Bubbly flows can be found in many applications in chemical, biological and power engineering. Reliable simulation tools of such flows that allow the design of new processes and optimization of existing one are therefore highly desirable. CFD-simulations applying the multi-fluid approach are very promising to provide such a design tool for complete facilities. In the multi-fluid approach, however, closure models have to be formulated to model the interaction between the continuous and dispersed phase. Due to the complex nature of bubbly flows, different phenomena have to be taken into account and for every phenomenon different closure models exist. Therefore, reliable predictions of unknown bubbly flows are not yet possible with the multi-fluid approach. A strategy to overcome this problem is to define a baseline model in which the closure models including the model constants are fixed so that the limitations of the modeling can be evaluated by validating it on different experiments. Afterwards, the shortcomings are identified so that the baseline model can be stepwise improved without losing the validity for the already validated cases. This development of a baseline model is done in the present work by validating the baseline model developed at the Helmholtz-Zentrum Dresden-Rossendorf mainly basing on experimental data for bubbly pipe flows to bubble columns, bubble plumes and airlift reactors that are relevant in chemical and biological engineering applications. In the present work, a large variety of such setups is used for validation. The buoyancy driven bubbly flows showed thereby a transient behavior on the scale of the facility. Since such large scales are characterized by the geometry of the facility, turbulence models cannot describe them. Therefore, the transient simulation of bubbly flows with two equation models based on the unsteady Reynolds-averaged Navier–Stokes equations is investigated. In combination with the before mentioned baseline model these transient simulations can reproduce many experimental setups without fitting any model. Nevertheless, shortcomings are identified that need to be further investigated to improve the baseline model. For a validation of models, experiments that describe as far as possible all relevant phenomena of bubbly flows are needed. Since such data are rare in the literature, CFD-grade experiments in an airlift reactor were conducted in the present work. Concepts to measure the bubble size distribution and liquid velocities are developed for this purpose. In particular, the liquid velocity measurements are difficult; a sampling bias that was not yet described in the literature is identified. To overcome this error, a hold processor is developed. The closure models are usually formulated based on single bubble experiments in simplified conditions. In particular, the lift force was not yet measured in low Morton number systems under turbulent conditions. A new experimental method is developed in the present work to determine the lift force coefficient in such flow conditions without the aid of moving parts so that the lift force can be measured in any chemical system easily.

<div>Studying the motion of cells and investigating their migration patterns in inertial microchannels have been of great interest among researchers because of their numerous biological applications such as sorting, separating, and filtering them. A great drawback in conventional microfluidics is the inability to focus extremely small biological particles and pathogens in the order of sub-micron and nanometers due to the requirement of designing an impractically elongated microchannel, which could be in the order of a few meters in extreme cases. This restriction is because of the inverse correlation between the cube of the particle size and the theoretically required channel length. Exploiting an oscillatory flow is one solution to this issue where the total distance that the particle needs to travel to focus is virtually extended beyond the physical length of the device. Due to the present symmetry in such flow, the directions of the lift forces acting on the particle remain the same, making the particle focusing feasible. </div><div><br></div><div>Here, we present results of simulation of such oscillatory flows of a single capsule in a rectangular microchannel containing a Newtonian fluid. A 3D front-tracking method has been implemented to numerically study the dynamics of the capsule in the channel of interest. Several cases have been simulated to quantify the influence of the parameters involved in this problem such as the channel flow rate, capsule deformability, frequency of oscillation, and the type of applied mechanism for inducing flow oscillations. In all cases, the capsule blockage ratio and the initial location are the same, and it is tracked until it reaches its equilibrium position. The capability to focus the capsule in a short microchannel with oscillatory flow has been observed for capsule deformabilities and mechanisms to induce the oscillations used in our study. Nevertheless, there is a limit to the channel flow rate beyond which, there is no single focal point for the capsule. Another advantage of having an oscillatory microchannel flow is the ability to control the capsule focal point by changing the oscillation frequency according to the cases presented in the current study. The capsule focusing point also depends on its deformability, flow rate, and the form of the imposed periodic pressure gradient; more deformable capsules with lower maximum velocity focus closer to the channel center. Also, the difference between the capsule equilibrium point in steady and oscillatory flows is affected by the capsule stiffness and the device flow rate. Furthermore, increasing the oscillation frequency, capsule rigidity, and system flow rate shorten the essential device length. </div><div><br></div><div>Although the oscillation frequency can provide us with new particle equilibrium positions, especially ones between the channel center and wall that can be very beneficial for separation purposes, it has the shortcoming of having a zero net throughput. To address this restriction, a steady component has been added to the formerly defined oscillatory flow to make it pulsating. Furthermore, this type of flow adds more new equilibrium points because it behaves similarly to a pure oscillatory flow with an equivalent frequency in that regard. They also enable the presence of droplets at high Ca or Re that could break up in the steady or a very low-frequency regime. Therefore, we perform new numerical simulations of a deformable droplet suspended in steady, oscillatory, and pulsating microchannel flows. We have observed fluctuations in the trajectory of the drop and have shown that the amplitude of these oscillations, the average of the oscillatory deformation, and the average migration velocity decrease by increasing the frequency. The dependence of the drop focal point on the shape of the velocity profile has been investigated as well. It has been explored that this equilibrium position moves towards the wall in a plug-like profile, which is the case at very high frequencies. Moreover, due to the expensive cost of these simulations, a recursive version of the Multi Fidelity Gaussian processes (MFGP) has been used to replace the numerous high-fidelity (or fine-grid) simulations that cannot be afforded numerically. The MFGP algorithm is used to predict the equilibrium distance of the drop from the channel center for a wide range of the input parameters, namely Ca and frequency, at a constant Re. It performs exceptionally well by having an average R^2 score of 0.986 on 500 random test sets.</div><div><br></div><div>The presence of lift forces is the main factor that defines the dynamics of the drop in the microchannel. The last part of this work will be dedicated to extracting the active lift force profiles and identify their relationships with the parameters involved to shed light on the underlying physics. This will be based on a novel methodology that solely depends on the drop trajectory. Assuming a constant Re, we then compare steady lift forces at different Ca numbers and oscillatory ones at the same constant Ca. We will then define analytical equations for the obtained lift profiles using non-linear regression and predict their key coefficients over a continuous range of inputs using MFGP.</div>

Part I: The present work is to calculate dimensionless three-dimensional universal phase and lift force diagrams for a microbubble or pendant drop in a static liquid on a solid surface or orifice. Studying microbubble dynamics is important due to its controlling mass, momentum, energy and concentration transfer rates encountered in micro- and nano-sciences and technologies. In this work, dimensionless phase and force diagrams are presented by applying an equation for microbubble shape to accuracy of the second order of small Bond number provided by O¡¦Brien (1991). Two dimensionless independent parameters, Bond number and contact angle (or base radius), are required to determine dimensionless phase and force diagrams governing static and dynamic states of a microbubble. The phase diagram divides the microbubble surface into three regions, the apex to inflection, inflection to neck, and neck to the edge of microbubble. The growth, collapse, departure and entrapment of a microbubble on a surface thus can be described. The lift forces include hydrostatic buoyancy, difference in gas and hydrostatic pressures at the microbubble base, capillary pressure and surface tension resulted from variation of circumference. The force to attach the microbubble to solid surface is the surface tension resulted from variation of circumference, which is not accounted for in literature. Adjusting the base radius to control static and dynamic behaviors of a microbubble is more effective than Bond number. Part II: Controlling states and growth of a microscale bubble (or pendant drop) in a static liquid on a surface by introducing general phase diagrams is proposed. Microbubbles are often used to affect transport phenomena in micro- and nano-technologies. In this work, a general phase diagram is provided by applying a perturbation solution of Young-Laplace equation for bubble shape with truncation errors of the second power of small Bond number. The three-dimensional phase diagram for a given Bond number is uniquely described by the dimensionless radius of curvature at the apex, contact angle and base radius of the microbubble. Provided that initial and end states are chosen, adjusting two of them gives the desired states and growth, decay and departure of the bubble described by path lines in the phase diagram. A universal three-dimensional phase diagram for a microbubble is also introduced.

Il est bien connu que les particules inertielles dans un écoulement périodique ont tendance à se focaliser sur des trajectoires privilégiées. Le but de ce travail de thèse est d'étudier l'influence de cette focalisation sur le transport et la sédimentation de particules dans une fracture plane à rugosité périodique. Tout d'abord, un écoulement monophasique dans une fracture est analysé asymptotiquement dans le cas de faible rugosité. Les résultats classiques de la théorie de la lubrification inertielle sont généralisés au cas de fractures avec des parois asymétriques. Les corrections non linéaires à la loi de Darcy sont calculées explicitement en fonction des facteurs géométriques de la fracture. Le transport de particules dans une fracture horizontal est étudié asymptotiquement dans le cas de particules de faible inertie. Les particules se focalisent sur une trajectoire attractrice, si le débit d'écoulement est assez fort par rapport à la gravité. Un diagramme complet de focalisation a été obtenu, qui prédit l'existence de l'attracteur en fonction du nombre de Froude et des facteurs géométriques de la fracture. Les paramètres quantitatifs du transport ont été calculés également. L'influence de la force de portance sur la migration de particules a été étudiée également. Dans un canal vertical, la portance (provoquée par la gravité) modifie le nombre d'attracteurs et leurs positions. En absence de gravité, la portance peut provoquer une dynamique chaotique des particules. En outre, le captage des particules par une paire de tourbillons a été étudié. Le diagramme d'accumulation obtenu démontre que toute paire de tourbillons peut être un piège à particules / It is well-known that inertial particles tend to focus on preferential trajectories in periodic flows. The goal of this thesis was to study the joint effect of particle focusing and sedimentation on their transport through a model 2D fracture with a periodic corrugation. First, single-phase flow though the fracture has been considered: the classical results of the inertial lubrication theory are revisited in order to include asymmetric fracture geometries. Cubic corrections to Darcy's law have been found analytically and expressed in terms of two geometric factors, describing channel geometry. For weakly-inertial particles in a horizontal channel it has been shown that, when inertia is strong enough to balance out the gravity forces, particles focus to some attracting trajectory inside the channel. The full trapping diagram is obtained, that predicts the existence of such attracting trajectory regime depending on the Froude number and on geometric factors. Numerical simulations confirm the asymptotic results for particles with small response times. The influence of the lift force on particle migration has also been studied. In a vertical channel the lift is induced by gravity and leads to complex trapping diagrams. In the absence of gravity the lift is caused by inertial lead/lag of particles and can lead to chaotic particle dynamics. Finally, for dust particles in a vortex pair it has been shown that particles can be trapped into one or two equilibrium points in a reference frame rotating with the vortices. A full trapping diagram has been obtained, showing that any pair of vortices can trap particles, independently of their strength ratio and the direction of rotation

Determining the hydrodynamic force acting on bubbles and particles moving parallel to a wall in a shear flow is a problem of fundamental importance, as this configuration is involved in a variety of technical and natural systems. The presence of the wall tends to increase the drag force, and more importantly causes a transverse lift force acting on the body. This thesis focuses on extending the current capability in predicting drag and lift forces on spherical bubbles and particles translating in a linear shear flow, primarily in the vicinity of a wall, and obtaining quantitative insight into the interaction mechanisms at work in the context of finite sphere Reynolds number. The investigations are performed through direct numerical simulation (DNS) using an accurate finite volume method. The first part of the thesis summarizes all expressions for the drag and lift forces available in the literature. A comprehensive review of existing results from analytical, experimental, and direct numerical simulation studies is given. The available correlations are critically assessed by comparison to data from these studies. Based on the comparison, recommendations are given which correlations to use including some new proposals, and gaps in the data are identified. The second part aims to fill the gaps mentioned above by means of DNS. Specifically, the three-dimensional flow around a non-rotating sphere translating steadily in a wall-bounded linear shear flow is investigated by solving the full Navier-Stokes equations. Numerical results and analytical expressions are combined to provide accurate semi-empirical expressions for the drag and lift forces at arbitrary Reynolds number and separation distance. Present numerical results help to rationalize and quantify the various mechanisms at work and the ways they interact. From a practical point of view, they also result in several closure models for the drag correction and transverse force, which are necessary inputs in the point-particle based Eulerian-Lagrangian simulations or in Eulerian-Eulerian simulations based on the interpenetrating continua concept.:1 INTRODUCTION 1.1 Background 1.2 Underlining mechanisms 1.3 State of the art 1.4 Motivation, goal and outline of the thesis 2 STATE OF THE ART 2.1 Statement of the problem 2.2 Overview of literatures 2.3 Unbounded linear shear flow 2.4 Linear shear flow with the wall lying in the inner region 2.5 Stagnant flow with the wall lying in the outer region 2.6 Linear shear flow with the wall lying in the outer region 2.7 Conclusions 3 NUMERICAL APPROACH AND PRELIMINARY TESTS 3.1 Numerical approach 3.2 Preliminary tests 4 CLEAN SPHERICAL BUBBLE IN WALL-BOUNDED FLOW 4.1 Characteristics of the flow field and fundamental mechanisms 4.2 Hydrodynamic forces on the bubble: fluid at rest at infinity 4.3 Hydrodynamic forces on the bubble: linear shear flow 4.4 Conclusions 5 RIGID SPHERE IN WALL-BOUNDED FLOW 5.1 Characteristics of the flow field and fundamental mechanisms 5.2 Hydrodynamic forces on the sphere: fluid at rest at infinity 5.3 Hydrodynamic forces on the sphere: Linear shear flow 5.4 Conclusions 6 CONCLUSIONS AND FUTURE WORK 6.1 Summary and conclusions 6.2 Future work 7 REFERENCE

Multiphase flows abound in nature and enterprises. Our daily interactions with fluids - washing, drinking, and cooking, for example - occur at a free surface and within the realm of multiphase flows. The applications of multiphase flows within the context of emulsions, which are caused by mixing two immiscible fluids, have been of interest since the nineteenth century: compartmentalizing one fluid in another is particularly of interest in applications in pharmaceutical, materials, microfluidics, chemical, and biological engineering. Even more control in compartmentalization and delivery can be obtained through the usage of double emulsions, which are emulsions of smaller drops (i.e., inner drop) within larger drops (i.e., outer drop). The goal of this work is to understand the dynamic behavior of compound drops in confined flow at low Reynolds numbers. These behaviors include the migration patterns, limit cycles, and equilibrium locations in confined flows such as channel flows.<br> <br>Firstly, we look at non-concentric compound drops that are subject to simple shear flows. The eccentricity in the inner drop is either within the place of shear, normal to the plane of shear, or mixed. We show unreported motions that persist throughout time regardless of the initial eccentricity, given that the deformations of the inner and outer drops are small. Understanding the temporal dynamics of compound drops within the simple shear flow, one of the simplest background flows that may be imposed, allows us to probe at the dynamics of more complicated background flows.<br> <br>Secondly, we look at the lateral migration of compound drops in a Poiseuille flow. Depending on the initial condition, we show that there are multiple equilibria. We also show that the majority of initial configurations results in the compound drop with symmetry about the short wall direction. We then show the time it takes for the interfaces to merge if a given initial configuration does not reach the aforementioned symmetry.<br> <br>Thirdly, while the different equilibria of compound drops offer some positional differences at different radii ratio, we show that the lift force profiles at non-equilibrium locations offer distinctly different results for compound drops with different radii ratio. We then look at how this effect is greater than changes that arise due to viscosity ratio changes, and offer insights on what may create such a change in the lift force profile.

Early in the ship design process, naval architects must often evaluate and compare multiple hull forms for a specific set of requirements. Analytical tools are useful for quick comparisons, but they usually specialize in a specific hull type and are therefore not adequate for comparing dissimilar hull types. Scale model hydrodynamic testing is the traditional evaluation method, and is applicable to most hull forms. Scale model tests are usually performed on the largest model possible in order to achieve the most accurate performance predictions. However, such testing is very resource intensive, and is therefore not a cost effective method of evaluating multiple hull forms. This thesis explores the testing of small scale models. It is hypothesized that although the data acquired by these tests will not be accurate enough for performance predictions, they will be accurate enough to rank the performance of the multiple hull forms being evaluated.

The aim of the dissertation thesis is the control of the tube bundle on the cyclic fatigue caused by the flow past tube bundle. Fatigue due to flow is caused by flow-induced vibrations. Examined vibrations are caused by the mutual interaction of two phases (solid and liquid). The present work is focused mainly on the interaction of tube bundles with fluid. The current level of knowledge in this field allows to predict mainly static respectively quazi-static loading. These predictions are based on methods of comparing key vibration variables such as frequencies, amplitudes or speeds (see TEMA [1]). In this way, it is possible to determine quickly and relatively precisely the occurrence of a vibrational phenomenon, but it is not possible to quantitatively assess the effect of these vibrations on the damage of to the tube beam and to predict its lifespan, which would require the determination of the temperature field and the distribution of forces from the fluid on the beam. The aim of the work is to evaluate the-state-of-the-art, to perform a numerical simulation of the flow of fluids in the area of shell side under the inlet nozzle. Current methods of numerical analyses very well solve this problem, but at the expense of computing time, devices and expensive licences. The benefit of this work is the use of user-defined function (UDF) as a method for simulating interaction with fluid and structure in ANSYS Fluent software. This work places great emphasis on using the current state of knowledge for verifying and validation. Verifying and validation of results include, for example, experimentally measured Reynolds and Strouhal numbers, the drag coefficients and for example magnitude of pressure coefficient around the tube. At the same time, it uses the finite element method as a tool for the stress-strain calculation of a key part on tube such as a pipe-tube joint. Another benefit of this work is the extension of the graphical design of heat exchanger according to Poddar and Polley by vibration damages control according to the method described in TEMA [1]. In this section, the author points out the enormous influence of flow velocity on both the tube side and the shell side for design of the heat exchanger to ensure faultless operation. As an etalon of damage, the author chose a heat exchanger designated 104 from the Heat Exchanger Tube Vibration Data Bank [3]. With this heat exchanger, vibrational damage has been proven to be due to cutting of the tubes over the baffles. The last part outlines the possibilities and limits of further work.