Particle laden turbulent flows find applications in many industrial processes such as energy conversion, air pollution control etc. In these types of flows, there are strong coupling between the turbulent fluctuations in the fluid velocity fields, and the fluctuating velocities of the particles. In order to analyze the stresses and the heat and mass transfer properties in turbulent suspensions, it is necessary to have a good understanding of not just the mean flow of the gas and particles, but also of the fluctuations in the two phases. The coupling is a two-way coupling; the fluid turbulence contributes to the velocity fluctuations in the particles, and conversely, the particle velocity fluctuations generate fluctuations in the fluid. Two-phase flow models capture these interactions only in an indirect way, usually through a ‘particle pressure’ term for the particle phase. In the present work the effect of fluid velocity fluctuations on the dynamics of the particles in a turbulent gas-solid suspension is analyzed in the low Reynolds number and high Stokes number limit, where the particle relaxation time is long compared to the correlation time for the fluid velocity fluctuations. The direct numerical simulation (DNS) is used for solving the Navier-Stokes equations for the fluid, the particles are modeled as hard spheres which undergo elastic collisions. A one-way coupling algorithm is used where the force exerted by the fluid on the particles is incorporated, but not the reverse force exerted by the particles on the fluid. This is because the main focus of our study is to examine the effect of the fluid turbulence on the particle fluctuations, and we are interested in examining whether a Langevin model with random forcing can accurately capture the effect of fluid turbulence on the particle phase. First, the turbulent flow in a plane Couette is analyzed. Though this is a model flow which is not encountered often in applications, it is easier to analyze because the turbulent velocity fluctuations are maximum at the center of the channel, in contrast to the Poiseuille flow, where the velocity fluctuations are maximum at a location between the center and the wall. Also, in a Couette flow, the wall-normal and the spanwise root mean square velocities are nearly a constant in the central region in the channel, and the percentage variation in the stream-wise velocity fluctuations is also less than that in a pressure driven Poiseuille flow. Therefore, it is possible to treat the central region as a region with homogeneous, but anisotropic, fluid velocity fluctuations and with a linear mean velocity variation. The particle mean and root mean square fluctuating velocities, as well as the probability distribution function for the fluid velocity fluctuations and the distribution of acceleration of the particles in the central region of the Couette, which comprises about 20% of the entire channel have been studied. It is found that the distribution of particle velocities is very different from a Gaussian, especially in the span-wise and wall-normal directions. However, the distribution of the acceleration fluctuation on the particles is found to be close to a Gaussian, though the distribution is highly anisotropic and there is a correlation between the fluctuations in the flow and gradient directions. The non-Gaussian nature of the fluid velocity fluctuations is found to be due to inter-particle collisions induced by the large particle velocity fluctuations in the flow direction. Another interesting result is a comparison of the distribution of the acceleration on a particle due to the fluid velocity fluctuation at the particle position, and the distribution of the ratio of fluid velocity fluctuation to the viscous relaxation time in the fluid. The comparison shows that these two distributions are almost identical, indicating that the fluid velocity fluctuations are not correlated over time scales comparable to the relaxation time of a particle. This result is important because it indicates that in order to model the fluctuating force on the particle, it is sufficient to obtain the variance of the force distribution from the variance of the fluid velocity distribution function. Finally, the correlation time for the acceleration correlations is calculated along the trajectory of a particle. The correlation time is found to be of the same magnitude as the correlation time for the fluid velocity in an Eulerian reference frame, and much smaller than the viscous relaxation time and the time between collisions of the particles. All of these results indicate that the effect of the turbulent fluid velocity fluctuations can be accurately represented by an anisotropic Gaussian white noise. The above results are used to formulate a ‘fluctuating force’ model for the particle phase alone, where the force exerted by the fluid turbulent velocity fluctuations is modeled as random Gaussian white noise, which is incorporated into the equation of motion for the particles. The variance of the distribution function for the fluctuating force distribution is obtained from the variance of the local turbulent fluid velocity fluctuations, assuming linear Stokes drag law. The force distribution is anisotropic, and it has a non-zero correlation between the flow and gradient directions. It is found that the results of the fluctuating force simulations are in quantitative agreement with the results of the complete DNS, both for the particle concentration and variances of the particle velocity fluctuations, at relatively low volume fractions where the viscous relaxation time is small compared to the time between collisions, as well as at higher volume fractions where the time between collisions is small compared to the viscous relaxation time. The simulations are also able to predict the velocity distributions in the center of the Couette, even in cases where the velocity distribution is very different from a Gaussian distribution. The fluctuating force model is applied to the turbulent flow of a gas-particle suspension in a vertical channel in the limit of high Stokes number. In contrast to the Couette flow analyzed the fluid velocity variances in the different directions in the channel are highly non-homogeneous, and they exhibit a significant variation across the channel. First, we analyze the fluctuating particle velocity and acceleration distributions at different locations across the channel using direct numerical simulation. The distributions are found to be non-Gaussian near the center of the channel, and they exhibit significant skewness. The time correlations of the fluid velocity fluctuations and the acceleration fluctuations on the particles are evaluated and compared. Unlike the case of Couette flow it is found that the time correlation functions for the fluid in the fixed Eulerian frame are not in agreement with the time correlation of the acceleration on the particles. However, the time correlations of the particle acceleration are in good agreement with the velocity time correlations in the fluid in a ‘moving Eulerian’ reference frame, moving with the mean velocity of the fluid. The fluctuating force simulations are used to model the particle phase, where the force on the particles due to the fluid velocity fluctuations are substituted by random white noise in the equations for the particle motion. The random noise is assumed to be Gaussian and anisotropic. The variances of the fluctuating force are calculated form the fluid velocity fluctuations in a moving Eulerian reference frame using DNS. The results from the fluctuating force simulations are then compared with the results obtained from DNS. Quantitative agreement between the two simulations are obtained provided the particle viscous relaxation time is at least five times larger than the fluid integral time. The interactions between the solid particles and the fluid turbulence have been investigated experimentally in a vertical fully developed channel flow of air and solid particles. Experiments are conducted at low volume fraction for which viscous relaxation time of the particle is expected to be lower than the particle particle collision time, as well as at moderately high volume fraction where the particle particle collision time is expected to be lower than the particle relaxation time. Velocity statistics of both the particle and gas phases are obtained using high spatial resolution Particle Image Velocimetry (PIV) system. It is observed that at low solid volume fraction, the particle root mean square velocities and the velocity distribution are in good agreement with those predicted by the fluctuating force simulation, provided the polydispersity in the particle size distribution is incorporated in the fluctuating force simulations. In this case, the modification of turbulence in the center of the channel due to the particles is small. At much higher volume fraction, the mean gas flow is significantly affected by the presence of particles, and the mean flow is no longer symmetric about the center line of the channel. Simultaneously, there is also a significant change in the volume fraction across the channel, and the volume fraction is also not symmetric about the center line. This seems to indicate that there is a spontaneous instability of the symmetric volume fraction and velocity profiles, giving rise to a region of high fluid velocity and high particle volume fraction coexisting with a region of low gas velocity and low particle volume fraction. There is some recirculation of the gas within the channel, and the gas phase turbulence intensity is significantly enhanced when the velocity and volume fraction profiles become asymmetric. As we have considered only one way coupling in the computation of the particle laden flow it is expected that the particle statistics obtained for this condition can not be predicted by our fluctuating force model due to modification of the gas phase statistics.

Polymer networks are an important class of materials that are ubiquitously found in natural, biological, and man-made systems. The complex mesoscale structure of these soft materials has made it difficult for researchers to fully explore their properties. In this dissertation, we introduce a coarse-grained computational model for permanently cross-linked polymer networks than can properly capture common properties of these materials. We use this model to study several practical problems involving dry and solvated networks. Specifically, we analyze the permeability and diffusivity of polymer networks under mechanical deformations, we examine the release of encapsulated solutes from microgel capsules during volume transitions, and we explore the complex tribological behavior of elastomers. Our simulations reveal that the network transport properties are defined by the network porosity and by the degree of network anisotropy due to mechanical deformations. In particular, the permeability of mechanically deformed networks can be predicted based on the alignment of network filaments that is characterized by a second order orientation tensor. Moreover, our numerical calculations demonstrate that responsive microcapsules can be effectively utilized for steady and pulsatile release of encapsulated solutes. We show that swollen gel capsules allow steady, diffusive release of nanoparticles and polymer chains, whereas gel deswelling causes burst-like discharge of solutes driven by an outward flow of the solvent initially enclosed within a shrinking capsule. We further demonstrate that this hydrodynamic release can be regulated by introducing rigid microscopic rods in the capsule interior. We also probe the effects of velocity, temperature, and normal load on the sliding of elastomers on smooth and corrugated substrates. Our friction simulations predict a bell-shaped curve for the dependence of the friction coefficient on the sliding velocity. Our simulations also illustrate that at low sliding velocities, the friction decreases with an increase in the temperature. Overall, our findings improve the current understanding of the behavior of polymer networks in equilibrium and non-equilibrium conditions, which has important implications for synthesizing new drug delivery agents, designing tissue engineering systems, and developing novel methods for controlling the friction of elastomers.

Behavior of particles in buoyancy driven turbulent flow at Ra ≈ 10º is investigated experimentally. The volume fraction of the particles is low enough for the inter particle influence to be neglected, the mass loading of particle is low enough that the turbulence as not modified, and the particles Reynolds numbers (Re p ) st are small enough that the wake effect can be neglected. The buoyancy driven turbulent flow is created by maintaining an unstable density difference, using NaCl dissolved in water, across the ends of a long vertical tube. There is no mean flow and the turbulence is axially homogeneous. A method for uniform introduction of the particles was devised. Glass particles (S.G=2.4-2.5) of different diameter ranges (50-400 µm) are introduced into this flow. The sizes of particles considered are less than the Kolmogrov length scale corresponding to the turbulence level. The turbulence intensity level was varied in order to match its characteristic time and velocity scale to those of the particles. The ratio of the timescales, the Stokes number; is in the range (0.01-0.55); Stokes number is defined as a ratio of the viscous relaxation time of the particle and a turbulent time scale, and represents the effect of the particle inertia in the interaction with the turbulence, Stk =τp/τk. Another important non-dimensional parameter is the velocity ratio, the k ratio of the particle settling velocity in still fluid to a characteristic turbulence velocity. The flow field is illuminated by a continuous Argon-ion laser and a PHOTRON high- speed digital camera is used for imaging. The raw images are processed to evaluate particle centers followed by their velocity measurements. The objective of the experiment is to check for the effect of the turbulent flow on the sedimentation rate of the heavy particles. This sedimentation rate is compared with the settling velocity obtained in still water. It is expected that within a certain range of Stokes numbers and velocity ratios the sedimentation rate would be substantially changed, and the spatial concentration distribution of the particles may become patchy implying that turbulence may actually inhibit rather than enhance mixing of particles. By varying the turbulence level and particle mean diameter we achieved a set of values for the particle parameters, namely St k. ≈ 0.01, 0.1, 0.14, 0.55 and velocity ratios[[Wp ] St]]≈ 0.2, .0, 0.5, 2.25 respectively. The w rms velocity ratio [[Wp ] St /wf defined as a ratio between the article terminal velocity [Wp ] St and a suitable flow velocity scale; it is a measure of the residence time of the particle in an eddy, in eddy turnover time units. In this study we have considered the turbulence r.m.s velocity for the flow velocity scale.The particle Reynolds number (Re p)st corresponding to these 4 cases were 0.2, 31.5, 4.0, 31.5. Some preliminary quantitative measurements were made only for the 150-200 µm particles and turbulence level w rms ≈ 4.0 cm/s,corresponding to Stk ≈0.14 [[Wp ] St] = 0.5. A quantitative picture was obtained for the other cases. Streak pictures for these four different groups of particles revealed that Stk and the velocity ratio [[Wp ] St ] were important in influencing the particle- w rms turbulence interaction not the Stk alone. The r.m.s velocity fluctuations of particles in both the lateral (utp) and vertical direction (wtp) measured were found to be different from those obtained in still-water case.(For equations, pl see the pdf file)

<p>We investigate the dielectrophoretic separation of microparticles. Two different models are formulated in two characteristic time scales. The first model mainly accounts for the orientation behavior and rotational motion of non-spheric microparticles. The concept of effective charge is suggested to calculate the finite size non-spheric particles. It is combined with the fluid particle dynamics method to calculate hydrodynamic as well as dielectrophoretic forces and torques. The translational motion and the particle-particle interaction are calculated also, but they take much longer time to be observed due to the different time scales of the rotational and translational motions By viewing the particle as spheres, the second model focus on the translational motion of spheres. The hydrodynamic force between particles and particle-particle electrostatic interactions are also taken into account. We check the relative magnitude ratio between these forces in order to determine the importance of these forces. To predict and guide the design of experimental dielectrophoretic separation, two numerical applications are carried out. The first calculation suggests optimum patterns to improve the trapping efficiency of<em> E.coli.</em> cells by applying superimposed AC electric fields. The second calculation finds out the mobility and separation rate of particles which differs in size and electric properties by a multi-step trapping-releasing strategy.</p>

Exposure to aerosol particles can cause health issues such as respiratory and cardiovascular diseases. Nevertheless, aerosol particle exposure with size-resolved information was seldom investigated in real-life European houses in the long term, especially for the ultrafine size range. In this work, indoor and outdoor measurements were conducted from December 2016 to March 2019. A standard of high-quality indoor and outdoor particle measurements in multi-homes was established for the first time. For more than 500 days, measurement data were collected in 40 German homes, including particle mass concentrations (PMC) of PM10, PM2.5, and PM1, particle number concentration (PNC) and size distribution, equivalent black carbon (eBC) mass concentration, CO2 concentration, and the residents’ activity log. With such novel dataset, representative patterns of diurnal and seasonal variation of particle concentration and size distribution as well as eBC mass concentration have been captured. In the warm season, diurnal cycles of indoor PMC and PNC showed weaker variation and less intense peaks (around the time of breakfast, lunch and dinner) than in the cold season, due to ventilation reducing the effect of indoor sources. To better understand the key dynamic processes of indoor particles (i.e. ventilation, building shell penetration, particle losses and emission), two commonly used quantitation methods, single parameter approach (SPA) and Indoor Aerosol Model approach (IAM), were compared and evaluated for the first time. Correction factors were derived to adjust the emission rates calculated from the simplified SPA approach, making emission rates derived from different levels of analysis mutually comparable. Results show that indoor source was the major contributor (56%) to indoor particle number exposure in investigated German residences. For the contribution of outdoors, penetration through the building envelope (26%) was higher than infiltration through open windows (15%). Burning candles and opening of window(s) led to seasonal differences in the contributions of indoor sources to residential exposure (70% and 40% in the cold and warm season, respectively). Indoor sources should be taken into consideration in future epidemiological studies and risk assessment of exposure to particle air pollution. / Aerosolpartikel können gesundheitliche Probleme wie Atemwegs- und Herz-Kreislauf-Erkrankungen verursachen. Jedoch wurde die Aerosolpartikelexposition, insbesondere für ultrafeine Partikel, in realen Europäischen Haushalten bisher nicht langfristig untersucht. Für diese Arbeit wurden Messungen im Innen- und Außenbereich von Dezember 2016 bis März 2019 durchgeführt. Erstmals wurde ein Standard für qualitativ hochwertige Innen- und Außenmessungen von Aerosolpartikeln in Wohnhäusern etabliert. An mehr als 500 Tagen wurden Messdaten verteilt über 40 deutsche Haushalte gesammelt, darunter die Partikelmassekonzentration (PMC) von PM10, PM2,5 und PM1, die Partikelanzahlkonzentration (PNC) und deren Größenverteilung, die Massekonzentration des äquivalenter schwarzer Kohlenstoff (eBC – equivalent Black Carbon) und ein Aktivitätsprotokoll der Bewohner. Mit diesem neuartigen Datensatz wurden repräsentative Muster der tages- und jahreszeitlichen Variation der Partikelkonzentration und Partikelgrößenverteilung sowie der eBC-Konzentration erstellt. Im Sommerhalbjahr zeigen sich schwächere Variationen und weniger intensive Spitzen (während der Frühstücks-, Mittags- und Abendessenszeit) in den Tageszyklen von PMC und PNC als im Winterhalbjahr. Dies ist auf die die Belüftung zurückzuführen, die den Effekt von Innenraumquellen reduziert. Zwei häufig verwendete Quantifizierungsmethoden der Schlüsselprozesse, der Single-Parameter-Ansatz (SPA) und der Indoor-Aerosol-Modellansatz (IAM), wurden zum ersten Mal quantitativ verglichen und bewertet. Es wurden Korrekturfaktoren abgeleitet, um die mit dem vereinfachten SPA-Ansatz berechneten Emissionsraten anzupassen, so dass die aus verschiedenen Analyseebenen abgeleiteten Emissionsraten miteinander vergleichbar sind. Die Ergebnisse zeigen, dass Quellen in Innenräumen den größten Beitrag (56%) zur Partikelanzahlexposition der Bewohner der untersuchten deutschen Haushalte leisten. Der Beitrag des Außenbereichs durch das Eindringen durch die Gebäudehülle (26%) ist höher als der durch offene Fenster (15%). Brennende Kerzen und das Öffnen von Fenster(n) hatten den größten Einfluss auf die saisonalen Unterschiede in der Partikelexposition. Innenraumquellen sollten in zukünftigen epidemiologischen Studien und bei der Risikobewertung Aerosolpartikelexposition berücksichtigt werden.

We present simulation results of a minimal life-like, artificial, molecular aggregate (i.e. protocell) that has been proposed by Steen Rasussen and coworkers and is currently pursued both experimentally and computationally in interdisciplinary international research projects. We develop a space-time continuous physically motivated simulation framework based on the method of dissipative particle dynamics (DPD) which we incrementally extend (most notably by chemical reactions) to cope with the needs of our model. The applicability of the method over the entire length scale of interest is reintroduced, by rejecting a concern that DPD introduces a freezing artifact for any model above the atomistic scale. This is achieved by deriving an alternative scaling procedure for interaction parameters in the model. We perform system-level simulations of the design which attempt to account for theoretical, and experimental knowledge, as well as results from other computational models. This allows us to address key issues of the replicating subsystems container, genome, and metabolism both individually and in mutual coupling. We analyze each step in the life-cycle of the molecular aggregate, and a finnal integrated simulation of the entire life-cycle is prepared. Our simulations confirm most assumptions of the theoretical designs, but also exhibit unanticipated system-level dynamics. These findings are used to revise the original design of the Los Alamos minimal protocell over the course of the analysis. The results support the hypothesis that self-replication and probably other life-like features can be achieved in systems of formerly unanticipated simplicity if these systems exploit physicochemical principles that are immanent to their physical scale.

We investigate the dielectrophoretic separation of microparticles. Two different models are formulated in two characteristic time scales. The first model mainly accounts for the orientation behavior and rotational motion of non-spheric microparticles. The concept of effective charge is suggested to calculate the finite size non-spheric particles. It is combined with the fluid particle dynamics method to calculate hydrodynamic as well as dielectrophoretic forces and torques. The translational motion and the particle-particle interaction are calculated also, but they take much longer time to be observed due to the different time scales of the rotational and translational motions By viewing the particle as spheres, the second model focus on the translational motion of spheres. The hydrodynamic force between particles and particle-particle electrostatic interactions are also taken into account. We check the relative magnitude ratio between these forces in order to determine the importance of these forces. To predict and guide the design of experimental dielectrophoretic separation, two numerical applications are carried out. The first calculation suggests optimum patterns to improve the trapping efficiency of E.coli. cells by applying superimposed AC electric fields. The second calculation finds out the mobility and separation rate of particles which differs in size and electric properties by a multi-step trapping-releasing strategy. / QC 20101118

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La tension interfaciale est une grandeur physico-chimique d'intérêt pour de nombreuses industries et notamment l'industrie pétrolière. Cette grandeur est l'un des paramètres qui permet d'optimiser le rendement d'un puits de pétrole. La difficulté liée à sa mesure dans les conditions réservoirs a amené à étudier les systèmes eau/tensioactif/huile par simulation moléculaire. Ce travail a permis de montrer que la Dynamique Particulaire Dissipative (DPD) était un outil adapté pour l’étude de systèmes eau/tensioactif/huile sous différents aspects, de la caractérisation de la structure des interfaces au calcul de la tension interfaciale. Cette thèse a permis la démonstration de l’influence non-négligeable de la variation des paramètres de la force harmonique, l’amplitude K et la distance d’équilibre r0 , sur le calcul de la tension interfaciale et sur la structure des interfaces à forte concentration en tensioactif. En effet, la structure des tensioactifs aux interfaces est le résultat d’une balance subtile entre les forces intra et inter moléculaire. L’étude d’une population modèle de tensioactifs non chargés a permis de montrer que la DPD reproduit bien l'évolution de la tension interfaciale en fonction de la concentration en tensioactif en solution et en fonction du coefficient de partage de tensioactifs modèles non chargés. Une méthodologie est proposée pour caractériser les systèmes contenant des interfaces et où la tension interfaciale est calculée.Des travaux prospectifs ont permis de montrer que la DPD permettait d'étudier des phénomènes liés à la tension interfaciale comme le mûrissement d'Ostwald dans les émulsions d'huile dans l'eau. Ces derniers travaux ouvrent la voie à l’étude d’autres systèmes d’intérêt pour le milieu pétrolier comme le décollement de gouttes de pétroles adsorbées sur des parois ou l’étude d’émulsions pétrolières. / The interfacial tension is a physical-chemical property that numerous industrial areas have an interest of especially the petroleum industry. This property is one of the many which helps to optimize production wells' rate of return. Measuring that property in reservoir's conditions (high pressure and temperature) is highly difficult and led to study water/surfactant/oil systems using molecular modeling. The difficulty to measure that specific physical-chemical property linked to the pressure and temperature conditions in the reservoirs led the scientists to study water/surfactant/oil systems using molecular modeling. This thesis establishes that the Dissipative Particle Dynamics (DPD) is able to study water/surfactant/oil systems. The study of the effect of the variation of the harmonic force's parameters, namely the force constant K and the equilibrium distance r0, demonstrated that their variation can heavily influence the interfacial tension computation. Actually, a subtle balance exists between the intra and inter-molecular interactions, which influences the local structure of the surfactants at the oil-water interface, modifies the interfacial tension and influences the interface stability. It was demonstrated that DPD reproduces the variation of interfacial tension with the bulk surfactant concentration and the effect of the variation of hydrophobicity of models of un-charged surfactants on interfacial tension by mean of their coefficient partition. We established a method to properly study systems containing interfaces where interfacial tension is computed. Prospective work showed that DPD was a good tool to study microscopic phenomenon which can be observed macroscopically like the Ostwald ripening in oil in water emulsions. This is a first step before studying others systems of interest for the petroleum industry such as oil/water emulsion or the adsorption of oil droplets on rock wall.