Wall Separation Research Articles

Parametric investigation of the pintle-perturbed conical nozzle flows

Thrust variation, flow structures and geometric factors of pintle-perturbed conical nozzle are studied experimentally and numerically under the condition of constant mass flow rate. Regulators and choking orifice are installed for constant mass flow rate and static pressure distributions along the nozzle wall, chamber pressure and thrust are measured. In order to prevent transfer of non-linear load to the load cell by the flexible tubes, the load cell for thrust measurement is calibrated. Roeʼs flux-difference splitting scheme is applied to resolve the flow discontinuities such as the lip shock, trailing shock and oblique shock typical in the pintle-perturbed nozzle flows. Three different turbulent models of k–ε, k–ω SST and Spalart–Allmaras are examined to assess their validity for resolving the turbulence. The predicted thrust by empirical method, which includes influence of deliverable thrust coefficient by flow separation, showed good agreement with measured thrust at the minimum and maximum throat area. Spalart–Allmaras model and k–ω SST model showed good agreement with experiment on predicting the thrust especially in the case that the flow separation occurs in the nozzle diverging section, and the Spalart–Allmaras model predicted wall separation point more accurately than the other models. Shock impinging point and the flow separation point induced by shock wave separation occur and move in the nozzle flow as the pintle inserted into the nozzle throat. Because, the dominant factor which determines thrust is chamber pressure, the pintle shape must be considered essentially in view of minimizing the force acting on the pintle.

Hypofractionated Whole-Breast Radiation Therapy: Does Breast Size Matter?

To evaluate the effects of breast size on dose-volume histogram parameters and clinical toxicity in whole-breast hypofractionated radiation therapy using intensity modulated radiation therapy (IMRT). In this retrospective study, all patients undergoing breast-conserving therapy between 2005 and 2009 were screened, and qualifying consecutive patients were included in 1 of 2 cohorts: large-breasted patients (chest wall separation>25 cm or planning target volume [PTV]>1500 cm3) (n=97) and small-breasted patients (chest wall separation<25 cm and PTV<1500 cm3) (n=32). All patients were treated prone or supine with hypofractionated IMRT to the whole breast (42.4 Gy in 16 fractions) followed by a boost dose (9.6 Gy in 4 fractions). Dosimetric and clinical toxicity data were collected and analyzed using the R statistical package (version 2.12). The mean PTV V95 (percentage of volume receiving>=95% of prescribed dose) was 90.18% and the mean V105 percentage of volume receiving>=105% of prescribed dose was 3.55% with no dose greater than 107%. PTV dose was independent of breast size, whereas heart dose and maximum point dose to skin correlated with increasing breast size. Lung dose was markedly decreased in prone compared with supine treatments. Radiation Therapy Oncology Group grade 0, 1, and 2 skin toxicities were noted acutely in 6%, 69%, and 25% of patients, respectively, and at later follow-up (>3 months) in 43%, 57%, and 0% of patients, respectively. Large breast size contributed to increased acute grade 2 toxicity (28% vs 12%, P=.008). Adequate PTV coverage with acceptable hot spots and excellent sparing of organs at risk was achieved by use of IMRT regardless of treatment position and breast size. Although increasing breast size leads to increased heart dose and maximum skin dose, heart dose remained within our institutional constraints and the incidence of overall skin toxicity was comparable to that reported in the literature. Taken together, these data suggest that hypofractionated radiation therapy using IMRT is a viable and appropriate therapeutic modality in large-breasted patients.

Optimization and performance of atmospheric Fused Hollow Cathodes

Experimental results on the atmospheric hollow cathode plasma generation and performance, using a special configuration with tunable wall separations, are presented. The influence of the gas and type of the power used for generation on the optimum size of the cathode slit is investigated. The experimental results are in agreement with the hollow cathode model. The plasma source/plasma reactor design is of utmost importance for control of plasma-chemical kinetics. An example of the atmospheric hollow cathode plasma application for the NOx conversion is given. The energy consumption and plasma characteristics are discussed.

Diblock copolymer–selective nanoparticle mixtures in the lamellar phase confined between two parallel walls: a mean field model

We present a mean field model for a mixture of AB diblock-copolymers and A-block selective nanoparticles confined between two identical non-selective walls. A horizontally symmetric lamellar structure of the nanocomposite is considered where nanoparticles are allowed to segregate between the polymer–wall interfaces. For a fixed value of wall separation, we study changes in the free energy as a function of the number of lamellar layers and the amount of nanoparticle uptake in the A-phase denoted by y = ϕx with 0 ≤ x ≤ 1 for a given value of ϕ, where ϕ is the overall nanoparticle volume fraction. The absorption isotherm for nanoparticle uptake in the A-phase as a function of ϕ shows saturation beyond a threshold value ϕs, and the optimal value of uptake y increases with increasing strength of monomer–nanoparticle attractive interaction. Increasing ϕ above ϕs produces a decrease in the optimal number of lamellar layers which is related to a jump-like transition of the chain extension. The effect of varying film thickness is also studied. By considering A-block selective walls we also investigated a wetting transition of the copolymer film and found the transition to be discontinuous. A corresponding phase diagram is constructed.

Shear-induced Electrokinetic Lift at Large Péclet Numbers

We analyze the problem of shear-induced electrokinetic lift on a particle freely suspended near a solid wall, subject to a homogeneous (simple) shear. To this end, we apply the large-Peclet-number generic scheme recently developed by Yariv et al. (J. Fluid Mech. , Vol. 685, 2011, p. 306). For a force- and torque-free particle, the driving flow comprises three components, respectively describing (i) a particle translating parallel to the wall; (ii) a particle rotating with an angular velocity vector normal to the plane of shear; and (iiii) a stationary particle in a shear flow. Symmetry arguments reveal that the electro-viscous lift, normal to the wall, is contributed by Maxwell stresses accompanying the induced electric field, while electro-viscous drag and torque corrections, parallel to the wall, are contributed by the Newtonian stresses accompanying the induced flow. We focus upon the near-contact limit, where all electro-viscous contributions are dominated by the intense electric field in the narrow gap between the particle and the wall. This field is determined by the gap-region pressure distributions associated with the translational and rotational components of the driving Stokes flow, with the shear-component contribution directly affecting only higher-order terms. Owing to the similarity of the corresponding pressure distributions, the induced electric field for equal particle–wall zeta potentials is proportional to the sum of translation and rotation speeds. The electro-viscous loads result in induced particle velocities, normal and tangential to the wall, inversely proportional to the second power of particle–wall separation.

Commensurate-incommensurate phase transition in chlorine monolayer chemisorbed on Ag(111): Direct observation of crowdion condensation into a domain-wall fluid

The commensurate-incommensurate phase transition in chlorine monolayer on Ag(111) has been studied with low-temperature scanning tunneling microscopy and low-energy electron diffraction. We report the observation of self-interstitial defects: two-dimensional (2D) crowdions formed at the initial stage of the ($\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$)R30${}^{\ensuremath{\circ}}$ lattice compression. After critical coverage of 0.34 monolayer (ML) the crowdions condense into a domain-wall fluid. As coverage further increases, the domain-wall crystal solidifies with average wall separation of 23 \AA{}.

Impact of surface roughness on diffusion of confined fluids

Using event-driven molecular dynamics simulations, we quantify how the self diffusivity of confined hard-sphere fluids depends on the nature of the confining boundaries. We explore systems with featureless confining boundaries that treat particle-boundary collisions in different ways and also various types of physically (i.e., geometrically) rough boundaries. We show that, for moderately dense fluids, the ratio of the self diffusivity of a rough wall system to that of an appropriate smooth-wall reference system is a linear function of the reciprocal wall separation, with the slope depending on the nature of the roughness. We also discuss some simple practical ways to use this information to predict confined hard-sphere fluid behavior in different rough-wall systems.

Early Results of a Novel Hypofractionated Schedule for Adjuvant Radiation Therapy in Early-stage Breast Cancer

Currently reported trials of conventional whole breast irradiation (WBI) or hypofractionated (HF) WBI in patients with early stage breast cancer have shown comparable clinical and cosmetic outcomes. These trials have informed the current ASTRO guidelines for HF-WBI. Unresolved questions include the appropriateness of excluding patients with large chest wall separations, use of a breast boost in this setting, and the suitability of DCIS patients and node-positive patients. We designed a prospective phase II trial of a novel HF-WBI schedule for early stage breast cancer that incorporates a boost and limits treatment to 3 weeks.

Low density mesostructures of confined dipolar particles in an external field

Mesostructures formed by dipolar particles confined between two parallel walls and subjected to an external field are studied by Monte Carlo simulations. The main focus of the work is the structural behavior of the Stockmayer fluid in the low density regime. The dependence of cluster thickness and ordering is estimated as a function of density and wall separation, the two most influential parameters, for large dipole moments and high field strengths. The great sensitivity of the structure to details of the short-range part of the interactions is pointed out. In particular, the attractive part of the Lennard-Jones potential is shown to play a major role in driving chain aggregation. The effect of confinement, evaluated by comparison with results for a bulk system, is most pronounced for a short range hard sphere potential. No evidence is found for a novel "gel-like" phase recently uncovered in low density dipolar colloidal suspensions [A. K. Agarwal and A. Yethiraj, Phys. Rev. Lett. 102, 198301 (2009)].

A fresh look at white line disease

White line disease occurs secondary to a hoof wall separation. Clinical signs may vary from not being lame to severe lameness with rotation of the distal phalanx depending on the extent of the disease affecting the inner hoof wall. The author has found that removal of the hoof wall overlying the diseased area combined with the appropriate farriery is the most important aspect of therapy.

Discussion notes on “Thoughts on some outstanding issues in the physics of equilibrium wetting and conceptual understanding of contact lines”, by K. Sefiane

Comments are provided for [10]. In this article, it is emphasized that “The two parameters in Young’s equation, which are amenable to experimental measurement, are the wetting contact angle and the liquid-vapor surface tension”. It is implied, that it is impossible to measure directly the difference between solid-vapor and solid-liquid interfacial tensions. Hence, it is implied that experimentally the validity of Young’s equation cannot be tested at all. In this remark I wish to point out that this difference, however, can be “measured” in computer experiments on simple model systems. One such computer experiment was recently performed for the Ising (Lattice gas) model of a fluid [1–3]. Another one dealt with a symmetrical binary Lennard-Jones mixture [4]. Another point of concern is the statement that the contact angle of nanoscopically small droplets can be satisfactorily accounted for without the need for a line tension. Again this statement does not apply to the simulations of sessile nanodroplets in the Ising model [1–3], where the need for a line tension contribution was shown. In this model, the line tension (for the case of a contact angle of 90 degrees) could be estimated independently, by considering a “liquid slab” bridging two walls, computing the interfacial excess free energy as a function of wall separation, and considering the extrapolation of the results to the thermodynamics limit [1]. The limiting value (surface tension of a planar interface) can be obtained independently from a study of a corresponding liquid slab without walls but with periodic boundary conditions. This method has also been used successfully for the binary Lennard-Jones mixture [4]. Note that for macroscopic planar liquid-vapor interfaces the pressure in equilibrium cannot differ from the coexistence pressure, unlike the case of nanodroplets. When one considers nanoscopic droplets the dependence of the vapor-liquid surface tension on the radius of curvature of the droplet may play a role. This problem often is discussed in terms of Tolman’s length [5]. Recent simulations for a a e-mail: Kurt.Binder@uni-mainz.de 162 The European Physical Journal Special Topics Lennard-Jones fluid [6] have given additional evidence that this length is negative but small (typically 10% of a molecular diameter only, although it is predicted to diverge at the critical point [7]). The curvature dependence of the surface free energy of a nanodroplet needs also to be considered when one tries to extract the line tension from contact angle observations taken from nanodroplets and this was done in the studies cited [1–4]. In addition, there are severe conceptual problems since on the molecular scale interfaces are not sharp but rather diffuse, and line tension contributions then depend on conventions how exactly the contact line is defined [8,9] (depending on different definitions for the positions of the various dividing interfaces between the three phases). So the judgement of this author is that although the line tension clearly is difficult to measure in experiments and also difficult to calculate from theoretical models, but it should not be altogether dismissed.

Fluctuation-induced forces in critical crosslinked polymer blends

Abstract In this paper, we report on the computation of the induced forces in crosslinked polymer blends, with immersed small colloidal particles (nanoparticles) or confined to two parallel plates (film). We assume that the particles or the walls prefer to be attracted by one polymer, close to the spinodal temperature where a microphase separation takes place. This is the so-called “critical adsorption”. As an assumption, the particle diameter or the film thickness is considered to be small enough in comparison with the size of microdomains (“mesh size”). The critical fluctuations of the crosslinked mixture induce a pair potential between particles located in the non-preferred phase or between the confining walls. The purpose is to recall how this Casimir pair potential can be determined, as a function of the interparticle distance or the walls separation. To achieve calculations, use is made of an extended de Gennes model that takes into account the colloid-polymer and polymer-wall interactions. Finally, the obtained results are compared to those relatively to uncrosslinked polymer blends in the same geometries, and the main conclusion is that the induced force is reduced by the presence of permanent crosslinks.

Motion of a sphero-cylindrical particle in a viscous fluid in confined geometry

The motion of a millimeter size spherocylinder particle settling in a very viscous oil in a closed container is measured by laser interferometry, with the goal to model the motion of a particle of this shape in a fluid at microscales. The container is a cylinder with vertical axis and closed at both ends by horizontal plates. The displacement of the particle along the container axis is recorded with a resolution of the order of 100 nm, that is much smaller than the particle–wall separation when in the lubrication regime. The particle friction coefficient, measured as a function of the particle–wall distance, is then used to test the theoretical predictions of an accurate hydrodynamic analysis. The Stokes flow problem is solved by using the hydromultipole method, that is in general appropriate for spheres but is extended here to a non-spherical particle by using a compound of overlapping spheres. The lateral wall effect is negligible but the two parallel horizontal end plane walls are accurately taken into account. The result of the theoretical model is in good quantitative agreement with experiment for the whole settling motion of the spherocylinder, that is for any position between the walls.

Molecular Simulations of Kinetic-Friction Modification in Nanoscale Fluid Layers

Molecular simulations are used to explore kinetic-friction modification in nanoscale fluid layers of oil and additive confined between sheared parallel walls. The molecules are represented by coarse-grained bead-spring models that reflect the essential solvophilic and solvophobic natures of the chemical groups. The degree of friction modification is surveyed as a function of wall separation, sliding velocity, additive molecular weight and architecture, and oil–additive composition. As a rule, the kinetic-friction coefficient is found to increase first linearly and then logarithmically with increasing sliding velocity. From the results for different additive molecules, some subtle but systematic effects are found that point towards an optimum molecular weight and architecture.

Photoelastic Analysis of Stress Distribution of Surgically Assisted Rapid Maxillary Expansion With and Without Separation of the Pterygomaxillary Suture

The aim of this study was to evaluate in vitro the mechanical effect over teeth and bone structures of surgically assisted rapid maxillary expansion (SARME) in photoelastic analogs by observing stress produced during Hyrax-type and Haas-type appliances activation. Two photoelastic analogs were fabricated by use of birefringent materials to simulate an adult skull that contained teeth, bone, and maxillary sinus. Hyrax-type and Haas-type appliances were applied to the anchor teeth and incrementally activated. SARME was simulated by subsequent cuts of the lateral maxillary wall and midpalatine suture and separation of the pterygomaxillary junction. After each osteotomy, the appliances were activated. Resulting stress patterns were recorded photographically in the field of a plane polariscope. Before any osteotomy, the activation of the appliances distributed stress through the anchoring teeth to the zygomaticomaxillary walls, concentrating at the zygomaticomaxillary and zygomaticofrontal sutures and at the midline of the maxilla. After midpalatal and maxillary wall osteotomies, a marked reduction of the stress at the zygomaticomaxillary walls was observed. With successive activations, the stresses increased in intensity at the molars, maxillary tuberosity, and pterygoid plates. After pterygomaxillary separation, the stress intensity at the molar area, maxillary tuberosity, and pterygoid plates decreased considerably. With successive activations, stress increased in intensity around the molars and maxillary tuberosity. There were no differences between stresses produced by the 2 appliances. In this biomechanical model, sectioning of all maxillary articulations, including separation of the pterygomaxillary junction, caused a decrease in stress over anchorage teeth. These data cannot be completely extrapolated to the clinical situation but indicate that electing to perform the SARME technique should be based on periodontal health and amount of expansion necessary for each case.

Trust and Risk based Access Control and Access Control Constraints

Access control in dynamic environments needs the ability to provide more access opportunities of information to users, while also ensuring protection information from malicious users. Trust and risk are essential factors and can be combined together in access control decision-making to meet the above requirement. In this paper, we propose the combination of the trust and risk in access control to balance information accessibility and protection. Access control decision is made on the basis of trustworthiness of users and risk value of permissions. We use potential relations between users and relations between permissions in access control. Our approach not only provides more access opportunities for trustworthy users in accessing permissions, but also enforces traditional access control constraints such as Chinese Wall policy and Separation of Duty (SoD) of Role-Based Access Control (RBAC) model in an effective way.

Effect of mechanical and thermal treatments on the microstructure and rheological properties of carrot, broccoli and tomato dispersions

The food industry has shown an increased interest in the manufacture of healthier and more natural food products. By tailored processing fruit and vegetables can be used as structurants thus reducing artificial gums and stabilisers. The effect of different thermal and mechanical treatments, including high-pressure homogenisation, on the microstructural and rheological properties of carrot, broccoli and tomato dispersions was studied. As part of the rheological characterisation small oscillatory deformation as well as shear flow measurements were performed. Carrot and broccoli showed a different behaviour from tomato under the conditions studied. Changing the order of thermal and mechanical treatment led to microstructures with different flow properties. The resulting microstructures differed in the manner of cell wall separation: either breaking across the cell walls or through the middle lamella. High-pressure homogenisation decreased the viscosity of carrot and broccoli dispersions, while it increased the viscosity of tomato. Cryo-scanning electron microscopy showed that the cell walls of carrot and broccoli remained as compact structures after homogenisation whereas tomato cell walls were considerably swollen. Based on the type of vegetable, the different processes applied led to microstructures with different rheological properties. This study shows that particle size distribution, morphology and phase volume are important parameters to explain the complex relationship between rheology and microstructure for these types of systems.

Shearing and compression behavior of end-grafted polyelectrolyte brushes with mono- and trivalent counterions: a molecular dynamics simulation

We investigate polyelectrolytes end-grafted on two apposing walls using molecular dynamics simulation techniques. Monovalent and trivalent counterions are explicitly treated. Under normal compression, the osmotic pressure is examined in detail by decomposing it into various virial terms. It has been found that at small wall separations the increase in the osmotic pressure can be ascribed to the increase in the short-range virial term. At large wall separations, a negative osmotic pressure is observed in trivalent systems. Moreover, we study the effect of lateral shear on the density profiles of monomers and counterions, the net charge distribution, the local pressure tensor, the degree of interpenetration and the friction coefficient. At large shear ratios, the electrostatic interactions are weakened at the interface between two brushes. It is worth noting that although the magnitudes of the normal and shear stress components for the trivalent case are significantly lesser than those for the monovalent case, the friction coefficient is larger in the trivalent systems.

Monte Carlo simulation of a confined hard ellipse fluid

The structural and thermodynamic properties of a confined hard ellipse fluid are studied using Monte Carlo simulation. The angular, average number densities and order parameters of hard ellipses confined between hard parallel walls are obtained for various bulk densities, aspect ratios and wall separations. The results show that the effect of the existence of the wall on the molecular fluid structure, either on their directions or their locations, with respect to the bulk, especially close to the walls, is significant. For this system the pressure is also obtained and it is shown that the average density at the wall is proportional to the pressure, β P = 〈 ρ w 〉 . Our simulation results show that the order parameters depend on the number of the particles in the box unless it exceeds thousand.

Electrophoretic motion of a colloidal cylinder near a plane wall

An analytical study is presented for the electrophoretic motion of a circular cylindrical particle in an electrolyte solution with a transversely imposed electric field near a large plane wall parallel to its axis in the quasisteady limit. The electric double layers at the solid surfaces are assumed to be thin relative to the particle radius and to the particle–wall gap width, but the polarization effect of the diffuse ions in the double layer surrounding the particle is incorporated. The presence of the confining wall causes two basic effects on the particle velocity: first, the local ionic electrochemical potential gradients on the particle surface are altered by the wall, thereby affecting the motion of the particle; secondly, the wall enhances the viscous retardation of the moving particle. Through the use of cylindrical bipolar coordinates, the transport equations governing this problem are solved and the wall effects on the electrophoresis of the cylinder are determined for various cases. The presence of the plane wall prescribed with the ionic electrochemical potentials consistent with the far-field distributions reduces the electrophoretic mobility of the particle, which depends upon the properties of the particle–solution system, the relative particle–wall separation distance, and the direction of the applied electric field relative to the plane wall. The direction of the electrophoretic migration of a cylindrical particle near a plane wall is different from that of the prescribed electric field, except when it is oriented parallel or perpendicular to the wall. The effects of the plane wall on the electrophoresis of a cylinder are found to be much more significant than those for a sphere at the same separation.