Abstract Background In recent years, many studies have evaluated the effects of noninvasive brain stimulation (NIBS) techniques for the treatment of several neurological and psychiatric disorders. ...Positive results led to approval of NIBS for some of these conditions by the Food and Drug Administration in the USA. The therapeutic effects of NIBS have been related to bi-directional changes in cortical excitability with the direction of change depending on the choice of stimulation protocol. Although after-effects are mostly short lived, complex neurobiological mechanisms related to changes in synaptic excitability bear the potential to further induce therapy-relevant lasting changes. Objective To review recent neurobiological findings obtained from in vitro and in vivo studies that highlight molecular and cellular mechanisms of short- and long-term changes of synaptic plasticity after NIBS. Findings Long-term potentiation (LTP) and depression (LTD) phenomena by itself are insufficient in explaining the early and long term changes taking place after short episodes of NIBS. Preliminary experimental studies indicate a complex scenario potentially relevant to the therapeutic effects of NIBS, including gene activation/regulation, de novo protein expression, morphological changes, changes in intrinsic firing properties and modified network properties resulting from changed inhibition, homeostatic processes and glial function. Conclusions This review brings into focus the neurobiological mechanisms underlying long-term after-effects of repetitive transcranial magnetic stimulation (rTMS) and transcranial direct current stimulation (tDCS) recently obtained from in vitro and in vivo studies, both in animals and humans.
Two phase direct particle-high speed compressible gas flow simulation techniques developed by the author are extended to include the effects of particle deformation and plasticity with a focus on ...high speed impact of metallic steel particles moving in high speed air. The first part of the study involves the development of the necessary modeling techniques. Using the results of fluid independent finite element analyses, normal force functions were devised to simulate the effect of a collision of two 1.5mm radius steel particles over a range of relative impact velocities from 12.5 to 200m/s with an elastic and then an elastic perfectly plastic material model. Methods were introduced to model the deformation of the particles/objects. Use of the collision force model and the inclusion of deformation in a simulation of the collision between two particles in air replicated the appropriate short term post collision velocities of the corresponding finite element analysis. The parametric studies conducted during this model development demonstrated the importance of utilizing the proper material model in predicting the motion of the particles. A longer contact time and increasingly comparable post collision velocities for the two colliding particles develop with an elastic perfectly plastic based collision model, with the differences from the elastic based model results growing with collision velocity. The modeling techniques were then applied to analyze the motion of fourteen steel particles placed ahead of a gas driven piston in a flow channel 19.5mm in width for three different piston driving pressures of 50, 75, and 100MPa, implementing the elastic and then the elastic plastic based collision force models. The results clearly indicate that the incorporation of plasticity and particle deformation causes the particles to remain closer together both inside and outside of the flow channel. The modeling methods developed provide the ability to incorporate the plasticity/deformation effects of multiple interacting particles together with the local flow induced force to simulate the coupled gas flow and particle motion. The techniques developed offer a tool to capture more of the physical phenomena occurring in the two phase flow regime of interest, extending the capabilities to simulate the motion of gas-particles to a wider range of particle and flow velocities and providing information to assist in the understanding of particle motion and interaction characteristics.
Display omitted
•Finite element based collision model for elastic and elastic-plastic materials.•Used in direct CFD based particle-flow coupled modeling method.•Method to allow for particle deformation developed and implemented.•Particle ejection from a flow channel by gas pressure driven piston.•Revealed importance of using the appropriate material property model.
Display omitted
•Motion, deformation, break-up of molten copper droplets in compressible flow.•Higher driving pressures transport more copper out of system.•Solidification requires proper cooper ...location and flow and surface temperatures.
In this work, the motion, deformation, break-up, and deposition of molten copper droplets transported by high speed, high pressure, and high temperature gas flow along a steel internal flow path are examined numerically using a volume of fluid based approach. The study of the related phenomena is motivated by the erosion and fouling patterns along flow paths caused by such droplets. Good correlation to literature was achieved for a single drop impact. Methods were applied to multiple drops moving with gas flow in a main flow channel with a by-pass with the strength of the flow varied by changing the initial pressure in the main channel gas by up to a factor of four. The amount of copper removed from the main flow path grows from 58% to 87% when the driving pressure is quadrupled. A greater portion of the copper in the by-pass section is solidified as well with 90% solidified for the higher pressure, compared to 60% at the lower pressure. Solidification of the copper and melting of the steel require the proper combination of the molten copper location, the temperatures in the copper, steel, and gas, and the velocity of the transporting gas flow. The knowledge gained from the study can be applied to mitigate the fouling and erosion that may develop along internal surfaces. Because of the need for a small element size to prevent copper mass loss, the use of this method with complex three dimensional geometries may be limited.
Display omitted
•New comprehensive multi-mode thermal interaction model for fluid flow with discretized particles.•Conductive contact model devised while maintaining a gap space for mesh topology ...preservation.•Convective energy exchange between fluids and solid surfaces locally determined.•Radiative heat transfer found from local temperature and geometry conditions.•Technique used to study thermal and mechanically interacting particle-fluid flow in CFD models.
A novel technique is proposed to simulate the thermal interactions occurring within a compressible fluid flow with particles. Thermal exchanges between particles, fluid, and flow paths including particle-particle and particle-solid body contact are included. Contact conduction with a contact resistance, convection, and radiation modes of heat transfer are accounted for without the need for heat transfer coefficient correlations or other simplifying assumptions since the particles are discretized within the fluid domain. The discretization of the particles in the flow allows for the local temperature and flow field data to be utilized in calculating the thermal interactions and facilitates the exploration of temperature dependent phenomena based on local instead of bulk interface conditions. So that these developed techniques can be applied to computational fluid dynamics simulations with moving objects, a fluid gap space is retained around the particles while the thermal conductive contact effects are simulated. The method is applied to an eleven fixed particle configuration with the flow driver strengths and limited radiative emissivities varied. The results demonstrate that for the conditions considered, the conductive contact dominates the temperature field development, with the radiative and convective cooling taking a lesser role. The new method clearly improves capabilities for the study of the thermal interaction in particle laden high speed gas flow and thus of temperature dependent phenomena for a more comprehensive simulation.
A novel computational technique is applied to investigate particle trapping in straight and bent channel flow paths with various groove configurations in high-speed compressible, particle laden flow. ...The technique is valid for particle sizes of the same order of magnitude as the groove dimensions and where the particle–flow path, particle–particle, and particle–flow interactions play significant roles in determining the particle motion. The sacrificial grooves within the flow path can remove particles from the flow to reduce particle impact-induced wear. The feasibility of the trapping grooves and the conditions for which they are most beneficial can be gleaned from analysis of the model results. Three groove configurations are studied: a straight groove, a flared groove, and a 45 degree angle groove, for the same groove entrance size, groove depth, and spacing in a straight channel and a channel with a 90 degree bend. A transient maximum of 22% of the particles were trapped for the flared groove for the bent channel and a transient maximum of 15% of the particles for the straight channel configuration. The second groove of the bent channel produces the greatest single groove particle holding of 8.25% of all of the particles for the flared grove configuration. The contributions of the groove positioning, groove shape, gas flow, and particle interaction conditions to the trapping characteristics can be readily obtained from examination of the model results since the modeling technique includes detailed treatment of particle–flow path and flow interactions, allowing for the study of the mechanisms acting to trap the particles within the grooves.
This work involves the investigation of the sensitivity of computational fluid dynamics based models of auto ignition of hydrogen gas escaping into the surroundings to the use of an ideal gas and a ...real gas Noble–Abel equation of state. Ensuring consistent modeling techniques when the real gas equation of state is implemented, real gas based thermodynamic properties, real gas based property mixture models, and real gas based chemical equilibrium constant formulations are utilized. Within the standard computational fluid dynamics models, a customized chemical kinetic equation integrator is employed. An LES based turbulence model is implemented. For tank pressures of 40, 80, and 120 MPa, differences in the gas conditions, including gas pressures, temperatures, velocities, flow rates, energy, and chemical species mass fractions, are compared. The relationships between the local and time varying gas conditions, chemical reaction indicators, the tank pressure, and the equation of state captured in the simulations are described in detail. The results clearly show the increasing deviation between the ideal gas and Noble–Abel based results as the tank pressure increases, indicating the importance of the use of the proper material model and chemical equilibrium formulation for the conditions of interest.
•Customized – chemical kinetics model developed.•Noble–Abel consistent material properties and chemical kinetic parameters.•Results show relationships between local flow and reaction characteristics.•Results indicate importance of use of equation of state appropriate to conditions.•Findings can be applied to hydrogen escape studies for improved predictions.
Maternal mortality in the United States is the highest among all developed nations, partly because of the increased prevalence of cardiovascular disease in pregnancy and beyond. There is growing ...recognition that specialists involved in caring for obstetric patients with cardiovascular disease need training in the new discipline of cardio-obstetrics. Training can include integrated formal cardio-obstetrics curricula in general cardiovascular disease training programs, and developing and disseminating joint cardiac and obstetric societal guidelines. Other efforts to help strengthen the cardio-obstetric field include increased collaborations and advocacy efforts between stakeholder organizations, development of US-based registries, and widespread establishment of multidisciplinary pregnancy heart teams. In this review, we present the current challenges in creating a cardio-obstetrics community, present the growing need for education and training of cardiovascular disease practitioners skilled in the care of obstetric patients, and identify potential solutions and future efforts to improve cardiovascular care of this high-risk patient population.
A comprehensive two-phase compressible gas-solid particle computational fluid dynamics-based modeling method has been developed and applied to study the erosion and fouling along flow path surfaces ...due to particle impact. Extending upon methods to predict particle interaction phenomena which incorporate rolling, twisting, sliding, and adhesion forces to capture conditions that frequently occur as particle laden flow passes over a flow path surface, the simulation methods have been specifically designed for high particle concentrations and large particle sizes. In the developed technique, individual particles are directly discretized in the computational mesh, and the particles move through the fluid, and interact with the flow, flow path boundaries, and other particles. With particle motion calculated from the direct two-phase flow simulations, the distribution and intensity of the mass loss or erosion conditions along a flow path surface resulting from particle impact is predicted. The developed method was applied to study the erosion of an aluminum wall due to impacting sand particles of different sizes, impact velocities, and counts. A more fluid-like particle motion and distribution upon impact were found as the particle size decreased. The method was also implemented to examine the erosion that develops for three common flow configurations which force particle-flow path surface interactions: a diverging/converging flow path, a bypass flow path, and a baffle configuration. These applications demonstrate the utility of the model to explore and better understand the relationships between the geometry, the flow, the particles, and the adhesion and erosion that develop as particle laden flow moves over the flow path surfaces.
Display omitted
•Employs computational direct two-phase solid particle-compressible gas flow interaction simulation to erosion prediction•Effectively incorporates rolling, twisting, sliding, adhesion effects in particle interaction•Indicates erosion intensity and distribution altered by particle size/count/impact velocity•Aids in understanding relationships between particle and flow conditions, flow path geometry, and erosion distribution
In this work, structural finite element analyses of particles moving and interacting within high speed compressible flow are directly coupled to computational fluid dynamics and heat transfer ...analyses to provide more detailed and improved simulations of particle laden flow under these operating conditions. For a given solid material model, stresses and displacements throughout the solid body are determined with the particle–particle contact following an element to element local spring force model and local fluid induced forces directly calculated from the finite volume flow solution. Plasticity and particle deformation common in such a flow regime can be incorporated in a more rigorous manner than typical discrete element models where structural conditions are not directly modeled. Using the developed techniques, simulations of normal collisions between two 1mm radius particles with initial particle velocities of 50–150m/s are conducted with different levels of pressure driven gas flow moving normal to the initial particle motion for elastic and elastic–plastic with strain hardening based solid material models. In this manner, the relationships between the collision velocity, the material behavior models, and the fluid flow and the particle motion and deformation can be investigated. The elastic–plastic material behavior results in post collision velocities 16–50% of their pre-collision values while the elastic-based particle collisions nearly regained their initial velocity upon rebound. The elastic–plastic material models produce contact forces less than half of those for elastic collisions, longer contact times, and greater particle deformation. Fluid flow forces affect the particle motion even at high collision speeds regardless of the solid material behavior model. With the elastic models, the collision force varied little with the strength of the gas flow driver. For the elastic–plastic models, the larger particle deformation and the resulting increasingly asymmetric loading lead to growing differences in the collision force magnitudes and directions as the gas flow strength increased. The coupled finite volume flow and finite element structural analyses provide a capability to capture the interdependencies between the interaction of the particles, the particle deformation, the fluid flow and the particle motion.
Coupled finite volume fluid flow and finite element structural analysis – particle collision in compressible gas. Display omitted
•Dynamic coupling of finite volume fluid flow and finite element structural analysis.•Elastic and elastic–plastic with isotropic strain hardening.•Range of collision velocities.•Range of fluid flow driving pressures.
A numerical investigation was conducted into an alternative method of natural convection enhancement by the transverse oscillations of a thin short plate, strategically positioned in close proximity ...to a rectangular heat source. The heat source is attached to a mounting board in a vertical channel. Two-dimensional laminar flow finite element studies were carried out with the oscillation parameters, the oscillating plate-heat source mean clearance spacing, and the oscillating plate position varied. Significant cooling was found for displacement amplitudes of at least one-third of the mean clearance together with frequencies
(
Re
/
Gr
)
of over 2π with the displacement being more critical to the cooling level. For the parameters investigated, up to a 52% increase in the local heat transfer coefficient relative to standard natural convection was obtained. The results indicate that this method can serve as a feasible, simpler, more energy and space efficient alternative to common methods of cooling for low power dissipating devices operating at conditions just beyond the reach of pure natural convection.
PDF
