Systematic Stochastic Reduction of Inertial Fluid-Structure Interactions subject to Thermal Fluctuations

We present analysis for the reduction of an inertial description of fluid-structure interactions subject to thermal fluctuations. We show how the viscous coupling between the immersed structures and the fluid can be simplified in the regime where this coupling becomes increasingly strong. Many descriptions in fluid mechanics and in the formulation of computational methods account for fluid-structure interactions through viscous drag terms to transfer momentum from the fluid to immersed structures. In the inertial regime, this coupling often introduces a prohibitively small time-scale into the temporal dynamics of the fluid-structure system. This is further exacerbated in the presence of thermal fluctuations. We discuss here a systematic reduction technique for the full inertial equations to obtain a simplified description where this coupling term is eliminated. This approach also accounts for the effective stochastic equations for the fluid-structure dynamics. The analysis is based on use of the Infinitesmal Generator of the SPDEs and a singular perturbation analysis of the Backward Kolomogorov PDEs. We also discuss the physical motivations and interpretation of the obtained reduced description of the fluid-structure system. Working paper currently under revision. Please report any comments or issues to [email protected]: 19 pages, 1 figure. arXiv admin note: substantial text overlap with arXiv:1009.564

Progress in particle-based multiscale and hybrid methods for flow applications

This work focuses on the review of particle-based multiscale and hybrid methods that have surfaced in the field of fluid mechanics over the last 20 years. We consider five established particle methods: molecular dynamics, direct simulation Monte Carlo, lattice Boltzmann method, dissipative particle dynamics and smoothed-particle hydrodynamics. A general description is given on each particle method in conjunction with multiscale and hybrid applications. An analysis on the length scale separation revealed that current multiscale methods only bridge across scales which are of the order of O(102)−O(103) and that further work on complex geometries and parallel code optimisation is needed to increase the separation. Similarities between methods are highlighted and combinations discussed. Advantages, disadvantages and applications of each particle method have been tabulated as a reference

Mesoscale Structures at Complex Fluid-Fluid Interfaces: a Novel Lattice Boltzmann / Molecular Dynamics Coupling

Complex fluid-fluid interfaces featuring mesoscale structures with adsorbed particles are key components of newly designed materials which are continuously enriching the field of soft matter. Simulation tools which are able to cope with the different scales characterizing these systems are fundamental requirements for efficient theoretical investigations. In this paper we present a novel simulation method, based on the approach of Ahlrichs and D\"unweg [Ahlrichs and D\"unweg, Int. J. Mod. Phys. C, 1998, 9, 1429], that couples the "Shan-Chen" multicomponent Lattice Boltzmann technique to off-lattice molecular dynamics to simulate efficiently complex fluid-fluid interfaces. We demonstrate how this approach can be used to study a wide class of challenging problems. Several examples are given, with an accent on bicontinuous phases formation in polyelectrolyte solutions and ferrofluid emulsions. We also show that the introduction of solvation free energies in the particle-fluid interaction unveils the hidden, multiscale nature of the particle-fluid coupling, allowing to treat symmetrically (and interchangeably) the on-lattice and off-lattice components of the system

Computational methods and software for the design of inertial microfluidic flow sculpting devices

The ability to sculpt inertially flowing fluid via bluff body obstacles has enormous promise for applications in bioengineering, chemistry, and manufacturing within microfluidic devices. However, the computational difficulty inherent to full scale 3-dimensional fluid flow simulations makes designing and optimizing such systems tedious, costly, and generally tasked to computational experts with access to high performance resources. The goal of this work is to construct efficient models for the design of inertial microfluidic flow sculpting devices, and implement these models in freely available, user-friendly software for the broader microfluidics community. Two software packages were developed to accomplish this: uFlow and FlowSculpt . uFlow solves the forward problem in flow sculpting, that of predicting the net deformation from an arbitrary sequence of obstacles (pillars), and includes estimations of transverse mass diffusion and particles formed by optical lithography. FlowSculpt solves the more difficult inverse problem in flow sculpting, which is to design a flow sculpting device which produces a target flow shape. Each piece of software uses efficient, experimentally validated forward models developed within this work, which are applied to deep learning techniques to explore other routes to solving the inverse problem. The models are also highly modular, capable of incorporating new microfluidic components and flow physics to the design process. It is anticipated that the microfluidics community will integrate the tools developed here into their own research, and bring new designs, components, and applications to the inertial flow sculpting platform

Direct Simulation Monte Carlo Analysis on Thrust Vectoring of a Supersonic Micro Nozzle using Bypass Mass Injection

Converging diverging micro nozzle is fundamentally intended for flow acceleration through the generation of kinetic energy for the advanced micro-propulsion systems. Such supersonic micro nozzles have significant applications in the launching, propulsion and rapid directional control of the micro-satellites for a better maneuver. Micro scale analysis of such flow devices is extended to the rarefied flow regime. Present study has addressed thrust vectoring in a planar converging-diverging supersonic micro nozzle by the bypass mass injection technique. Direct Simulation Monte Carlo (DSMC) method has been used for numerical modelling. Primary focus is given on the thrust vectoring control of the micro nozzle with a throat height of 20 micro meter and an expansion ratio of 1.7. For the secondary injection, a rectangular channel of 5 different bypass widths (2 - 12 micro meter) is considered for two different outlet pressures (Pout = 10 kPa and 40 kPa) while keeping the inlet pressure (Pin) and temperature (Tin) fixed at 1 atm and 300 k respectively. The physical behavior of the micro nozzle is acknowledged through the analysis of Mach, pressure, temperature and density contours. Numerical results reveal that the secondary flow injection is adapted into the primary flow through the formation of a pressure bump in the diverging section. Moreover, the total mass flow rate, secondary flow percentage, thrust force, the thrust coefficient and specific impulse increase with the bypass channel width. A change in thrust direction is obtained which in turn produces a considerable vectoring effect in the supersonic micro nozzle. The vectoring angle for Pout of 40 kPa peaks at 6 micro meter bypass channel whereas a gradual increase of the vectoring angle with the bypass channel width is observed for Pout = 10 kPa.Comment: Journal paper and it's under revie

Applications of CFD Simulations on Studying the Multiphase Flow in Microfluidic Devices

Microfluidics has been extensively investigated as a unique platform to synthesize nanoparticles with desired properties, e.g., size and morphology. Compared to the conventional batch reactors, wet-chemical synthesis using continuous flow microfluidics provides better control over addition of reagents, heat and mass transfer, and reproducibility. Recently, millifluidics has emerged as an alternative since it offers similar control as microfluidics. With its dimensions scaled up to millimeter size, millifluidics saves fabrication efforts and potentially paves the way for industrial applications. Good designs and manipulations of microfluidic and millifluidic devices rely on solid understanding of fluid dynamics. Fluid flow plays an important role in heat and mass transfer; thereby, it determines the quality of the synthesized nanoparticles. Computational fluid dynamics (CFD) simulations provide an effective approach to understand various effects on fluid flows without carrying out complicated experiments. The goal of this dissertation is to utilize CFD simulations to study flow behaviors inside microfluidic and millifluidic systems. Residence time distribution (RTD) analysis coupled with TEM characterization was applied to understand the effect of reagent flow rates on particle sizes distribution. Droplet-based microfluidics, as a solution to the intrinsic drawbacks associated with single-phase microfluidics, depends on proper manipulation of the flow to generate steady droplet flow with desired droplet / slug sizes. The droplet/slug formation process inside a millifluidic reactor was investigated by both experiments and numerical simulations to understand the hydrodynamics of slug breaking. Geometric optimization was carried out to analyze the dependency of slug sizes on geometric dimensions. Numerical simulations were also performed to quantify the mixing efficiency inside slugs with the aid of mixing efficiency index. In some circumstances, the droplet sizes are difficult to control via manipulating the flow rates. By applying external electric field to the conventional droplet-based microfluidic systems, the electric force induced on the fluid interface can reduce the droplet sizes effectively. This work provides insight to understand fluid flow inside microfluidic and millifluidic systems. It may benefit the design and operations of novel microfluidic and millifluidic systems

Stochastic Reductions for Inertial Fluid-Structure Interactions Subject to Thermal Fluctuations

We present analysis for the reduction of an inertial description of fluid-structure interactions subject to thermal fluctuations. We show how the viscous coupling between the immersed structures and the fluid can be simplified in the regime where this coupling becomes increasingly strong. Many descriptions in fluid mechanics and in the formulation of computational methods account for fluid-structure interactions through viscous drag terms to transfer momentum from the fluid to immersed structures. In the inertial regime, this coupling often introduces a prohibitively small time-scale into the temporal dynamics of the fluid-structure system. This is further exacerbated in the presence of thermal fluctuations. We discuss here a systematic reduction technique for the full inertial equations to obtain a simplified description where this coupling term is eliminated. This approach also accounts for the effective stochastic equations for the fluid-structure dynamics. The analysis is based on use of the Infinitesmal Generator of the SPDEs and a singular perturbation analysis of the Backward Kolomogorov PDEs. We also discuss the physical motivations and interpretation of the obtained reduced description of the fluid-structure system. Working paper currently under revision. Please report any comments or issues to [email protected]