Analysis of Meshfree Methods for Lagrangian Fluid-Structure Interaction

In this dissertation a new numerical method for solving Fluid-Structure Interaction (FSI) problems in a Lagrangian framework is developed, where solids of different constitutive laws can suffer very large deformations and fluids are considered to be newtonian and incompressible. For that, we first introduce a meshless discretization based on local maximum-entropy interpolants. This allows to discretize a spatial domain with no need of tessellation, avoiding the mesh limitations. Later, the Stokes flow problem is studied. The Galerkin meshless method based on a max-ent scheme for this problem suffers from instabilities, and therefore stabilization techniques are discussed and analyzed. An unconditionally stable method is finally formulated based on a Douglas-Wang stabilization. Then, a Langrangian expression for fluid mechanics is derived. This allows us to establish a common framework for fluid and solid domains, such that interaction can be naturally accounted. The resulting equations are also in the need of stabilization, what is corrected with an analogous technique as for the Stokes problem. The fully Lagrangian framework for fluid/solid interaction is completed with simple point-to-point and point-to-surface contact algorithms. The method is finally validated, and some numerical examples show the potential scope of applications.

Study of Tsunami-Induced Fluid and Debris Load on Bridges using the Material Point Method

Thesis (Ph.D.)--University of Washington, 2016-08

Solid-liquid interactions in microscale structures and devices

Liquid-solid interactions become important as dimensions approach mciro/nano-scale. This dissertation focuses on liquid-solid interactions in two distinct applications: capillary driven self-assembly of thin foils into 3D structures, and droplet wetting of hydrophobic micropatterned surfaces. The phenomenon of self-assembly of complex structures is common in biological systems. Examples include self-assembly of proteins into macromolecular structures and self-assembly of lipid bilayer membranes. The principles governing this phenomenon have been applied to induce self-assembly of millimeter scale Si thin films into spherical and other 3D structures, which are then integrated into light-trapping photovoltaic (PV) devices. Motivated by this application, we present a generalized analytical study of the self-folding of thin plates into deterministic 3D shapes, through fluid-solid interactions, to be used as PV devices. This study consists of developing a model using beam theory, which incorporates the two competing components — a capillary force that promotes folding and the bending rigidity of the foil that resists folding into a 3D structure. Through an equivalence argument of thin foils of different geometry, an effective folding parameter, which uniquely characterizes the driving force for folding, has been identified. A criterion for spontaneous folding of an arbitrarily shaped 2D foil, based on the effective folding parameter, is thus established. Measurements from experiments using different materials and predictions from the model match well, validating the assumptions used in the analysis. As an alternative to the mechanics model approach, the minimization of the total free energy is employed to investigate the interactions between a fluid droplet and a flexible thin film. A 2D energy functional is proposed, comprising the surface energy of the fluid, bending energy of the thin film and gravitational energy of the fluid. Through simulations with Surface Evolver, the shapes of the droplet and the thin film at equilibrium are obtained. A critical thin film length necessary for complete enclosure of the fluid droplet, and hence successful self-assembly into a PV device, is determined and compared with the experimental results and mechanics model predictions. The results from the modeling and energy approaches and the experiments are all consistent. Superhydrophobic surfaces, which have unique properties including self-cleaning and water repelling are desired in many applications. One excellent example in nature is the lotus leaf. To fabricate these surfaces, well designed micro/nano- surface structures are often employed. In this research, we fabricate superhydrophobic micropatterned Polydimethylsiloxane (PDMS) surfaces composed of micropillars of various sizes and arrangements by means of soft lithography. Both anisotropic surfaces, consisting of parallel grooves and cylindrical pillars in rectangular lattices, and isotropic surfaces, consisting of cylindrical pillars in square and hexagonal lattices, are considered. A novel technique is proposed to image the contact line (CL) of the droplet on the hydrophobic surface. This technique provides a new approach to distinguish between partial and complete wetting. The contact area between droplet and microtextured surface is then measured for a droplet in the Cassie state, which is a state of partial wetting. The results show that although the droplet is in the Cassie state, the contact area does not necessarily follow Cassie model predictions. Moreover, the CL is not circular, and is affected by the micropatterns, in both isotropic and anisotropic cases. Thus, it is suggested that along with the contact angle — the typical parameter reported in literature quantifying wetting, the size and shape of the contact area should also be presented. This technique is employed to investigate the evolution of the CL on a hydrophobic micropatterned surface in the cases of: a single droplet impacting the micropatterned surface, two droplets coalescing on micropillars, and a receding droplet resting on the micropatterned surface. Another parameter which quantifies hydrophobicity is the contact angle hysteresis (CAH), which indicates the resistance of the surface to the sliding of a droplet with a given volume. The conventional methods of using advancing and receding angles or tilting stage to measure the resistance of the micropatterned surface are indirect, without mentioning the inaccuracy due to the discrete and stepwise motion of the CL on micropillars. A micronewton force sensor is utilized to directly measure the resisting force by dragging a droplet on a microtextured surface. Together with the proposed imaging technique, the evolution of the CL during sliding is also explored. It is found that, at the onset of sliding, the CL behaves as a linear elastic solid with a constant stiffness. Afterwards, the force first increases and then decreases and reaches a steady state, accompanied with periodic oscillations due to regular pinning and depinning of the CL. Both the maximum and steady state forces are primarily dependent on area fractions of the micropatterned surfaces in our experiment. The resisting force is found to be proportional to the number of pillars which pin the CL at the trailing edge, validating the assumption that the resistance mainly arises from the CL pinning at the trailing edge. In each pinning-and-depinning cycle during the steady state, the CL also shows linear elastic behavior but with a lower stiffness. The force variation and energy dissipation involved can also be determined. This novel method of measuring the resistance of the micropatterned surface elucidates the dependence on CL pinning and provides more insight into the mechanisms of CAH.

Multiphase flow in the vascular system of wood: From microscopic exploration to 3-D Lattice Boltzmann experiments

This paper provides insights into liquid free water dynamics in wood vessels based on Lattice Boltzmann experiments. The anatomy of real wood samples was reconstructed from systematic 3-D analyses of the vessel contours derived from successive microscopic images. This virtual vascular system was then used to supply fluid-solid boundary conditions to a two-phase Lattice Boltzmann scheme and investigate capillary invasion of this hydrophilic porous medium. Behavior of the liquid phase was strongly dependent on anatomical features, especially vessel bifurcations and reconnections. Various parameters were examined in numerical experiments with ideal vessel bifurcations, to clarify our interpretation of these features. (c) 2010 Elsevier Ltd. All rights reserved.

The kinetics of NO and N2O reduction over coal chars in fluidised-bed combustion

This paper presents a comprehensive and critical review of the mechanisms and kinetics of NO and N2O reduction reaction with coal chars under fluidised-bed combustion conditions (FBC). The heterogeneous reactions of NO and N2O with char/carbon surface have been well recognised as the most important processes in reducing both NOx and N2O in situ FBC. Compared to NO-carbon reactions in FBC, the reactions of N2O with chars have been relatively less understood and studied. Beginning with the overall reaction schemes for both NO and N2O reduction, the paper extensively discusses the reaction mechanisms including the effects of active surface sites. Generally, NO- and N2O-carbon reactions follow a series of step reactions. However, questions remain concerning the role of adsorbed phases of NO and N2O, and the behaviour of different surface sites. Important kinetics factors such as the rate expressions, kinetics parameters as well as the effects of surface area and pore structure are discussed in detail. The main factors influencing the reduction of NO and N2O in FBC conditions are the chemical and physical properties of chars, and the operating parameters of FBC such as temperature, presence of CO, O-2 and pressure. It is shown that under similar conditions, N2O is more readily reduced on the char surface than NO. Temperature was found to be a very important parameter in both NO and N2O reduction. It is generally agreed that both NO- and N2O-carbon reactions follow first-order reaction kinetics with respect to the NO and N2O concentrations. The kinetic parameters for NO and N2O reduction largely depend on the pore structure of chars. The correlation between the char surface area and the reactivities of NO/N2O-char reactions is considered to be of great importance to the determination of the reaction kinetics. The rate of NO reduction by chars is strongly enhanced by the presence of CO and O-2, but these species may not have significant effects on the rate of N2O reduction. However, the presence of these gases in FBC presents difficulties in the study of kinetics since CO cannot be easily eliminated from the carbon surface. In N2O reduction reactions, ash in chars is found to have significant catalytic effects, which must be accounted for in the kinetic models and data evaluation. (C) 1997 Elsevier Science Ltd.

Reactivity of chars and carbons: New insights through molecular modeling

A new model proposed for the gasification of chars and carbons incorporates features of the turbostratic nanoscale structure that exists in such materials. The model also considers the effect of initial surface chemistry and different reactivities perpendicular to the edges and to the faces of the underlying crystallite planes comprising the turbostratic structure. It may be more realistic than earlier models based on pore or grain structure idealizations when the carbon contains large amounts of crystallite matter. Shrinkage of the carbon particles in the chemically controlled regime is also possible due to the random complete gasification of crystallitic planes. This mechanism can explain observations in the literature of particle size reduction. Based on the model predictions, both initial surface chemistry and the number of stacked planes in the crystallites strongly influence the reactivity and particle shrinkage. Its test results agree well with literature data on the air-oxidation of Spherocarb and show that it accurately predicts the variation of particle size with conversion. Model parameters are determined entirely from rate measurements.

Percolative fragmentation of char particles during gasification

Percolative fragmentation was confirmed to occur during gasification of three microporous coal chars. Indirect evidence obtained by the variation of electrical resistivity (ER) with conversion was supported by direct observation of numerous fragments during gasification. The resistivity increases slowly at low conversions and then sharply after a certain conversion value, which is a typical percolation phenomenon suggesting the occurrence of internal fragmentation at high conversion. Two percolation models are applied to interpret the experimental data and determine the percolation threshold. A percolation threshold of 0.02-0.07 was found, corresponding to a critical conversion of 92-96% for fragmentation. The electrical resistivity variation at high conversions is found to be very sensitive to diffusional effects during gasification. Partially burnt samples with a narrow initial particle size range were also observed microscopically, and found to yield a large number of small fragments even when the particles showed no disintegration and chemical control prevailed. It is proposed that this is due to the separation of isolated clusters from the particle surface. The particle size distribution of the fragments was essentially independent of the reaction conditions and the char type, and supported the prediction by percolation theory that the number fraction distribution varies linearly with mass in a log-log plot. The results imply that perimeter fragmentation would occur in practical combustion systems in which the reactions are strongly diffusion affected.

A modified pore-filling isotherm for liquid-phase adsorption in activated carbon

This article modifies the usual form of the Dubinin-Radushkevich pore-filling model for application to liquid-phase adsorption data, where large molecules are often involved. In such cases it is necessary to include the repulsive part of the energy in the micropores, which is accomplished here by relating the pore potential to the fluid-solid interaction potential. The model also considers the nonideality of the bulk liquid phase through the UNIFAC activity coefficient model, as well as structural heterogeneity of the carbon. For the latter the generalized adsorption integral is used while incorporating the pore-size distribution obtained by density functional theory analysis of argon adsorption data. The model is applied here to the interpretation of aqueous phase adsorption isotherms of three different esters on three commercial activated carbons. Excellent agreement between the model and experimental data is observed, and the fitted Lennard-Jones size parameter for the adsorbate-adsorbate interactions compares well with that estimated from known critical properties, supporting the modified approach. On the other hand, the model without consideration of bulk nonideality, or when using classical models of the characteristic energy, gives much poorer bts of the data and unrealistic parameter values.

Multicomponent supercritical adsorption in microporous activated carbon materials

In this paper, a new technique for predicting multicomponent adsorption equilibria of supercritical fluids in microporous carbons is presented. In difference from adsorption on a surface, which is a function of the fluid-solid interaction only, adsorption in porous media is influenced by the proximity of the pore walls, resulting in the enhancement in adsorption affinity. The degree of this enhancement is different for different adsorbates, and it increases with a decrease in pore size. The theory is applied to a number of carbonaceous systems with good success.

Modeling of phase equilibrium and vapor adsorption on carbon black based on a combination of a lattice theory and equation of state

A thermodynamic approach is developed in this paper to describe the behavior of a subcritical fluid in the neighborhood of vapor-liquid interface and close to a graphite surface. The fluid is modeled as a system of parallel molecular layers. The Helmholtz free energy of the fluid is expressed as the sum of the intrinsic Helmholtz free energies of separate layers and the potential energy of their mutual interactions calculated by the 10-4 potential. This Helmholtz free energy is described by an equation of state (such as the Bender or Peng-Robinson equation), which allows us a convenient means to obtain the intrinsic Helmholtz free energy of each molecular layer as a function of its two-dimensional density. All molecular layers of the bulk fluid are in mechanical equilibrium corresponding to the minimum of the total potential energy. In the case of adsorption the external potential exerted by the graphite layers is added to the free energy. The state of the interface zone between the liquid and the vapor phases or the state of the adsorbed phase is determined by the minimum of the grand potential. In the case of phase equilibrium the approach leads to the distribution of density and pressure over the transition zone. The interrelation between the collision diameter and the potential well depth was determined by the surface tension. It was shown that the distance between neighboring molecular layers substantially changes in the vapor-liquid transition zone and in the adsorbed phase with loading. The approach is considered in this paper for the case of adsorption of argon and nitrogen on carbon black. In both cases an excellent agreement with the experimental data was achieved without additional assumptions and fitting parameters, except for the fluid-solid potential well depth. The approach has far-reaching consequences and can be readily extended to the model of adsorption in slit pores of carbonaceous materials and to the analysis of multicomponent adsorption systems. (C) 2002 Elsevier Science (USA).

Mixed gas equilibrium adsorption on zeolites and energetic heterogeneity of adsorption volume

A model for binary mixture adsorption accounting for energetic heterogeneity and intermolecular interactions is proposed in this paper. The model is based on statistical thermodynamics, and it is able to describe molecular rearrangement of a mixture in a nonuniform adsorption field inside a cavity. The Helmholtz free energy obtained in the framework of this approach has upper and lower limits, which define a permissible range in which all possible solutions will be found. One limit corresponds to a completely chaotic distribution of molecules within a cavity, while the other corresponds to a maximum ordered molecular structure. Comparison of the nearly ideal O-2-N-2-zeolite NaX system at ambient temperature with the system Of O-2-N-2-zeolite CaX at 144 K has shown that a decrease of temperature leads to a molecular rearrangement in the cavity volume, which results from the difference in the fluid-solid interactions. The model is able to describe this behavior and therefore allows predicting mixture adsorption more accurately compared to those assuming energetic uniformity of the adsorption volume. Another feature of the model is its ability to correctly describe the negative deviations from Raoult's law exhibited by the O-2-N-2-CaX system at 144 K. Analysis of the highly nonideal CO2-C2H6-zeolite NaX system has shown that the spatial molecular rearrangement in separate cavities is induced by not only the ion-quadrupole interaction of the CO2 molecule but also the significant difference in molecular size and the difference between the intermolecular interactions of molecules of the same species and those of molecules of different species. This leads to the highly ordered structure of this system.

Schistosomal myeloradiculopathy in a low-prevalence area: 27 cases (14 autochthonous) in Campinas, São Paulo, Brazil

Schistosomal myeloradiculopathy (SMR) is a form of schistosomiasis that is not linked with a high worm burden but rather is found in patients who have been sporadically exposed to Schistosoma mansoni. This paper aims to determine the occurrence of SMR in a low-endemic area with urban transmission in Campinas, São Paulo, Brazil. A retrospective study was performed, identifying confirmed cases in the two largest public hospitals on the region. Patients were diagnosed with SMR using standardised criteria, common clinical parameters, evidence of schistosomal infection and exclusion of other causes of myelopathy. A total of 27 patients were identified; 19 (85.2%) were men and four (14.8%) were women, ranging from 13-57 years of age (mean = 31.2; standard deviation = 12.8). Patients were classified as autochthonous (n = 14; 51.9%) or allochthonous (n = 11; 40.7%) and epidemiological data could not be obtained for two patients (7.4%). The clinical parameters of these patients were not different from previous studies. The sensitivity of serum immune reactions, cerebrospinal fluid immune reactions and parasitological stool examinations in identifying infected individuals was 87.5%, 93.8% and 40%, respectively. The epidemiological importance of these findings and their relationship with the control policies of schistosomiasis are discussed.

A pseudo-spectral method for the simulation of poro-elastic seismic wave propagation in 2D polar coordinates using domain decomposition

We present a novel numerical approach for the comprehensive, flexible, and accurate simulation of poro-elastic wave propagation in 2D polar coordinates. An important application of this method and its extensions will be the modeling of complex seismic wave phenomena in fluid-filled boreholes, which represents a major, and as of yet largely unresolved, computational problem in exploration geophysics. In view of this, we consider a numerical mesh, which can be arbitrarily heterogeneous, consisting of two or more concentric rings representing the fluid in the center and the surrounding porous medium. The spatial discretization is based on a Chebyshev expansion in the radial direction and a Fourier expansion in the azimuthal direction and a Runge-Kutta integration scheme for the time evolution. A domain decomposition method is used to match the fluid-solid boundary conditions based on the method of characteristics. This multi-domain approach allows for significant reductions of the number of grid points in the azimuthal direction for the inner grid domain and thus for corresponding increases of the time step and enhancements of computational efficiency. The viability and accuracy of the proposed method has been rigorously tested and verified through comparisons with analytical solutions as well as with the results obtained with a corresponding, previously published, and independently bench-marked solution for 2D Cartesian coordinates. Finally, the proposed numerical solution also satisfies the reciprocity theorem, which indicates that the inherent singularity associated with the origin of the polar coordinate system is adequately handled.

Aerodynamical instability of web

Diplomityön tavoitteena oli tutkia miten ilman turbulenttisuus vaikuttaa tasaisesti liikkuvan rainan tilaan. Yhtenä sovelluskohteena teollisuudessa voidaan mainita esimerkiksi leiju-kuivain. Tiedetään, että konenopeuksien kasvu ja siitä johtuva ilmavirran nopeuden kasvu aiheuttaa voimavaikutuksia rainaan ja voi aiheuttaa lepatusta. Lepatus johtaa dynaamiseen epästabiilisuuteen, joka voidaan havaita, kun lineaarinen systeemi tulee epävakaaksi ja joh-taa epälineaariseen, rajoitettuun värähtelyyn. Lepatus huonontaa tuotteiden laatua ja voi johtaa ratakatkoihin. Työssä on esitetty tietoa ilman ja rainan vuorovaikutuksesta, jota hyödyntämällä voidaan kehittää yksinkertaistettu malli, jonka avulla liikkuvaa rainaa voidaan simuloida kuivaimes-sa. Kaasufaasin virtausyhtälöt on ratkaistu eri turbulenttimalleja käyttäen. Myös viskoelas-tisen rainan muodonmuutosta on tarkasteltu. Koska rainalle ei ole kirjallisuudesta saatavilla tarkkoja fysikaalisia ja mekaanisia arvoja, näitä ominaisuuksia testattiin eri arvoilla, jotta rainan käyttäytymistä jännityksen alaisena voidaan tarkastella. Näiden ominaisuuksien tun-teminen on ensiarvoisen tärkeää määritettäessä rainan aeroviskoelastista käyttäytymistä. Virtaussimulointi on kallista ja aikaa vievää. Tämä tarkoittaa uusien tutkimusmenetelmien omaksumista. Tässä työssä vaihtoehtoisena lähestymistapana on esitetty yksinkertaistettu malli, joka sisältää ilman ja rainan vuorovaikutusta kuvaavat ominaisuudet. Mallin avulla saadaan tietoa epälineaarisuuden ja turbulenssin vaikutuksesta sekä monimutkaisesta yh-teydestä stabiilisuuden ja ulkoisesti aikaansaadun värähtelyn sekä itse aiheutetun värähtelyn välillä. Työn lopussa on esitetty havainnollinen esimerkki, jolla voidaan kuvata olosuhteita, jossa rainan tasainen liike muuttuu epävakaaksi. Kun turbulenttisuudesta johtuva painevaih-telu ylittää tietyn rajan, rainan värähtely kasvaa muuttuen satunnaisesta järjestäytyneeksi. Saaduttulokset osoittavat, että turbulenttisuudella on suuri vaikutus eikä sitä voi jättää huomioimatta. Myös rainan viskoelastiset ominaisuudet tulee huomioida, jotta rainan käyt-täytymistä voidaan kuvata tarkasti.

Atomikerroskasvatuksen soveltaminen kemiantekniikassa

Tässä kirjallisuustyössä tutkittiin atomikerroskasvatuksen (ALD) soveltamista kemiantekniikassa. Työn alussa kerrottiin atomikerroskasvatuksesta, sen toimintaperiaatteista ja prosessitekniikasta. Tämän jälkeen tutkittiin viittä eri kemiantekniikan sovellusta, jotka olivat polymeerien pinnoittaminen, heterogeenisten katalyyttien syntetisointi, membraanien modifiointi, korroosionesto ja kaasunilmaisimet. ALD on ohutkalvotekniikka, jolla voidaan valmistaa nanometrin tai jopa Ångströmin (1 Å = 0.1 nm) tarkkuudella epäorgaanisia materiaalikerroksia, jotka yleensä ovat metallioksideja, kuten alumiinioksidi. ALD perustuu kaasu-kiintoainereaktioihin, joissa kaasumaiset kemialliset prekursorit reagoivat vuorotellen kasvualustan kanssa. Tyypilliset prekursorit ovat metalliligandi ja vesi, joka on yleisin hapen lähde ALD-reaktioissa. ALD−reaktiot suoritetaan yleensä matalassa paineessa (100−200 Pa) ja korkeassa lämpötilassa (200–400 °C) suljetussa reaktorikammiossa. ALD-prosesseissa voidaan hyödyntää myös plasmaa alentamaan reaktiolämpötiloja. Plasman avulla prekursoreista luodaan hyvin reaktiivisia radikaaleja, jotka voivat reagoida jopa huoneenlämmössä. Lämpöherkkiä polymeerejä voidaan pinnoittaa ohutkalvoilla, joilla voidaan lisätä esimerkiksi pakkausmateriaalien suojaa happea ja vesihöyryä vastaan. ALD:llä voidaan syntetisoida tarkasti nanomittakaavan heterogeenisiä katalyyttejä, joilla on korkea dispersio tukimateriaalin pinnalla. ALD:n avulla voidaan säästää katalyyttimateriaalia menettämättä katalyytin aktiivisuutta, mikä on tärkeää monien katalyyttisovellusten taloudellisuuden kannalta, esimerkiksi polttokennot. ALD soveltuu hyvin membraanien modifiointiin, koska kaasumaiset prekursorit leviävät tasaisesti membraanin huokosiin. Membraanien pinnoittamisella pyritään vaikuttamaan, selektiivisyyteen, hydrofiilisyyteen, liuotinkestävyyteen, huokoskokoon ja sen jakaumaan. Lisäksi membraaneja voidaan pinnoittaa katalyyttisillä ohutkalvoilla, mikä on tärkeää nanoreaktoreiden kehityksen kannalta. ALD:llä voidaan pinnoittaa esimerkiksi terästä, ja vähentää täten teräksen korroosiota. Puolijohtavia metallioksideja voidaan käyttää kaasunilmaisimina, joiden valmistuksessa ALD:n tarkkuudesta on suurta hyötyä.