Unity Aspect Ratio Research Articles

EN 206 Conformity Testing for Concrete Strength in Compression

Concrete strength in compression is the common characteristic used in the design of concrete structures. However, in general, there are two different types of concrete specimens specified for the determination of concrete strength in compression. They are the standard 150mm diameter by 300mm length cylinders and the standard 150mm cubes. In the case where the maximum size of coarse aggregate is less than 40mm (1½ in.), 100mm diameter by 200mm length cylinders and 100mm cubes may be adopted. Cylinder specimens with aspect ratio (length/diameter) of 2 are likely to be closer to the uniaxial compressive strength as end friction effects at the platens tend to induced a more complex state of stress compared to that of a cubical specimen with aspect ratio of unity. Hence, the two types of test specimens do not provide the same measured strength for the same concrete. For the purpose of conformity testing, the consistency of test results is of greater interest than the magnitude of the measured strength of the concrete specimens. This paper reviews the issues concerning the two types of specimens, (cylinder or cube) and the size of the specimen in relation to maximum size of coarse aggregate used. Some experimental data for compressive strength at 3 strength classes for which slightly over 100 batches produced over several consecutive calendar months were tested. Three types of specimens, 150mm diameter by 300mm cylinders, 150mm cubes and 100mm cubes were adopted for testing at the age of 28- days. Maximum size aggregate of 20mm and CEM I cement were used for strength classes of C32/40, C50/60 and C65/80 achieved with added silica fume. The findings of these test regimes are summarised with recommendations for conformity testing based on EN 206 and its complementary British Standard BS 8500.

Pollutant removal mechanism in two-dimensional street canyons: a laboratory study

Velocity and concentration fields have been measured simultaneously in the water channel to quantify the turbulent dispersion of a passive tracer emitted at street level by a line source within an idealised urban canyon. The experiment has been conducted for an arrangement of two-dimensional obstacles with aspect ratio unity. Statistical moments of velocity and concentration have been calculated with high spatial resolution. Furthermore, the simultaneous measure of velocity and concentration at the same point made it possible to determine the tracer flux in the whole domain as well as to quantify the removal and re-entrainment of the pollutant through the canyon top.

On the evolution of flow topology in turbulent Rayleigh-Bénard convection

Small-scale dynamics is the spirit of turbulence physics. It implicates many attributes of flow topology evolution, coherent structures, hairpin vorticity dynamics, and mechanism of the kinetic energy cascade. In this work, several dynamical aspects of the small-scale motions have been numerically studied in a framework of Rayleigh-Bénard convection (RBC). To do so, direct numerical simulations have been carried out at two Rayleigh numbers Ra = 108 and 1010, inside an air-filled rectangular cell of aspect ratio unity and π span-wise open-ended distance. As a main feature, the average rate of the invariants of the velocity gradient tensor (QG, RG) has displayed the so-called “teardrop” spiraling shape through the bulk region. Therein, the mean trajectories are swirling inwards revealing a periodic spin around the converging origin of a constant period that is found to be proportional to the plumes lifetime. This suggests that the thermal plumes participate in the coherent large-scale circulation and the turbulent wind created in the bulk. Particularly, it happens when the plumes elongate substantially to contribute to the large-scale eddies at the lower turbulent state. Supplementary small-scale properties, which are widely common in many turbulent flows have been observed in RBC. For example, the strong preferential alignment of vorticity with the intermediate eigenstrain vector, and the asymmetric alignment between vorticity and the vortex-stretching vector. It has been deduced that in a hard turbulent flow regime, local self-amplifications of straining regions aid in contracting the vorticity worms, and enhance the local interactions vorticity/strain to support the linear vortex-stretching contributions. On the other hand, the evolution of invariants pertained to the traceless part of velocity-times-temperature gradient tensor has also been considered in order to determine the role of thermals in the fine-scale dynamics. These new invariants show an incorporation of kinetic and thermal gradient dynamics that indicate directly the evolution and lifetime of thermal plume structures. By applying an identical approach, the rates of the new invariants have shown a symmetric cycling behaviour decaying towards two skew-symmetric converging origins at the lower Ra number. The trajectories near origins address the hot and cold coherent plumes that travel as an average large-scale heat flux in the sidewall vicinities, and denote a periodic spin period close to the plumes lifetime. At the hard turbulent case, the spiraling trajectories travel in shorter tracks to reveal the reduced lifetime of plumes under the dissipative and mixing effects. The turbulent background kinetic derivatives get self-amplified and the trajectories converge to a zero-valued origin indicating that there is no contribution from the plumes to the average coherent large scales of heat flux. These and other peculiar scrutinies on the small-scale motions in RBC have been enlightened, and may have a fruitful consequence on modelling approaches of buoyancy-driven turbulence.

New subgrid-scale models for large-eddy simulation of Rayleigh-Bénard convection

At the crossroad between flow topology analysis and the theory of turbulence, a new eddy-viscosity model for Large-eddy simulation has been recently proposed by Trias et al.[PoF, 27, 065103 (2015)]. The S3PQR-model has the proper cubic near-wall behaviour and no intrinsic limitations for statistically inhomogeneous flows. In this work, the new model has been tested for an air turbulent Rayleigh-Benard convection in a rectangular cell of aspect ratio unity and n span-wise open-ended distance. To do so, direct numerical simulation has been carried out at two Rayleigh numbers Ra = 108 and 1010, to assess the model performance and investigate a priori the effect of the turbulent Prandtl number. Using an approximate formula based on the Taylor series expansion, the turbulent Prandtl number has been calculated and revealed a constant and Ra-independent value across the bulk region equals to 0.55. It is found that the turbulent components of eddy-viscosity and eddy-diffusivity are positively prevalent to maintain a turbulent wind essentially driven by the mean buoyant force at the sidewalls. On the other hand, the new eddy-viscosity model is preliminary tested for the case of Ra = 108 and showed overestimation of heat flux within the boundary layer but fairly good prediction of turbulent kinetics at this moderate turbulent flow.

Improvement of a stochastic backscatter model and application to large‐eddy simulation of street canyon flow

A stochastic backscatter (SB) subgrid‐scale (SGS) model is applied, for the first time, to large‐eddy simulation (LES) of street canyon flow. We model a ‘skimming flow’ regime under a neutrally stratified atmosphere, in which the approaching wind is perpendicular to the along‐street axis of a street canyon of unity aspect ratio. Previous LESs of this type have shown an under‐prediction of the intensity of the primary eddy (PE) that forms within the street canyon, indicating a lack of momentum transfer across the roof‐level shear layer. The SB model, however, acts to increase this momentum transfer, bringing the simulated PE intensity significantly closer towards that observed in a corresponding wind‐tunnel experiment. A metric for the PE intensity, ωPE, based on the two‐dimensional vorticity field, is increased from around 70% of the wind‐tunnel ωPE value (with the Smagorinsky SGS model) to as much as 90% (with the SB model). Calculation of the air exchange rate at roof‐level confirms that the rate of entrainment into the street canyon is increased with the inclusion of backscatter.We also outline an improvement to the SB model prior to its application. In its previous version, a constraint on the magnitude of the backscatter acceleration variances ensured a theoretically appropriate level of additional grid‐scale (backscattered) energy. Here, a further constraint on the magnitude of the main covariance term also facilitates a better representation of grid‐scale vertical momentum flux. This new constraint alone can help to increase the simulated ωPE value by as much as 10% of the wind‐tunnel ωPE value, and requires almost no additional computational effort. The effect of varying the magnitude and length‐scale of the imposed backscatter (via the backscatter coefficient and length of the filter used to generate the backscatter acceleration fields, respectively) is also investigated.

Effect of Periodic Bottom Plate Heating on Large Scale Flow in Turbulent Rayleigh-Bénard Convection

We report analysis of temperature fluctuations measured inside a Rayleigh-Benard convection cell of aspect ratio unity with steady and time-varying heating of the bottom boundary. The working fluid is cryogenic helium gas at Rayleigh number Ra = 10 13 , for which a large scale coherent flow (mean wind) exists. We observe the wind to occasionally vanish under both steady and time-varying heating conditions. However, with applied periodic modulation of the lower boundary temperature at the frequency of the wind, we can observe destruction of the periodic temperature variation at the cell mid-plane, due to relative changes in phase between the mean wind and the propagating temperature wave. The wind speed is not observed to change with modulation, demonstrating that it is set by mean properties of the system. Finally we propose that the frequency of the mean wind – and hence the large scale Reynolds number – could be better resolved by finding resonance with applied forcing.

Flow characteristics and discharge rate of ellipsoidal particles in a flat bottom hopper

Hopper flow characteristics are significantly affected by particle shape. In this work, ellipsoidal particles which can represent a large number of shapes are used to investigate the shape effect on granular flow in a cylindrical hopper. Numerical experiments are conducted by use of the discrete element method, with its validity verified by comparison with the results from physical experiments. The results indicate that particle shape can make a significant effect on the flow pattern. In particular, the increase of deviation from spheres can decrease the mixed region adjacent to the side wall, and increase the stagnant zone at the bottom corner. It may also lead to decreased wall stress. Furthermore, particle shape has a significant effect on the discharge rate. Spheres of unity aspect ratio have the highest flow rate, and the lower or higher aspect ratio, the smaller the flow rate. Based on the numerical results, the Beverloo equation is modified, where parameters C and k in the equation are respectively formulated as a function of aspect ratio.

On the pollutant removal, dispersion, and entrainment over two-dimensional idealized street canyons

Pollutant dispersion over urban areas is not that well understood, in particular at the street canyon scale. This study is therefore conceived to examine how urban morphology modifies the pollutant removal, dispersion, and entrainment over urban areas. An idealized computational domain consisting of 12 two-dimensional (2D) identical street canyons of unity aspect ratio is employed. The large-eddy simulation (LES) is used to calculate the turbulent flows and pollutant transport in the urban boundary layer (UBL). An area source of uniform pollutant concentration is applied on the ground of the first street canyon. A close examination on the roof-level turbulence reveals patches of low-speed air masses in the streamwise flows and narrow high-speed downdrafts in the shear layer. Different from the flows over a smooth surface, the turbulence intensities are peaked near the top of the building roughness. The pollutant is rather uniformly distributed inside a street canyon but disperses quickly in the UBL over the buildings. Partitioning the vertical pollutant flux into its mean and turbulent components demystifies that the pollutant removal is mainly governed by turbulence. Whereas, mean wind carries pollutant into and out of a street canyon simultaneously. In addition to wind speed promotion, turbulent mixing is thus required to dilute the ground-level pollutants, which are then removed from the street canyon to the UBL. Atmospheric flows slow down rapidly after the leeward buildings, leading to updrafts carrying pollutants away from the street canyons (the basic pollutant removal mechanism).

Dynamics of the large-scale circulation in high-Prandtl-number turbulent thermal convection

AbstractWe report experimental investigations of the dynamics of the large-scale circulation (LSC) in turbulent Rayleigh–Bénard convection at high Prandtl number $\mathit{Pr}= 19. 4$ (and also $\mathit{Pr}= 7. 8$) and Rayleigh number $\mathit{Ra}$ varying from $8. 3\times 1{0}^{8} $ to $2. 9\times 1{0}^{11} $ in a cylindrical convection cell with aspect ratio unity. The dynamics of the LSC is measured using the multithermal probe technique. Both the sinusoidal-fitting (SF) and the temperature-extrema-extraction (TEE) methods are used to analyse the properties of the LSC. It is found that the LSC in high-$\mathit{Pr}$ regime remains a single-roll structure. The azimuthal motion of the LSC is a diffusive process, which is the same as those for $\mathit{Pr}$ around 1. However, the azimuthal diffusion of the LSC, characterized by the angular speed $\Omega $ is almost two orders of magnitude smaller when compared with that in water. The non-dimensional time-averaged amplitude of the angular speed $\langle \vert \Omega \vert \rangle {T}_{d} $ (${T}_{d} = {L}^{2} / \kappa $ is the thermal diffusion time) of the LSC at the mid-height of the convection cell increases with $\mathit{Ra}$ as a power law, which is $\langle \vert \Omega \vert \rangle {T}_{d} \propto {\mathit{Ra}}^{0. 36\pm 0. 01} $. The $\mathit{Re}$ number based on the oscillation frequency of the LSC is found to scale with $\mathit{Ra}$ as $\mathit{Re}= 0. 13{\mathit{Ra}}^{0. 43\pm 0. 01} $. It is also found that the normalized flow strength $\langle \delta \rangle / \mrm{\Delta} T\times \mathit{Ra}/ \mathit{Pr}\propto {\mathit{Re}}^{1. 5\pm 0. 1} $, with the exponent in good agreement with that predicted by Brown & Ahlers (Phys. Fluids, vol. 20, 2008, p. 075101). A wealth of dynamical features of the LSC, such as the cessations, flow reversals, flow mode transitions, torsional and sloshing oscillations are observed in the high-$\mathit{Pr}$ regime as well.

Large Eddy Simulation of Fire Smoke Re-circulation in Urban Street Canyons of Different Aspect Ratios

Critical re-circulation phenomenon happens when a fire occurs in an urban street canyon, in which the uprising smoke plume is re- circulated back into the base of the street canyon by the wind-induced flow. This happens when the inertia of the wind flow with a certain so-called critical re-circulation velocity balances the buoyancy force of the fire smoke. The fire smoke dispersion and critical re- circulation wind velocity have been revealed in previous works for urban street canyons with aspect ratio (W/H) of unity. This paper presents an investigation of the effect of street canyon aspect ratio on the above behaviour. Large Eddy Simulation (LES) was performed by Fire Dynamics Simulation (FDS) for a 5 MW fire corresponding to the Heat Release Rate of a burning car. The simulated street canyon has height of 18 m and different width of 9 m, 12 m, 15 m, 18 m, 21 m, 24 m, 27 m, 30 m and 36 m respectively, with aspect ratios varied from 0.5 to 2.0. The smoke soot and CO concentration distribution were presented for different street canyon aspect ratios. It was found that the variation of critical re-circulation wind velocity with street canyon aspect ratio can be divided into three zones. The critical re-circulation wind velocity firstly increases at the first zone, then remains constant at the second zone, and finally increases again at the third zone with the increasing of the street canyon aspect ratio. This special behaviour was explained based on the flow characteristics in the street canyon.

Materials for Drug Delivery: Innovative Solutions to Address Complex Biological Hurdles

Materials for Drug Delivery: Innovative Solutions to Address Complex Biological Hurdles

A new application of a multifrequency submillimeter radiometer in determining the microphysical and macrophysical properties of volcanic plumes: A sensitivity study

In this paper, the potential of a multifrequency submillimeter radiometer to characterize ash plumes in the near‐field of a volcanic eruption is evaluated. The radiometer's sensitivity to mass concentration and particle effective dimension is shown to depend most critically on aerosol altitude and ejected water vapor concentration. There is also some dependence on temperature, aerosol shape and complex refractive index. For this study, the volcanic aerosols are assumed to be randomly oriented solid hexagonal silicates of aspect ratio unity. The T‐matrix method is used to calculate the single‐scattering properties of the aerosols at 36 frequencies between 90 GHz and 880 GHz, and the aerosol bulk scattering properties are derived assuming lognormal size distribution functions. A midlatitude standard summer atmosphere and a perturbed midlatitude summer atmosphere are used to quantify the sensitivity, using the delta‐Eddington two‐stream approximation, of the radiometer to the presence of aerosol. It is shown that at 34 frequencies, between 113 GHz and 880 GHz, the sensitivity to aerosol is a maximum if the following four conditions are satisfied: (i) The altitude of the aerosol layer should be ≫ 3 km, (ii) 0.1 g m−3 < mass concentration < 30 g m−3 (iii) the aerosol effective dimension, De, 20 μm < De < 1000 μm and (iv) water vapor ejected by a volcano into the atmosphere should be < 1000 times greater than the background water vapor concentration. The paper demonstrates the potential usefulness of using spectrally resolved submillimeter measurements in the near‐field of volcanic eruptions to characterize plume properties.

Observations of electron phase-space holes driven during magnetic reconnection in a laboratory plasma

This work presents detailed experimental observations of electron phase-space holes driven during magnetic reconnection events on the Versatile Toroidal Facility. The holes are observed to travel on the order of or faster than the electron thermal speed, and are of large size scale, with diameter of order 60 Debye lengths. In addition, they have 3D spheroidal structure with approximately unity aspect ratio. We estimate the direct anomalous resistivity due to ion interaction with the holes and find it to be too small to affect the reconnection rate; however, the holes may play a role in reining in a tail of accelerated electrons and they indicate the presence of other processes in the reconnection layer, such as electron energization and electron beam formation.

Laminar Forced Convection Heat Transfer From Isothermal Bodies With Unity Aspect Ratio in Coaxial Air Flow

In this article a semianalytical approach is employed to obtain dimensionless heat transfer correlations for forced convection over three geometries—sphere, cone, and cylinder with unity aspect ratio in laminar axial air flow. The comparison of the present results for a sphere with the previous work shows very good agreement. For example, the average difference between the results of the present model and those of Ahmed and Yovanovich and of McAdams are 4.3% and 0.8%, respectively. Validation of the results for cone and cylinder was done based on available data on drag coefficients. Finally, a general correlation is developed for a rough estimate of forced convection from isothermal bodies of unity aspect ratio and coaxial with the airflow.

Large-eddy simulation of reactive pollutant dispersion for the spatial instability of photostationary state over idealised 2D urban street canyons

Dispersion of chemically reactive pollutants in the urban canopy layer (UCL) over twelve idealised two-dimensional (2D) street canyons of unity aspect ratio in isothermal conditions is examined. The reversible NO x -O 3 mechanism is integrated into a large-eddy simulation (LES) model. The ground-level NO emission in the first street canyon and the background O 3 in the prevailing wind initiate the NO 2 production. The chemical equilibrium is measured by the photostationary state (PSS) which is a function of the time scales of turbulent mixing and chemical reactions. PSS of the first street canyon increases with increasing NO emission. For small amount of ground-level NO emission and background O 3 in the prevailing flows, (say NO/O 3 = 1/1), PSS increases gradually from the second to last street canyons. It increases faster if the NO-to-O 3 ratio is raised, say 1,000/30. Further increasing the NO-to-O 3 ratio, say 10,000/1, PSS exhibits a non-linear behaviour in which a trough is observed in the fifth street canyon.

Effects of Wall Heating on Flow Characteristics in a Street Canyon

We develop a large-eddy simulation (LES) model based on a meteorological numerical model for a real scale street-canyon flow with rough building facets heated by a given temperature. The model is applied to a canyon with the aspect ratio of unity for two idealized heating scenarios: (1) the roof and the entire upstream wall are heated, named as ‘assisting cases’, and (2) the roof and the entire downstream wall are heated, named as ‘opposing cases’. These facets were heated up to 15 K above the air temperature. A wall function for temperature is proposed for a rough facet with an assumption that the thermal roughness length, z0T, is much smaller than the aerodynamic roughness length, z0. It is demonstrated that the sensible heat flux and canyon-air temperature are significantly influenced by the near-facet process that is parametrized by z0T as the primary factor; other processes such as in-canyon mixing and roof-level exchange are secondary. This new finding strongly suggests that it is vital to choose an appropriate value of z0T in a numerical simulation of street-canyon flows with the facet-air exchange processes of heat or any scalar. The finding also raises an awareness of the demand for carefully designed laboratory or field experiments of quantifying z0T values for various urban surfaces. For the opposing cases, an unsteady penetrating narrow updraft zone appears occasionally along the heated wall and this feature is consistent field observations. The unique result indicates the superior capability of LES. The results of this study can be used to guide the parametrization of turbulent processes inside the urban canopy layer.

Modelling diffraction by facetted particles

A method to approximate azimuthally resolved light scattering patterns and phase functions due to diffraction and external reflection by strongly absorbing facetted particles is demonstrated for a cube and compared with results from an exact method, T-matrix. A phase function averaged over a range of orientations of a strongly absorbing hexagonal column of aspect ratio unity has been calculated and tested against Discrete Dipole Approximation (DDA) results for a size parameter of 50.

Large-Eddy Simulation of Flow and Pollutant Transports in and Above Two-Dimensional Idealized Street Canyons

A large-eddy simulation (LES) model, using the one-equation subgrid-scale (SGS) parametrization, was developed to study the flow and pollutant transport in and above urban street canyons. Three identical two-dimensional (2D) street canyons of unity aspect ratio, each consisting of a ground-level area source of constant pollutant concentration, are evenly aligned in a cross-flow in the streamwise direction x. Theflow falls into the skimming flow regime. A larger computational domain is adopted to accurately resolve the turbulence above roof level and its influence on the flow characteristics in the street canyons. The LES calculated statistics of wind and pollutant transports agree well with other field, laboratory and modelling results available in the literature. The maximum wind velocity standard devi- ations σi in the streamwise (σu), spanwise (σv) and vertical (σw) directions are located near the roof-level windward corners. Moreover, a second σw peak is found at z ≈ 1.5h (h is the building height) over the street canyons. Normalizing σi by the local friction velocity u∗ ,i t is found that σu/u∗ ≈ 1.8, σv/u∗ ≈ 1. 3a ndσw/u∗ ≈ 1.25 exhibiting rather uniform values in the urban roughness sublayer. Quadrant analysis of the vertical momentum flux u �� w �� shows that, while the inward and outward interactions are small, the sweeps and ejections dominate themomentumtransportoverthestreetcanyons.Inthe x direction,thetwo-pointcorrelations of velocity Rv,x and Rw,x drop to zero at a separation larger than h but Ru,x (= 0.2) persists even at a separation of half the domain size. Partitioning the convective transfer coefficient � T of pollutant into its removal and re-entry components, an increasing pollutant re-entrain- ment from 26. 3t o 43.3% in the x direction is revealed, suggesting the impact of background pollutant on the air quality in street canyons.

Effect of nanofluid variable properties on natural convection in enclosures

In this work, the heat transfer enhancement in a differentially heated enclosure using variable thermal conductivity and variable viscosity of Al 2O 3–water and CuO–water nanofluids is investigated. The results are presented over a wide range of Rayleigh numbers ( Ra = 10 3–10 5), volume fractions of nanoparticles (0 ≤ φ ≤ 9%), and aspect ratios (½ ≤ A ≤ 2). For an enclosure with unity aspect ratio, the average Nusselt number of a Al 2O 3–water nanofluid at high Rayleigh numbers was reduced by increasing the volume fraction of nanoparticles above 5%. However, at low Rayleigh numbers, the average Nusselt number was slightly enhanced by increasing the volume fraction of nanoparticles. At high Rayleigh numbers, CuO–water nanofluids manifest a continuous decrease in Nusselt number as the volume fraction of nanoparticles is increased. However, the Nusselt number was not sensitive to the volume fraction at low Rayleigh numbers. The Nusselt number demonstrates to be sensitive to the aspect ratio. It was observed that enclosures, having high aspect ratios, experience more deterioration in the average Nusselt number when compared to enclosures having low aspect ratios. The variable thermal conductivity and variable viscosity models were compared to both the Maxwell-Garnett model and the Brinkman model. It was found that at high Rayleigh numbers the average Nusselt number was more sensitive to the viscosity models than to the thermal conductivity models.

Compact Physics-Based Circuit Models for Graphene Nanoribbon Interconnects

Physics-based equivalent circuit models are presented for armchair and zigzag graphene nanoribbons (GNRs), and their conductances have been benchmarked against those of carbon nanotubes and copper wires. Atomically thick GNRs with smooth edges can potentially have smaller resistances compared with copper wires with unity aspect ratios for widths below 8 nm and stacks of noninteracting GNRs can have substantially smaller resistivities compared to Cu wires. It is shown that rough edges can increase the resistance of narrow GNRs by an order of magnitude. This fact highlights the need for patterning methods that can produce relatively smooth edges to fabricate low resistance GNR interconnects.