Cheuk Ming Mak

Numerical investigation of wind‐induced airflow and interunit dispersion characteristics in multistory residential buildings

Compared with the buoyancy-dominated upward spread, the interunit dispersion of pollutants in wind-dominated conditions is expected to be more complex and multiple. The aim of this study is to investigate the wind-induced airflow and interunit pollutant dispersion in typical multistory residential buildings using computational fluid dynamics. The mathematical model used is the nonstandard k-ε model incorporated with a two-layer near-wall modification, which is validated against experiments of previous investigators. Using tracer gas technique, the reentry of exhaust air from each distinct unit to other units on the same building, under different practical conditions, is quantified, and then, the possible dispersion routes are revealed. The units on the floor immediately below the source on the windward side, and vertically above it on the leeward side, where the reentry ratios are up to 4.8% and 14.9%, respectively, should be included on the high-infection list. It is also found that the presence of balconies results in a more turbulent near-wall flow field, which in turn significantly changes the reentry characteristics. Comparison of the dispersion characteristics of the slab-like building and the more complicated building in cross (#) floorplan concludes that distinctive infectious control measures should be implemented in these two types of buildings.

The effect of sound on office productivity

This article reports the results of a questionnaire survey examining the effects of sound on office productivity and assessing the relationship between changes in office productivity and noise sources as well as five environmental and office design factors, namely temperature, air quality, office layout, sound and lighting. The convenience sample for the survey comprised 259 office workers in 38 air-conditioned offices in Hong Kong. The subjects were requested to complete the questionnaires themselves. The results show that among the five environmental and office design factors examined, sound and temperature were the principal factors affecting office productivity. A strong and significant correlation was also found between changes in office productivity and sound, temperature and office layout. Participants were separated into low- and high-productivity groups using the mean productivity score of all participants as the cut-point. The three most annoying noise sources, including conversation, ringing phones and machines, differed little in mean annoyance scores for the low- and high-productivity participants, indicating that they had a significant negative impact on all participants. The results also indicate that low-productivity participants were easily influenced by noises such as background noise, closing doors, and human activity, as well as those coming from both inside and outside the office. Practical applications: This study evaluates the effects of sound and other environmental and office design factors on office productivity. It suggests that sound is a principal factor affecting office productivity in modern air-conditioned offices.

From street canyon microclimate to indoor environmental quality in naturally ventilated urban buildings: Issues and possibilities for improvement

Abstract Many buildings in urban areas are more or less naturally ventilated. A good understanding of the current status and issues of indoor environmental quality (IEQ) in naturally ventilated urban buildings and the association with urban microclimate is fundamental for improving their IEQ. This paper reviews past studies on (a) the microclimate in urban street canyons, (b) the potential influence of such microclimate on IEQ of nearby naturally ventilated buildings, and (c) the real-life IEQ status in these buildings. The review focuses mainly on studies conducted by on-site measurements. The microclimate in urban street canyons is characterized by low wind speed, high surface temperature difference, high pollutant concentration, and high noise level. Insufficient ventilation rates and excessive penetration of outdoor pollutants are two key risks involved in naturally ventilated urban buildings. Existing knowledge suggests that reasonable urban planning and careful building envelope design are the primary methods to ensure acceptable IEQ and maximize the utilization of natural ventilation. However, quantitative studies of both microclimate in street canyons and IEQ in buildings are still highly insufficient in many aspects, which make cross comparison and influencing factors analysis currently impossible. Based on the limitations of previous studies and the current issues of naturally ventilated urban buildings, suggestions are made for future studies to better understand and improve IEQ in naturally ventilated urban buildings.

A power transmissibility method for assessing the performance of vibration isolation of building services equipment

Abstract Force transmissibility is commonly adopted in building services engineering to assess the performance of vibration isolation. However, it neglects the effect of floor mobility on structure-borne sound power transmitted from a vibratory machine to the floor/roof and the interactions among several contact points between the vibratory machine and the floor/roof. The problem that motivated this study is the occasional occurrence of unsatisfactory performance of vibration isolators observed in building services engineering. This problem may be due to the over-simplification of the vibratory problem in the usual definition of the un-damped force transmissibility or isolation efficiency commonly used in engineering practice. In this paper, use of a “ power transmissibility ”, which includes the effect of floor mobility and the interaction of all dynamic forces transmitted to the floor through the vibration isolators, is proposed.

Flow noise from spoilers in ducts

Measurements of flow noise produced by strip spoilers in the air duct of a ventilation system and radiated from an open exhaust termination unit into a reverberation chamber have been made. The results agree with the previous work of Nelson and Morfey [J. Sound Vib. 79, 263-289 (1981)]. Prediction of flow noise produced by multiple spoilers requires the values of the ratio of the mean drag forces that act on the spoilers, the phase relationship between the fluctuating drag forces that act on the spoilers, and the coherence function of the noise sources. The latter is empirically derived from the measured results, where the predicted results agree well with the experimental results within 3 dB at most frequencies except for very high frequencies.

Large eddy simulation of wind-induced interunit dispersion around multistory buildings

Abstract Previous studies regarding interunit dispersion used Reynolds‐averaged Navier–Stokes (RANS) models and thus obtained only mean dispersion routes and re‐entry ratios. Given that the envelope flow around a building is highly fluctuating, mean values could be insufficient to describe interunit dispersion. This study investigates the wind‐induced interunit dispersion around multistory buildings using the large eddy simulation (LES) method. This is the first time interunit dispersion has been investigated transiently using a LES model. The quality of the selected LES model is seriously assured through both experimental validation and sensitivity analyses. Two aspects are paid special attention: (i) comparison of dispersion routes with those provided by previous RANS simulations and (ii) comparison of timescales with those of natural ventilation and the survival times of pathogens. The LES results reveal larger dispersion scopes than the RANS results. Such larger scopes could be caused by the fluctuating and stochastic nature of envelope flows, which, however, is canceled out by the inherent Reynolds‐averaged treatment of RANS models. The timescales of interunit dispersion are comparable with those of natural ventilation. They are much shorter than the survival time of most pathogens under ordinary physical environments, indicating that interunit dispersion is a valid route for disease transmission.

Measurement and prediction of road traffic noise at different building floor levels in Hong Kong

In Hong Kong, approximately one million people are affected by severe road traffic noise. It is crucial to estimate the vertical distribution of traffic noise levels at different levels in high-rise buildings during the planning and design stages for new residential buildings. The Calculation of Road Traffic Noise (CRTN) has been adopted in Hong Kong to estimate traffic noise from the road. However, there have been criticisms of the CRTN model’s accuracy and suitability for predicting road traffic noise in Hong Kong. This study examines and evaluates the accuracy of the CRTN method in predicting the vertical distribution of traffic noise level LA10 at different floor levels of a 20-storey residential building in Hong Kong. Also, measurements have been conducted of the traffic noise levels at each floor level of the building. Both the predicted and measured LA10 show a similar trend: the higher the floor level, the lower the traffic noise levels. However, the predicted LA10 at the building façade has a tendency of overestimation, especially at the higher floor levels, with a mean difference of +2.0 dBA between the predicted and measured results. A correlation coefficient (R2) of 0.9331 between the predicted and the measured LA10 indicates that the predicted levels correlate closely with the measured levels. The CRTN is therefore a useful tool in predicting traffic noise levels at different floor levels during the building planning stage. Practical application: This study provides an evaluation of the accuracy of the CRTN method in predicting the vertical distribution of traffic noise level at different floor levels of a residential building. It suggests that the CRTN is a useful tool in predicting traffic noise levels at different floor levels during the building planning stage.

Wave propagation in a duct with a periodic Helmholtz resonators array

Helmholtz resonator is often used to reduce noise in a narrow frequency range. To obtain a broader noise attenuation band, combing several resonators is a possible way. This paper presents a theoretical study of sound propagation in a one-dimensional duct with identical side-branch resonators mounted periodically. The analysis of each resonator was based on a distributed-parameter model that considered multi-dimensional wave propagation in its neck-cavity interface. This model provided a more accurate prediction of the resonant frequency of the resonator than traditional lumped-parameter model. Bloch wave theory and the transfer matrix method were used to investigate wave propagation in these spatially periodic resonators. The results predicted by the theory fit well with the computer simulation using a three-dimensional finite element method and the experimental results. This study indicates that the wave coupling in this periodic system results in the dispersion of the frequency band into the stop and the pass bands. The long-term significance is that periodic resonators may more effectively control noise in ducts by broadening the bandwidth they attenuate and increasing the magnitude of sound attenuation.

Development of a prediction method for flow-generated noise produced by duct elements in ventilation systems

Abstract Flow-generated noise generated on the quiet side of the primary attenuators of a ventilation system is the result of interaction between air flow and duct discontinuities. It is of engineering importance to predict the flow-generated noise caused by air duct elements in ventilation systems at the design stage. However, all prediction methods are based upon an isolated in-duct element that is very different from a real ventilation system. Until recently, Mak and Yang have produced a prediction method for flow-generated noise produced by the interaction of two elements in air ducts. In this paper, an attempt has been made to modify their equations so that their predictive equations can possibly be used to predict noise produced by “real” duct discontinuities. By comparing their predictive values with the experimental results of Oldham and Ukpoho, their validity can be proved. The modified Mak–Yang predictive equations, therefore, provide a basis for permitting a more accurate prediction of flow-generated noise produced by various configurations of two in-duct elements and duct dimensions.

Modeling of coupled urban wind flow and indoor air flow on a high-density near-wall mesh: Sensitivity analyses and case study for single-sided ventilation

Coupled urban wind flow and indoor air flow is an important flow problem that is associated with many environmental processes. This paper provides detailed sensitivity analyses of some important computational parameters that may influence the prediction accuracy of such a flow problem. The CFD prediction of single-sided ventilation rate is taken as a case study. Based on both the RANS and LES turbulence models, the most commonly used predictive methods, namely the integration and tracer gas decay methods, are examined. A range of wind directions are considered, since the characteristics of both building aerodynamics and ventilation mechanics are distinctive under different wind directions. The performance of numerical model is thoroughly evaluated, including validation against field measurements. Specific attention is paid to sensitivity analyses of the near-wall mesh density. The implications for accurate CFD prediction of the single-sided ventilation rate are summarized, which are also applicable to other coupled flows.