Microscopic Motion of Particles Flowing through a Porous Medium

We use Stokesian Dynamics simulations to study the microscopic motion of particles suspended in fluids passing through porous media. We construct model porous media with fixed spherical particles, and allow mobile ones to move through this fixed bed under the action of an ambient velocity field. We first consider the pore scale motion of individual suspended particles at pore junctions. The relative particle flux into different possible directions exiting from a single pore, for two and three dimensional model porous media is found to approximately equal the corresponding fractional channel width or area. Next we consider the waiting time distribution for particles which are delayed in a junction, due to a stagnation point caused by a flow bifurcation. The waiting times are found to be controlled by two-particle interactions, and the distributions take the same form in model porous media as in two-particle systems. A simple theoretical estimate of the waiting time is consistent with the simulations. We also find that perturbing such a slow-moving particle by another nearby one leads to rather complicated behavior. We study the stability of geometrically trapped particles. For simple model traps, we find that particles passing nearby can ``relaunch'' the trapped particle through its hydrodynamic interaction, although the conditions for relaunching depend sensitively on the details of the trap and its surroundings.Comment: 16 pages, 19 figure

MODELING AND SIMULATION OF MIXING LAYER FLOWS FOR ROCKET ENGINE FILM COOLING

Film cooling has been selected for the thermal protection of the composite nozzle extension of the J-2X engine which is currently being developed for the second stage of NASA's next generation launch vehicle, the Ares I rocket. However, several challenges remain in order to achieve effective film cooling of the nozzle extension and to ensure its safe operation. The extreme complexity of the flow (three-dimensional wakes, lateral flows, vorticity, and flow separation) makes predicting film cooling performance difficult. There is also a dearth of useful supersonic film cooling data available for engineers to use in engine design and a lack of maturity of CFD tools to quantitatively match supersonic film cooling data. This dissertation advances the state of the art in film cooling by presenting semi-empirical analytical models which improve the basic physical understanding and prediction of the effects of pressure gradients, compressibility and density gradients on film cooling effectiveness. These models are shown to correlate most experimental data well and to resolve several conflicts in the open literature. The core-to-coolant stream velocity ratio, R, and the Kays acceleration parameter, KP, are identified as the critical parameters needed to understand how pressure gradients influence film cooling performance. The convective Mach number, Mc, the total temperature ratio, Ω0, and the Mach number of the high speed stream, MHS, are shown to be important when explaining the effects of compressibility and density gradient on film cooling effectiveness. An advance in the simulation of film cooling flows is also presented through the development of a computationally inexpensive RANS methodology capable of correctly predicting film cooling performance under turbulent, subsonic conditions. The subsonic simulation results suggest that it in order to obtain accurate predictions using RANS it is essential to thoroughly characterize the turbulent states at the inlet of the coolant and core streams of the film cooling flow. The limitations of this approach are established using a Grid Convergence Index (GCI) Test and a demonstration of the extension of this RANS methodology to supersonic conditions is presented

INVESTIGATION OF FUEL-AIR MIXING IN A MICRO-FLAMEHOLDER FOR MICRO-POWER AND SCRAMJET APPLICATIONS

This thesis presents a first principles model of the fuel-air mixing process in a micro-flameholder. This model is used to identify key design parameters involved in fuel-air mixing and to characterize how mixing performance scales with the Reynolds number. The results of this analysis show that fuel-air mixing in micro-flameholders occurs primarily at low Reynolds numbers (1<Re<5x103) traditionally associated with the laminar to transitional flow regime. Mixing lengths in micro-flameholders based solely on molecular diffusion are also predicted using a modified Burke-Schumann model. The predicted mixing lengths indicate that less distance is required for fuel-air mixing as micro-flameholders get smaller. Axisymmetric CFD simulations are performed to validate the predictions of the Burke-Schumann model, and to investigate the importance of axial diffusion and viscous effects. The results of these simulations suggest that viscous shear at the wall and at the fuel-air interface can significantly impact mixing lengths in micro-flameholders

System and method for analysis of the upper airway and a respiratory pressure support system

A system for analysis of the upper airway has a sensor arrangement with at least two sensor positions provided along a flow path leading to the mouth and/or nose of a user (4). A relation is derived between sensor signals at the two locations, and this is interpreted to detect at least the presence of upper airway obstructions, and preferably also the location and/or extent of such obstructions. The system is adapted to distinguish between inhalation and exhalation using the acoustic arrangement signals at the first and second locations

Method and device for the non-invasive monitoring and identification of drug effects and interactions

A method for use in detecting and monitoring effects experienced by a subject taking a first drug, drug A, and at least one other substance, substance B, the method comprising: (a) obtaining a plurality of measured values of a physiological characteristic of the subject over a time period; (b) comparing the measured values to a predefined signature, the drug A signature, associated with the first drug, and thereby calculating a measure, Adiff, of the difference between the measured values and the drug A signature; (c) comparing the measured values to a predefined signature, the substance B signature, associated with the at least one other substance, and thereby calculating a measure, Bdiff, of the difference between the measured values and the substance B signature; (d) comparing the measured values to a predefined signature, the DO signature, associated with a desired physiological state of the subject, and thereby calculating a measure, DOdiff, of the difference between the measured values and the DO signature; and (e) combining Adiff and DOdiff to produce an output

Laminar flow of two miscible fluids in a simple network

When a fluid comprised of multiple phases or constituents flows through a network, non-linear phenomena such as multiple stable equilibrium states and spontaneous oscillations can occur. Such behavior has been observed or predicted in a number of networks including the flow of blood through the microcirculation, the flow of picoliter droplets through microfluidic devices, the flow of magma through lava tubes, and two-phase flow in refrigeration systems. While the existence of non-linear phenomena in a network with many inter-connections containing fluids with complex rheology may seem unsurprising, this paper demonstrates that even simple networks containing Newtonian fluids in laminar flow can demonstrate multiple equilibria. The paper describes a theoretical and experimental investigation of the laminar flow of two miscible Newtonian fluids of different density and viscosity through a simple network. The fluids stratify due to gravity and remain as nearly distinct phases with some mixing occurring only by diffusion. This fluid system has the advantage that it is easily controlled and modeled, yet contains the key ingredients for network non-linearities. Experiments and 3D simulations are first used to explore how phases distribute at a single T-junction. Once the phase separation at a single junction is known, a network model is developed which predicts multiple equilibria in the simplest of networks. The existence of multiple stable equilibria is confirmed experimentally and a criteria for their existence is developed. The network results are generic and could be applied to or found in different physical systems

Going beyond 20 μm-sized channels for studying red blood cell phase separation in microfluidic bifurcations

Despite the development of microfluidics, experimental challenges are considerable for achieving a quantitative study of phase separation, i.e., the non-proportional dis- tribution of Red Blood Cells (RBCs) and suspending fluid, in microfluidic bifurca- tions with channels smaller than 20lm. Yet, a basic understanding of phase separation in such small vessels is needed for understanding the coupling between microvascular network architecture and dynamics at larger scale. Here, we present the experimental methodologies and measurement techniques developed for that pur- pose for RBC concentrations (tube hematocrits) ranging between 2% and 20%. The maximal RBC velocity profile is directly measured by a temporal cross-correlation technique which enables to capture the RBC slip velocity at walls with high resolu- tion, highlighting two different regimes (flat and more blunted ones) as a function of RBC confinement. The tube hematocrit is independently measured by a photometric technique. The RBC and suspending fluid flow rates are then deduced assuming the velocity profile of a Newtonian fluid with no slip at walls for the latter. The accuracy of this combination of techniques is demonstrated by comparison with reference measurements and verification of RBC and suspending fluid mass conservation at individual bifurcations. The present methodologies are much more accurate, with less than 15% relative errors, than the ones used in previous in vivo experiments. Their potential for studying steady state phase separation is demonstrated, highlight- ing an unexpected decrease of phase separation with increasing hematocrit in symmetrical, but not asymmetrical, bifurcations and providing new reference data in regimes where in vitro results were previously lacking. Published by AIP Publishin