Local Flow Analysis Research Articles

Numerical investigation on flow characteristics and heat transfer of mixed convection in a packed bed

Flow characteristic and heat transfer of mixed convection in the packed bed were investigated numerically comparing with the previous experimental studies. For the fixed packed bed and sphere diameters of 0.06 m and 0.01 m, the ratios of bed height to sphere diameter and Reynolds numbers were varied from 5 to 20, and from 1 to 300, respectively for both buoyance aided and opposed flows. The temperatures difference between fluid and sphere was 78.85 K. The packed bed structure was modeled by generating location coordinates through the random number with an in-house code and computational analyses were performed using ANSYS FLUENT software. Laminar mixed convection behaviors were observed: heat transfer enhancement for buoyancy aided flow and impairment for buoyancy opposed flow. For larger bed heights, even under a laminar flow condition, turbulent behaviors were observed: heat transfer impairment for buoyance aided flow and enhancement for buoyance aided flow. The same trend was observed in the analysis of turbulence intensity. Local flow analysis within the packed bed confirmed that this is due to the vortices and the wakes resulting from the interaction between mixed convection flows and the randomly stacked spheres in the packed bed. This study unveiled mixed convection phenomena accompanied by the geometric effects of the packed bed on vortices and wakes, which exhibit turbulence-like behavior.

Local nonsimilar solution and heat analysis of mixed convective flow across whole spherical shape with nonisothermal surface temperature

This study investigates the behavior of a mixed convective boundary layer flowing around a rigid spherical shape with non-isothermal wall temperature. The fluid assumed in this examination exhibits both incompressibility and viscosity. The major purpose is to examine the impact of the non-uniform surface temperature on the flow patterns, including velocity outlines and temperature outlines. An examination is conducted on the modified conservation equations of the boundary layer flow utilizing the local non-similarity method. The MATLAB built-in code bvp4c is used to find computational solutions. The outcomes are then shown for both air and water, specifically at a temperature of 21°C. The influence of several factors, such as the mixed convective variable [Formula: see text] and the exponent [Formula: see text] in the wall temperature function [Formula: see text]), is shown in velocity and temperature profiles. In addition, the study computes the local frictional force factor and local wall heat transport factor and compares these values with results from earlier research. Significantly, the study expands upon data made around the lower stagnating point to include other sites within the sphere, thereby offering a thorough comprehension of the whole flowing field.

Can tourism support resource circularity in small islands? On-field analysis and intervention proposals in Madagascar.

Open dumping and burning of solid waste are common practices in low-income countries. On small and touristic islands, the problem is exacerbated due to the additional volume of waste amount generated by tourists. This article presents how, using Nosy Be island in Madagascar as a case study, waste recovery and recycling can be fostered by tourism to tackle the waste challenge. About 95% of the waste of Nosy Be is openly dumped, discarded to sea or openly burned. Field analysis, interviews with local stakeholders and waste flow analysis served as methodological tools to assess the current solid waste management (SWM) system of Nosy Be. Stakeholder mapping and involvement as well as first exchange among local and international actors provided the basis to identify key practice and opportunities. Research findings highlight the importance of active participation and involvement of local partners supported by international experts, to suggest how touristic centres can serve as core of circular approaches. The article presents potential circular models to be implemented in Nosy Be, taking tourism as the entry point including the aspects of financial support, separated waste flows and the interest in 'green tourism marketing'. This case study underlines how international cooperation, touristic activities and common efforts can potentially help low-income communities improve their SWM practices.

The role and safety of UVA and UVB in UV-induced skin erythema.

Different wavelengths of ultraviolet (UV) light cause skin damage through different mechanisms. Minimal erythema dose (MED) is usually used to clinically evaluate skin sensitivity to ultraviolet radiation by inducing skin erythema using ultraviolet B (UVB) or ultraviolet A (UVA) + UVB. In this study, we detected changes in the blood flow at the MED erythema caused by UVB and UVA + UVB radiation through optical coherence tomography (OCT) to explain the role of different bands of ultraviolet rays in erythema induction. Two MED irradiation areas on the subjects' back were irradiated with UVB alone or UVA + UVB (UVA: UVB = 8:1). The absolute energy of UVB remained the same in UVB and UVA+UVB. At 24 h after the irradiation, the changes in the blood flow in the MED area were detected using OCT. Compared with the blank control, the maximum blood flow depth, blood flow peak, and total blood flow of UVB-MED and UVA+UVB-MED were significantly increased. Notably, the maximum blood flow depth and blood flow peak of UVB-MED were higher than UVA+UVB-MED. There was no significant difference in total blood perfusion between UVA+UVB-MED and UVB-MED. Under the same UVB energy, the skin erythema caused by UVA + UVB was weaker than UVB alone. The analysis of local blood flow by OCT showed that the peak and maximum depth of local blood flow caused by UVB alone were significantly higher than UVA + UVB.

Analyzing Socio-Metabolic Vulnerability: Evidence from the Comoros Archipelago

Small island developing states are often characterized as vulnerable owing to their unique geographies of smallness and remoteness, resource insecurity, and more recently from the impacts of climate change. These vulnerabilities are often manifested in resource insecurity, significant imports, poor waste management, and the inability to develop economies of scale. In effect, sustaining small islands in an era of global environmental change is a task both scholars and policy makers are increasingly grappling with. Can small islands be sustainable? This research examines the social metabolism of an island system, and introduces the concept of “socio-metabolic vulnerability”. As such, this research provides novel insights into the linkages between patterns of resource-use, systemic risks and vulnerability. Results from a local material and energy flow analysis (local-MEFA) for the island of Ndzuwani (Comoros) suggest a very low level of resource-use but at the same time heavy reliance on critical imports that cover vast distances, that are vulnerable to price and climate shocks. Informal activities in resource extraction play an important role in lending both vulnerability and resilience to Ndzuwani. This study adds to the scarce body of literature that argues that small island economies would need to leverage resource-use patterns to build system resilience, along with bold policies and institutions that support material circularity, engage communities and fosters frugal innovation.

Research on the Evolution Mechanism of Congestion in the Entrances and Exits of Parking Facilities Based on the Improved Spatial Autoregressive Model

The entrance and exit area of parking facilities has the characteristics of high concentration of urban traffic and prominent traffic intertwining phenomenon, which easily induces rapid congestion of mixed heterogeneous traffic at specific times and local locations and quickly spreads to the entire road section or even a larger area. In order to better understand the congestion distribution characteristics and propagation effects of access section of the parking entrance and exit from the mid and microperspective, a 5 m ∗ lane width pixel grid is used to divide the frontage road research. It also proposes a spatially robust autoregressive model and complex network tools suitable for analysis of local traffic flow to analyze it. The results show that as spatial scale increases, the congestion propagation decreases sharply and spatial adjacency within the fourth order can account for more than 90% of the propagation; the frontage road to the entrance and exit is the place where the congestion first happens, and the congestion gradually attenuates as it propagates to the inner lane and the upstream of the road segments; the lateral congestion propagation attenuates faster, so the area affected by congestion is mainly distributed in the outermost lane. This paper can provide theoretical guidance for alleviating traffic congestion in the entrance and exit areas of parking facilities and has theoretical and empirical significance.

Fuel Stratification Influence on NOx Emission in a Premixed Axial Reacting Jet-in-Crossflow at High Pressure

Abstract Three reacting jet-in-crossflow (JiC) methane/air flames were numerically investigated in a lean axially staged combustor at a pressure of five atmospheres. A detailed chemistry Star-CCM+ computational fluid dynamics (CFD) model was used with 53 species considered and the result of turbulence-governed finite-rate modeling was validated with in-house experimental data. An optically accessible test section features three side windows, allowing local flow and flame analysis with particle image velocimetry (PIV) and CH* chemiluminescence as well as pressure, temperature, and species exit measurements. The research objective was to predict and verify NOx formation of the premixed 12.7 mm axial jet. Three headend temperature levels were investigated along with three premixed jets at lean (φJet = 0.75), near-stoichiometric (φJet = 1.07), and rich (φJet = 1.78) axial fuel line equivalence ratio. Based on the matching exit emission concentration, global emission benefits were investigated by adjustment of the fuel stratification. The perfectly premixed methane/air flames of this study were shown to ignite at the lee-side of the jet. For the elevated headend temperature level T = 1800 K, the flame extended beyond the windward jet trajectory and caused high axial NO production. For industry application, a firing temperature of 1920 K was achieved with a NOx optimized fuel split of 25%, combining a lean headend (φHeadend = 0.61) with a rich (φJet = 1.78) jet equivalence ratio. This operating point allowed minimization of the combustor residence time at temperatures above 1700 K as well as combustion in a compact flame at the jet lee-side along the counter rotating vortex pair.

Double-Layer Metal Foams for Further Heat Transfer Enhancement in a Channel: An Analytical Study

A local thermal non-equilibrium analysis of heat and fluid flow in a channel fully filled with aluminum foam is performed for three cases: (a) pore density of 5 PPI (pore per inch), (b) pore density of 40 PPI, and (c) two different layers of 5 and 40 PPI. The dimensionless forms of fully developed heat and fluid flow equations for the fluid phase and heat conduction equation for the solid phase are solved analytically. The effects of interfacial heat transfer coefficient and thermal dispersion conductivity are considered. Analytical expressions for temperature profile of solid and fluid phases, and also the channel Nusselt number (NuH) are obtained. The obtained results are discussed in terms of the channel-based Reynolds number (ReH) changing from 10 to 2000, and thickness ratio between the channel height and sublayers. The Nusselt number of the channel with 40 PPI is always greater than that of the 5 PPI channel. It is also greater than the channel with two-layer aluminum foams until a specific Reynolds number then the Nusselt number of the channel with two-layer aluminum foams becomes greater than the uniform channels due to the higher velocity in the outer region and considerable increase in thermal dispersion.

Supercritical CO2 conjugate heat transfer and flow analysis in a rectangular microchannel subject to uniformly heated substrate wall

In this paper, a conjugate heat transfer and fluid flow analysis for supercritical CO2 (sCO2) was numerically investigated in a rectangular microchannel subject to uniform heat flux on the bottom of the substrate wall. The study was conducted to explore and quantify the effect of a sharp variation of sCO2 transport properties, Reynolds number and operational pressure conditions on the fluid flow and heat transfer in the pseudo-critical region and within close range of the Critical Point (CP). The numerical results indicate that the large variation of sCO2 properties within close range of the CP affects the heat transfer and flow along the channel. The local flow and heat transfer analyses demonstrate that the wall shear stress and heat flux are an increasing function of operating conditions especially for operational pressures higher than the critical pressure. The results show that the large variation of Nusselt number disappears for high operating pressures. It was also shown that the Nusselt Number is not affected by the Reynolds number in the laminar regime at high operating pressures.

Local blood flow analysis and visualization from RGB-video sequences

Abstract The extraction of heart rate and other vital parameters from video recordings of a person has attracted much attention over the last years. In this paper, we examine time differences between distinct spatial regions using remote photoplethysmography (rPPG) in order to extract the blood flow path through human skin tissue in the neck and face. We can show that the visualization of the blood flow path corresponds to the physiologically defined path.

Computational simulation of turbulent flows around a marine propeller by solving the partially averaged Navier–Stokes equation

This study concerns the characteristics of the partially averaged Navier–Stokes method for local flow analysis around a rotating propeller. Partially averaged Navier–Stokes, resolving crucial large-scale structures of turbulent flow at a given computational grid resolution, is a bridging turbulence closure model between the Reynolds-averaged Navier–Stokes equation and the direct numerical simulation. A detailed comparison between partially averaged Navier–Stokes and Reynolds-averaged Navier–Stokes models is made to achieve a better understanding of partially averaged Navier–Stokes characteristics for predicting the coherent structures in turbulent flow. The two-equation k-ω shear stress transport model and the seven-equation Reynolds stress model are selected for Reynolds-averaged Navier–Stokes computations. The problem of interest is the flow around a rotating KP505 propeller in open water conditions at an advance ratio of 0.7. Near the leading edge, the partially averaged Navier–Stokes results are similar to those of Reynolds stress model in terms of the vortical structures. Vorticity predicted by different turbulence models, however, shows significant differences. For a more detailed analysis, the velocity gradient constituting the vorticity is identified at the leading edge. It is proven that partially averaged Navier–Stokes is able to capture the anisotropic characteristics of the flow at the leading edge, where both the geometric and flow characteristics change abruptly.

Entropy generation analysis for the cavitating head-drop characteristic of a centrifugal pump

Cavitation is a challenging flow abnormality that leads to undesirable effects on the hydraulic behaviour of centrifugal pumps. This study analyses the cavitating flow at a normal flow rate using the shear stress transport k-ω turbulence model and the Zwart cavitation model to capture the cavitation development and spatial distribution of vapour structures within the pump. A general description of the pump head-drop phenomenon was analysed through the study of local and global flow fields. The relationship between vapour distribution and entropy generation fields was mainly investigated through the local flow analysis method. Results show that the tendency of total entropy generation rate within the impeller coincides with the pump head-drop curve. The entropy generation rate changes slightly at the early stage of the cavitation development whereas rapidly increases when the cavitation has fully developed. Cavitation development enhances the hydraulic losses and flow unsteadiness in the impeller. These hydraulic losses are located substantially at the interface of the cavity, especially at the rear portion of the cavity region.

A computational analysis of local flow for reacting Diesel sprays by means of an Eulerian CFD model

An implementation and validation of the coupled Σ-Y ADF model is presented in this work for reacting Diesel spray CFD simulations under a RANS turbulence modeling approach. An Approximated Diffusion Flamelet (ADF) model Michel et al. (2008) implemented in the OpenFOAM CFD open-source library by Winklinger (2014) is fed with the spray description, i.e. mixing formation process, provided by the Σ-Y Eulerian atomization model Garcia-Oliver et al. (2013). In the present investigation, the Engine Combustion Network Spray A reference configuration is used for validation. Specifically, the model can provide accurate predictions of typical reacting spray metrics, such as the ignition delay and the lift-off length. Moreover, the internal structure is also fairly reproduced in terms of quasi-steady spatial distribution of formaldehyde and OH, related with low and high temperature reactions respectively. Additionally, modeling results have been compared to recent Particle image velocimetry (PIV) measurements Garcia-Oliver et al. (2017) under both inert and reacting conditions. Flow response to heat release is quantitatively predicted by the model, both in terms of local velocity increase as well as radial dilation. The model has been used to understand combustion-induced reduction in entrainment, in particular around the lift-off length location. Flow confinement does not seem to influence the global flame behaviour, even though some changes in the local flow hint can be observed when moving from an open to a closed domain.

Validation and flow structure analysis in a turbofan stage at windmill

In the present study, the flow through the fan stage of a high bypass ratio turbofan at windmill is studied numerically. First, steady mixing plane simulations are validated against detailed experimental engine test-bed measurements, at several locations within the fan stage and close to the core/bypass flow splitter. Good agreement between the numerical and experimental results is obtained for the global operating point in the fan map. For the two windmilling points considered, it appears that as the flight Mach number is reduced, the windmilling operating point moves closer to the stability limit of the fan. For the radial profiles, good agreement is found downstream of the rotor. Downstream of the stator, it is shown that the steady approach fails to reproduce some important feature of the flow. A local flow analysis is proposed, evidencing several characteristics of the flow in windmilling: in the rotor, the size of the separation zone is found to increase from hub to tip, due to a solidity effect. In the stator, massive flow separation occurs at mid-span, which leads to the formation of two streamwise counter-rotating vortices. Then, the nonlinear harmonic method is applied to a section (at 70% of the relative span) of the fan stage. A modal analysis is performed, showing a specific behavior at windmill: the massively separated flows in the rotor and the stator entail strong rotor/stator interaction modes. Finally, the unsteady flow pattern is examined: the velocity defect of the rotor wake, which periodically increases the flow angle on the stator, is shown to trigger a periodic movement of the reattachment point at the trailing edge of the stator, associated with vortex shedding from the lower side of the vane. The implication of this qualitative flow behavior on the method to extract computational fluid dynamics results for comparisons with experiments is discussed.

Local vs global heat transfer and flow analysis of hydrocarbon complete condensation in plate heat exchanger based on infrared thermography

Plate heat exchangers (PHE), compact and energy-efficient, are used as condensers in various industrial applications to recover and recycle heat energy. The condenser optimization still represents a challenge. Due to complex thermo-hydraulic couplings inside three-dimensional PHE geometry, most of the literature studies are correlatives based on hypothesis as uniform heat flux or heat transfer coefficient along the condenser. In this article, some new information on the analysis of condensation heat transfer along the PHE is highlighted. The experimental study focuses on complete condensation of saturated pentane inside a PHE of 4.4mm hydraulic diameter placed vertically with a descending flow of the refrigerant. The global and local thermo-hydraulic characteristics, as the vapor quality, the heat flux density and the heat transfer coefficient, were identified along the PHE based on the infrared thermography, and the effect of mass flux, between 9 and 30kgm−2s−1, on these characteristics is analysed. The results show a significant variation, of the heat transfer coefficient and the heat flux density, between the inlet and the outlet of the condensation region for most of the mass fluxes. In our operating range, the heat flux rate decreases till 400% between the PHE inlet and outlet, while the condensation heat transfer coefficient decreases by 5–10 times. The PHE mean heat transfer coefficients, calculated from the local values are then 10–20% higher than the ones calculated from the literature assumption of uniform heat fluxes or the assumption of constant heat transfer coefficient. Moreover, the variation of the mean heat transfer coefficient with pentane mass flux allowed the identification of two condensation regimes, from gravity mode to a mix gravity/convection mode, with a transition limit around 15kgm−2s−1. Condensation flow analysis was conducted based on pressure drop measurements and calculations. Hence the global pressure drop measured experimentally and the local profile deduced from infrared images, are compared to the results obtained by models from the literature. The comparison shows that the homogeneous model and the Lockhart–Martinelli (1949)’s model predict with good agreement the experimental results.

Tissue motion tracking at the edges of a radiation treatment field using local optical flow analysis

This paper investigates the feasibility and accuracy of tracking the motion of an intruding organ-at-risk (OAR) at the edges of a treatment field using a local optical flow analysis of electronic portal images. An intruding OAR was simulated by modifying the portal images obtained by irradiating a programmable phantom's lung tumour. A rectangular treatment aperture was assumed and the edges of the beam's eye view (BEV) were partitioned into clusters/grids according to the width of the multi-leaf collimators (MLC). The optical flow velocities were calculated and the motion accuracy in these clusters was analysed. A velocity error of 0.4 ± 1.4 mm/s with a linearity of 1.04 for tracking an object intruding at 10mm/s (max) was obtained.

On contact point modifications for forced convective heat transfer analysis in a structured packed bed of spheres

The present paper systematically investigated the appropriateness of different contact point modification approaches for forced convective heat transfer analysis in structured packed beds of spheres. The three-dimensional Navier–Stokes equations and RNG k–ɛ turbulence model with scalable wall function are adopted to model the turbulent flow inside the pores. Both macroscopic and local flow and heat transfer characteristics for different packing forms (simple cubic, body center cubic and face center cubic packing forms) and contact treatments (gaps, overlaps, bridges and caps modifications) are carefully examined. In particular, the effects caused by the bridge size for the bridges treatment are discussed, and the numerical results are compared with available experiments in literature. It is found that the effects of contact treatments on the pressure drops are remarkable for different structured packing forms, especially when the porosity is relatively low, while such effects on the Nusselt numbers are relatively small. Among the four different contact modifications, the bridges method would give the most reasonable pressure drops for all the structured packing forms studied and this method is also proved to be suitable for predicting the Nusselt numbers. The local flow and heat transfer characteristics in the structured packed bed are sensitive to the methodology of contact modifications. The gaps and caps treatments would distort the local flow and temperature distributions in the packed bed, especially near the contact zones. While the local flow and temperature distributions from the overlaps and bridges treatments would be more reasonable and close to those in the original packing with points contact. Based on both the macroscopic and local flow and heat transfer analyses, the bridges treatment is recommended. The effects caused by the bridge size in the bridges treatment are also remarkable. It is noted that too small or too large bridge size would lead to unreasonable results for both the macroscopic and local flow and heat transfer analyses. A reasonable range of bridge diameter is found to be from 16% dp to 20% dp.

Inertial effects at moderate Reynolds number in thin-film rimming flows driven by surface shear

In this paper, we study two-dimensional thin-film flow inside a stationary circular cylinder driven by an imposed surface shear stress. Modelling is motivated by a need to understand the cooling and film dynamics provided by oil films in an aero-engine bearing chamber characterised by conditions of very high surface shear and additional film mass flux from oil droplets entering the film through the surface. In typical high-speed operation, film inertial effects can provide a significant leading-order mechanism neglected in existing lubrication theory models. Inertia at leading-order is included within a depth-averaged formulation where wall friction is evaluated similar to hydraulic models. This allows key nonlinear inertial effects to be included while retaining the ability to analyse the problem in a mathematically tractable formulation and compare with other approaches. In constructing this model, a set of simplified mass and momentum equations are integrated through the depth of the film yielding a spatially one-dimensional depth-averaged formulation of the problem. An a priori assumed form of velocity profile is needed to complete the system. In a local Stokes flow analysis, a quadratic profile is the exact solution for the velocity field though it must be modified when inertial effects become important. Extension of the velocity profile to a cubic profile is selected enabling specification of a wall friction model to include the roughness of the cylinder wall. A modelling advantage of including the inertia term, relevant to the applications considered, is that a smooth progression in solution can be obtained between cases of low Reynolds number corresponding to lubrication theory, and high Reynolds number corresponding to uniform rimming-flow. Importantly, we also investigate the effect of inertia on some typical solutions from other studies and present a greater insight to existing and new film solutions which arise from including inertia effects.

CFD verification and validation of added resistance and motions of KVLCC2 with fixed and free surge in short and long head waves

The motions and added resistance of KVLCC2 at Fr=0.142 and 0.25 with free and fixed surge in short and long head waves are predicted using URANS and validated against EFD datafor Fr=0.142. Verification studies show the results are fairly insensitive to the grid size and time step. CFD indicates no significant difference between free and fixed surge while EFD pitch motion and added resistance are affected. Added resistance was largest when the bow relative motion has largest amplitude and is about 180° out of phase with the waves. The decomposition of forces and moments works well for first harmonics but not for higher harmonics originated from radiation in long waves and diffraction in very short waves. Maximum responses occur near the resonance condition and near the maximum wave excitation force which is at λ/L=1.33 and long waves for surge/pitch and heave, respectively. Potential flow predictions for motions and the added resistance are further from the data than CFD. Local flow analyses show that added resistance is mainly induced by high pressure on the upper bow which is correlated with bow relative motion. The unsteady wave pattern is analyzed and the wake flow is compared with PIV measurements.

Global flow impacts time-to-passage judgments based on local motion cues

We assessed the effect of the coherence of optic flow on time-to-passage judgments in order to investigate the strategies that observers use when local expansion information is reduced or lacking. In the standard display, we presented a cloud of dots whose image expanded consistent with constant observer motion. The dots themselves, however, did not expand and were thus devoid of object expansion cues. Only the separations between the dots expanded. Subjects had to judge which of two colored target dots, presented at different simulated depths and lateral displacements would pass them first. Image velocities of the target dots were chosen so as to correlate with time-to-passage only some of the time. When optic flow was mainly incoherent, subjects’ responses were biased and relied on image velocities rather than on global flow analysis. However, the bias induced by misleading image velocity cues diminished as a function of the coherence of the optic flow. We discuss the results in the context of a global tau mechanism and settle a debate whether local expansion cues or optic flow analysis are the basis for time-to-passage estimation.