Lattice-Boltzmann Method for Non-Newtonian Fluid Flows

We study an ad hoc extension of the Lattice-Boltzmann method that allows the simulation of non-Newtonian fluids described by generalized Newtonian models. We extensively test the accuracy of the method for the case of shear-thinning and shear-thickening truncated power-law fluids in the parallel plate geometry, and show that the relative error compared to analytical solutions decays approximately linear with the lattice resolution. Finally, we also tested the method in the reentrant-flow geometry, in which the shear-rate is no-longer a scalar and the presence of two singular points requires high accuracy in order to obtain satisfactory resolution in the local stress near these points. In this geometry, we also found excellent agreement with the solutions obtained by standard finite-element methods, and the agreement improves with higher lattice resolution

A Geometric Representation for the Proca Model

The Proca model is quantized in an open-path dependent representation that generalizes the Loop Representation of gauge theories. The starting point is a gauge invariant Lagrangian that reduces to the Proca Lagrangian when certain gauge is selected.Comment: 10 pages, Late

Simulation of the velocity field in compound channel flow using different closure models

1st European IAHR Congress,6-4 May, Edinburg, ScotlandIn this study a comparison of three turbulence closure models (two isotropic and one anisotropic) with experimental data is performed. The interaction between the main channel (MC) flow and the floodplain (FP) generates a complex flow structure. A shallow mixing layer develops between the MC flow and the slower FP flow generating a high horizontal shear layer, streamwise and vertical vortices, momentum transfer and other phenomena, related to velocity retardation and acceleration. This phenomenon dissipates part of the kinetic energy and contributes to the reduction of the velocity differences between the MC and the FP. The large scale vortices that are generated in the shear layer are anisotropic, provoking the formation of secondary flow cells that influence the primary velocity distribution. These threedimensional turbulent structures can be reasonable well reproduced by a simple anisotropic model (Algebraic Stress Model). The isotropic models are capable of simulating the boundary layer, especially the model base in k-ω equations, but cannot simulate the shear layer that develops at the interface

The scenario of two-dimensional instabilities of the cylinder wake under EHD forcing: A linear stability analysis

We propose to study the stability properties of an air flow wake forced by a dielectric barrier discharge (DBD) actuator, which is a type of electrohydrodynamic (EHD) actuator. These actuators add momentum to the flow around a cylinder in regions close to the wall and, in our case, are symmetrically disposed near the boundary layer separation point. Since the forcing frequencies, typical of DBD, are much higher than the natural shedding frequency of the flow, we will be considering the forcing actuation as stationary. In the first part, the flow around a circular cylinder modified by EHD actuators will be experimentally studied by means of particle image velocimetry (PIV). In the second part, the EHD actuators have been numerically implemented as a boundary condition on the cylinder surface. Using this boundary condition, the computationally obtained base flow is then compared with the experimental one in order to relate the control parameters from both methodologies. After validating the obtained agreement, we study the Hopf bifurcation that appears once the flow starts the vortex shedding through experimental and computational approaches. For the base flow derived from experimentally obtained snapshots, we monitor the evolution of the velocity amplitude oscillations. As to the computationally obtained base flow, its stability is analyzed by solving a global eigenvalue problem obtained from the linearized Navier–Stokes equations. Finally, the critical parameters obtained from both approaches are compared

Verification and validation of computational fluid dynamics simulations of compound channel

In this study the verification and validation of a 2nd order turbulence closure model is performed for an experimental compound channel flow, where the velocity field was measured by a Laser Doppler Velocimeter. Detailed Explicit Algebraic Reynolds Stress Model (EARSM) simulation is reported. The Grid Convergence Index (GCI) approach proposed by Roache (1998) was adopted to evaluate the uncertainty associated to grid resolution. The velocity components, the turbulent kinetic energy (TKE) and the dissipation rate were used as variables of interest. The GCI results present low values for the streamwise velocity, TKE and dissipation rate, but higher values in what concerns vertical and spanwise velocities. This indicates that the mean primary flow has converged but the secondary flow field still depends on grid resolution. Based on GCI values distribution, the mesh was locally refined. Comparison of numerical and experimental results shows good agreemen

Verification and validation of computational fluid dynamics simulations of compound channel

In this study the verification and validation of a 2nd order turbulence closure model is performed for an experimental compound channel flow, where the velocity field was measured by a Laser Doppler Velocimeter. Detailed Explicit Algebraic Reynolds Stress Model (EARSM) simulation is reported. The Grid Convergence Index (GCI) approach proposed by Roache (1998) was adopted to evaluate the uncertainty associated to grid resolution. The velocity components, the turbulent kinetic energy (TKE) and the dissipation rate were used as variables of interest. The GCI results present low values for the streamwise velocity, TKE and dissipation rate, but higher values in what concerns vertical and spanwise velocities. This indicates that the mean primary flow has converged but the secondary flow field still depends on grid resolution. Based on GCI values distribution, the mesh was locally refined. Comparison of numerical and experimental results shows good agreemen

Isochronous island bifurcations driven by resonant magnetic perturbations in Tokamaks

Recent evidences show that heteroclinic bifurcations in magnetic islands may be caused by the amplitude variation of resonant magnetic perturbations in tokamaks. To investigate the onset of these bifurcations, we consider a large aspect ratio tokamak with an ergodic limiter composed of two pairs of rings that create external primary perturbations with two sets of wave numbers. An individual pair produces hyperbolic and elliptic periodic points, and its associated islands, that are consistent with the Poincar\'e-Birkhoff fixed point theorem. However, for two pairs producing external perturbations resonant on the same rational surface, we show that different configurations of isochronous island chains may appear on phase space according to the amplitude of the electric currents in each pair of the ergodic limiter. When one of the electric currents increases, isochronous bifurcations take place and new islands are created with the same winding number as the preceding islands. We present examples of bifurcation sequences displaying (a) direct transitions from the island chain configuration generated by one of the pairs to the configuration produced by the other pair, and (b) transitions with intermediate configurations produced by the limiter pairs coupling. Furthermore, we identify shearless bifurcations inside some isochronous islands, originating nonmonotonic local winding number profiles with associated shearless invariant curves

A Genomic Signature and the Identification of New Sporulation Genes

Bacterial endospores are the most resistant cell type known to humans, as they are able to withstand extremes of temperature, pressure, chemical injury, and time. They are also of interest because the endospore is the infective particle in a variety of human and livestock diseases. Endosporulation is characterized by the morphogenesis of an endospore within a mother cell. Based on the genes known to be involved in endosporulation in the model organism Bacillus subtilis, a conserved core of about 100 genes was derived, representing the minimal machinery for endosporulation. The core was used to define a genomic signature of about 50 genes that are able to distinguish endospore-forming organisms, based on complete genome sequences, and we show this 50-gene signature is robust against phylogenetic proximity and other artifacts. This signature includes previously uncharacterized genes that we can now show are important for sporulation in B. subtilis and/or are under developmental control, thus further validating this genomic signature. We also predict that a series of polyextremophylic organisms, as well as several gut bacteria, are able to form endospores, and we identified 3 new loci essential for sporulation in B. subtilis: ytaF, ylmC, and ylzA. In all, the results support the view that endosporulation likely evolved once, at the base of the Firmicutes phylum, and is unrelated to other bacterial cell differentiation programs and that this involved the evolution of new genes and functions, as well as the cooption of ancestral, housekeeping functions.FCT grant: (PEst-OE/EQB/LA0004/2011), FCT Ph.D. fellowship: (SFRH/BPD/36328/2007), FCT postdoc fellowship: (SFRH/BPD/65605/2009), Instituto Gulbenkian de Ciência research fellowship

A conceptual model for sheet-flow drawn from rapid granular flow theories

33rd IAHR Congress: Water Engineering for a Sustainable EnvironmentThis paper is aimed at presenting i) a simple, yet sound, conceptual model applicable to the simulation of erosion, deposition and transport of cohesionless sediment in stratified flows under high shear stresses and ii) numerical solutions in idealized unsteady flow non-equilibrium transport situations. The conceptual model for the granular phase comprises 2DV mass and momentum and energy equations and constitutive equations, all derived within the dense limit of the Chapman-Enskog kinetic theory. 1D shallow-flow conservation and closure equations are derived for the fluid-granular mixture. Formulas for the average velocity in the transport layers, the vertical net flux of sediment mass and the thickness of the transport layer are thus obtained. Numerical solutions for dam-break flows over cohesionless mobile beds in prismatic and non-prismatic channels are obtained and discussed

Ion species fractions in the far-field plume of a high-specific impulse Hall thruster

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76418/1/AIAA-2003-5001-731.pd