Array Of Cubes Research Articles

Experimental study of wind-induced ventilation in urban building of cube arrays with various layouts

Urban airflow and dispersion processes have become matters of great concern from the viewpoints of urban heat island phenomena, thermal comfort of pedestrians, hazard control for toxic gases, wind-induced ventilation of urban buildings, and so on. In this study, the authors measured the spatial distribution of the pressure drag acting on the walls of rectangular block arrays in a wind tunnel. The arrays were arranged with staggered, square, and diamond layouts under different conditions of roughness and packing density. The total drag and wind profiles of the arrays were preliminarily measured. On the basis of experimental results, the contribution of pressure drag to the total surface drag on staggered cube arrays was estimated at greater than 95%. In addition, the bulk pressure coefficients Cp for the arrays defined by mean velocity at a height of H, 2H and 20H were derived (H refers to the height of a block), and the clear relation between Cp and the block packing densities were shown for rough estimation of wind-induced ventilation in urban buildings. In addition, the authors present a simple mathematical expression to explain the relationship.

Effect of urban vegetation on outdoor thermal environment: Field measurement at a scale model site

The goal of this research is to bridge the gap between numerical studies and field measurements on thermal environment of a real urban street and to present information on the effects of urban vegetation suitable for use by designers and planners. Outdoor measurements were conducted at a scale model site consisting of an array of concrete cubes each 1.5 m high. Eight urban street units with various vegetation conditions were reproduced to examine the quantitative effects of vegetation along the sidewalk and in median strips on the thermal environment in summer. The results can be summarized as follows. The presence of four sidewalk trees reduces the wind speed inside the canopy by up to 51%. Trees along the sidewalk also decrease the globe temperature; the reduction is attributed mainly to the decrease in radiation flux resulting from the shade they cast. Moreover, thermal mitigation due to vegetation persists even when an area is shaded. In contrast, the mitigating effect of a vegetated median strip is not remarkable. A sidewalk facing a southwestern wall exhibited the most significant thermal mitigation.

Geometric Dependence of the Scalar Transfer Efficiency over Rough Surfaces

We performed a series of wind-tunnel experiments under neutral conditions in order to create a comprehensive database of scalar transfer coefficients for street surfaces using regular block arrays representing an urban environment. The objective is to clarify the geometric dependence of scalar transfer phenomena on rough surfaces. In addition, the datasets we have obtained are necessary to improve the modelling of scalar transfer used for computational simulations of urban environments; further, we can validate the results obtained by numerical simulations. We estimated the scalar transfer coefficients using the salinity method. The various configurations of the block arrays were designed to be similar to those used in a previous experiment to determine the total drag force acting on arrays. Our results are summarized as follows: first, the results for cubical arrays showed that the transfer coefficients for staggered and square layouts varied with the roughness packing density. The results for the staggered layout showed the possibility that the mixing effect of air can be enhanced for the mid-range values of the packing density. Secondly, the transfer coefficients for arrays with blocks of non-uniform heights were smaller than those for arrays with blocks of uniform height under conditions of low packing density; however, as the packing density increased, the opposite tendency was observed. Thirdly, the randomness of rotation angles of the blocks in the array led to increasing values of the transfer coefficients under sparse packing density conditions when compared with those for cubical arrays.

Spatio-Temporal Surface Shear-Stress Variability in Live Plant Canopies and Cube Arrays

This study presents spatiotemporally-resolved measurements of surface shear-stress τs in live plant canopies and rigid wooden cube arrays to identify the sheltering capability against sediment erosion of these different roughness elements. Live plants have highly irregular structures that can be extremely flexible and porous resulting in considerable changes to the drag and flow regimes relative to rigid imitations mainly used in other wind-tunnel studies. Mean velocity and kinematic Reynolds stress profiles show that well-developed natural boundary layers were generated above the 8 m long wind-tunnel test section covered with the roughness elements at four different roughness densities (λ = 0, 0.017, 0.08, 0.18). Speed-up around the cubes caused higher peak surface shear stress than in experiments with plants at all roughness densities, demonstrating the more effective sheltering ability of the plants. The sheltered areas in the lee of the plants are significantly narrower with higher surface shear stress than those found in the lee of the cubes, and are dependent on the wind speed due to the plants ability to streamline with the flow. This streamlining behaviour results in a decreasing sheltering effect at increasing wind speeds and in lower net turbulence production than in experiments with cubes. Turbulence intensity distributions suggest a suppression of horseshoe vortices in the plant case. Comparison of the surface shear-stress measurements with sediment erosion patterns shows that the fraction of time a threshold skin friction velocity is exceeded can be used to assess erosion of, and deposition on, that surface.

Experimental Investigation of Thermal Deformation in Thermoelectric Coolers

Abstract: From high‐tech industry to consumer electronics, thermoelectric cooler (TEC) has been widely used. Basically, TEC is a sandwich structure. An array of small bismuth telluride cubes is placed between two ceramic plates and bonded to them. When a DC is applied, thermal deformation occurs because of the temperature gradient produced between the two ceramic plates of the device. To ensure the safety of the TEC, it is therefore important to investigate the thermal deformation induced. However, because of the complexity of the TEC structure, numerical simulation cannot be easily performed. In this study, digital speckle pattern interferometry (DSPI) was employed to measure the real‐time full‐field thermal deformation in TECs. Variations of thermal deformations versus magnitudes of DC were obtained. The obtained experimental results will be very useful for building the numerical model.

Large-Eddy Simulation of Coherent Flow Structures within a Cubical Canopy

Instantaneous flow structures “within” a cubical canopy are investigated via large-eddy simulation. The main topics of interest are, (1) large-scale coherent flow structures within a cubical canopy, (2) how the structures are coupled with the turbulent organized structures (TOS) above them, and (3) the classification and quantification of representative instantaneous flow patterns within a street canyon in relation to the coherent structures. We use a large numerical domain (2,560 m × 2,560 m × 1,710 m) with a fine spatial resolution (2.5 m), thereby simulating a complete daytime atmospheric boundary layer (ABL), as well as explicitly resolving a regular array of cubes (40 m in height) at the surface. A typical urban ABL is numerically modelled. In this situation, the constant heat supply from roof and floor surfaces sustains a convective mixed layer as a whole, but strong wind shear near the canopy top maintains the surface layer nearly neutral. The results reveal large coherent structures in both the velocity and temperature fields “within” the canopy layer. These structures are much larger than the cubes, and their shapes and locations are shown to be closely related to the TOS above them. We classify the instantaneous flow patterns in a cavity, specifically focusing on two characteristic flow patterns: flushing and cavity-eddy events. Flushing indicates a strong upward motion, while a cavity eddy is characterized by a dominant vortical motion within a single cavity. Flushing is clearly correlated with the TOS above, occurring frequently beneath low-momentum streaks. The instantaneous momentum and heat transport within and above a cavity due to flushing and cavity-eddy events are also quantified.

Wind-Direction Effects on Urban-Type Flows

Practically all extant work on flows over obstacle arrays, whether laboratory experiments or numerical modelling, is for cases where the oncoming wind is normal to salient faces of the obstacles. In the field, however, this is rarely the case. Here, simulations of flows at various directions over arrays of cubes representing typical urban canopy regions are presented and discussed. The computations are of both direct numerical simulation and large-eddy simulation type. Attention is concentrated on the differences in the mean flow within the canopy region arising from the different wind directions and the consequent effects on global properties such as the total surface drag, which can change very significantly—by up to a factor of three in some circumstances. It is shown that for a given Reynolds number the typical viscous forces are generally a rather larger fraction of the pressure forces (principally the drag) for non-normal than for normal wind directions and that, dependent on the surface morphology, the average flow direction deep within the canopy can be largely independent of the oncoming wind direction. Even for regular arrays of regular obstacles, a wind direction not normal to the obstacle faces can in general generate a lateral lift force (in the direction normal to the oncoming flow). The results demonstrate this and it is shown how computations in a finite domain with the oncoming flow generated by an appropriate forcing term (e.g. a pressure gradient) then lead inevitably to an oncoming wind direction aloft that is not aligned with the forcing term vector.

Spatial Visualization Tasks To Support Students’ Spatial Structuring In Learning Volume Measurement

Many prior researches found that most of students in grade five tended to have difficulty in fully grasping the concept of volume measurement because they have to build their competence in spatial structuring. The unit of volume packing measurement must be integrated and coordinated in three-dimension. On the other hand, it is revealed the errors that students made on the volume measurement tasks with three dimensional cube arrays are related to some aspects of spatial visualization, such as the skill to "read off" two-dimensional representation of solid objects. For those reasons, this research is aimed to develop classroom activities with the use of spatial visualization tasks to support students' spatial structuring in learning volume measurement. Consequently, design research was chosen as an appropriate means to achieve this research goal. In this research, a sequence of instructional activities is designed and developed based on the hypothesis of students' learning processes. This research was conducted in grade 5 of SD Pupuk Sriwijaya Palembang, Indonesia. Keywords : volume measurement, spatial structuring, spatial visualization, design research.

A fast model for pollutant dispersion at the neighbourhood scale

This paper is devoted to the development and evaluation of a fast three-dimensional Eulerian model for dispersion inside and above the urban canopy layer. Spatially averaged wind and diffusivity coefficient profiles obtained from the commercial Computational Fluid Dynamics (CFD) code FLUENT are used as input in the developed model. This model is numerically solved by means of a finite volume method and mean concentration outputs are compared with the corresponding results from FLUENT. We considered several canopies made of arrays of cubes laid in staggered position. Results from the comparison suggest the potential of this type of simple modelling approach. As the spatially averaged wind and diffusivity profiles are strongly dependent from the mean morphometry properties of the urban canopy, these results, though preliminary, highlight the necessity of using specific turbulence closure models where building effects at the neighbourhood scale are taken into account.

Representing complex urban geometries in mesoscale modeling

AbstractReal cities are comprised of a diverse, random arrangement of building positions, shapes, and sizes. Yet most of the urban parameterisations thus far developed share the assumption that a city is made up of either a regular array of parallelepipeds or infinitely long canopies. The inputs to these models, which include street width, building width, building density and a statistical representation of building heights, are generally obtained through quantitative field surveys (which are very slow and time consuming to perform) or qualitative estimates from Digital Elevation Model. But in performing this geometric abstraction there is no way to ensure that the total built surfaces and volumes of the simplified geometry match those of the actual city, or more importantly, that the energy and momentum exchanges are equivalent. In this paper, we aim to test the central hypothesis that cities can be accurately represented by a regular array of parallelepipeds or canopies. For this, we investigate, for a particular scenario, the effects of complexity in urban geometry on the spatially averaged drag forces and shortwave radiation exchange. For drag computation, we used the Immersed Surface Technique, while for computing the incident radiation we used the Simplified Radiosity Algorithm. After testing the above hypothesis, we propose a new approach for fitting an array of cubes to any complex (realistic) geometry, so that new or existing urban parameterisation schemes can be used with confidence. Copyright © 2010 Royal Meteorological Society

Analysis and fabrication of corner cube array based on laser direct writing technology

A detailed analysis of corner cube array (CCA) structure is carried out using the commercially available ray-tracing software ZEMAX. The retroreflective properties of CCA for short-range and long-range propagation are compared. An improved CCA structure with a relatively low structural depth (compared with rectangular CCA) is designed and analyzed. Higher retroreflectance (compared with triangular CCA) at a specific incident angle is proved by ray tracing. Two kinds of CCA structures, including the improved CCA, have been fabricated using laser direct writing technology. The fabrication results are qualified by three-dimensional shape measurement and ray tracing.

Wind Conditions in Idealized Building Clusters: Macroscopic Simulations Using a Porous Turbulence Model

Simulating turbulent flows in a city of many thousands of buildings using general high-resolution microscopic simulations requires a grid number that is beyond present computer resources. We thus regard a city as porous media and divide the whole hybrid domain into a porous city region and a clear fluid region, which are represented by a macroscopic k–\({\varepsilon}\) model. Some microscopic information is neglected by the volume-averaging technique in the porous city to reduce the calculation load. A single domain approach is used to account for the interface conditions. We investigated the turbulent airflow through aligned cube arrays (with 7, 14 or 21 rows). The building height H, the street width W, and the building width B are the same (0.15 m), and the fraction of the volume occupied by fluid (i.e. the porosity) is 0.75; the approaching flow is parallel to the main streets. There are both microscopic and macroscopic simulations, with microscopic simulations being well validated by experimental data. We analysed microscopic wind conditions and the ventilation capacity in such cube arrays, and then calculated macroscopic time-averaged properties to provide a comparison for macroscopic simulations. We found that the macroscopic k–\({\varepsilon}\) turbulence model predicted the macroscopic flow reduction through porous cube clusters relatively well, but under-predicted the macroscopic turbulent kinetic energy (TKE) near the windward edge of the porous region. For a sufficiently long porous cube array, macroscopic flow quantities maintain constant conditions in a fully developed region.

Channel flow over large cube roughness: a direct numerical simulation study

Computations of channel flow with rough walls comprising staggered arrays of cubes having various plan area densities are presented and discussed. The cube height h is 12.5% of the channel half-depth and Reynolds numbers (uτh/ν) are typically around 700 – well into the fully rough regime. A direct numerical simulation technique, using an immersed boundary method for the obstacles, was employed with typically 35 million cells. It is shown that the surface drag is predominantly form drag, which is greatest at an area coverage around 15%. The height variation of the axial pressure force across the obstacles weakens significantly as the area coverage decreases, but is always largest near the top of the obstacles. Mean flow velocity and pressure data allow precise determination of the zero-plane displacement (defined as the height at which the axial surface drag force acts) and this leads to noticeably better fits to the log-law region than can be obtained by using the zero-plane displacement merely as a fitting parameter. There are consequent implications for the value of von Kármán's constant. As the effective roughness of the surface increases, it is also shown that there are significant changes to the structure of the turbulence field around the bottom boundary of the inertial sublayer. In distinct contrast to two-dimensional roughness (longitudinal or transverse bars), increasing the area density of this three-dimensional roughness leads to a monotonic decrease in normalized vertical stress around the top of the roughness elements. Normalized turbulence stresses in the outer part of the flows are nonetheless very similar to those in smooth-wall flows.

Organized Structure of Active Turbulence Over an Array of Cubes within the Logarithmic Layer of Atmospheric Flow

We investigate the coherent structure of atmosphere turbulence over very large roughness within a fully rough, high Reynolds number turbulent flow. The horizontal distributions of coherent turbulence were determined by multipoint measurements of velocity fluctuations using sonic anemometers in a comprehensive outdoor scale model experiment for urban climate (COSMO). COSMO is made up of 512 cubical obstacles, each 1.5 m on a side, arranged in a rectangular pattern on a flat 50 m × 100 m concrete plate. A total of 15 sets of sonic anemometers were aligned horizontally within the logarithmic layer above this site. The velocity fluctuations observed in COSMO were decomposed into active and inactive contributions by applying a spatial-filtering method, and which used a simple moving average along the spanwise direction of the predominant flow as a filter function. The size of the filter should be between the sizes of the active and inactive fluctuations. This method potentially eliminates the considerable portion of low frequency modes included in the horizontal velocity fluctuation, while preserving well the Reynolds stress. The structural characteristics of the active turbulence were qualitatively similar to those measured over various surface configurations. Overall, the observed structures of the active turbulence are composed of very large streaks of low momentum fluid elongated in the streamwise direction with some sub-structures included in the streaks. The sub-structures were the main cause of the ejections, which accompany horizontal vortices. The active motion, including the streaky structures, did not reproduce the lower frequency peak of the bi-modal distribution of the horizontal velocity spectra, but reproduced the higher frequency mode that robustly follows inner-layer similarity (i.e. Monin–Obukhov similarity).

INVESTIGATION ON TURBULENT CHARACTERISTICS WITHIN URBAN CANOPY LAYER BASED ON OUTDOOR EXPERIMENT USING FINE WIRE THERMOCOUPLES

The authors performed a field measurement on the turbulent characteristics of an urban canopy layer over a cubical array using numerous fine thermocouples and nine ultra sonic anemometers. The time and spatially fluctuations of air temperature were discussed comparing the instantaneous velocity under the unstable condition. The result clearly showed that the passage of the microfront of temperature above the canopy, which are correlate the sweep and ejection motions due to a turbulent organized structure (TOS) developed within an inertial sublayer, in contrast, the temperature fluctuation below the canopy height is less sensitive to the TOB.

Spatial versus object visualisation: The case of mathematical understanding in three-dimensional arrays of cubes and nets

This paper investigates the relations of students’ spatial and object visualisation with their analytic, creative and practical abilities in three-dimensional geometry. Fifty-three 11-year-olds were tested using a Greek modified version of the Object–Spatial Imagery Questionnaire (OSIQ) ( Blajenkova, Kozhevnikov, & Motes, 2006) and two mathematics tests, one on three-dimensional arrays of cubes and one on nets. The results suggest that spatial visualisation was related to students’ practical abilities in three-dimensional arrays of cubes, whereas object visualisation was related to students’ creative abilities in nets. Furthermore, high and low spatial visualisers differed in their practical abilities in three-dimensional arrays of cubes and in their analytic abilities in nets, whereas, high and low object visualisers differed in their creative abilities in nets.

Animal-eyeball vs. road-sign retroreflectors

The retroreflective characteristics of ex-vitro cow and deer eyeballs were compared to those of man-made materials used in road signs and bicycle-style reflectors. Reflected intensities were measured using a goniometer that consists of a green He-Ne laser as the light source, and a photomultiplier tube as the detector. It was found that the best quality road-sign reflector, made from a 200-micron-scale, close-packed array of corner cubes, is approximately six times more efficient than a cow eyeball at returning light in the direction of the incoming beam. Less expensive man-made retroreflectors, utilizing 35-micron glass beads (as in mailbox decals) or millimeter-scale arrays of corner cubes (bicycle-style reflectors) are, however, less efficient than the cow eye. The high quality of animal eyeball optics is evidenced by their extremely tight angular spread (full width half maximum congruent with 1 degrees) of retroreflected intensity about the incident path. Moreover, as the reflector itself is rotated relative to the incident beam, the eyeballs preserve their efficiency of retroreflection better than the man-made materials. Interference-diffraction patterns were observed in the retroreflected beams from the small-scale corner cubes, but were not observed in eyeball retroreflection.

Lateral channeling within rectangular arrays of cubical obstacles

Water channel experiments were conducted with the goal of obtaining better understanding of flows through urban-like arrays of buildings. Particle Image Velocimetry (PIV) was used for comprehensive flow measurements within a modeled simple urban setup. Building arrays were modeled using acrylic blocks whose refractive index is the same as that of salty water. Such a setup allowed for undisturbed laser sheet illumination through the obstacles enabling detailed flow measurements between the obstacles/buildings. Building array size, measurement plane and flow conditions were varied. A novel flow feature, lateral channeling, observed and quantitatively measured, within regular 3×3 and 5×5 arrays of cubes is reported here. A sideways mean outflow from the building array is observed behind the first row of buildings followed by the mean inflow in the lee of all succeeding rows of buildings. When the central building in a 3×3 array is replaced by a building of double height, due to the strong downdraft caused by this tall building, the lateral outflow becomes significantly more intense. When the central building in a 5×5 array is replaced by a building of double height, the building downdraft blocks the lateral inflow to the array. This is the first time that such detailed measurements are available for a mock urban array of finite size—a real three-dimensional case. The newly identified mean flow pattern may be accountable for the initial plume spread within an array of obstacles.

High resolution interferometer with multiple-pass optical configuration

An interferometer having fourteen times higher resolution than a conventional single-pass interferometer has been developed by making multiple-pass optical path. To embody the multiple-pass optical configuration, a two-dimensional corner cube array block was designed, and its symmetric structure minimized the measurement error. The effect from the alignment error and the imperfection of corner cube is calculated as picometer level. An experiment proves that the suggested interferometer has about 45 nm of optical resolution and its nonlinearity is about 0.5 nm in peak-to-valley.

Temporal cortex participates in spontaneous perceptual reversal

The Necker cube is perceived as two distinct three-dimensional forms; participants experience alternation between two mutually exclusive perceptions. Perceptual dominance for one form tends to be maintained when the visual stimulus is intermittently removed. The effect is enhanced with the Necker lattice (an array of Necker cubes). Neural processes underlying perceptual reversal and stabilization are unknown. Functional MRI was used to investigate the brain regions involved. Regional activation differed between endogenous and stimulus-driven perceptual reversals, and between reversal and stabilization. Our results indicated that the right anterior portion of superior temporal sulcus is likely to be involved in perceptual stabilization (perceptual memory), whereas reversal is modulated by destabilizing influences from the right frontal lobe.