Numerical study of aerodynamic characteristics of a square prism in a uniform flow

Aerodynamic characteristics of a square prism in a uniform flow for various angles of attack have been investigated using LES turbulence model. The results show that mean aerodynamic coefficients, surface pressures, and flow patterns for all angles of attack agree favorably with experiments. In addition, spanwise length of computational domain is found to slightly influence the mean aerodynamic coefficients whereas it shows strong impacts on fluctuating aerodynamic coefficients, which motivates a development of grid-independent estimation method for quantitative evaluations of fluctuations. The predicted power spectral densities for fluctuating aerodynamic coefficients are in good agreements with experiments in primary frequencies, which can be explained by the flow patterns. Strouhal numbers obtained from the simulations also agree well with experiments and acute change observed around 14° is successfully captured. Finally, a method for estimating grid-independent fluctuating aerodynamic coefficients is proposed by using a systematic elongation of spanwise length and is validated by the numerical tests.     

Air pumping action of a plume in a room fire

Air pumping effect of a fire plume to give higher intake rate through vertical openings in a post-flashover room fire will be discussed in this paper. The thermal balance equation was set up with known fire phenomena in a room. The hydrostatic model was applied to study the air intake rate through vertical openings. An equation relating heat release rate to room air temperature rise with empirical constants was then justified by reported experimental data on post-flashover room fire. The heat release rate was measured by the oxygen consumption method in that experiment. The predicted heat release rate from the empirical equation reported in the literature was observed to be proportional to the room air temperature rise as derived from hydrostatics. However, the proportionality constant is lower than the experimental value. A possible explanation is due to neglecting another fire phenomenon on air pumping action of the fire plume in a real room fire. Higher pressure differences across the door would give higher airflow rates across an opening. This would supply more air to give higher heat release rate as observed in the experiment. In this paper, the pressure due to air pumping of the fire plume is taken as a proportion of the hydrostatic pressure due to temperature differences between the upper hot layer and lower cool layer. Comparing the measured heat release rate with the estimated heat release rate due only to hydrostatics will give the air pumping action. The possible increase in heat release rate in a post-flashover fire can then be estimated accordingly.     

Simulation of mixed convection flow in a room with a two-layer turbulence model

Most indoor airflows are mixed convection. In order to simulate mixed convection accurately and efficiently, this paper uses a two-layer turbulence model. The two-layer model combines a one-equation model for near wall flow together with the standard k-epsilon model for outer-wall flow. The model has been used to predict the mixed convection by displacement ventilation in an office. The computed results agree well with the corresponding airflow pattern and the distributions of air temperature, air velocity, air velocity fluctuation, and tracer-gas concentration. The model can predict correctly heat transfer from a wall where the standard k-epsilon model and re-normalization group (RNG) k-epsilon model with wall functions often fails. The computing cost required by the two-layer model is comparable to that of the standard k-epsilon model and RNG k-epsilon model and is significantly less than that by a low-Reynolds number model.     

Mixing characteristics in a ventilated room with non-isothermal ceiling air supply

A two-dimensional turbulence k–ε model is used to predict distribution of air velocity, temperature and turbulence kinetic energy in an air-conditioned room using ceiling air supply. Mixing characteristics of the airflow are analyzed under different air supply velocities and temperatures. A modified Archimedes number is correlated with the parameters characterizing heat transfer, ventilation system, and turbulence kinetic energy of room air flow. Significant correlations have been shown. It is found that the linear ceiling air supply air-conditioning system with a high level return air provides excellent air mixing across a wide range of Archimedes numbers. The results are useful for air-conditioning design and thermal comfort study.     

Wall pressures of separation—reattachment flow on a square prism in uniform flow

Mean and fluctuating pressures on a two-dimensional square prism in uniform flow at a Reynolds number of 1.4 × 10⁵ were measured to investigate the nature of the separation—reattachment phenomenon. Measurements were taken on the side-wall face and in the wake region to quantify the behavior of the fluid in the reattachment zone. The experiment was performed with and without a turbulence producing grid for angles of approach flow ranging from ?50° to +90°.     

矢流洁净室二维流场的大涡模拟

采用大涡模拟技术对矢流洁净室二维流场进行了数值模拟，分析了模拟结果，确定了空态洁净室的送风口半径、回风口高度、送风速度等参数，指出湍流大涡模拟能准确得到洁净室流场中旋涡的位置和尺度，适用于对洁净空调多种工作环境的数值模拟。     

北京某数据机房空调节能设计

该项目由办公楼改造而成,在无法利用原有建筑空调系统、使用空间十分紧张及制冷设备机房面积十分有限的情况下,采用水冷加自然冷却双冷源机房专用空调,在保证数据机房“安全、可靠、稳定”的前提下,实现数据机房节能运行的空调设计.     

An excitation model for a square prism with leading edge flaps in low turbulent flow

A new digital pressure scanning system has been developed and used to obtain pressure data on a 2D square prism with leading edge flaps. Experiments were performed with angles of attack between −5° and +5°. The component of pressure fluctuation that was caused by the motion of the leading edge flaps was extracted from the fluctuating pressures. It has been demonstrated that within this range of angles of attack, the pressure fluctuations due to the flap motion can be modelled as a function of the flap operating frequency, amplitude, and the angle of attack, and hence the overall load can be evaluated. The equivalent viscous damping ratio due to this flap-induced load was derived in order to benchmark the effectiveness of a leading edge flap system against existing solutions to structural vibrations.     

洁净室空气热湿处理能耗分析

分析了一次回风加再热系统、二次回风系统的空气处理过程,并以山西大学光电楼的一间100级实验室为例,对这两个方案的空气热湿处理过程进行了能耗分析,结果表明采用二次回风系统的方案最节能。     

Numerical simulation of flow past a square cylinder using Partially-Averaged Navier-Stokes model

In this study, we carry out a two-stage partially averaged Navier-Stokes (PANS) simulation past a square cylinder to examine the effect of a grid system on the predicted results. The important focus in PANS is how the control parameter which is the ratio of unresolved-to-total turbulent kinetic energy is determined in the flow field. In this work, the parameter is evaluated by using an equation that we propose. The model equation utilizes the turbulent length scale, the Kolmogorov length scale and the cut-off grid length. We first perform RANS simulation to approximately evaluate the turbulent length scale and the Kolmogorov length scale. Then we construct five different grid systems based on these length scales and perform PANS simulation in which the length scales to determine control parameter are also updated as the simulation proceeds. Simulation results are compared with experimental data and the other LES and DES results. The comparison suggests that PANS approach is very effective in simulating separated turbulent flows in the sense that accurate predictions can be obtained by using a much coarser grid system than is required for LES. We then suggest the grid spacing requirements for a proper PANS simulation.     

Air flow and particle control with different ventilation systems in a classroom

Most ventilation and air conditioning systems are designed without much concern about how settling particles behave in ventilation air flows. For displacement ventilation systems, designers normally assume that all pollutants follow the buoyant air flow into an upper zone, where they are evacuated. This is, however, not always true. Previous studies show that high concentrations of settling respirable particles can be found in the breathing zone, and that the exposure rates can be a health hazard to occupants. The emphasis here is on how ventilation systems should be designed to minimize respirable airborne particles in the breathing zone. The supply and exhaust conditions of the ventilation air flow are shown to play an important role in the control of air quality. Computer simulation programs of computational fluid dynamics (CFD) type are used. Particle concentrations, thermal conditions and modified ventilation system solutions are reported.     

Interaction of square prisms in two flow fields

A single square prism was tested at several angles of attack in a wind tunnel in smooth and in turbulent flow. A second square prism of equal size was placed upstream with various separation distances and orientations. Proper combination of signals from sensitive pressure transducers yielded the fluctuating aerodynamic forces and moments. The mean forces and moments were determined using conventional piezometer openings in conjunction with a liquid manometer. Some error is inevitable since only three transducers were used to a side. Comparisons with available information show reasonable correspondence with the mean and fluctuating forces. Little information appears available for moments.Although the upstream prism effectively shields the downstream prism in many cases, particularly for the mean forces and moments, there are in all cases, significant values for the coefficients of fluctuating force and moment. When the two objects are offset both vertically and horizontally, particularly large mean moments occur.Use of a turbulent flow field is to be recommended for testing. Consideration of moment measurement should be of prime importance in wind sensitive structures.     

Numerical simulation of transient flow development in a naturally ventilated room

Numerical simulations were conducted to model the transient flow development in a naturally ventilated space containing a centrally located localized source of heat. The simulations were compared with a series of small-scale laboratory experiments and existing theoretical models. The aim of the work was to benchmark CFD models for time-dependent buoyancy-driven natural ventilation against previously published experimental results and theoretical models. The simulations agree well with experimental results during the initial development of the room stratification. The CFD results accurately predict the maximum depth of the hot buoyant layer at the top of the room as well as the steady-state interface height which separates the warm upper buoyant layer from the cooler air below. The simulations also predict well the time taken for the buoyant upper layer to reach its maximum depth. However, at longer times the results diverge. This may be due to thermal diffusion and mixing at the interface between the upper and lower layers due to the inflow from the floor level vents.     

Studying the features of air flow induced by a room ceiling-fan

Creating a breeze in a space is necessary to enhance convective heat transfer and accordingly body heat dissipation. In most tropical countries, ceiling fans are extensively used to create an indoor breeze, improve the space air distribution and, hence enhance the feeling of comfort. The fan speed, diameter, number of blades, blade angle, and location all play an important role in determining the induced flow pattern features in the space.Few previous studies have investigated fan induced flow and its characteristics under different geometric and operating conditions. In this study, an analytical as well as a computational model was used to predict the flow pattern induced by a ceiling fan into a selected space. The numerical and mathematical results were compared to published experimental data. The results showed that the flow pattern has different features as it leaves the fan and proceeds towards the floor. It has a divergence angle of almost 150°. Two correlations are introduced to help predicting the air velocity distribution due to using a ceiling fan.     

济宁医学院附院层流手术室若干设计问题的探讨

以济宁医学院附院手术室为例，简要介绍了层流手术室在净化等级，气流组织等方面的特点，并分析了设计中净化标准的确定，参数的选取，机房的布置等问题。     

旋转流风量分配器空气动力性能研究

研究了用于等截面圆形直管均匀送风的旋流风量分配器，及其沿程阻力和局部阻力问题。通过测试优化出一种局部阻力系数显著降低的旋流风量分配器，并针对其特点给出了不同流量下的单位摩擦阻力变化曲线，为工程应用提供了参考依据。     

高诱导比低温风口在工程中的应用

简要介绍了用于低温空调工程中的高诱导比低温风口的工作原理，对实际工程中应用的这种低温风口的送风性能作了实测，分析了测试数据，认为这种风口可获得满意的气流组织和均匀的温度场，而且在低温工况下送风口未出现结露现象。