Numerical simulations of viscoelastic interfacial flows

While several experimental and numerical studies for Newtonian sprays have been conducted, the exploration of their non-Newtonian counterparts has received comparatively little attention. Achieving a fundamental understanding of the physical phenomena governing spray formation of this type of flow remains a challenge. The numerical simulations of the spray formation of a non-Newtonian fluid still offer substantial challenges, but it is reflective of industrial applications (i.e. spray-drying) and can lead to the optimisation of spray processes containing complex fluids. This thesis aims to provide the basis for the numerical examination of non-Newtonian atomisation and spray systems. We begin with axisymmetric simulations of an impulsively-started viscoelastic jet exiting a nozzle and entering a stagnant gas phase using the open-source code Basilisk. This code allows for efficient computations through an adaptively-refined volume-of-fluid technique that can accurately capture the interface. We use the FENE-P constitutive equation to describe the viscoelasticity of the fluid and employ the log-conformation transformation, which provides stable solutions for the conformation tensor. For the first time, the entire jetting and breakup process of a viscoelastic fluid is simulated, including the flow through the nozzle, which results in an inhomogeneous initial radial stress distribution that affects the subsequent breakup dynamics. The evolution of the velocity field and the elastic stresses in the nozzle are validated against analytical solutions, and the early-stage dynamics of the jet are compared favourably to the predictions of linear stability theory. We explore the effect of flow inside the nozzle on the thinning dynamics of the viscoelastic jet, which develops distinctive "beads-on-a-stringstructures", via analysis of the spatiotemporal evolution of the polymeric stresses. We also systematically investigate the dependence of the filament thinning and breakup characteristics on the axial momentum of the jet and the extensibility of the dissolved polymer chains. We also probe how the secondary droplet formation can be controlled by the finite extensibility of the polymeric chains, as well as the wavenumber of the forced oscillation of the injected liquid at the nozzle inlet. In addition, we study numerically the thinning and breakup in a Dripping-onto-Substrate (DoS) rheometry. The DoS is a conceptually-simple, but dynamically-complex, probe of the extensional rheology of low-viscosity non-Newtonian fluids. It exploits the capillary-driven thinning of a liquid bridge, produced by a single drop as it is dispensed from a syringe pump and spreads laterally onto a solid substrate. By following the filament thinning process, the extensional viscosity and relaxation time of the sample can be determined. Importantly, DoS rheometry allows experimentalists to measure the extensional properties of solutions with lower viscosity than is possible with commercially-available capillary break-up extensional rheometers. Understanding the fluid mechanics underlying the operation of DoS is essential for optimising and extending the performance of this protocol. To achieve this, we employ a computational rheology approach using adaptively-refined axisymmetric numerical simulations with the Basilisk code. The volume-of-fluid technique is used to resolve the moving interface, and the log-conformation transformation provides a stable and accurate solution of the viscoelastic constitutive equation that describes the rheology of the thinning liquid filament. Here, we focus on understanding the role of elasticity and finite chain extensibility in controlling the elasto-capillary (EC) regime, as well as the perturbative effects that gravity and the substrate wettability play in establishing the evolution of the self-similar thinning and pinch-off dynamics. To illustrate the interplay of these different forces, we construct a simple one-dimensional model that captures the initial rate of thinning when the interplay of inertia and capillarity dominates; the model also captures the structure of the transition region to the nonlinear EC regime where the rapidly growing elastic tensile stresses in the thread balance the capillary pressure as the filament thins towards breakup. Finally, we develop and test a rheological model for avoiding the numerical challenges associated with the commonly-used constitutive equations for viscoelastic extensional flows, which accounts for the changes in the fluid viscosity based on the principal invariants of the deviatoric stress tensor. We validate the predictions of the model against a free-filament thinning and a jetting flow configuration of a FENE-P fluid, highlighting its capability to account for a substantial increase in viscosity under elongation. The model, however, fails to exhibit all of the characteristic viscoelastic flow regimes observed in our FENE-P-based simulation results. This highlights the need for further model improvement incorporating the flow kinematics history, a distinctive characteristic of viscoelasticity, which will be the subject of future work.Open Acces

Transition to elasto-capillary thinning dynamics in viscoelastic jets

We perform simulations of an impulsively-started, axisymmetric viscoelastic jet exiting a nozzle and entering a stagnant gas phase using the open-source code Basilisk. This code allows for efficient computations through an adaptively-refined volume-of-fluid technique that can accurately capture the deformation of the liquid-gas interface. We use the FENE-P constitutive equation to describe the viscoelasticity of the liquid and employ the log-conformation transformation, which provides stable solutions for the evolution of the conformation tensor as the jet thins down under the action of interfacial tension. For the first time, the entire jetting and breakup process of a viscoelastic fluid is simulated, including the pre-shearing flow through the nozzle, which results in an inhomogeneous initial radial stress distribution in the fluid thread that affects the subsequent breakup dynamics. The evolution of the velocity field and the elastic stresses in the nozzle are validated against analytical solutions where possible, and the early-stage dynamics of the jet evolution are compared favourably to the predictions of linear stability theory. We study the effect of the flow inside the nozzle on the thinning dynamics of the viscoelastic jet (which develops distinctive "beads-on-a-string" structures) and on the spatio-temporal evolution of the polymeric stresses in order to systematically explore the dependence of the filament thinning and breakup characteristics on the initial axial momentum of the jet and the extensibility of the dissolved polymer chains

On Supporting Dual-Mode HiperLAN/2: Architecture and Overhead

The IST BroadWay project, [1], introduces the ad-hoc networking paradigm in the traditional 5 GHz HiperLAN/2. The ad-hoc networking paradigm is employed at the 60 GHz frequency band, which allows for a high transmission rate communication. The dual mode of operation primarily aims at offloading the 5 GHz HiperLAN/2 cell in very dense urban deployments (high traffic needs and number of users), while the peculiarities of the BroadWay system induce modifications on the existing HiperLAN/2 system and make the development of a routing scheme a rather challenging task. The Centralized Ad-hoc Network Architecture (CANA) is an efficient way to support dual mode systems, [2], by implementing specific changes and algorithms in the standard HiperLAN/2 protocol stack. In this paper, the new enhanced protocol stack of HiperLAN/2 is presented as well as the algorithms required for the efficient utilization of the system resources. Simulations provide insight into the performance issues regarding the dual mode of operation and the parameters that affect the efficiency of the described network model. I

Layered Architecture and Modules of CANA Supporting Dual Mode HiperLAN/2

(CANA) is an efficient way to support dual mode systems, [1]. A dual mode of operation has been introduced in the traditional 5 GHz HiperLAN/2 to allow data off-loading by using resources of an ad-hoc network created at 60 GHz (the dual mode concept was introduced by the IST BroadWay project, IST-2001-32686). The off-loading is possible using CANA but certain changes in the standard HiperLAN/2 protocol stack are required and certain algorithms need to be incorporated. In this paper, the new enhanced protocol stack of HiperLAN/2 that supports CANA is presented as well as key algorithms that need to be developed and whose performance is crucial for the efficient utilization of the system resources. These algorithms can also be implemented for any other WLAN in order to support a second mode of operation. Simulations shed light into the trade-offs induced by the dual mode of operation and the way the network resources may efficiently be utilized under certain conditions

Increasing Capacity in Dual-Band WLANs through Ad-hoc Networking

Abstract- The ad-hoc networking paradigm – originally conceived to cope with infrastructureless military and emergency situations – is also being considered to support the ever-evolving user requirements for higher data rates and enhance the capabilities of traditional networks. The Centralized Ad-Hoc Network Architecture (CANA) proposed in this article aims at increasing substantially the capacity of traditional Wireless Local Area Networks (WLANs). It is based on a dual frequency system in which the operation in the original WLAN frequency supports the centralization of some of the traditionally distributed and problematic adhoc functionalities enabled at the new frequency; higherrate, shorter-range, peer-to-peer and multi-hop transmissions are possible at the new frequency, resulting in a significant increase of the WLAN capacity. In order to take advantage of the extra capacity, modifications are defined and described by exploring the HiperLAN/2 standard. The gain of the induced dual mode of operation depends on several parameters that define the performance of CANA. This article explores the performance issues that CANA arises by providing both analytical and simulation results. The overhead of CANA is rather low if one takes into account the profits of such architecture

The Effect of Touch-Cure Polymerization on the Conversion and Hardness of Core Build-Up Resin Composites: A Laboratory Study.

To improve the self-curing capacity and interfacial strength with dentine of dual-cured composite materials, touch-cure activators have been introduced. The aim of the study was to evaluate the effect of these activators on the hardness and conversion of dual-cured resin composite core build-up restoratives. The materials tested were Clearfil DC Core Plus (CF) and Gradia Core (GC) with the corresponding adhesives Clearfil S(3) Bond Plus (for CF) and G-Premio Bond/G-Premio DCA activator (for GC). Disk-shaped specimens (n = 6/group) were prepared for the following groups: dual-cured, self-cured and self-cured in contact with the adhesive activators at the bottom surface. After a 3-week storage period (dark/dry/37 °C) the Martens hardness (HM) and degree of conversion (DC%) were determined for the previously mentioned groups and the top surfaces of groups in contact with the adhesives. A statistical analysis was performed by a one-way ANOVA and Holm-Sidak test per material and a Pearson’s correlation analysis (HM vs. DC%) at an α = 0.05. The self-cured specimens resulted in significantly lower HM and DC% values from the dual-cured group, as expected. However, in the presence of the adhesives with touch-cure activators, the conversion of the self-cured groups showed insignificant differences in HM and DC% from the dual-cured in both composite materials. The improvements on the bottom composite surfaces in contact with the adhesives did not extend to the entire specimen length. Nevertheless, improved interfacial curing may improve interfacial durability