Statics, dynamics, rheology and scission-recombination kinetics of self-assembling linear micelles are investigated at equlibrium state and under shear flow by computer simulations using a newly proposed mesoscopic model. We model the micelles as linear sequences of Brownian beads whose space-time evolution is governed by Langevin dynamics. A Monte Carlo algorithm controls the opening of a bond or the chain-end fusion. A kinetic parameter omega modelling the effect of a potential barrier along a kinetic path, is introduced in our model.<p>For equilibrium state we focus on the analysis of short and long time behaviors of the scission and recombination mechanisms. Our results show that at time scales larger than the life time of the average chain length, the kinetics is in agreement with the mean-field kinetics model of Cates. By studying macroscopic relaxation phenomena such as the average micelle length evolution after a T-jump, the monomer diffusion, and the zero shear relaxation function, we confirm that the effective kinetic constants found are indeed the relevant parameters when macroscopic relaxation is coupled to the kinetics of micelles.<p>For the non-equilibrium situation, we study the coupled effects of the shear flow and the scission-recombination kinetics, on the structural and rheological properties of this micellar system. Our study is performed in semi-dilute and dynamically unentangled regime conditions. The explored parameter omega range is chosen in order for the life time of the average size chain to remain shorter than its intrinsic (Rouse) longest relaxation time. Central to our analysis is the concept of dynamical unit of size Lambda, the chain fragment for which the life time tau_Lambda and the Rouse time are equal. Shear thinning, chain gyration tensor anisotropy, chain orientation and bond stretching are found to depend upon the reduced shear rate Beta_Lambda=gamma dot*tau_Lambda while the average micelle size is found to decrease with increasing shear rate, independently of the height of the barrier of the scission-recombination process. / Doctorat en sciences, Spécialisation physique / info:eu-repo/semantics/nonPublished

<p>This thesis deals with fluid dynamic mechanisms related to papermaking, specif- ically: the initial dewatering mechanisms during roll-forming and fibre motion in sedimentation and in shear flow. </p><p>Pressure and wire position measurements have been conducted in a model resembling the forming zone and the measured pressure distributions are shown to have more complex patterns than the simple model p / T/R (where T is the wire tension and R is the roll radius). It is shown that an increase in wire tension has a similar effect as a decrease in flow-rate on the shape of the pressure distribution. In addition, it is shown that the drainage has a stabilizing effect on the dewatering pressure. </p><p>The flow around the forming roll has also been modelled with the assump- tion that the wire is impermeable. A non-linear equation for the position of the wire is derived that clearly shows that the Weber number, We, is an im- portant parameter. The equation is linearized around the trivial solution and has a standing wave solution with a specific wavelength that scales with the We-number. </p><p>Motion of non-Brownian fibre settling in a Newtonian fluid at a small but finite Reynolds number has been studied experimentally. Two different regimes of sedimentation were identified. For dilute suspensions, fibres gener- ally fall without flipping and may travel at velocities larger than that of an isolated particle. In the semi-dilute regime we found the settling process to be dominated by large-scale fluctuations. The velocity fluctuations scale with the suspension volume concentrationφ according toφ1/3, which is similar to the findings for settling spheres. </p><p>The influence of shear on fibre orientation in the near wall region was studied in cellulose acetate fibre suspensions. At low concentration and low aspect ratio fibres were observed to orient perpendicular to the streamwise direction (named rollers) in the near wall region whereas the orientation further into the suspension was unchanged. As the concentration and aspect ratio increased the fraction of rollers decreased. </p><p>Finally, an evaluation of a commercial Ultra Velocity Profiler unit in fibre suspensions are presented. The idea was to determine the velocity and characterise the turbulence from ultra sound echoes from particles in the fluid. However, the spatial and/or temporal resolution of the measurements did not permit turbulence characterisation. These limitations might be possible to overcome and some procedures are proposed and evaluated.</p>

A poluição sonora é um problema central de uma grande diversidade de aplicações industriais. Na engenharia, podemos citar diversos casos que geram ruído, como exemplos os trens, automóveis, rotores de helicópteros e o ruído aerodinâmico das aeronaves, o qual se divide em ruído gerado pelos motores a jato e a estruturas da aeronave. No presente momento o ruído dos motores aeronáuticos, principalmente os jatos, atingiu níveis de ruídos semelhantes às estruturas da aeronave, como por exemplo, eslates, flaps e trens de pouso. Desta forma, as autoridades de transporte aéreo estão exigindo também redução no ruído das estruturas. O presente trabalho apresenta a verificação das potencialidades e limitações do software CFD++, programa este adquirido pela EMBRAER e inserido como parte do projeto Aeronave Silenciosa, para assim poder compreender de uma melhor maneira o fenômeno da aeroacústica, e deste modo, poder contribuir para a redução do ruído externo das aeronaves. Para verificar as potencialidades e limitações do CFD++, foi proposto investigar o mecanismo de som do eslate. Tal fenômeno é devido ao deslocamento da camada limite no intradorso do eslate a partir de onde se desenvolve a camada de mistura, foco do presente trabalho. / Noise pollution is a central problem of a wide variety of industrial applications. In engineering, cite several cases that generate noise, as examples trains, automobiles, rotors of helicopters and the noise generated by aircraft, which is divided into noise generated by jet engines and airframe. At present the noise of aircraft engines, largely the jets reached noise levels similar structures, such as slat, flaps and landing gear. Thus, the air transport authorities are also demanding a reduction in noise of the structures of airframe. This report presents the verification of potentialities and limitations of CFD++, a program acquired by EMBRAER and inserted as part of the Silent Aircraft, so they can understand better how the phenomenon of aeroacoustics, and thus able to reduce contribute external noise from aircraft. To check the potentialities and limitations of CFD++, was proposed to investigate the mechanism of sound generated by the slat. This phenomenon is due to the displacement of the boundary layer on the lower surface of the slat from which the mixed layer develops. The mixing layer is the focus of this work.

The main goal of this work was to develop numerical models for studying the behaviour of fibres in an accelerated flow. This is of special interest for e.g. papermaking. The early stage of the paper manufacturing process determines most of the final properties of a paper sheet. The complexity of studying the flow of fibre suspensions both experimentally and numerically emphasises a need for new ideas and developments. By means of solving the evolution of a convective-dispersion equation, i.e. the Fokker-Planck equation, a fully 3D approach with respect to the position and the two fibre angles, polar and azimuthal angles, following a streamline is presented. As an input to the fibre orientation model the turbulent flow field is solved by Computational Fluid Dynamics (CFD) with second-order closure in the turbulence model. In this work two new hypotheses have been presented for the variation of the non-dimensional rotational diffusivity with non-dimensional fibre length, Lf /η and the Reynolds number based on the Taylor micro-scale of the turbulence, Reλ Parameters for the two new hy- potheses and earlier models are determined with the aim of achieving a general relation and a value of the rotational dispersion coeffcient of stiff fibres in an anisotropic turbulent fluid flow. Earlier modelling work has been focused on solving the planar approach, i.e. assuming all fibres to be in one plane. This planar approach is discussed and compared with the fully 3D approach and its validity is evaluated. The optimization of parameters for the different hypotheses correlated on a central streamline, showed a good agreement with an independent experimental result in the undisturbed region. Moreover, it is particularly interesting that the boundary layer region and the wake region are predicted fairly well and the phenomena are well described, which has not been the case earlier. It seems that the new hypothesis based on the variation of the non-dimensional fibre length, Lf /η gives the best correlation in these shear-layer regions. Further- more it was established that the planar approach fails to predict shear layers, i.e. the boundary layer and the wake regions. As emphasized in the theory section, the planar formulation is strictly valid only if all fibres are oriented in one plane, which is not the case in the shear layers. In the undisturbed region, the 3D and the planar approaches, agree in their results. This leads to the conclusion that both approaches are suitable when shear layers are not studied. / QC 20100812

This thesis deals with fluid dynamic mechanisms related to papermaking, specif- ically: the initial dewatering mechanisms during roll-forming and fibre motion in sedimentation and in shear flow. Pressure and wire position measurements have been conducted in a model resembling the forming zone and the measured pressure distributions are shown to have more complex patterns than the simple model p / T/R (where T is the wire tension and R is the roll radius). It is shown that an increase in wire tension has a similar effect as a decrease in flow-rate on the shape of the pressure distribution. In addition, it is shown that the drainage has a stabilizing effect on the dewatering pressure. The flow around the forming roll has also been modelled with the assump- tion that the wire is impermeable. A non-linear equation for the position of the wire is derived that clearly shows that the Weber number, We, is an im- portant parameter. The equation is linearized around the trivial solution and has a standing wave solution with a specific wavelength that scales with the We-number. Motion of non-Brownian fibre settling in a Newtonian fluid at a small but finite Reynolds number has been studied experimentally. Two different regimes of sedimentation were identified. For dilute suspensions, fibres gener- ally fall without flipping and may travel at velocities larger than that of an isolated particle. In the semi-dilute regime we found the settling process to be dominated by large-scale fluctuations. The velocity fluctuations scale with the suspension volume concentrationφ according toφ1/3, which is similar to the findings for settling spheres. The influence of shear on fibre orientation in the near wall region was studied in cellulose acetate fibre suspensions. At low concentration and low aspect ratio fibres were observed to orient perpendicular to the streamwise direction (named rollers) in the near wall region whereas the orientation further into the suspension was unchanged. As the concentration and aspect ratio increased the fraction of rollers decreased. Finally, an evaluation of a commercial Ultra Velocity Profiler unit in fibre suspensions are presented. The idea was to determine the velocity and characterise the turbulence from ultra sound echoes from particles in the fluid. However, the spatial and/or temporal resolution of the measurements did not permit turbulence characterisation. These limitations might be possible to overcome and some procedures are proposed and evaluated. / QC 20101021

Regions containing gradients of vertical flow are often associated with sharp changes in hydrographic and biochemical water properties in coastal marine ecosystems. Often these are sites of dense plankton aggregations of critical ecological importance. In this study, a recirculating flume apparatus with a laminar, planar free jet (the Bickley jet) was used to create finescale gradients of fluid velocity (shear) in both upwelling and downwelling configurations for zooplankton behavioral assays. Particle image velocimetry (PIV) was used to fully resolve the velocity fields allowing us to fine-tune experimental parameters to match fluid mechanical conditions commonly measured in the field. Zooplankton behavioral assays with two tropical calanoid copepods, Acartia negligens and Clausocalanus furcatus, an estuarine mysid, Neomysis americana, and the larvae of an estuarine mud crab, Panopeus herbstii, were conducted in control (stagnant), upwelling, and downwelling flow configurations. Statistical analyses (ANOVA) of individual zooplankton trajectories revealed the potential for individual behavioral responses to persistent finescale vertical shear layers to produce population scale aggregations, which is proposed here as a mechanism of patchiness in coastal marine ecosystems. Results from behavioral analyses reveal species-specific threshold shear strain rates that trigger individual behavioral responses. Furthermore, results show statistically significant changes in behavior (relative swimming speed, turn frequency, heading) for all species tested in response to a coherent shear structure in the form of finescale upwelling and downwelling jets. The results show that changes in individual behavior can increase Proportional Residence Time (PRT = percent time spent in the jet structure). On a population scale, the increase in PRT can lead to dense aggregations around persistent flow features, which is consistent with numerous field studies. These dense, patchy aggregations of zooplankton have profound trickle-up ecological consequences in coastal marine ecosystems.

Separating and reattaching flows are important in a large number of engineering configurations. The flow through a sudden expansion (backward-facing step) represents a conceptually simple case of this class of flows and hence has been the subject of numerous studies. The present study focuses on the effect of the expansion ratio (defined as the ratio of downstream channel height to upstream channel height) on the unsteady flow features in the reattachment region and further downstream. It is known that this flow demonstrates two different instabilities; the Kelvin-Helmholtz shear layer instability, which scales with the shear layer thickness, and the instability associated with the separation bubble, which scales with the step height and has similarities to K´arman vortex shedding behind a cylinder.In addition to these, there is also a possibility of the presence of the ‘preferred’ mode of the jet issuing from the inlet channel of the sudden expansion, especially at high expansion ratios, where the flow resembles a wall jet. The aim of this study is to investigate experimentally the changes in the instability of the separation bubble, as the expansion ratio is changed, and its possible interactions with the other instabilities in the flow.One might expect some changes in the flow with expansion ratio, as at low expansion ratios, the configuration represents a simple backward-facing step geometry, while at high expansion ratios, the geometry approaches that of a wall jet. A variable expansion ratio backward-facing step facility has been developed in an open circuit wind tunnel.This facility permits continuous variation of the expansion ratio from 1 to around 6. Attention is focused on the turbulent regime of the flow, where the flow structure has been found in previous studies to be relatively insensitive to the Reynolds number. The inlet conditions have been kept constant with a thin turbulent boundary layer at the step, the boundary layer thickness at separation being approximately 14 % of the inlet channel height. The Reynolds number based on the inlet channel height, H, is kept constant at Re=48,000 and the expansion ratio is varied by changing the channel height downstream of the step. Detailed hot wire measurements have been made to characterize the spatial variation of the dominant frequencies in the flow at different expansion ratios. The expansion ratio has been varied from a low value of 1.14 to a high value of 3.25, and detailed measurements are obtained for five expansion ratios of 1.14, 1.3, 1.5, 2.0 and 3.0. Further, to elucidate the dominant vortical structures in the flow, Particle Image Velocimetry measurements have been undertaken simultaneously with hot wire measurements for the case of expansion ratio 1.5, which have permitted the conditional averaging of vorticity fields.These investigations have brought forth some interesting features of the flow over a backward-facing step. Results for the time-mean properties of the flow indicate that the shear layer separating from the step deviates from a free mixing layer behaviour away from the step, possibly due to its interaction with the wall and the recirculation region underneath it. At any given streamwise location, the shear layer momentum thickness, θ, is seen to increase with the expansion ratio. Further, upto reattachment, the momentum thickness of the shear layer is seen to scale with the step height, h, independent of its initial thickness at separation, θo, as long as the boundary layer at separation is sufficiently thin as compared to the step height. Investigations for the unsteady flow features show that the frequency of the dominant peak in the velocity spectrum, supposed to represent the passage frequency (Strouhal number, S, based on the step height, h, and the inlet velocity, U) of the vortical structures, varies in the cross stream (y) direction, in addition to its expected variation in the streamwise (x) direction. The variation of the Strouhal number in the cross stream direction is seen to scale with the local momentum thickness of the shear layer, except for locations very close to the step. To characterize the development of the dominant frequency in the streamwise direction, the maximum value of the Strouhal number at a streamwise location is taken to be the representative value for that streamwise location. The Strouhal number is seen to decrease in the streamwise direction, from a very high value near the step, to a value of approximately 0.08 in the reattachment region, and remains constant further downstream. This value, supposed to represent the large scale structures shed from the reattachment region, is seen to remain very close to 0.08 for all Expansion ratios investigated. Conditional averaging of the vorticity fields in the reattachment region is done for an expansion ratio of 1.5, to get a detailed picture of the unsteady flow field. The hot wire signal at the outer edge of the shear layer in the reattachment region, which represents the non-dimensional structure passage frequency of S=0.08, is used as the conditioning signal. Results seem to indicate that the recirculation region, or the ‘bubble’ divides into two cells, and sheds the downstream cell quasi-periodically. The passage of these structures through the reattachment region seems to be concomitant With a local vertical motion of the shear layer. Further, the streamwise development of the local Strouhal number, Sθ, based on the local momentum thickness of the shear layer, and the local free stream velocity, Umax, indicates a possibility of a coupling between the shear layer and the structures shed from the reattachment region.

The desire for repair of tissue defects and injury is the major need prompting research into tissue engineering. Engineering of anisotropic tissues requires production of ordered substrates that orient cells preferentially and support cell viability and differentiation. Towards this goal, this thesis investigated methodologies to align extracellular matrix structures in vitro to guide stem/progenitor cell behaviour for tissue regeneration. Aligned collagen fibrils were deposited on planar substrates from collagen solutions streaming through a microfluidic channel system. Collagen solution concentration, degree of gelation, shear rate and pre-coating of the substrate were demonstrated to determine the orientation and density of the immobilized fibrils. The degree of collagen fibril orientation increased with increasing flow rates of the solution while the matrix density increased at higher collagen solution concentrations and on hydrophobic polymer pre-coatings. Additionally, the length of the immobilized collagen fibrils increased with increasing solution concentration and gelation time. Aligned collagen matrices were refined by incorporating the glycosaminoglycan heparin to study multiple extracellular matrix components in a single system. Multilineage (osteogenic/adipogenic/chondrogenic) differentiation of mesenchymal stem and progenitor cells was maintained by the aligned structures. Most noticeable was the observation that during osteogenesis, aligned collagen substrates choreographed ordered matrix mineralization. Likewise, myotube assembly of C2C12 cells was profoundly influenced by aligned topographic features resulting in enhanced myotube organization and length. Neurites from neural stem cells were highly oriented in the direction of the underlying fibrils. Neurite outgrowth was enhanced on aligned collagen compared to non-aligned collagen or poly-D-lysine substrates, while neural differentiation and cell survival were not influenced by the type of substrate. Using the new method to align collagen type I, the interior walls of cellulose hollow fiber membranes were coated with longitudinally aligned collagen fibrils to fabricate an advanced guidance conduit for nerve regeneration. First cell culture experiments showed that the tubes coated with aligned collagen supported viability and adherence of spinal cord-derived neurospheres. Together, these results demonstrate the feasibility of aligned collagen matrices as a versatile platform to control cell behaviour towards tissue regeneration. Ultimately, the new method to align collagen fibrils and to coat hollow membranes may become an integral component of tissue engineering, working synergistically with other emerging techniques to promote functional tissue replacements.

The instabilities of shallow shear flows are analyzed to study exchanges processes across shear flows in inland and coastal waters, coastal and ocean currents, and winds across the thermal-and-moisture fronts. These shear flows observed in nature are driven by gravity and governed by the shallow water equations (SWE). A highly accurate, and robust, computational scheme has been developed to solve these SWE. Time integration of the SWE was carried out using the fourth-order Runge-Kutta scheme. A third-order upwind bias finite difference approximation known as QUICK (Quadratic Upstream Interpolation of Convective Kinematics) was employed for the spatial discretization. The numerical oscillations were controlled using flux limiters for Total Variation Diminishing (TVD). Direct numerical simulations (DNS) were conducted for the base flow with the TANH velocity profile, and the base flow in the form of a jet with the SECH velocity profile. The depth across the base flows was selected for the' balance of the driving forces. In the rotating flow simulation, the Coriolis force in the lateral direction was perfectly in balance with the pressure gradient across the shear flow during the simulation. The development of instabilities in the shear flows was considered for a range of convective Froude number, friction number, and Rossby number. The DNS of the SWE has produced linear results that are consistent with classical stability analyses based on the normal mode approach, and new results that had not been determined by the classical method. The formation of eddies, and the generation of shocklets subsequent to the linear instabilities were computed as part of the DNS. Without modelling the small scales, the simulation was able to produce the correct turbulent spreading rate in agreement with the experimental observations. The simulations have identified radiation damping, in addition to friction damping, as a primary factor of influence on the instability of the shear flows admissible to waves. A convective Froude number correlated the energy lost due to radiation damping. The friction number determined the energy lost due to friction. A significant fraction of available energy produced by the shear flow is lost due the radiation of waves at high convective Froude number. This radiation of gravity waves in shallow gravity-stratified shear flow, and its dependence on the convective Froude number, is shown to be analogous to the Mach-number effect in compressible flow. Furthermore, and most significantly, is the discovery from the simulation the crucial role of the radiation damping in the development of shear flows in the rotating earth. Rings and eddies were produced by the rotating-flow simulations in a range of Rossby numbers, as they were observed in the Gulf Stream of the Atlantic, Jet Stream in the atmosphere, and various fronts across currents in coastal waters.

A computational model of thrombus initiation and aggrandizement was proposed. The model separated the thrombotic process into three mechanisms, including shear enhanced diffusivity, platelet margination, and platelet adhesion. The model indicates that transport mechanisms may be the rate limiting condition of thrombus formation at physiological shear rates and that at higher shear rates; platelet binding becomes the rate limiting condition. Additionally a wall shear rate of 20000 s-1 and above should be considered as a new criterion for prophylactic treatment of an atherosclerotic lesion.