Nearby Wall Research Articles

Development and Experimental Validation of a New Off-Grid Thermoelectric Fancoil for Domestic Heating

Conventional heating systems are based either on radiators or on fan coils and the heat transfer process is carried out by convection phenomena in both cases, but with some fundamental differences. Traditional radiators can be considered less efficient compared to fancoils due to the fact that their performance relies on natural convection, a less efficient heat transfer process than forced convection. However, in contrast to fan coils, conventional radiators can operate without the need for mains supply. In this paper, a new heating device based on thermoelectricity, that combines the advantages of both radiators and fancoils, is presented. This new system exploits the heat coming from the heating system to generate electricity, which is consumed in situ to power a fan, eliminating the need for a nearby wall outlet. For this purpose, a laboratory prototype has been built and experimental characterization has been carried out.

Numerical simulation of acoustic droplet vaporization near a wall

Numerical simulations are performed for acoustic-driven vaporization of a volatile droplet near a wall by introducing two level-set functions to track the droplet and bubble surfaces. The level-set method is combined with the ghost-fluid and semi-implicit pressure correction methods to implement the effects of liquid-vapor phase change and bubble compressibility. The computations demonstrate the bubble growth in the whole acoustic vaporization process of a dodecafluoropentane droplet and its interaction with a nearby wall. The effects of acoustic pulsing amplitude and frequency as well as initial droplet location and size on the bubble growth and wall shear stress are quantified.

Does setup on rectal wall improve rectal cancer boost radiotherapy?

BackgroundRectal cancer patients that show a pathological complete response (pCR) after neo-adjuvant chemo-radiotherapy, have better prognosis. To increase pCR rates several studies escalate the tumor irradiation dose. However, due to lacking tumor contrast on online imaging techniques, no direct tumor setup can be performed and large boost margins are needed to ensure tumor coverage.The purpose of this study was to evaluate the feasibility of performing a setup on rectal wall for rectal cancer boost radiotherapy, thereby using rectal wall nearby the tumor as tumor position surrogate.MethodsFor sixteen patients, daily MRI’s were performed during 1 week of radiotherapy. On each of these images, tumor and rectum were delineated. Residual displacements were determined per surface voxel after setup on bony anatomy or nearby rectal wall and setup errors for both setups were compared. Furthermore for every rectal wall voxel nearby the tumor, displacement was compared with the closest tumor point and correlation was determined.ResultsMean (SD) setup error was 2.7 mm (3.3 mm) and 2.2 mm (3.2 mm) after setup on bony anatomy and rectal wall respectively. Nevertheless, similar PTV-margin estimates i.e. 95th percentile distances, were found; 8.0 mm. Also, a merely moderate correlation; ρ = 0.66 was found between rectal wall and tumor displacement. Further investigation into tumor and rectal mobility differences showed that the rectal wall lacks appropriate anatomical landmarks to find true displacements, especially to capture motion along the rectal wall.ConclusionsSetup on rectal wall slightly reduces mean setup errors but requires a similar PTV-margin as compared to setup on bony anatomy. Rectal mobility might be similar to tumor mobility, but due the absence of anatomical landmarks in the rectum, displacements along the rectal wall are not detected on current online imaging. Therefore, to further reduce tumor position uncertainties, direct or indirect online tumor visualization is needed.

Dynamics of a colloidal particle near a thermoosmotic wall under illumination.

The effects of light on colloidal particles in solution are multiple, including transfer of photonic linear/angular momentum and heating of the particles or their surroundings. The temperature increase around colloidal particles due to light heating can drive a thermoosmotic flow along a nearby boundary wall, which significantly influences the motion of the particles. Here we perform mesoscopic dynamics simulations to study two typical scenarios, where thermoosmosis is critical. In the first scenario, we consider a light-absorbing colloidal particle heated by uniform illumination. Depending on the fluid-wall interactions, the thermoosmotic flow generated by the wall can exert a long-ranged hydrodynamic attraction or repulsion on the "hot" Brownian particle, which leads to a strong accumulation/depletion of the particle to/from the boundary. In the second scenario, we investigate the motion of a colloidal particle confined by an optical tweezer in a light-absorbing solution. In this case, thermoosmosis can induce a unidirectional rotation of the trapped particle, whose direction is determined by thermoosmotic properties of the wall. We show that colloidal particles near a thermoosmotic wall exhibit rich dynamics. Our results can be applied for the manipulation of colloidal particles in microfluidics.

Numerical study of acoustically driven bubble cloud dynamics near a rigid wall

The dynamics of a bubble cloud excited by a sinusoidal pressure field near a rigid wall is studied using a novel Eulerian/Lagrangian two-phase flow model. The effects of key parameters such as the amplitude and frequency of the excitation pressure, the cloud and bubble sizes, the void fraction, and the initial standoff distance on the bubbles’ collective behavior and the resulting pressure loads on the nearby wall are investigated. The study shows that nonlinear bubble cloud dynamics becomes more pronounced and results in higher pressure loading at the wall as the excitation pressure amplitude increases. The strongest collective bubble behavior occurs at a preferred resonance frequency. At this resonance frequency, pressure peaks orders of magnitudes higher than the excitation pressure result from the bubble interaction when the amplitude of the pressure excitation is high. The numerically obtained resonance frequency is significantly different from the reported natural frequency of a spherical cloud derived from linear theory, which assumes small amplitude oscillations in an unbounded medium. At high amplitudes of the excitation, the resonance frequency decreases almost linearly with the ratio of excitation pressure amplitude to ambient pressure until the ratio is larger than one.

Dielectrophoretic motions of a pair of particles in the vicinity of a planar wall under a direct-current electric field

This paper presents direct simulations on the two-dimensional dielectrophoretic (DEP) motion of a pair of particles in a viscous fluid, interacting with a nearby planar wall, to further understand the DEP interaction among multiple particles and a wall. Results show that, under an external direct-current electric field parallel to the wall-fluid interface, the nearby wall has significant effects on the DEP motion of both particles including their revolution, alignment and aligned movement. Regardless of their particle conductivity, the wall being less (more) conductive than the fluid pushes (draws) both particles to move away from (toward) it.

Computations of incompressible fluid flow around a long square obstacle near a wall: laminar forced flow and thermal characteristics

Computations of incompressible fluid flow and heat transfer around a square obstacle with a nearby adiabatic wall have been performed in a horizontal plane. The ranges of dimensionless control parameters considered are Prandtl number (Pr) = 10–100, Reynolds number (Re) = 1–150 and gap ratio (G) = 0.25–1. The steady-flow regime is observed up to Re = 121 for G = 0.5, and beyond this Re, time-periodic regime is observed. The shift to a time-periodic regime from a steady regime occurred at greater Re than that for an unconfined square obstacle. With increasing Pr, increase in average Nusselt number values is recorded for all Re and G studied. The heat transfer augmentation is approximately 1332% at Re = 150 (Pr = 100, G = 0.25) with regard to the corresponding values at Re = 1. Lastly, a correlation for j h factor is determined for the preceded conditions.

Dynamics of a flexible helical filament rotating in a viscous fluid near a rigid boundary

The effects of a no-slip wall on flagellar propulsion are investigated by experiments with macroscopic-scale model flagella and simulations based on algorithms from computer graphics. A nearby wall shifts the threshold for buckling of the flexible flagellum and increases the propulsive force.

Triassic salt sheets of Mezzouna, Central Tunisia: New comments on Late Cretaceous halokinesis and geodynamic evolution of the northern African margin

Two discrete Triassic salt sheets have been discovered within the Coniacian–Santonian series near the salt wall of Mezzouna, central Tunisia. The structure and the lithology of these sheets suggest two halokinetic episodes giving respectively 1) Triassic evaporitic rocks flows over a sloped basin floor resulting in probable salt glacier, and 2) redeposition of erosional debris from the nearby salt wall of Mezzouna, transported and then deposited next to the wall. This finding is used to precise the halokinetic events and the geodynamic evolution of the northern African margin near the Pelagian block between southeastern Tunisia and Tripolitania during Late Cretaceous. A discussion of the halokinesis-related structures is also attempted with emphasize of their genetic mechanisms and temporal development as inferred from geological mapping and new field data.

Monitoring the accumulated water soluble airborne compounds deposited on surfaces of showcases and walls in museums, archives and historical buildings

BackgroundCultural heritage objects are subject to the deposition of a multitude of airborne pollutants even when kept inside museums, archives, historical buildings or showcases. Some of the pollutants are quickly deposited onto any available surface, including the interior walls and CH objects themselves. This might make the compounds seem absent from analyses of indoor air samples.Context and purpose of the studyA new method of detecting water soluble pollutants without taking samples from the interior walls or from the CH objects themselves has been developed. The method involves sampling the pollutants accumulated on a surface near the CH object, e.g. a nearby wall or an interior glass surface of a showcase. The samples were obtained by gently flushing the surface with deionised water to collect the ions readily removed from the surface. The method was tested on a variety of surfaces.Results and main findingsThe flushed water were analysed with IC (Ion Chromatography) and ICP-MS (Inductively Coupled Plasma Mass Spectrometry) and included ions of Al, As, Ba, Ca, CH3COOH, Cl, Co, (COOH)2, Cr, Cu, F, Fe, K, Mg, Mn, Na, NH4, Ni, Pb, S, Sb, Si, Sn, Sr, Ti and Zn, and the ions NO3−, PO43−, and SO42−. The resulting concentrations were converted to µEq, providing the ionic balance and the relative amounts of the ions and elements present. Ionic balance was observed on smooth and inert surfaces, but in some cases the chemistry of the wall contributed to the flush water. Solid samples of some of the surfaces have been analysed by XRF to clarify these more complex situations.ConclusionsClear results appear from smooth or well defined surfaces, whereas more complex situations arise when the underlying surface itself contributes to the flush water. The method is working very well and is easy and cheap to implement by curators and conservators, who can the send the flush water to specialized laboratories.Brief summaryA new methodology capable of monitoring the accumulated airborne deposits on surfaces in showcases and historic buildings is presented and tested. The method is cheap and is easy to implement by curators and conservators and allows the assessment of threats to the CH objects which are not always observed by analyses of the indoor air. Graphical abstractA novel method is presented detecting water soluble airborne pollutants without taking samples of the cultural heritage objects themselves. Left: schematic diagram depicting the sampling method. a Handheld squeeze bottle; b funnel made from aluminum foil to collect the flushed water; c plastic bottle collecting the flushed water; right: applying the flush water on the interior glass wall of a show case (Photo: Kaare Lund Rasmussen)

Boundary-induced autophoresis of isotropic colloids: anomalous repulsion in the lubrication limit

When suspended in a liquid solution, chemically active colloids may self-propel due to an asymmetry in either particle shape or the interfacial distribution of solute absorption. We here consider a chemically homogeneous spherical particle which undergoes self-diffusiophoresis due to the presence of nearby inert wall. In particular, we focus upon the near-contact limit where it was recently observed (Yariv,Phys. Rev. Fluids, vol. 1 (3), 2016, 032101) that the solute-concentration profile within the narrow gap separating the particle and the wall cannot be uniquely determined by a gap-scale analysis. We here revisit this near-contact limit using matched asymptotic expansions, the inner region being the gap domain and the outer region being on the particle scale. Asymptotic matching with the Hankel-transform representation of the outer distribution of solute concentration serves to determine both the scaling and magnitude of the corresponding inner profile. The ensuing gap-scale pressure field, set by a lubrication mechanism, gives rise to an anomalous particle–wall interaction, scaling as an irrational power of the gap clearance.

Electrophoretic and Electroosmotic Motion of a Charged Spherical Particle within a Cylindrical Pore Filled with Debye-Bueche-Brinkman Polymeric Solution.

Electrophoretic and electroosmotic motion of a charged spherical particle within a cylindrical pore filled with a Debye-Bueche-Brinkman (DBB) polymeric solution is investigated theoretically, which is of high relevance in capillary electrophoresis as well as micro- and nanofluidic applications involving polymeric solutions in a micro- or nanopore. The DBB model describes the rheological response of a polymeric solution with a linear polymer dissolved in a homogeneous solvent. It is a well-known non-Newtonian model in liquid physics based on rigorous theoretical derivations. By Debye and Bueche, corresponding governing fundamental electrokinetic equations are solved numerically with a patched pseudo-spectral method based on Chebyshev polynomials. We found that the double-layer polarization effect reduces the particle mobility severely when the Debye parameter, κa, is around unity, especially in narrow pores. This is attributed to the extra confinement effect from the nearby wall, which tends to sweep the predominant counterions within the double layer to the wake of the moving particle, resulting in a motion-deterring induced electric field. The electrophoretic mobility in a polymer solution is smaller than that in an aqueous electrolyte solution in general as a result of the much stronger viscous drag effect in a polymer solution. Moreover, electroosmotic flow (EOF) as a result of a charged pore wall is found to exhibit a highly non-Newtonian behavior. Unlike the corresponding plug-like flow for a Newtonian solution, an axisymmetric flow with a large local maximum in the velocity profile in the region near the pore wall is observed. This radial-varying velocity profile offers a potential extra separation mechanism, which favors the elution of smaller particles in general. The results obtained here provide fundamental understandings and insights of the electrophoresis and electroosmosis phenomena in a cylindrical pore filled with polymeric solution.

Numerical modeling of gravity-driven bubble flows with account of polydispersion

The present study is focused on the investigation of motion of bubble-liquid medium inside the bubble columns or vertical pipes with account of polydisperse phenomena by the means of numerical simulation. The underlying mathematical model is based on the Euler- Euler approach with interphase interaction described by the momentum and mass transfer between phases, along with the k-w-SST turbulence model which includes turbulence generation by the bubble motion and bubble path dispersion. Polydispersion is taken into account by the multi-class model with piecewise-constant distribution of bubble sizes per cell. Simulation of downward flow inside the straight vertical pipe resulted in maximum of the bubble void fraction close to the pipe center which is in good correlation with the experimental data. Simulation of multiphase flow inside rectangular bubble column with off-center sparger resulted in vertical bubble-liquid jet which is biased towards nearby wall with the correct prediction of attachment point location.

How walls affect the dynamics of self-phoretic microswimmers

We study the effect of a nearby planar wall on the propulsion of a spherical phoretic micro-swimmer driven by reactions on its surface. An asymmetric coverage of catalysts on its surface which absorb reactants and generate products gives rise to an anisotropic interfacial flow that propels the swimmer. We analyse the near-wall dynamics of such a self-phoretic swimmer as a function of the asymmetric catalytic coverage of the surface. By an analysis of the fundamental singularities of the flow and concentration or electrostatic potential gradients generated we are able to obtain and rationalise a phase diagram of behaviours as a function of the characteristics of the swimmer surface. We find a variety of possible behaviours, from "bound states" where the swimmer remains near the wall to "scattering" or repulsive trajectories in which the swimmer ends far from the wall. The formation of some of the bound states is a purely wall-phoretic effect and cannot be obtained by simply mapping a phoretic swimmer to a hydrodynamic one.

Near-wall diffusion tensor of an axisymmetric colloidal particle.

Hydrodynamic interactions with confining boundaries often lead to drastic changes in the diffusive behaviour of microparticles in suspensions. For axially symmetric particles, earlier numerical studies have suggested a simple form of the near-wall diffusion matrix which depends on the distance and orientation of the particle with respect to the wall, which is usually calculated numerically. In this work, we derive explicit analytical formulae for the dominant correction to the bulk diffusion tensor of an axially symmetric colloidal particle due to the presence of a nearby no-slip wall. The relative correction scales as powers of inverse wall-particle distance and its angular structure is represented by simple functions in sines and cosines of the particle's inclination angle to the wall. We analyse the correction for translational and rotational motion, as well as the translation-rotation coupling. Our findings provide a simple approximation to the anisotropic diffusion tensor near a wall, which completes and corrects relations known from earlier numerical and theoretical findings.

An Eulerian-Lagrangian study of cloud dynamics near a wall

The dynamics of bubble clouds is studied using an Eulerian-Lagrangian model treating the two-phase medium as a continuum and modeling the microbubbles as discrete sources and dipoles tracked in a Lagrangian fashion. These two are coupled through the local void fractions associated with the instantaneous bubble volumes and locations. Resonance of the bubble cloud, resulting in the highest pressure at the nearby rigid wall, is deduced from simulations with the same initial bubble distribution while varying the excitation frequency and amplitude. This resonance frequency deviates significantly from the classical linearized solution as the relative amplitude of driving pressure increases. It gradually drops as the excitation amplitude increases until reaching a limit value. In the high amplitude driving pressure regime, the peak pressure generated at the wall has a maximum for an optimum value of the initial bubble radius for the same cloud radius. This occurs when the ratio of maximum over initial bubble size is maximum. Too weak or too strong bubble interactions in the cloud inhibit strong collective effects. For the same initial void fraction and for different initial bubble sizes (changing the bubble number), an optimum bubble size also exists, resulting in the highest collective pressure.

Non-reacting flow visualization of supersonic combustor based on cavity and cavity–strut flameholder

Nano-particle planer laser scattering and particle image velocimetry technology are employed to observe the flow field of scramjet combustors based on cavity and cavity–strut flameholder. Density field and velocity distribution inside combustors are obtained. Mainstream fluid enters into cavity nearby side wall in experimental observation because side wall shock waves interact with bottom wall boundary layer. Cavity fluid is entrained into mainstream in the middle of combustor meanwhile. Flow past cavity displays obvious three dimensional characteristics in both combustors. But cavity–strut combustor displays asymmetrical flow field because of strut configuration. Mass exchange between mainstream and cavity fluid is evaluated by statistic mass flow rate into cavity. Mass flow rate near side wall is raised to 6.62 times of the value in the middle of cavity combustor while it is 5.1 times in cavity–strut combustor. Further study is needed to injection strategies and realistic flow characteristics on condition of combustion.

비전도성 벽과의 상호작용에 따른 한 쌍 입자의 직류 유전영동 운동

The present study numerically investigated two-dimensional dielectrophoretic motions of a pair of particles suspended freely in a viscous fluid, interacting with a nearby nonconducting planar wall, under an external uniform electric field. The results show that the motions depend strongly on the set of two electric conductivity signs and the particles-wall separation gap. When both particles have the same sign, they revolve and finally align parallel to the electric field. In contrast, with different signs, they revolve in the opposite direction and finally align perpendicular to the field. Simultaneously, they are repelled to move farther away from the wall regardless of their conductivity set. With further separation from the wall, the particles-wall interaction effect diminishes and tthe particle-particle effect dominates.

The dynamics of a self-phoretic Janus swimmer near a wall

We study the effect of a nearby planar wall on the propulsion of a phoretic Janus micro-swimmer driven by asymmetric reactions on its surface which absorb reactants and generate products. We show that the behaviour of these swimmers near a wall can be classified based on whether the swimmers are mainly absorbing or producing reaction solutes and whether their swimming directions are such that the inert or active face is at the front. We find that the wall-induced solute gradients always promote swimmer propulsion along the wall while the effect of hydrodynamics leads to re-orientation of the swimming direction away from the wall.

Growth-and-collapse dynamics of small bubble clusters near a wall

The violent collapse of bubble clusters is thought to damage adjacent material in both engineering and biomedical applications. Yet the complexities of the root mechanisms have restricted theoretical descriptions to significantly simplified configurations. Reduced-physics models based upon either homogenization or arrays of idealized spherical bubbles do reproduce important gross cluster-scale features. However, these models neglect detailed local bubble–bubble interactions, which are expected to mediate damage mechanisms. To describe these bubble-scale interactions, we simulate the expansion and subsequent collapse of a hemispherical cluster of 50 bubbles adjacent to a plane rigid wall, explicitly representing both the asymmetric dynamics of each bubble within the cluster and the compressible-fluid mechanics of bubble–bubble interactions. Results show that the collapse propagates inward, as visualized in experiments, and that geometric focusing generates high impulsive pressures. This gross behaviour is nearly independent of the specific arrangement of bubbles within the cluster and matches predictions from the corresponding particle and homogenized models we consider. The peak pressure in the detailed simulations is associated with the centremost bubble, which causes a corresponding peak pressure on the nearby wall. However, the peak pressures in all cases are a small fraction – over a factor of ten times smaller in many cases – of those predicted in the corresponding reduced models. This is due to the enhanced focusing in the homogeneous model and the spherical constraint on each bubble in the particle models assessed. These would be important factors to consider in any subsequent predictions of wall damage based upon reduced models.