Wall Proximity Research Articles

Sedimentation and migration dynamics of a spherical particle in an elastoviscoplastic fluid near a wall

The sedimentation of a spherical particle in an elastoviscoplastic fluid in proximity of a flat wall is investigated by direct numerical simulations. The governing equations under inertialess conditions are solved by the finite element method with an Arbitrary Lagrangian-Eulerian formulation to manage the particle motion. The fluid is modeled with the Giesekus constitutive equation modified as proposed by Saramito (2007).

Effect of Wall Proximity and Shape on Bubble Enhanced Heat Transfer from a Vertical Heated Surface

<div class="section abstract"><div class="htmlview paragraph">Numerical simulation has been performed to study the heat transfer enhancement from the vertical heated wall surfaces with the help of rising bubbles due to the buoyancy force. The effect of wall proximity and bubble shapes are investigated for three wall shapes such as plane wall, wavy wall and triangular wall. Numerical solution is obtained by solving both the thermos-fluid governing equations and the Volume of Fluid (VOF) advection equation along with the Piecewise-Linear Interface Construction (PLIC) algorithm available in ANSYS-Fluent, an FVM based commercial CFD code. The results observed in the three types of wall geometries were showing the heat transfer differently for the 3 mm bubble. For the plane wall from the rise of the bubble to 0.3 seconds the temperature gradient is 10 K whereas for the curved and triangular wavy walls these gradients are 9.6 K and 17.23 K respectively. and after 0.6 seconds, this gradient is almost the same for all the wall shapes. In the latter two cases, the heat transfer by the effect of the wall was changing because of the alternating widening and narrowing channel. The research also discusses the varying distance of the injection of the bubble from the heated wall viz. 6 mm, 12 mm, 8 mm, 14 mm. It is observed that in addition to the well-known wake enhanced heat transfer by rising bubble, the contribution in the heat transfer enhancement by the wall modification from plane surface to wavy/triangular surfaces is substantial. It is also found that, as expected, this enhancement is significantly decreased as the wall proximity is increased.</div></div>

Numerical investigation of flexible Purcell-like integrated microfluidic pumps.

Integrating miniature pumps within microfluidic devices is crucial for advancing point-of-care diagnostics. Understanding the emergence of flow from novel integrated pumping systems is the first step in their successful implementation. A Purcell-like elasto-magnetic integrated microfluidic pump has been simulated in COMSOL Multiphysics and its performance has been investigated and evaluated. An elastic, cilia-like element contains an embedded magnet, which allows for actuation via a weak, uniaxial, sinusoidally oscillating, external magnetic field. Pumping performance is correlated against a number of variables, such as the frequency of the driving field and the proximity of the pump to the channel walls, in order to understand the emergence of the pumping behavior. Crucially, these simulations capture many of the trends observed experimentally and shed light on the key interactions. The proximity of the channel walls in the in-plane direction strongly determines the direction of net fluid flow. This characterization has important implications for the design and optimization of this pump in practical applications.

Simultaneous side-wall-schlieren and -emission imaging of autoignition phenomena in conventional and constrained-reaction-volume shock-tube experiments

Simultaneous schlieren and emission imaging through the side wall of a round shock tube is reported for application to experimental autoignition studies. A round, optically accessible shock tube featuring windows designed as aberration-corrected cylindrical lenses provides a large side-wall field of view compatible with both emission and schlieren imaging. Autoignition experiments are reported for non-dilute propane–oxygen–argon mixtures (ϕ=1 C3H8 in 21% O2, 79% Ar) at temperatures (T5) spanning the range 1,203–1,438 K and pressures near 1 atm. Experiments are performed using both conventional and constrained-reaction-volume (CRV) filling, allowing for the most detailed comparison to date of ignition dynamics between these two types of experiments. When T5 exceeds 1,290 K, strong ignition is observed to initiate in the immediate proximity of the end wall regardless of the filling strategy. In lower-T5 experiments, mild ignition is observed to initiate spontaneously at remote locations, which often exceeding the imaging FOV in conventional experiments but are constrained within about 5 cm of the end wall in CRV experiments. Measured ignition delay times (IDTs) are compared to predictions from three kinetic mechanisms. IDTs simulated using the San Diego and USC Mech II mechanisms are of the same magnitude as the measurements but exhibit a lower activation energy; the propane submechanism from NUIG 1.1, meanwhile, systematically over predicts observed IDTs by roughly a factor of two. Comparisons are also made between the experimental measurements and predictions from a recently introduced 1-D axial-temperature-gradient model describing remote ignition in shock tubes. Considering the effect of a supposed dT5/dz=150 K/m gradient, the 1-D model functionally predicts the remote ignition observed in T5< 1,290 K experiments and suggests the use of CRV filling may significantly reduce the error introduced into IDT measurements by remote ignition at low-T5 conditions.

Path Loss Measurements and Model Analysis in an Indoor Corridor Environment at 28 GHz and 38 GHz.

This paper examines the large-scale path loss models for an indoor corridor environment at frequencies of 28 and 38 GHz. The measurement environment consists of an indoor corridor with both line-of-sight (LOS) and non-line of sight (NLOS) scenarios using vertical–vertical (V–V) and vertical–horizontal (V–H) antenna polarizations. The single-frequency close-in (CI), floating intercept (FI), free space large-scale path loss models and measured data from the measurement campaign were used to evaluate the performance analysis. The paper also focuses on various parameters, such as standard deviation, path loss exponent (PLE), accuracy, simplicity, and stability of the models. The analysis focuses on the peculiarity of the effect of the wall proximity on the path loss parameters as well as comparisons with the parameters in some of the reviewed literature studies. The FI and CI models produce comparable results for both antenna polarizations and clearly fit with the measured path loss. The PLE, with the highest value of 3.33 at 38 GHz (V–H), is much higher in the NLOS scenario with V–H polarization due to the signal degradation along the path from the transmitter (Tx) to the receiver (Rx). This is because there is no direct LOS between the Tx and Rx antennas. The Rx only relies on signal diffractions and reflections from obstacles as it transmits through the path from the Tx antenna. The path loss measurements and model analysis presented here are useful in designing 5G wireless communication systems for indoor environments, particularly for power budget calculations.

Spacing effect on the vortex-induced vibrations of near-wall flexible cylinders in the tandem arrangement

The vibration responses and flow dynamics in the vortex-induced vibrations of two near-wall flexible cylinders in the tandem arrangement are investigated through three-dimensional direct numerical simulations with the spacing ratio s/D =1.5–6 (D = diameter of the cylinder), gap ratio G/D = 0.8, cylinder length of 25D, and Reynolds number of 500. The in-line (IL) and crossflow (CF) vibrations are predominated by the first-order mode along the span. The upstream cylinder oscillates at a higher CF amplitude than the downstream one, and the maximum IL and CF vibration amplitudes of the tandem cylinders are both smaller than those of the single cylinder. The dominant frequencies of IL and CF oscillation are identical for the tandem cylinders, and they are larger than that of the single cylinder. The smaller mean drag and larger rms drag occur on the downstream cylinder than in the upstream counterpart. The difference between the spanwise rms lift of the two cylinders reduces as the s/D increases. Different flow types are observed along the flexible cylinders: at s/D = 1.5–2, an “extended-body regime” and a “reattachment regime” are excited near the two-ends and the middle regions along the span, respectively; at s/D = 3, a reattachment regime and a “co-shedding regime” appear; at s/D = 4–6, the co-shedding regime is observed but with different vorticities related to the vibration amplitudes. At s/D = 3, the wall proximity induces multi-frequencies in both the IL and CF oscillations, compared to single-frequency oscillations in wall-free conditions. The lower IL and higher CF vibration amplitudes are excited in the near-wall conditions. Weak “2S” and the typical 2S vortex shedding patterns are observed in the near-wall and wall-free conditions, respectively.

Evaluating suitability of confined impinging jet reactor for exothermic reactions: Hydrodynamics, residence time distribution, and heat transfer

AbstractEffect of confinement (wall proximity) of a confined impinging jet reactor (CIJR) on the flow field, residence time distribution and heat transfer are explored, through experiments and CFD simulations. Hydrodynamic characteristics are evaluated for different parameters namely confinement, impinging jet velocity, temperature gradient, and so on. For 2 mm confinement, highest values of dispersion number and overall heat transfer coefficient are observed due to interaction of turbulent eddies followed by the effect of reactor wall proximity. For the CIJR having confinements above 10 mm, jet velocity need to be greater than 3 m/s to achieve both, excellent mixing efficiency and high heat removal rate. Empirical correlations for Dispersion and Nusselt numbers as a function of Rej and L/D are obtained, over a range of and , which correspond to jet velocity of 0.5–3 m/s. The present study gives a basis for designing CIJR suitable for rapid, homogeneous, exothermic reactions.

T Stage and Pretreatment Standardized Uptake Values Predict Tumor Recurrence With 5-Fraction SABR in Early-Stage Non-Small Cell Lung Cancer

PurposeFive-fraction stereotactic ablative radiotherapy (SABR) regimens are frequently used to treat centrally located early-stage non-small cell lung cancer or disease in the proximity of the chest wall as a means of optimizing tumor control and reducing treatment toxicity. However, increasing these SABR regimens to 5 fractions may reduce tumor control outcomes. We sought to identify the clinical parameters predictive of treatment failures with these 5-fraction courses. MethodsNinety patients with T1-2 non-small cell lung cancer were treated with 50 or 60 Gy in 5 fractions. Failure over time was modeled using cumulative incidences of local, regional, or distant failure, with death as a competing risk. Cox proportional hazards analysis for incidences of failure was performed to control for patient variables. ResultsOf 90 patients, 24 of 53 patients with T1 tumors and 19 of 37 patients with T2 tumors received 50 Gy SABR, and the other 47 patients received 60 Gy. Two-year overall survival and progression-free survival for the whole cohort were 75.8% and 59.3%, respectively. Total SABR dose (50 vs 60 Gy) did not influence survival nor failure rates at 2 and 5 years. Within 2 years of treatment, 7.8% of all patients developed local failure. For all patient and tumor characteristics evaluated, only T stage and pretreatment positron emission tomography standardized uptake values served as predictors of local, regional, and distant failure at 2 and 5 years posttreatment on univariate and multivariable analysis. ConclusionsFive-fraction SABR provides excellent in-field control. T2 and high fluorodeoxyglucose uptake tumors have increased failure rates, suggesting the potential need for adjuvant therapies, which are being assessed in randomized phase 3 trials.

Experimental investigation on the alteration of natural frequency of a flexible pipe adjacent to a bottom plane with the consideration of pipe-plane impact

The obtainment of natural frequencies of near-wall submarine spanning pipelines is the essential basis of the prediction of vibration response and fatigue life. Nevertheless, published literature has not yet taken into account the pipe–wall impact in free attenuation tests and has not offered a method to identify the impact frequency from natural ones. Therefore, the present work experimentally investigates the free attenuation response of a horizontal flexible pipe positioned adjacent to and parallel to a bottom plane with the consideration of pipe–wall collision, aiming to figure out the influences of wall proximity and wall impact on natural frequencies and to provide a measurement method for attenuation tests of such fluid–flexible-structure–solid interaction issues. The experimental results indicate that setting a horizontal initial displacement is easier than setting a vertical initial displacement to get the vertical natural frequencies of a near-wall flexible pipe. The natural frequency of near-wall flexible pipe is a function of the added mass coefficient, decreasing with the reduction of pipe-to-wall distance. Three main alterations of natural frequencies are introduced in the presence of pipe–wall impact. With the imposition of a horizontal initial displacement, the impact and natural frequencies can be distinguished from the wavelet time-frequency contours because of their participations during different time periods.

Experimental investigation of the effect of the wall proximity on the mode transition of a vortex-induced vibrating flexible pipe and the evolution of wall-impact

Experimental investigation of the effect of the wall proximity on the mode transition of a vortex-induced vibrating flexible pipe and the evolution of wall-impact

Three-dimensional flow past a circular cylinder in proximity to a stationary wall

In this paper, we present direct numerical simulations (DNS) of the three-dimensional flow around a circular cylinder near a stationary wall at the Reynolds number of 500. The gap (G) between the cylinder surface and the stationary wall varies in the range of 0–3.0D where D is the cylinder diameter. We observe that the proximity wall significantly affects the flow topology around the cylinder, which is characterized by the asymmetric wake, upward gap flow, and strong interactions of the cylinder vortices with the wall-generated boundary layer. Three flow regimes are identified with variations of the fluid forces and vortex dynamics. (i) Steady regime: at G/D < 0.3, vortex shedding is suppressed in the near wake, and scattered streamwise vortices form in the far wake where the upper side shear layer meets the stationary wall. No fluctuating forces act on the cylinder. (ii) Biased unsteady regime: at 0.3 ≤ G/D < 1.5, the gap flow deflects upward and the vortices shed from the gap side are significantly suppressed. The cylinder has enlarged fluctuating drag and lift forces with identical dominant frequencies. (iii) Parallel unsteady regime: at G/D ≥ 1.5, vortices shed from both sides of the cylinder are of almost identical intensities while the gap flow is approximately parallel to the wall. The vortex dynamics approaches that of an isolated circular cylinder in unconfined flows. Flow features of each regime are illuminated through the flow fields, vorticity contours, statistics of the enstrophies and gap flow, and pressure distribution on the cylinder surface. Finally, we investigate the critical gap ratio for the onset of the vortex-shedding of the upper shear layer and discuss the underlying physics. Moreover, we observe that the occurrence of the second harmonic lift and fundamental drag frequencies are resulted from the asymmetric wake because of the proximity of the stationary wall.

The role of particle-electrode wall interactions in mobility of active Janus particles driven by electric fields

HypothesisThe interaction of active particles with walls can explain discrepancies between experiments and theory derived for particles in the bulk. For an electric field driven metallodielectric Janus particle (JP) adjacent to an electrode, interaction between the asymmetric particle and the partially screened electrode yields a net electrostatic force – termed self-dielectrophoresis (sDEP) - that competes with induced-charge electrophoresis (ICEP) to reverse particle direction. ExperimentsThe potential contribution of hydrodynamic flow to the reversal is evaluated by visualizing flow around a translating particle via micro-particle image velocimetry and chemically suppressing ICEP with poly(l-lysine)-g-poly(ethylene glycol) (PLL-PEG). Mobility of Polystyrene-Gold JPs is measured in KCl electrolytes of varying concentration and with a capacitive SiO2 coating at the metallic JP surface or electrode. Results are compared with theory and numerical simulations accounting for electrode screening. FindingsPLL-PEG predominantly suppresses low-frequency mobility where propulsive electro-hydrodynamic jetting is observed; supporting the hypothesis of an electrostatic driving force at high frequencies. Simulations and theory show the magnitude, direction and frequency dispersion of JP mobility are obtained by superposition of ICEP and sDEP using the JP height and capacitance as fitting parameters. Wall proximity enhances ICEP and sDEP and manifests a secondary ICEP charge relaxation time dominating in the contact limit.

Experimental and numerical investigations of thermal and flow characteristics of a shear-thinning non-Newtonian fluid in a differentially heated cavity

The thermal and flow characteristics of carboxymethyl cellulose (CMC) were investigated by experimental and numerical means, in order to assess the shear-thinning behaviour of a non-Newtonian fluid inside a differentially heated square cavity. A novel particle image velocimetry technique was used to analyse natural convective flow in a non-Newtonian fluid. The square cavity was filled with an aqueous solution of CMC, which exhibits non-Newtonian shear-thinning behaviour. The present study carried out a comparative analysis of several different numerical models, including the Power-law model, the Cross model, and the Carreau model, with the aim of producing a model that can be used to approximate viscosity distributions for non-Newtonian fluids. Three Rayleigh numbers were considered, Ra=1.22 × 104, 2.29 × 104, and 3.33 × 104. The patterns and magnitudes of velocity obtained from the experimental and numerical analyses were in close agreement in the proximity of the cavity walls and at the core of the natural convective flow. For CMC, the mean Nusselt number (Nu) showed a direct correlation with Ra. The outcomes provide some confidence in terms of their use as benchmark data for the simulation of a non-Newtonian fluid medium. For the first time, the proposed system was applied to non-Newtonian shear-thinning (pseudoplastic) flow, and quantities such as the distribution of mean Nu along the hot wall and the velocity field were determined concurrently for different Rayleigh numbers. Of the three alternative models, the Carreau model is the most effective, and is capable of predicting the thermal and flow performances within the thermal system described here with precision.

УСТРОЙСТВО СКВОЗНЫХ ПРОЕМОВ В КАМЕННЫХ СВОДАХ ИСТОРИЧЕСКИХ ЗДАНИЙ

The article is devoted to the problem of arranging through openings in the stone vaults of reconstructed building structures. The need for such openings arises in connection with the use of new communication and engineering systems in opening cases and laying, for example, elevators, plumbing equipment, water supply systems, ventilation, etc. An analysis is made of the severity of the state of a stone wall with a rectangular and checked openings, as well as a study of anisotropy the strength of the stone lining on its bearing capacity in the presence of openings. It is noted that the use of the device of openings is possible with a compromise between the requirements of the project, the allowable sizes, shapes and locations of openings in the vaults, and the likelihood of their use. For this purpose, a detailed analysis of the complex acute condition of the vaults using modern software systems is required. In this case, special attention should be paid to minimizing tensile strains that cause cracks, especially at the corners of rectangular openings. There are cases associated with the detection of violations in difficult conditions of the stress state of blood coagulation in the presence of openings. Use examples of the practical implementation of through openings in the main cylindrical and cross stone vaults with a detailed analysis of their state of sharpness, as well as a study of the bearing capacity. It is emphasized that the most common manifestations of openings are minor tense sections of vaults, for example, stripping due to the proximity of walls under them. The elimination of vaults weakened by openings is analyzed. Preference is given to their reinforcement or reinforcement using reinforced concrete belts. Technological aspects of making openings are discussed. Attention is also drawn to monitoring the progress of openings.

Effect of wall proximity on the wake of a rotating and translating sphere

Effect of wall proximity on the wake of a rotating and translating sphere

Hybrid cube-in-cup nanoantenna: towards ordered photonics

The most significant goal of nanophotonics is the development of high-speed quantum emitting devices operating at ambient temperature. In this regard, plasmonic nanoparticles-on-mirror are potential candidates for designing high-speed photon sources. We introduce a novel hybrid nanoantenna (HNA) with CdSe/CdS colloidal quantum dots (QDs) based on a silver nanocube in a metal cup that presents a nanoparticle-in-cavity coupled with an emitters system. We use focused ion beam nanolithography to fabricate an ordered array of cups, which were then filled with colloidal nanoparticles using the most simple drop-casting and spin coating methods. The spectral and time-resolved studies of the samples with one or more nanocubes in the cup reveal a significant change in the radiation characteristics of QDs inside the nanoantenna. The Purcell effect causes an increase in the fluorescence decay rate (≥30) and an increase in the fluorescence intensity (≥3) of emitters in the HNA. Using the finite element method simulations, we have discovered that the proximity of the cups wall affects the oscillation modes of the gap plasmon, which, in turn, leads to changes in the electric field enhancement inside the nanoantenna gap. Additionally, substantial variations in the behavior of the gap plasmons at different polarizations of the exciting radiation have been revealed. The proposed nanoantenna can be useful in the development of plasmonic sensors, display pixels, and single-photon sources.

Tunable, Anisotropic Permeability and Spatial Flow of SLM Manufactured Structures.

In this study, we report on a novel approach to produce defined porous selectively laser molten structures with predictable anisotropic permeability. For this purpose, in an initial step, the smallest possible wall proximity distance for selectively laser molten structures is investigated by applying a single line scan strategy. The obtained parameters are adapted to a rectangular and, subsequently, to a more complex honeycomb structure. As variation of the hatch distance directly affects the pore size, and thus the resulting porosity and finally permeability, we, in addition, propose and verify a mathematical correlation between selective laser melting process parameters, porosity, and permeability. Moreover, a triangular based anisotropic single line selectively laser molten structure is introduced, which offers the possibility of controlling the three-dimensional flow ratio of passing fluids. Basically, one spatial direction exhibits unhindered flow, whereas the second nearly completely prohibits any passage of the fluid. The amount to which the remaining orientation accounts for is controlled by spreading the basic triangular structure by variation of the included angle. As acute angles yield low passage ratios of 0.25 relative to continuous flow, more obtuse angles show increased ratios up to equal bidirectional flow. Hence, this novel procedure permits (for the first time) fabrication of selective laser molten structures with adjustable permeable properties independent of the applied process parameters.

Experimental investigation on vortex-induced vibration and solid-structure impact of a near-bottom horizontal flexible pipeline in oblique shear flow

Vortex-induced vibrations and solid-structure impacts of a horizontal flexible pipeline in the vicinity of a bottom wall boundary are experimentally investigated in a water flume. A neutrally-buoyant submerged pipe, with a length-to-diameter ratio of about 87 and fixed end supports, is subject to a shear flow with an oblique angle of 30°. The initial gap-to-diameter ratio between the wall and the pipe is equal to 0.2. A non-intrusive measurement with high-speed cameras is employed to simultaneously capture the space–time varying in-line and cross-flow vibrations in the normal flow reduced velocity range of 3–15 with a maximum Reynolds number of about 2780. Experimental results highlight new pipe equilibrium profiles and modal response branches being significantly broadened due to the wall proximity. Two solid-pipe impact features are discussed, including single-segment and two-segment alternating impacts depending on the dominant oscillation mode of the flexible pipe. Both the response amplitude and frequency increase with the reduced velocity, resulting in a more frequent impact with a greater impact force. A temporal evolution from a single point contact to a segment contact is observed for the two-segment alternating impact. Nevertheless, the lengths of the two contact segments are different as a result of the asymmetric response profile. In all cases, the pipe transversely impacts the bottom wall introducing a perturbation transmission along the pipe, which leads to the multi-mode, multi-frequency and asynchronous in-line and cross-flow oscillations with a 1:1 and 2:1 resonance. Due to a momentum energy consumption, the pipe experiences a slow post-impact bouncing phenomenon, contributing to a longer temporal period of the fluid–pipe interaction with a wall impact.

Wall Distance Effects on Transition to Turbulence in Low-Reynolds-Number Separated Flows

Airfoils operating at suffer from laminar separation that degrades aerodynamic efficiency. Transition to turbulence improves performance but becomes difficult to achieve at low Reynolds numbers. Experiments at separation Reynolds numbers, , with a rounded leading edge and a sharp back-step, were performed to distinguish between the effects of the wall proximity and Reynolds number on the shear layer transition to turbulence. When the shear layer is sufficiently far from the wall (), the transition distance, normalized by the model chord, is proportional to the Reynolds number at separation. The growth of instabilities are in good general agreement with both linear stability calculations based on the measured velocity profiles, and the predictions obtained using the computational tool XFOIL. For shape factors larger than 20, high growth rates occur, even for as low as 5. For smaller shape factors, instabilities are greatly reduced or eliminated, and the wall stabilizing effect becomes more significant as the Re number decreases. Guidelines for the design of turbulator devices are discussed.

Main results of the first Data Assimilation Challenge

This work presents the main results of the first Data Assimilation (DA) challenge, conducted within the framework of the European Union’s Horizon 2020 project HOMER (Holistic Optical Metrology for Aero-Elastic Research), grant agreement number 769237. The challenge was jointly organised by the research groups of DLR, ONERA and TU Delft. The same synthetic test case as in the Lagrangian Particle Tracking (LPT) challenge (also presented in this symposium) was considered, reproducing the flow in the wake of a cylinder in proximity of a flat wall. The participants were provided with three datasets containing the measured particles locations and their trajectories identification numbers, at increasing tracers concentrations from 0.04 to 1.4 particles/mm3 . The requested outputs were the three components of the velocity, the nine components of the velocity gradient and the static pressure, defined on a Cartesian grid at one specific time instant. The results were analysed in terms of errors of the output quantities and their distributions. Additionally, the performances of the different DA algorithms were compared with that of a standard linear interpolation approach. Although the velocity errors were found to be in the same range as those of the linear interpolation algorithm, typically between 3% and 12% of the bulk velocity, the use of the DA algorithms enabled an increase of the measurement spatial resolution by a factor between 3 and 4. The errors of the velocity gradient were of the order of 10-15% of the peak vorticity magnitude. Accurate pressure reconstruction was achieved in the flow field, whereas the evaluation of the surface pressure revealed more challenging.