Static Walls Research Articles

The effect of including dynamic imaging derived airway wall motion in CFD simulations of respiratory airflow in patients with OSA

Obstructive sleep apnea (OSA) is an airway disease caused by periodic collapse of the airway during sleep. Imaging-based subject-specific computational fluid dynamics (CFD) simulations allow non-invasive assessment of clinically relevant metrics such as total pressure loss (TPL) in patients with OSA. However, most of such studies use static airway geometries, which neglect physiological airway motion. This study aims to quantify how much the airway moves during the respiratory cycle, and to determine how much this motion affects CFD pressure loss predictions. Motion of the airway wall was quantified using cine MRI data captured over a single respiratory cycle in three subjects with OSA. Synchronously-measured respiratory airflow was used as the flow boundary condition for all simulations. Simulations were performed for full respiratory cycles with 5 different wall boundary conditions: (1) a moving airway wall, and static airway walls at (2) peak inhalation, (3) end inhalation, (4) peak exhalation, and (5) end exhalation. Geometric analysis exposed significant local airway cross-sectional area (CSA) variability, with local CSA varying as much as 300%. The comparative CFD simulations revealed the discrepancies between dynamic and static wall simulations are subject-specific, with TPL differing by up to 400% between static and dynamic simulations. There is no consistent pattern to which static wall CFD simulations overestimate or underestimate the airway TPL. This variability underscores the complexity of accurately modeling airway physiology and the importance of considering dynamic anatomical factors to predict realistic respiratory airflow dynamics in patients with OSA.

Heat and mass transfer analysis of non‐miscible couple stress and micropolar fluids flow through a porous saturated channel

AbstractThis study examines the flow rate, Bejan number transportation, concentration distribution and thermal characteristics of an immiscible couple stress‐ micropolar fluids within a porous channel. The authors focus on the effects of heat radiation and an angled magnetic field on the thermal dispersion, concentration distribution and entropy formation of two different types of incompressible immiscible micropolar and couple stress fluids inside a porous channel. Here, the static walls of the channel are isothermal, and the pressure gradient in the flow domain's entrance zone is constant. In this flow problem, we tried to simulate thermal radiation in the energy equation by applying Rosseland's diffusion approximation. To solve the problem, the authors have used no‐slip conditions at the channel's immovable walls, a continuity of temperature profile, shear stresses, thermal flux, linear velocity, and micro‐rotational velocity over the fluid‐fluid interface. The equations that govern the flow of immiscible fluids are solved using a well‐defined methodology and both the temperature and flow field are then evaluated using a closed‐form solution. The mathematical results of the thermal distribution and flow velocity are used to derive the Bejan number distribution and the entropy generation number. Graphical discussions are used to illustrate the impact of different emerging factors on the model's flow and thermal properties, which describe the major impact of the proposed model. These variables involve the micropolarity parameter, Reynolds number, inclination angle parameter, radiation parameter, and Hartmann number. The outcomes of the present models are corroborated by previously established results available in the literature.

Heating and energy performances of a dynamic Trombe wall incorporating phase change materials under different operation modes

Dynamic Trombe wall incorporating PCMs (DTWP) is promising for effectively utilizing solar energy and latent heat storage to achieve building energy saving and different operating modes of the DTWP have different functions corresponding to different thermal and energy performances. However, the suitable operation mode for this system is still unclear. Therefore, in this research, the heating and energy performances of the DTWP system with different operation modes and companion static concrete and PCM Trombe walls under a typical winter day in a cold climate were comprehensively investigated using the corresponding CFD model validated by experimental data. The main results are: (1) The DCNC mode(fully closed vents of the thermal storage wall during daytime and nighttime) showed significant decreases in heat loss and good heating performance for the DTWP system; (2) The DCNC mode could extend the time lag and the thermal comfort duration, and showed significantly improved energy saving by 97.6 %; (3) Compared to the DONO mode (opened vents of the thermal storage wall during daytime and nighttime), the DCNC mode could decrease the indoor temperature stratification by 53.7 % and 67.7 % at noon and midnight, respectively; (4) The air velocity near the outer surface of the energy storage wall and the inner surface of the glazing layer is higher than in the other regions at noon and midnight, respectively. The findings would provide guidance for the application and practical operation optimization of DTWP in cold climate regions.

Domain walls in Horndeski gravity

The Horndeski gravity yields domain wall solutions that connect asymmetric vacua for a certain region of parameters. In the thin wall limit there exist BPS domain walls that connect stable vacua. That is, the false vacua in this theory follows the same effect obtained in supergravity vacua, i.e., it does not decay according to Coleman-de Luccia theory and the BPS solutions interpolating different vacua, e.g., non-degenerate AdS vacua, are static infinite planar domain walls separating vacua associated with different spacetimes.

Scale resolving simulations of the effect of glottis motion and the laryngeal jet on flow dynamics during respiration

Background and ObjectiveThe movement of the respiratory walls has a significant impact on airflow through the respiratory tract. The majority of computational fluid dynamics (CFD) studies assume a static geometry which may not provide a realistic flow field. Furthermore, many studies use Reynolds Averaged Navier-Stokes (RANS) turbulence models that do not resolve turbulence structure. Combining the application of advanced scale-resolving turbulence models with moving respiratory walls using CFD will provide detailed insights into respiratory flow structures. MethodsThis study simulated a complete breathing cycle involving inhalation and exhalation in a nasal cavity to trachea geometry that incorporated moving glottis walls. A second breathing cycle was simulated with static glottis walls for comparison. A recently developed hybrid RANS-LES turbulence model, the Stress-Blended Eddy Simulation (SBES), was incorporated to resolve turbulent flow structures in fine detail for both transient simulations. Transient results were compared with steady-state RANS simulations for the same respiratory geometry. ResultsGlottis motion caused substantial effects on flow structure through the complete breathing cycle. Significant flow structure and velocity variations were observed due to glottal motion, primarily in the larynx and trachea. Resolved turbulence structures using SBES showed an intense mixing section in the glottis region during inhalation and in the nasopharynx during expiration, which was not present in the RANS simulations. ConclusionTransient simulations of a realistic breathing cycle uncovered flow structures absent in simulations with a constant flow rate. Furthermore, the incorporation of glottis motion impacted airflow characteristics that suggest rigid respiratory walls do not accurately describe respiratory flow. Future research in respiratory airflow should be conducted using transient scale-resolving models in conjunction with moving respiratory walls to capture flow structures in detail.

Thermally adaptive wall to enhance indoor comfort and energy performance

In the present work, a new environment-adaptive wall is proposed, based on an inside and outside radiant panel with pipes drowned on the panels themselves and hosting a heat-carrying liquid pushed by a pump. The purpose consists of transferring heat across the thickness of the wall, in the direction required for energy saving and enhancement of inner indoor thermal comfort. A Computational Fluid Dynamic (CFD) analysis is reported and the results are applied on a single zone box-shaped building, where a whole-year study is implemented by means of the transient simulation tool TRNSYS. The efficiency of this solution respect to the state-of the-art static walls is finally discussed. A concept of a laboratory setup of an adoptive wall along with first result are presented and discussed.

Mixing of powders under periodic shear stress: effect of initial filling level

Powder Mixing continues being of great importance in the field of particle technology because of the challenge to get homogeneous mixtures at a large scale. However, it is still moderately understood because of the complexity of designing 3D experiments that consider all the factors affecting the powder flow and powder mixing, hindering the acquisition of enough data that would provide a basic conception of powder behavior. This study involves a simple granular material mixing system of low friction that has five walls, where two of them are high friction moving walls following a cyclic function, and the others are non-friction static walls. The purpose was to create a complete set of 3D experiments of powder distribution to have a better insight into the inside mixing behavior as a function of the initial powder filling level. Experiments at different filling heights were run, obtaining the best mixing at the highest filling level. It was obtained a non-mixing zone with a triangle shape highly affected by the column particle weight. The increase of the vertical component of the resultant force produced by the acrylic motion walls causes a high momentum transfer in the x, y and z directions resulting in a decrease in the no-mixing zone. It was found an asymptotic behavior of the triangle height as a function of the initial filling height of the powders which indicates that values above a limited initial filling level will not have a significant effect on the triangle. A filling height increment of 100 % from 3.81 to 7.62 cm causes an increase in the mixing of 4.44 %. DEM simulations for mixing in the low-shear equipment were run to compare the experimental work

Heat and mass transfer analysis for MHD non‐miscible micropolar and Newtonian fluid flow in a rectangular porous channel

AbstractThe aim of the present work is to examine the entropy production characteristics, thermal profile, and flow behaviour of two non‐miscible natures of Newtonian and micropolar fluids, which take place through a rectangular porous enclosure channel. The flow region is divided into two distinct porous zones of the channel and is subjected to a constant oriented magnetic field. The Eringen's micropolar fluid is taking place in the upper porous zone, whereas in the lower porous zone, the Newtonian fluid is flowing. The wall surface of a rectangular porous channel is isothermal, and the flow of immiscible fluid through a porous channel takes place because of a constant pressure gradient. No slip condition is imposed on the static walls and continuity of vorticity, velocity, shear stress component, thermal distribution, and thermal flux are prescribed at the interface. Here, the production of entropy due to fluid friction and thermal exchange for non‐miscible Newtonian and micropolar fluids is evaluated. The characteristics of various estimated parameters on thermal and flow properties, such as Bejan number distribution, flow velocity, entropy production, and thermal profile, are discussed. The obtained results show that entropy production is directly proportional to viscous dissipation and Reynolds number, whereas it has a reverse nature with micropolarity parameter, inclination angle parameter, and Hartman number. Our results corroborate with previous published results.

Elastic softening and hardening at intersections between twin walls and surfaces in ferroelastic materials

Surfaces play a key role during ferroelastic switching and define the interactions of materials with ionic species and biological systems. Here, we perform molecular dynamics simulations and identify ridges and valleys with rounded singularities around the intersections between twin walls and surfaces. Two dominant length scales stem from the elastic bending of the surface layer (>30 lattice units) and local atomic reshuffles (some five lattice units). For static twin walls, which do not shift laterally under external stress, the intrinsic change in Young’s modulus involves softening near valleys and hardening near ridges. The boundary-induced changes in the surface Young’s modulus are of the order of 0.7%.

Bubble and particle dynamics in a thin rectangular bubbling fluidized bed under emulated marine instabilities

Floating fluidized beds are affected by swell/sea waves, limiting their effectiveness in seagoing applications as an alternative to fixed beds for the removal of exhaust pollutants. To address this, experiments were conducted using a rectangular fluidized bed mounted on a hexapod to emulate marine operation. The bed was subjected to dynamic perturbations, and Digital Image Analysis and Particle Image Velocimetry were used to analyze the effects of motion on fluidization, bubble growth/shape, void/velocity fields, and gas/solids circulations. Our investigation showed that non-vertical orientations resulted in a decrease in minimum fluidization velocity, but the most significant effect was seen in the trajectory and shape of the bubbles near both static and dynamic walls. This resulted in a slug regime with a high degree of gas bypass. These outcomes have important implications for the design/operation of marinized fluidized beds, and for process intensification solutions to address the constraints associated with ship emissions.

A puzzling insensitivity of magnon spin diffusion to the presence of 180-degree domain walls

We present room-temperature measurements of magnon spin diffusion in epitaxial ferrimagnetic insulator MgAl0.5Fe1.5O4 (MAFO) thin films near zero applied magnetic field where the sample forms a multi-domain state. Due to a weak uniaxial magnetic anisotropy, the domains are separated primarily by 180° domain walls. We find, surprisingly, that the presence of the domain walls has very little effect on the spin diffusion – nonlocal spin transport signals in the multi-domain state retain at least 95% of the maximum signal strength measured for the spatially-uniform magnetic state, over distances at least five times the typical domain size. This result is in conflict with simple models of interactions between magnons and static domain walls, which predict that the spin polarization carried by the magnons reverses upon passage through a 180° domain wall.

ANALYSIS OF ENTROPY PRODUCTION OF IMMISCIBLE MICROPOLAR AND NEWTONIAN FLUIDS FLOW THROUGH A CHANNEL: EFFECT OF THERMAL RADIATION AND MAGNETIC FIELD

This paper aims to analyze the thermal characteristics, entropy production, flow velocity and Bejan number profile for immiscible nature of micropolar and Newtonian viscous fluid within a channel. Here, the authors emphasize the influence of thermal radiation and oriented magnetic field on the thermal profile and entropy generation of two different types of non-miscible and incompressible micropolar and Newtonian fluids in a channel. The viscous dissipation and thermal radiation effect are also considered in the thermal energy equation. In this work, the entropy production is analyzed within a channel due to oriented magnetic field and thermal radiation. A constant pressure gradient acts on the entry zone of flow domain and static walls of the channel are isothermal. In this problem, we tried to simulate thermal radiation in energy equation by adopting the Rosseland’s diffusion approximation. According to geometrical configuration of the problem, the conditions of no-slip at the walls of the channel and continuity of thermal exchange, microrotation, shear stress, flow velocity and heat flux at the interface of immiscible fluids are used. The governing equations for the flow of immiscible fluids are solved by reliable technique and exact solution for thermal characteristics and flow field are evaluated. The mathematical results of thermal profile and flow characteristics are used to obtain the Bejan number profile as well as the entropy production number profile. The influence of various thermo-physical governing parameters such as radiation parameter, Reynolds number, inclination angle parameter, viscous dissipation parameter, micropolarity parameter and Hartmann number, which describe the physical significance of the present model, on the flow and thermal characteristics of the model are discussed graphically. The newly obtained results of this study are verified with previous published results.

Interacting with actuated walls: Exploring applications and input types

Previous literature on small-scale shape displays demonstrated the benefit of shape-changing interfaces and adaptive environments. However, research with large-scale implementations is limited to specific equipment. Thus, there is little understanding of how they can enrich user experience in a larger scope, especially from an interactive perspective. We proposed using pin-array large-scale implementations made of linear actuators matrix as vertical structures to replace static and planar walls and conducted a two-fold exploration to extend the understanding of potential applications and showcase users’ behaviors and preferences to inspire researchers. We summarized 14 potential applications for shape-changing walls, and presented users’ interaction preferences with such a wall of different sizes and interaction distances. We concluded with recommendations and guidelines for practitioners to design large-scale actuated walls.

Analysis of the Impact of Passive Design Strategies on Energy Consumption of a Building in Composite Climate Zone

The demand for buildings that can deal with current challenges such as environmental conservation, climate change, and sustainability etc., has increased as cities have grown more rapidly. Retrofitting of existing buildings utilizing ideas such as sustainability can enhance the energy performance of new and existing infrastructure while taking economic and cultural issues into account. Buildings account for over 40% of global power demand as well as 40% of CO2 emissions. This paper presents a thorough analysis of thermal performance parameters for composite building walls, including thermal transmittance. Renewable energy sources provide an endless supply of power. Solar energy can be used passively or actively to complement a building’s energy demands. It is possible to minimize energy consumption for a building’s heating, cooling, and lighting needs by using a climate-sensitive strategy in the design of architectural elements such as static sunshades, walls, and roofs. It has been observed that the basecase consumes 93134.4 kWh, whereas the proposed-case consumes 68317.7 kWh. There is a huge 26.65% reduction in the annual energy consumption by adopting passive design strategies.

Structural transformation of spin nanoclusters in low-dimensional anisotropic ferromagnets under applied magnetic field

Noncollinear discrete domain walls in the Heisenberg anisotropic ferromagnetic chain under applied magnetic field and their small excitation spectra are studied analytically and numerically in the framework of the Takeno-Homma equation. The intersecting frequency dependences of localized excitations and continuous spectrum oscillations and the removal of the degeneracy by the magnetic field are revealed. The variational approach is proposed to describe the domain walls and to investigate their stability. It is shown that the obtained analytical expressions fit very well the numerical solutions. The total energy of static discrete domain walls and the Peierls energy barrier between them are found explicitly. The stability diagram for noncollinear domain walls on the plane of parameters of the exchange and the magnetic field is calculated, and it looks like the alternating stripes structure of stability regions of the bond-centered and site-centered discrete domain walls. This diagram feature is explained by the oscillating dependence of the Peierls energy barrier on the exchange and the magnetic field parameters.

A new cardiac phantom for dynamic SPECT

BackgroundIn recent years, with the advance of myocardial blood flow (MBF) measurement capability in dynamic single photon emission computerized tomography (SPECT) systems, significant effort has been devoted to validation of the new capability. Unfortunately, the mechanical phantoms available for the validation process lack essential features—they either have a constant radiotracer concentration or they have rigid (static) walls unable to simulate cardiac beating. Methods and ResultsWe have developed a mechanical cardiac phantom that is able to mimic physiological radiotracer variation in the left ventricle (LV) cavity and in the myocardium (M), while performing beating-like motion. We have also developed a mathematical model of the phantom, allowing a description of the radiotracer concentrations in both regions (LV, M) as a function of time, which served as a tool for experiment planning and to accurately mimic physiological-like time-activity curves (TACs). A net retention model for the phantom was also developed, which served to compute the theoretical (i.e., expected) MBF of the phantom from measured quantities only, and thus validate the MBF reported by the SPECT system. In this paper, phantom experiments were performed on a GE Discovery NM 530c SPECT system. ConclusionsA novel dynamic cardiac phantom for emission tomography has been developed. The new phantom is capable of producing a wide range of TACs that can mimic physiological (and potentially in the future, pathological) curves, similar to those observed in dynamic SPECT systems. SPECT-reported MBF values were validated against known (measured) activity of the injected radiotracer from phantom experiments, which allowed to determine the accuracy of the GE Discovery 530c SPECT system.

An inclined magnetic field effect on entropy production of non-miscible Newtonian and micropolar fluid in a rectangular conduit

The present work concerns with an entropy production of two-phase non-miscible Newtonian and micropolar fluid in a rectangular conduit. In this model, the flow regime is divided in two distinct parts in such a way that Newtonian fluid takes place in the lower portion of the channel whereas non-Newtonian micropolar fluid flows in upper half of the channel. The walls of rectangular channel are kept at constant temperature and a constant pressure gradient is applied at the entrance zone of the rectangular conduit. The no slip boundary conditions are imposed on static walls of the channel and linear velocity, shear stress, microrotational velocity and thermal flux are considered continuous at the interface part of the channel. An analytical solution of the concerned governing partial differential equation which has been reduced in an ordinary differential equation for the considered problem, is obtained. Moreover, the momentum and energy equations are utilized to obtain the entropy production number and Bejan number. However, the impact of numerous hydrodynamic and thermal parameters such as inclination angle parameter, Reynolds number, micropolarity parameter, Hartmann number, viscous dissipation parameter on the fluid flow, thermal exchange, entropy production number and Bejan number are figure out graphically. The findings is verified with the previously estabished results. The present analysis is utilized in petroleum industries.

Conformal scale factor inversion for domain walls and holography

We describe a correspondence between domain wall solutions of Einstein gravity with a single scalar field and self-interaction potential. The correspondence we call conformal scale factor inversion (CSFI) is a map comprising the inversion of the scale factor in conformal coordinates, and a transformation of the field and its potential which preserves the form of the Einstein equations for static and isotropic domain walls. By construction, CSFI maps the asymptotic anti--de Sitter boundary to the vicinity of a naked singularity in a theory with a special Liouville (exponential) scalar field potential; it is also a map in the parameter space of exponential potentials. The correspondence can be extended to linear fluctuations, being akin to an S-duality, and can be interpreted in terms of ``SUSY quantum mechanics'' for the fluctuation modes. The holographic implementation of CSFI relates the UV and IR regimes of a pair of holographic renormalization group flows; in particular, it is a symmetry of the GPPZ flow.

Domain wall contributions to piezoelectricity in relaxor-lead titanate single crystals

It is demonstrated that the dielectric permittivity and piezoelectric coefficients in relaxor-PbTiO3 single crystals close to the morphotropic phase boundary (MPB) can be augmented by contributions from domain walls. Landau-Ginzburg-Devonshire models, incorporating both polarization and strain gradients through the domain walls, show that wall contributions in domain engineered single crystals originate from enhanced, field-induced polarization rotation in static domain walls, unlike ceramics, in which piezoelectricity is enhanced by domain wall translation. For 71° domain walls in 0.7 Pb(Mg1/3Nb2/3)O3 – 0.3PbTiO3 the piezoelectric charge coefficient d33 at the center of the wall ranges from 5000 to >30,000 pC N−1 depending on the wall width. Thus, a sufficiently high domain wall density can account for the experimentally observed augmentation in the measured properties compared to single domain models. The symmetry of the domain walls explains both the variety of average symmetries observed close to the MPB and the experimentally observed switching of the [001]-oriented crystals into the tetragonal phase via a symmetry-improbable MC phase. For a crystal of rhombohedral ground state, the presence of domain walls will impart monoclinic symmetry, the predominance of which increases with increasing domain wall density.

Photodetachment dynamics of negative ion confined in a time-dependent quantum well

We investigate the photodetachment dynamics of negative ion confined in a time-dependent trap created by a quantum well with an oscillating wall for the first time. Compared with the photodetachment of negative ion in a quantum well with static walls, the movement of the detached electron becomes substantially complicated and interesting as one wall in the quantum well oscillates with time. The position–time plot of the detached electron in the quantum well shows a fractal structure of asymptotic self-similarity due to the influence of the oscillating wall. The returning kinetic energy of the detached electron may have been accelerated to high energy by colliding with the oscillating wall. By calculating the photodetachment cross section of this system, we find that the oscillating wall in the quantum well functions as an external driving potential for the photodetachment of negative ion. The photodetachment cross section becomes increasingly complicated due to the influence of the oscillating wall. In addition, the amplitude, initial phase, and the initial position of the oscillating wall all affect the photodetachment cross section of this system. Our study may guide future experimental investigations on the photodetachment dynamics in the ion trap.