The effect of tree planting on traffic pollutant dispersion in an urban street canyon using large eddy simulation with a recycling and rescaling inflow generation method

The influence of various tree planting configurations on tracer gas dispersion in an urban street canyon is studied numerically, using OpenFOAM Large Eddy Simulation (LES). A recycling and rescaling turbulent inflow generation method is implemented and validated against a canonical backwards-facing step case. The street canyon simulations are based on the CODASC experiments, where the tracer gas was emitted from line sources at street level. The effect of tree stand density on the pollutant dispersion is studied for the first time using LES. Performance metric analysis of the CODASC simulations shows that the influence of tree crown porosity, and tree stand density, on the concentration field within the canyon can be captured by LES. The simulations show that tree crowns reduce the effectiveness of the canyon vortex at ventilating the street, and enhance the mean tracer gas concentration at pedestrian level on the leeward wall. A novel feature of the research arises from concentration probability density functions at pedestrian height in the canyon. Pedestrians can be exposed to pollutant concentrations almost three times the mean value at the leeward wall. The results have implications for urban greening strategies in city streets with heavy traffic

Numerical analysis on the thermal performance of microchannel heat sinks with Al2O3 nanofluid and various fins

The hydraulic and thermal performance of microchannel heat sink configurations for high performance electronic cooling applications is investigated by numerical modelling. Conjugate heat transfer simulations are obtained through the silicon walls and the fluid domain of a square base prism heat sink traversed by 50 parallel rectangular cooling ducts, under a 150 W/cm2 constant heat flux input through the base. Al2O3 nanofluid coolant with a nanoparticle volume fraction ranging from 0 to 3% is supplied at 298 K, over the Reynolds number range 100 to 350, modelled as a single-phase homogeneous medium. Rectangular, twisted, and zig-zag fins are inserted into the plain rectangular duct to enhance the heat transfer rate. The zig-zag fin and 3% Al2O3 nanofluid provide the best thermal performance, with a 6.44 K lower average heated wall contact temperature, 60% higher Nusselt number, and 15% higher second law efficiency than without fins and plain water cooling. Twist in the microchannel fin unexpectedly reduced the microchannel pressure drop by 2% to 15% compared to a straight fin, possibly due to the more evenly distributed axial mass flux across the microchannel

An Aeroacoustic Numerical Model of the Transonic Flow Past a Sphere

  Transonic flow past a sphere is investigated by Large Eddy Simulations. The Reynolds number based on the diameter of the sphere and on the free-stream quantities is 1000, while the free-stream Mach number is 0.9. This regime generates a weakly supersonic flow behind the sphere, with a stationary compression wave a few diameters downstream which traverses the developing turbulent wake. The focus of this study is the sound radiated from the wake turbulent structures and their interaction with the stationary compression wave. This direct aeroacoustic computation shows a spiral pressure wave radiating from the base flow precessing about the free-stream unit vector passing through the centre of the sphere. The main acoustically active region is the stationary compression wave-wake interaction one. The localised supersonic flow over the sphere prevents the upstream radiation in a cone of silence type effect within a narrow solid angle upstream of the sphere, which could be exploited for passive noise control.</p

Numerical investigation of various twisted tapes enhancing a circular microchannel heat sink performance

The continuous power increase and miniaturization of modern electronics require increasingly effective thermal management systems. The thermo-hydraulic performance of water-cooled L×L square-base silicon microchannel heat sinks is investigated by a conjugate heat transfer and computational fluid dynamics model over the Reynolds number range 100 to 500. Water at a constant inlet temperature of 298 K runs through 33 parallel tubes, extracting heat from the bottom wall that has a 100 W/cm2 constant heat flux input. Hydro-thermal performance-enhancing tape inserts are numerically tested featuring (i) radial gaps between the tape and the tube, (ii) tape twist with axial pitch distances of ∞, L/2, or L/4, (iii) zero, one, or two 90-degree angular steps between consecutive tape segments, (iv) alternating clockwise and anti-clockwise consecutive twisted tape segments, and combinations of these features. The radial gaps produce both a hydraulic and a thermal performance loss. All combinations of tape twist, angular steps, and twist direction reversal produced better thermal performance gains to hydraulic loss trade-offs than the baseline microchannel configuration with no tape. The microchannel heat sinks with four L/4 alternating pitch consecutive helical tape segments provided the lowest bottom wall average temperature, 16.13 K below that with not tape, at the same Reynolds number of 500. This predicted temperature drop is a significant achievement towards conditioning electronic components so they may be longer-lasting, use less energy, and have a reduced environmental impact.</p

Physically Consistent Implementation of the Mixture Model for Modelling Nanofluid Conjugate Heat Transfer in Minichannel Heat Sinks

As much as two-phase mixture models resolve more physics than single-phase homogeneous models, their inconsistent heat transfer predictions have limited their use in modelling nanofluid cooled minichannel heat sinks. This work investigates, addresses, and solves this key shortcoming, enabling reliable physically sound predictions of minichannel nanoflows, using the two-phase mixture model. It does so by applying the single-phase and the two-phase mixture model to a nine-passages rectangular minichannel, 3 mm deep and 1 mm wide, cooled by a 1% by volume suspension of Al2O3 nanoparticles in water, over the Reynolds number range 92 to 455. By varying the volume fraction αnf of the second phase between 2% and 50%, under a constant heat flux of 16.67 W/cm2 and 30 Celsius coolant inflow, it is shown that the two-phase mixture model predicts heat transfer coefficient, pressure loss, friction factor, exergy destruction rate, exergy expenditure rate, and second law efficiency values converging to the single-phase model ones at increasing αnf. A two-phase mixture model defined with 1% second phase volume fraction and 100% nanoparticles volume fraction in the second phase breaks the Newtonian fluid assumption within the model and produces outlier predictions. By avoiding this unphysical regime, the two-phase mixture model matched experimental measurements of average heat transfer coefficient to within 1.76%. This has opened the way for using the two-phase mixture model with confidence to assess and resolve uneven nanoparticle dispersion effects and increase the thermal and mass transport performance of minichannels

Large Eddy Simulation with ModelledWall-Stress for Complex Aerodynamics Applications and Stall Prediction

The aerodynamics of aircraft high-lift devices at near-stall conditions is particularly difficult to predict numerically. The computational requirements for accurate wall-resolved large-eddy simulations are currently prohibitive, whereas Reynolds-averaged Navier–Stokes (RANS) models are generally reliable only for low angles of attack with fully attached boundary layers. Methods such as detached-eddy simulation resolve unsteadiness of the outer boundary layer and can predict separation, but they rely upon a thick RANS layer and highly stretched cells that damp the resolved turbulent fluctuations near the wall. An alternative approach, adopted here, is to extend the LES down to the wall, employing a relatively large near-wall normal grid spacing and avoiding grid stretching and high aspect ratios near the wall. A boundary condition then applies the correct wall shear stress as provided by a semiempirical wall model. An adaptive formulation of this wall-modeled large-eddy simulation is presented here and validated using realistic test cases. Validation using a channel flow case at a range of Reynolds numbers demonstrates accurate results with a seamless transition between fully resolved (y+≈2) and wall resolved (y+≈50). Predictions of the MD-30P/30N airfoil using a modest grid with y+≈100 give excellent agreement with experiments and correctly predict CLmax. Finally, the method is demonstrated for the NASA High-Lift Common Research Model providing surface pressure coefficients and velocity profiles. The predictions using a 50-million-cell mesh (for a full aircraft half-model) are in good agreement with considerably finer-grid RANS solutions. The presented method has considerable potential because it can produce accurate solutions to challenging engineering problems involving separation with modest grid and computational requirements while being robust to variations in near-wall grid spacing

Local heat transfer on a finite width surface with laminar boundary layer flow

The effect of a lateral discontinuity in the thermal boundary conditions in two dimensional laminar flow on a flat plate is investigated with numerical and analytical modeling. When the thermal and momentum boundary layers start at the same location, the resulting self-similar two dimensional boundary layer equations were solved numerically. For flow with an unheated starting length, three dimensional numerical simulations were required. For both the two and three dimensional thermal simulations, the Blasius solution for a two dimensional momentum boundary layer was assumed. It is found that all the Nusselt numbers collapse to a single curve when graphed as a function of a spanwise similarity variable. Simple correlations for the local Nusselt number on a rectangular flat plate are presented for a variety of boundary conditions

An open-source coupled method for aeroacoustics modelling

 In the present work, an open-source fast coupling interface for high-order acoustic propagation is presented. This interface is implemented between two open-source codes, OpenFOAM and Nektar++. The coupling interface is validated on a 2D cylinder case showing a very good agreement with the analytical solution and demonstrating its high efficiency as compared to traditional file-based coupled methods. A more realistic rod-airfoil wake interaction case is also studied to verify the applicability of the methodology in more complex configurations. Although the interface implementation is only realised for the chosen two open-source codes, its adaptation to other similar solvers/packages is expected to be straightforward. </p

Further results on the mean mass transfer and fluid flow in a turbulent round jet

The present paper reports new numerical results of the mean mass transfer in a turbulent submerged round jet. A series of Large Eddy Simulations (LES) are presented at four Reynolds (Re) numbers (Re = 2492, Re = 4491, Re = 9994, Re = 19,988) and laminar Schmidt (Sc) number equal to Sc = 10. The numerical results are specially focused on their patterns in the Undisturbed Region of Flow (URF), Potential Core Region (PCR) and Fully Developed Region (FDR). The present results are compared with the previous literature concerning a two-dimensional and a three-dimensional jet, issuing from an infinitely wide slot, with the conclusion that the turbulence moments decay faster in a round jet configuration.</p

Numerical Simulation and Self-Similarity of the Mean Mass Transfer in Turbulent Round Jets

The paper investigates the mean mass transfer/passive scalar spreading in turbulent submerged round jets. Two regions of flow are present in the jet evolution: the Near-Field Region (NFR) and the Fully Developed Region (FDR). This group of research investigates from some years the mean evolution of turbulent rectangular jets with the new physical finding that two sub-regions (not a single one) are present in the NFR. The first region of the two is the newly discovered Undisturbed Region of Flow (URF), while the second one is the known Potential Core Region (PCR). In a recent paper we showed that the flow evolution of turbulent round jets, as far as momentum spreading is concerned, is self-similar also in the NFR. Literature shows that mass transfer spreading is self-similar only in FDR. The present paper presents new mean mass transfer results of the numerical Large Eddy Simulation (LES) in turbulent round jets. Four Reynolds numbers, from 2492 to 19,988, and two laminar Schmidt numbers, 1 and 10, are investigated. The first novel result of this paper is that mass transfer is self-similar in the NFR. The second result is that two new analytical models describe the passive scalar spreading in the URF and PCR. The third result is that two new self-similar laws describe the passive scalar spreading in the FDR. The fourth result states that the well-known power-law relationship, between passive scalar and axial momentum in the FDR, holds regardless of the modeling of turbulent viscosity and turbulent Schmidt number