiv2024

Bubble curtains for noise mitigation: one vs. two

Bubble curtains are widely used to protect marine life from exposure to noise during offshore construction. However, operating a bubble curtain is costly. Therefore optimizing the acoustic effect of the available air is important. An interesting approach is to split the airflow rate into two separate bubble curtains, rather than one single curtain. This concept is tested experimentally and numerically. The experiments and the model show an increase in performance of the compressed air when it is split between two manifolds. An increased insertion loss of up to 11dB is measured. This increase in performance is possibly due to the fact that the reflective properties of the bubble curtains are maintained when halving the airflow rate. In effect, by splitting the airflow a second acoustic barrier is added. Additionally, the variations in the bubble curtain performance between individual measurements are shown to be largely caused by temporal variations in the air distribution. The applicability of equivalent fluid models for bubble curtains is discussed, and it is shown that accounting for a gap in the bubble curtain, close to the manifold where the bubble curtain is not fully developed, results in better agreement between the modelled and the measured insertion loss.

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arXiv2024

The effect of gravity on bubble-particle collisions in turbulence

Bubble-particle collisions in turbulence are key to the froth flotation process that is widely employed industrially to separate hydrophobic from hydrophilic materials. In our previous study (Chan et al., J. Fluid Mech., vol. 959, 2023, A6), we elucidated the collision mechanisms and critically reviewed the collision models in the no-gravity limit. In reality, gravity may play a role since ultimately separation is achieved through buoyancy-induced rising of the bubbles. This effect has been included in several collision models, which have remained without a proper validation thus far due to a scarcity of available data. We therefore conduct direct numerical simulations of bubbles and particles in homogeneous isotropic turbulence with various Stokes, Froude, Reynolds numbers, and particle density ratios using the point-particle approximation. Generally, turbulence enhances the collision rate compared to the pure relative settling case by increasing the collision velocity. Surprisingly, however, for certain parameters the collision rate is lower with turbulence compared to without, independent of the history force. This is due to turbulence-induced bubble-particle spatial segregation, which is most prevalent at weak relative gravity and decreases as gravitational effects become more dominant, and reduced bubble slip velocity in turbulence. The existing bubble-particle collision models only qualitatively capture the trends in our numerical data. To improve on this, we extend the model by Dodin & Elperin (Phys. Fluids, vol. 14, no. 8, 2002, pp.2921-2924) to the bubble-particle case and found excellent quantitative agreement for small Stokes numbers when the history force is negligible and segregation is accounted for.

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arXiv2024

Electrolyte spraying within H$_2$ bubbles during water electrolysis

Electrolytically generated gas bubbles can significantly hamper the overall electrolysis efficiency. Therefore it is crucial to understand their dynamics in order to optimise water electrolyzer systems. Here we demonstrate a distinct transport mechanism where coalescence with microbubbles drives electrolyte droplets, resulting from the fragmentation of the Worthington jet, into the gas phase during hydrogen evolution reaction, both in normal and microgravity environments. This indicates that the H$_2$ bubble is not only composed of hydrogen gas and vapor but also includes electrolyte fractions. Reminiscent of bursting bubbles on a liquid-gas interface, this behavior results in a flow inside the bubble, which is further affected by Marangoni convection at the gas-electrolyte interface, highlighting interface mobility. In the case of electrode-attached bubbles, the sprayed droplets form electrolyte puddles at the bubble-electrode contact area, affecting the dynamics near the three-phase contact line and favoring bubble detachment from the electrode. The results of this work unravel important insights into the physicochemical aspects of electrolytic gas bubbles, integral for optimizing gas-evolving electrochemical systems. Besides, our findings are essential for studying the limits of jet formation and rupture relevant to acid mist formation in electrowinning, generation of sea spray aerosols, impact of droplets on liquid surfaces, etc.

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arXiv2024

How turbulence increases the bubble-particle collision rate

We study the effect of turbulence on collisions between a finite-size bubble and small inertial particles based on interface-resolved simulations. Our results show that the interaction with the flow field around the bubble remains the dominant effect. Nonlinear dependencies in this process can enhance the turbulent collision rate by up to 100\% compared to quiescent flow. Fluctuations in the bubble slip velocity during the interaction with the particle additionally increase the collision rate. We present a frozen-turbulence model that captures the relevant effects providing a physically consistent framework to model collisions of small inertial particles with finite-sized objects in turbulence.

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arXiv2024

Life beyond Fritz: On the detachment of electrolytic bubbles

We present an experimental study on detachment characteristics of hydrogen bubbles during electrolysis. Using a transparent (Pt or Ni) electrode enables us to directly observe the bubble contact line and bubble size. Based on these quantities we determine other parameters such as the contact angle and volume through solutions of the Young-Laplace equation. We observe bubbles without ('pinned bubbles') and with ('spreading bubbles') contact line spreading, and find that the latter mode becomes more prevalent if the concentration of HClO4 is greater than or equal to 0.1 M. The departure radius for spreading bubbles is found to drastically exceed the value predicted by the well-known formula of W. Fritz (Physik. Zeitschr. 1935, 36, 379-384) for this case. We show that this is related to the contact line hysteresis, which leads to pinning of the contact line after an initial spreading phase at the receding contact angle. The departure mode is then similar to a pinned bubble and occurs once the contact angle reaches the advancing contact angle of the surface. A prediction for the departure radius based on these findings is found to be consistent with the experimental data.

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arXiv2024

Performance enhancement of electrocatalytic hydrogen evolution through coalescence-induced bubble dynamics

The evolution of electrogenerated gas bubbles during water electrolysis can significantly hamper the overall process efficiency. Promoting the departure of electrochemically generated bubbles during (water) electrolysis is therefore beneficial. For a single bubble, a departure from the electrode surface occurs when buoyancy wins over the downward-acting forces (e.g. contact, Marangoni, and electric forces). In this work, the dynamics of a pair of H$_2$ bubbles produced during hydrogen evolution reaction in 0.5 M H$_2$SO$_4$ using dual platinum micro-electrode system is systematically studied by varying the electrode distance and the cathodic potential. By combining high-speed imaging and electrochemical analysis, we demonstrate the importance of bubble-bubble interactions for the departure process. We show that bubble coalescence may lead to substantially earlier bubble departure as compared to buoyancy effects alone, resulting in considerably higher reaction rates at constant potential. However, due to continued mass input and conservation of momentum repeated coalescence events with bubbles close to the electrode may drive departed bubbles back to the surface beyond a critical current, which increases with the electrode spacing. The latter leads to the resumption of bubble growth near the electrode surface, followed by buoyancy-driven departure. While less favourable at small electrode spacing, this configuration proves to be very beneficial at larger separations increasing the mean current up to 2.4 times compared to a single electrode under the conditions explored in this study.

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arXiv2024

Planar bubble plumes from an array of nozzles: Measurements and modelling

Bubble curtains are widely used for sound mitigation during offshore pile driving to protect marine life. However, the lack of well validated hydrodynamic models is a major factor in the inability to predict the sound attenuation of a bubble curtain a priori. We present a new dataset resulting from bubble curtain measurements carried out in a 10m deep and 31m wide freshwater tank. The data describe the evolution of the void fraction profile and the bubble size distribution along the height of the bubble curtain. On this basis, a new relationship is developed for the dependence of the entrainment parameter of the bubble curtain on the air flowrate. In addition, we have extended a recently developed integral model for round bubble plumes to seamlessly capture the transition from initially individual round plumes to a planar plume after their merger. With additional modifications to the entrainment relation, the effective slip velocity and the initial condition for the bubble size distribution, the new model is found to be in good agreement with the data. In particular, the bubble size distribution sufficiently distant from the source is found to be independent of the gas flowrate, both in the data and in the model.

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arXiv2024

Rising and settling 2D cylinders with centre-of-mass offset

Rotational effects are commonly neglected when considering the dynamics of freely rising or settling isotropic particles. Here, we demonstrate that particle rotations play an important role for rising as well as for settling cylinders in situations when mass eccentricity, and thereby a new pendulum timescale, is introduced to the system. We employ two-dimensional simulations to study the motion of a single cylinder in a quiescent unbounded incompressible Newtonian fluid. This allows us to vary the Galileo number, density ratio, relative moment of inertia, and Centre-Of-Mass offset (COM) systematically and beyond what is feasible experimentally. For certain buoyant density ratios, the particle dynamics exhibit a resonance mode, during which the coupling via the Magnus lift force causes a positive feedback between translational and rotational motions. This mode results in vastly different trajectories with significantly larger rotational and translational amplitudes and an increase of the drag coefficient easily exceeding a factor two. We propose a simple model that captures how the occurrence of the COM offset induced resonance regime varies, depending on the other input parameters, specifically the density ratio, the Galileo number, and the relative moment of inertia. Remarkably, depending on the input parameters, resonance can be observed for centre-of-mass offsets as small as a few percent of the particle diameter, showing that the particle dynamics can be highly sensitive to this parameter.

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arXiv2023

Mass transport at gas-evolving electrodes

Direct numerical simulations are utilised to investigate mass transfer processes at gas-evolving electrodes that experience successive formation and detachment of bubbles. The gas-liquid interface is modeled employing an Immersed Boundary Method. We simulate the growth phase of the bubbles followed by their departure from the electrode surface in order to study the mixing induced by these processes. We find that the growth of the bubbles switches from a diffusion-limited mode at low to moderate fractional bubble-coverages of the electrode to reaction-limited growth dynamics at high coverages. Furthermore, our results indicate that the net transport within the system is governed by the effective buoyancy driving induced by the rising bubbles and that mechanisms commonly subsumed under the term `microconvection' do not significantly affect the mass transport. Consequently, the resulting gas transport for different bubble sizes, current densities, and electrode coverages can be collapsed onto one single curve and only depends on an effective Grashof number. The same holds for the mixing of the electrolyte when additionally taking the effect of surface blockage by attached bubbles into account. For the gas transport to the bubble, we find that the relevant Sherwood numbers also collapse onto a single curve when accounting for the driving force of bubble growth, incorporated in an effective Jakob number. Finally, linking the hydrogen transfer rates at the electrode and the bubble interface, an approximate correlation for the gas-evolution efficiency has been established. Taken together, these findings enable us to deduce parametrizations for all response parameters of the systems.

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arXiv2023

Diffusive and convective dissolution of carbon dioxide in a vertical cylindrical cell

The dissolution and subsequent mass transfer of carbon dioxide gas into liquid barriers plays a vital role in many environmental and industrial applications. In this work, we study the downward dissolution and propagation dynamics of CO2 into a vertical water barrier confined to a narrow vertical glass cylinder, using both experiments and direct numerical simulations. Initially, the dissolution of CO2 results in the formation of a CO2-rich water layer, which is denser in comparison to pure water, at the top gas-liquid interface. Continued dissolution of CO2 into the water barrier results in the layer becoming gravitationally unstable, leading to the onset of buoyancy driven convection and, consequently, the shedding of a buoyant plume. By adding sodium fluorescein, a pH-sensitive fluorophore, we directly visualise the dissolution and propagation of the CO2 across the liquid barrier. Tracking the CO2 front propagation in time results in the discovery of two distinct transport regimes, a purely diffusive regime and an enhanced diffusive regime. Using direct numerical simulations, we are able to successfully explain the propagation dynamics of these two transport regimes in this laterally strongly confined geometry, namely by disentangling the contributions of diffusion and convection to the propagation of the CO2 front.

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arXiv2022

In-situ measurements of void fractions and bubble size distributions in bubble curtains

We report the development of a novel measurement system designed to measure bubble properties in bubble curtains (i.e. planar bubble plumes) in-situ alongside acoustical measurements. Our approach is based on electrical, contact-based needle sensors in combination with an optical system. The latter is used for calibration and validation purposes. Correcting for the insensitive distance of the needle tips yields very good agreement between the two approaches in terms of the local void fraction and bubble size distributions. Finally, the system is employed to study bubble plumes evolving from three different hose types. All hoses display consistent self-similar behaviour with spreading rates increasing with increasing gas flow. The spreading is further found to be significantly higher when the bubble plumes originated from a porous hose compared to the two other hose types featuring either discrete holes or nozzle elements.

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arXiv2022

Bubble-particle collisions in turbulence: insights from point-particle simulations

Bubble-particle collisions in turbulence are central to a variety of processes such as froth flotation. Despite their importance, details of the collision process have not received much attention yet. This is compounded by the sometimes counter-intuitive behaviour of bubbles and particles in turbulence, as exemplified by the fact that they segregate in space. Although bubble-particle relative behaviour is fundamentally different from that of identical particles, the existing theoretical models are nearly all extensions of theories for particle-particle collisions in turbulence. The adequacy of these theories has yet to be assessed as appropriate data remain scarce to date. In this investigation, we study the geometric collision rate by means of direct numerical simulations of bubble-particle collisions in homogeneous isotropic turbulence using the point-particle approach over a range of the relevant parameters, including the Stokes and Reynolds numbers. We analyse the spatial distribution of bubble and particles, and quantify to what extent their segregation reduces the collision rate. This effect is countered by increased approach velocities for bubble-particle compared to monodisperse pairs, which we relate to the difference in how bubbles and particles respond to fluid accelerations. We found that in the investigated parameter range, these collision statistics are not altered significantly by the inclusion of a lift force or different drag parametrisations, or when assuming infinite particle density. Furthermore, we critically examine existing models and discuss inconsistencies therein that contribute to the discrepancy.

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