Evaluation of buoyancy-driven ventilation in atrium buildings using computational fluid dynamics and reduced-scale air model

This research focuses on developing a reliable methodology for predicting the performance of buoyancy-driven ventilation in atrium buildings during the design stage using both computational fluid dynamics (CFD) and scale model tests. The results show several features. First, the agreement between CFD simulation and measurement results in the heated zone is better with rng k–? and zero-equation turbulent schemes; whereas, in the atrium space, the laminar and zero-equation CFD models provide better results. Second, the external ambient temperature has a larger effect on the temperature distribution in the atrium space than the thermal load inside the building. Third, the position of the stack openings that create a direct ventilation path can improve the internal thermal environment. The size of the stack openings also affects the temperature distribution in the atrium space. Lastly, due to the small temperature difference in hot and humid climates, a buoyancy-only ventilation strategy is not very effective in such a situation. That is, when a low-rise atrium building is situated in a hot and humid environment, additional efforts such as wind-driven ventilation, wind-buoyancy ventilation or mechanically driven ventilation will be necessary to achieve the thermal comfort desired.     

Rapid Profiling of an Evolving Bed Form Using Planar Laser Sheet Illumination

Traditional methods of measuring the profile of a scour hole or bed form have poor temporal resolution and may require the temporary cessation of the flow in order to be executed. These are undesirable characteristics since many hydraulic flows have unsteady water-sediment interfaces that can display considerable differences between their “dynamic” (flow on) and “static” (flow off) states. The present technical note discusses the application of planar laser sheet illumination to the erosion of a cohesionless granular bed due to a planar turbulent wall jet. The measurements allow for the quantitative study of the evolution of the bed in its earliest stages. In some cases an ephemeral bed form is observed to form prior to the development of the main bed form. Additionally, the measurements illustrate the oscillatory nature of the bed form under certain conditions. The experiments therefore demonstrate the great potential of this experimental technique for gaining previously inaccessible information on scour processes.     

Application of Proper Orthogonal Decomposition Method in Wind Field Simulation for Roof Structures

The weighted-amplitude wave superposition method and the proper orthogonal decomposition (POD) technique are first introduced. Then the POD technique combined with a wind field reconstruction method for a flat roof is studied. In the meantime, wind field extension using B-spline interpolation and the wind field reconstruction are verified. On this basis, a three-step simulation algorithm to generate wind field is presented for roof structures. In the proposed algorithm, the weighted-amplitude wave superposition method, the POD technique, and the B-spline interpolation technique are combined together. In contrast to the existing algorithms in frequency-domain-based simulation, the proposed one can reduce the computation cost remarkably. In order to demonstrate the capability and efficiency of the proposed algorithm, an engineering example, the wind field simulation for a large-scale roof of the Foshan Century Lotus Stadium, is illustrated. The results show that the proposed algorithm is very efficient with sufficient precision. Therefore, it is advantageous in wind resistance analysis for large-scale roofs.     

Pressure loss reduction in hydrogen pipelines by surface restructuring

This paper concerns the reduction of pressure losses during pipeline hydrogen transportation, as the cost of hydrogen compression is a significant obstacle for efficient hydrogen pumping on a large-scale basis. The use of organized micro-structures on pipeline walls is proposed to obtain lower values of pressure losses with respect to smooth walls. Three-dimensional micro-structures of a sinusoidal shape are investigated as potentially more efficient counterparts to conventional two-dimensional structures (riblets) developed in aerospace industry. Aerodynamic performance of three-dimensional structures is investigated computationally in terms of both skin friction and pressure drag, two constituents of the total drag. Three-dimensional structures are shown to provide larger total drag reduction than two-dimensional structures for some range of geometrical parameters (14.5% versus 11%). Parametric dependence of both pressure and skin friction drag on structure geometry is analyzed, and an optimum configuration maximizing the total drag reduction is proposed.     

Modeling of the fluctuating component in the form of the sum of an infinite number of random quantities. Part 2. Model of transfer of turbulent stresses and turbulent heat fluxes

The paper presents an approach to construction of turbulence models that allows modeling of the fluctuating component in the form of the sum of an infinite number of random quantities. The first part of the paper deals with the technique of construction of the k-ε type model. In the second part of the paper, construction of the model of transfer of turbulent stresses and heat fluxes is considered. It is shown that the technique does not depend on the choice of the dissipative variable.     

Simulation of the flow field and scour evolution by turbulent wall jets under a sluice gate

This numerical study of scour process tested the skills of computational fluid dynamics in modeling the unsteady flow field during the scour development stage by two-dimensional turbulent wall jets under a sluice gate. The modeling was found to well describe the experimentally observed flow patterns, that is, the main jet diverged to a returning jet and a tail jet. The model also correctly predicts the evolution of the scour depth and length. We examined the self-similarity of the profiles of scour bed and overlying velocities throughout the entire scour development and equilibrium stages. We found self-preserved profiles of velocities and scour beds using local jet parameters. Four growth curves were compared in describing the temporal evolution of scour depth. Finally, non-dimensional scaling of the equilibrium maximal scour depth was investigated. We used the theory of wall jet, and suggested that a modified jet Froude number can be used to predict the equilibrium scour depth, which accounts for the attenuation of the jet velocities along the apron.     

Optimum placement of condensing units of split-type air-conditioners by numerical simulation

Split-type air-conditioners used in residential or office buildings often have the outdoor condensing units installed at the sidewalls or on the roofs. Installation distance from the supporting wall for the first group and the height of installation for the second group are two factors that affect the condenser efficiency. In this study, a CFD code is used to calculate the effect of distance from the supporting wall on the entrance air temperature and on the on-coil temperature of condenser installed between two walls. In the case of condenser installed on the roof, the effect of installation height of the condenser from the finished roof on on-coil temperature is investigated and the minimum recommended height of installation is determined.     

TURBULENT VARIABLE PROPERTY GAS FLOWS

The surface renewal model of wall turbulence is coupled with the classical eddy diffusivity/mixing length approach for the turbulent core to analyze turbulent variable property gas flows with moderate heating and cooling. The analysis indicates that the mean frequency of wall turbulence is decreased by heating and increased by cooling, with the heating effect being significantly greater than the cooling effect. The analysis also gives rise to mean velocity and temperature distributions, friction factors and Stanton numbers that are consistent with most of the experimental data for both heating and cooling.     

On the Transition to Turbulence of Oscillatory Flow of Liquid Helium-4

Oscillating solid bodies have frequently been used for studying the properties of normal and superfluid helium. In particular, the transition from laminar flow to turbulence has attracted much interest recently. The purpose of this note is to review several central features of this transition in oscillatory flow, which have been inaccurately formulated in some recent work.      

Suction Removal of Sediment from between Armor Blocks. II: Waves

When a stone/armor layer on a sand bed is exposed to flow, the sand underneath will be agitated by the flow turbulence. When the flow velocity reaches a critical value, the sand will be sucked (winnowed out) from between the armor blocks. In a previous investigation, we studied suction removal of sediment in steady currents. The present study is an extension of our previous investigation to waves. The critical condition for the onset of suction is determined. It is found that the onset of suction is governed by three parameters: (1) the sediment mobility number (based on the sediment size); (2) the ratio of sediment size to stone size, d/D; and (3) the Keulegan-Carpenter (KC) number, based on the armor block/stone size. The variation of the critical mobility number for suction as a function of d/D and KC is determined for the ranges of the parameters 0.001相似文献