This paper focuses on the effectiveness of trees at dispersing road traffic emissions on a city scale. CFD simulations of air-pollutant concentrations were performed using the OpenFOAM software ...platform using the k-ε model. Results were validated against the CODASC wind tunnel database before being applied to a LIDAR database of buildings and trees representing the City of Leicester (UK). Most other CFD models in the literature typically use idealised buildings to model wind flow and pollution dispersion. However, the methodology used in this study uses real buildings and trees data from LIDAR to reconstruct a 3D representation of Leicester City Centre. It focuses on a 2×2 km area which is on a scale larger than those usually used in other CFD studies. Furthermore, the primary focus of this study is on the interaction of trees with wind flow dynamics. It was found that in effect, trees have a regionally beneficial impact on road traffic emissions by increasing turbulence and reducing ambient concentrations of road traffic emissions by 7% at pedestrian height on average. This was an important result given that previous studies generally concluded that trees trapped pollution by obstructing wind flow in street canyons. Therefore, this study is novel both in its methodology and subsequent results, highlighting the importance of combining local and regional scale models for assessing the impact of trees in urban planning.
•We model the effectiveness of trees at dispersing road traffic emissions.•City scale CFD simulations were performed under the OpenFOAM software.•Trees increase turbulence and vertical velocity at pedestrian height.•Trees reduce concentrations of road traffic emissions by 7% at pedestrian height.•We propose combining local and regional scales for planting trees in city planning.
•Field tests of methane-air explosion in a large-scale tube were conducted.•Numerical model of methane-air explosions in the large-scale tube was developed.•Effects of concentration and venting ...configurations on gas explosion loads were discussed.•The effect of high-velocity zone on the overpressure distribution was revealed.
12 batches of premixed methane-air mixture explosion tests were conducted in a 30 m long testing tube with section dimensions of 0.8 m × 0.8 m to explore the loading characteristics of gas explosion inside the tube. The pressure-time histories and flame travel distance of each test were recorded and analyzed. A 3D numerical model was developed and numerical simulations were carried out by using the open-source Computation Fluid Dynamic (CFD) toolbox OpenFOAM. Based on testing and numerical results, the characteristics of gas explosion loading inside the large-scale tube were revealed. It is found that the existence of vents can reduce the peak pressure by 13%–91% and increase the flame travelling distance inside the tube. The peak pressure increases with the rising concentration when the gas concentration range is 7.5%–9.5% and decreases with the rising concentration when the gas concentration ranges 9.5%–11.5%. End venting can reduce the peak pressure along the tube by 17%–69%. With the decrease of vent number, the gas explosion inside the tube is enhanced and the side vents closed to the ignition can reduce the gas explosion loads inside the tube effectively.
•A CFD study of multiphase GDI internal flow and near-field spray is described.•Excellent agreement to experimental ROI is achieved with transient needle motion.•Qualitative agreement to experimental ...imaging in the near-field is shown.•Complex internal nozzle flow is shown to influence ROI and spray angle.•Unsteady vortices cause string flash-boiling and expansion of the near-field spray.
A computational study was performed to investigate the influence of transient needle motion on gasoline direct injection (GDI) internal nozzle flow and near-field sprays. Simulations were conducted with a compressible Eulerian flow solver modeling liquid, vapor, and non-condensable gas phases with a diffuse interface. Variable rate generation and condensation of fuel vapor were captured using the homogeneous relaxation model (HRM). The non-flashing (spray G) and flashing (spray G2) conditions specified by the Engine Combustion Network were modeled using the nominal spray G nozzle geometry. Transient needle lift and wobble were based upon ensemble averaged X-ray imaging preformed at Argonne National Lab. The minimum needle lift simulated was 5 µm and dynamic mesh motion was achieved with Laplacian smoothing. The results were qualitatively validated against experimental imaging and the experimental rate of injection profile was captured accurately using pressure boundary conditions and needle motion to actuate the injection. Low needle lift is shown to result in vapor generation near the injector seat. Finally, the internal injector flow is shown to be highly complex, containing many transient and interacting vortices which result in perturbations in the spray angle and fluctuations in the mass flux. This complex internal flow also results in intermittent string flash-boiling when a strong vortex is injected and the resulting swirling spray contains a thermal non-equilibrium vapor core.
•Unsteady multi-scale cavitating flow in the wake of wedge-shaped bluff body are studied using a two-way coupling Eulerian-Lagrangian solver in OpenFOAM.•Bubble dynamic effects on the transient ...multi-scale cavitating flow structures are illustrated.•The physics for the bubble dynamic effects on cavitation-vortex-turbulence interaction are clarified.
Unsteady cavitating flow around the bluff body widely existed in practical engineering fields. This paper aims to investigate the physics involved in the multi-scale cavitating flow in the wake of a wedge-shaped bluff body, with special emphasis on the bubble effects. Numerical simulation is conducted using the traditional Eulerian-Eulerian (E-E) and newly developed Eulerian-Lagrangian (E-L) methods in OpenFOAM. It is found that, compared with the experimental results, the E-L method can predict more accurate cavitation characteristics than the E-E method due to the contribution of discrete bubbles. Bubbles can significantly influence the behaviors of the multi-scale cavitating flow. Specially, analysis of pressure fluctuations indicates that the bubbles induce a stronger power spectral density in the middle- and high-frequency regions. With the bubble effects considered, the vortex structure is stretched and more extensive in the near wake regions. The strength of time-averaged vorticity distribution decreases in the far wake region. Further analysis of the vortex transport equation shows that bubble dynamics significantly increase the intensity of the stretching term by enhancing the spanwise velocity fluctuations. The intensity of the baroclinic torque term predicted by the E-L method is higher than that by the E-E method near the center line due to the influence of bubbles. On the other hand, the analysis of turbulent kinetic energy distribution indicates that bubbles can induce turbulence by increasing cross-stream velocity fluctuations. The increment of the shear Reynold stress, u′xu′y‾, suggests that the bubbles intensify the coupling between the streamwise and cross-stream velocity fluctuations.
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In the last two decades, the use of OpenFOAM as a multi-physics library for nuclear applications has grown from a sporadic use for exploratory studies to a widespread application for the analysis of ...innovative reactor concepts and highly complex problems. This review paper provides an overview of the past and current development efforts in the field and summarizes some of the lessons learned during 10 years of R&D activities participated by the authors. The objective is to provide readers with an understanding of the benefits and challenges of this approach, thus facilitating an informed decision about its potential adoption for future studies.
The acquisition of complex fluxes inside a Tapered Roller Bearing (TRB) via Particle Image Velocimetry (PIV) is an experimental challenge. This can be successfully performed by exploiting a special ...test rig having the outer ring manufactured with sapphire. In the present paper, the velocity field in the region between cage, rollers and outer race have been captured via PIV in a fully flooded lubricated TRB. The experimental conditions have been reproduced numerically via Computational Fluid Dynamics (CFD). The comparison of PIV results with CFD ones showed excellent consistency. It has been observed that, in the target domain, the tangential velocity of the lubricant is greater than those of the cage. In addition, in the proximity of the edges of the rollers, squeezing effects due to high gradients of pressure have been recorded. The distribution of flow rates due to the pumping effect in different regions of the TRB have been estimated.
•Lubricant fluxes in a tapered bearing were studied experimentally and numerically.•Rotational speeds up to 2500 min-1 in fully flooded lubrication were tested via PIV.•Velocity fields obtained via PIV and CFD showed excellent consistency.•Squeezing and edge effects were obtained by means of CFD and observed through PIV.•The total flow rate in fully flooded lubrication conditions was estimated via CFD.
•Enhanced radiation transport model validated in commonly used photoreactors.•Isotropic, parallel and cone-shaped emission tested, representing common UV sources.•Increase of precision reached for ...sun direct light simulations.•Great improvement in LEDs simulations, from finer cone radiation and power cosine.
This work presents the enhanced numerical simulation of the radiation transport in three different types of photocatalytic reactor using a novel Discrete Ordinate Method model recently developed for the open-source Computational Fluid Dynamics (CFD) platform OpenFOAM. The photoreactors represent commonly used geometries and illumination sources in the field of heterogeneous photocatalysis: an annular reactor illuminated by a mercury fluorescent lamp, a tubular reactor coupled to a compound parabolic collector illuminated by sunlight, and a tubular reactor illuminated by LEDs. Simulations were carried out for different photocatalyst concentrations, considering absorption and anisotropic scattering, showing differences smaller than 2.4% with respect to the results obtained by commercial CFD software for systems with isotropic emission, such as fluorescent lamps. Moreover, the model was able to improve the simulation of solar reactors and dramatically outperformed the simulation of LED sources, due to the combined effect of quadrature rotation and cone-limit fitting for cone-shaped sources, as well as a LED-specific power-cosine light distribution. The developed model has been thoroughly validated and is now available to the open-source CFD community. It allows a comprehensive numerical simulation of radiation transport using any type of light source, with applications in numerous engineering fields where optical phenomena affect the performance of the process.
•A two-way coupling Eulerian-Lagrangian strategy is presented to assess cavitation erosion risk on the Delft twisted hydrofoil.•The distribution and intensity characteristics of the high impulsive ...pressures are analyzed quantitatively in detail.•The erosion sensitive zones at different stages of one cloud cavitation cycle are determined and their formation mechanism is clarified.
Cavitation damage is a major threat to the life span of fluid machinery and has been a hot research issue in engineering for a long time. The current work aims to show the ability to assess cavitation erosion risk on the twisted hydrofoil using a hybrid Eulerian-Lagrangian solver. The volume of fluid method is used for the liquid-vapor interface of resolved vapor structures, while the discrete bubble model is utilized to track micro-scale unresolvable bubbles. A two-way coupling approach is introduced to enable the transition between resolved cavities and bubbles based on their volume, relative location, and shape. A Lagrangian erosion model considering the effect of asymmetric bubble collapse is proposed to predict cavitation erosion risk. Compared with the experimental result, the current model can accurately identify the high erosion risk regions on the hydrofoil surface. The distribution and intensity of the high impact pressures emitted from bubbles is quantitatively evaluated. In addition, the erosion sensitive zones at different stages of the one cloud cavitation cycle are determined and the hydrodynamic mechanisms of aggressive collapse events are analyzed in detail. The results reveal that the potential erosion risk is highest in areas where primary shedding occurs, which causes the macroscopic cavity to roll up. Bubbles with high impact pressures are mainly focused around the edges of the shedding cavity. The secondary shedding contributes to erosive structure in a limited middle angle of attack area, such that when the primary shedding U-shaped structure evolves and begins to collapse, it becomes less erosive, and only isolated points of erosion is found. Further investigation demonstrates that this is due to a transformation from U-shaped vortex to O-shaped vortex, moving the bubbles gradually away from the hydrofoil surface, thereby reducing the cavitation erosion risk.
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The imbalance of normal stress around a particle induces its transverse migration in pressure-driven viscoelastic flow, offering possibilities for particle manipulation in microfluidic devices. ...Theoretical predictions align with experimental evidence of particles migrating towards the center-line of the flow. However, these arguments have been challenged by both experimental and numerical investigations, revealing the potential for a reversal in the direction of migration for viscoelastic shear-thinning fluids. Yet, a significant property of viscoelastic liquids that remains largely unexplored is the ratio of solvent viscosity to the sum of solvent and polymer viscosities, denoted as β. We computed the lift coefficients of a freely flowing cylinder in a bi-dimensional Poiseuille flow with Oldroyd-B constitutive equations. A transition from a negative (center-line migration) to a positive (wall migration) lift coefficient was demonstrated with increasing β values. Analogous to inertial lift, the changes in the sign of the lift coefficient were strongly correlated with abrupt (albeit small) variations in the rotation velocity of the particle. We established a scaling law for the lift coefficient that is proportional, as expected, to the Weissenberg number, but also to the difference in rotation velocity between the viscoelastic and Newtonian cases. If the particle rotates more rapidly than in the Newtonian case, it migrates towards the wall; conversely, if the particle rotates more slowly than in the Newtonian case, it migrates towards the center-line of the channel. Finally, experiments in microfluidic slits confirmed migration towards the wall for viscoelastic fluids with high viscosity ratio.
•Lift coefficient (CL) of a free cylinder in a Poiseuille flow of an Olroyd-B fluid is studied.•CL depends on the Weissemberg number and the distance to the center.•CL depends also on the solvent viscosity ratio (β).•CL is proportional to the angular velocity difference between the Newtonian case.•For high β, particle migration is toward the center. For low β, it shifts outwards
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