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Helmholtz resonators (HR) are widely used in aero-engine systems for noise reduction. By connecting a pair of HRs in series (neck-cavity-neck-cavity), a dual HRs system is formed. This study investigated the influence of neck length, cavity volume and flow Mach number on the noise attenuation performance of a dual HRs system. A three-dimensional numerical simulation was performed to calculate the transmission loss results. The transmission loss (TL) results indicated that the second neck length can influence the second resonance frequency and TL max. Changing the cavity volume significantly influences the noise attenuation ability under lower flow rate conditions compared to higher flow rate conditions. The flow Mach number had a more significant impact on the first TL peak than on the second TL peak. This study shows the relationship between the geometric parameters, grazing flow and noise attenuation performance of a dual HRs system and could provide guidance in designing suitable dual HRs for aero-engine systems.

This study investigated effects of incident wind angles on wind velocity distributions in wakes of two generic building configurations, namely, ‘T’- and ‘+’-shaped, and the air pressure distributions along their leeward walls by using computational fluid dynamics simulations. Results show that when the wind approaches laterally (90°) (vs. when the wind is direct (0°)), the downwind length and maximum bilateral width of the low-wind velocity zone in the wake of ‘T’-shaped building decrease by 11.5% and 37.9%, respectively. When the incident wind is oblique (45°) (vs. when it is direct), the length and width of this low-wind velocity zone in the wake of ‘+’-shaped building decrease by 15.0% and 30.9%, respectively. Furthermore, results show that the air pressure on the leeward walls of the ‘T’- and ‘+’-shaped buildings gradually decreases along with the building height. The resulting low-wind conditions on upper floors of buildings reduce the fresh air intake of their leeward units utilizing natural ventilation. It is particularly apparent in the case of direct approaching wind. Thus, the appropriate selection of building configurations and their orientations allows for the most effective use of wind to enhance ventilation in indoor and urban environments.

In this paper, a new combination method for sound field prediction is proposed. An optimization approach based on the genetic algorithm is employed for optimizing the transition frequency of the combined sound field prediction method in classrooms. The selected optimization approach can identify the optimal transition frequency so that the combined sound field prediction can obtain more efficient and accurate prediction results. The proposed combined sound field prediction method consists of a wave-based method and geometric acoustic methods that are separated by the transition frequency. In low frequency domain (below the transition frequency), the sound field is calculated by the finite element method (FEM), while a hybrid geometric acoustic method is employed in the high frequency domain (above the transition frequency). The proposed combined prediction models are validated by comparing them with previous results and experimental measurements. The optimization approach is illustrated by several examples and compared with traditional combination results. Compared to existed sound field prediction simulations in classrooms, the proposed combination methods take the sound field in low frequencies into account. The results demonstrate the effectiveness of the proposed model. Practical applications: This study proposes a combined sound field prediction method separated by transition frequency. A genetic algorithm optimization method is employed for searching the optimal transition frequency. The outcomes of this paper are essential for acoustical designs and acoustical environmental assessments.

The configuration of an urban street canyon plays an important role in determining local microclimate including pedestrian level wind environment (PLWE). The semi-closed U-type street canyon has been widely used in high-density trans-oriented-development urban design for providing private space. However, a quantitative evaluation on PLWE of the U-type street canyon is still absent to date. This study quantitatively assesses PLWE of the U-type canyon via particle image velocimetry tests in a wind tunnel. A range of canyon aspect ratios (the ratio of the canyon height to width) and length ratios (the ratio of the canyon length to height) were investigated. The wind flow inside and in the vicinity of the street canyons was measured and analysed under the perpendicular, oblique and parallel approaching wind directions. It was found that the U-type canyon shows a higher PLWE inside canyon on the open side while a lower PLWE inside canyon on the closed side. Compared to parallel canyon, there is a significant enhancement of PLWE in the centre of the U-type canyon and in some spots on the leeward side of the canyon. This is further strengthened with increasing the canyon aspect ratio and length ratio. Nevertheless, the U-type canyon exhibits a lower wind speed at the pedestrian level than that of the parallel canyon both inside and in the vicinity of the street canyons in general especially under parallel wind direction. It is believed that these findings are greatly beneficial to build sustainable cities with a high quality PLWE.

Warm and hot days account for most of the time in Hong Kong. Outdoor thermal comfort studies in Hong Kong should give its first consideration to warm and hot days. This study presents investigations about thermal comfort through 1600 human subject responses from the onsite survey with concurrent meteorological parameter measurements. Probit analysis was used for searching the thermal neutral range of Hong Kong residents in a year span. Logistic regression was used for locating the meteorological parameter ranges for thermal neutral and comfort conditions. It is shown that people had difficulties defining their actual thermal feelings near the thermal neutral status when being asked to use the nine-point thermal sensation scale. Obvious thermal adaptation effect for thermal neutral conditions were observed among Hong Kong residents over the seasons in a year. The transitional seasons had a wider thermal neutral range than that of winter and summer. Summer had the narrowest thermal neutral range. Wind and solar radiation had an interaction effect with air temperature in determining thermal sensation and thermal comfort. Wind can effectively offset the negative effect of solar radiation in summer when the air temperature was lower than 31 °C. The thermal comfort condition allowed a higher limit of solar radiation than the thermal neutral condition when the air temperature was lower than 31 °C. The investigations in this study provide some unique insights into the way to assess urban thermal comfort in the building design stage.

The Reynolds number ( R e ) independence criteria for flow and dispersion around purely outdoor environments has been examined in many previous studies, but little attention has been paid to the coupled outdoor and indoor environments. This study investigated the R e independence criteria of flow and dispersion in coupled outdoor and indoor environments using Computational Fluid Dynamics (CFD) with Large Eddy Simulation (LES) model. Two geometrical arrangements were considered, namely an isolated multi-story building, and an array of multi-story buildings. Four parameters including the non-dimensional air velocity, pollutant concentration, ventilation rate, and re-entry ratio were used to assess the R e independence criteria. The tracer gas decay method was used to predict the ventilation rate and to quantify the re-entry ratio for each room. Using the quantitative indicator, the deviation rate (DR), of the non-dimensional velocity below 5%, two sets of critical values were proposed: R e H based on the building height equal to 4.8 × 10 4 for the outdoor environment, and R e W based on the opening height equal to 1.4 × 10 4 for the indoor environment. The concentration field was more difficult to meet the R e -independent requirement. Using the DR of the non-dimensional concentration below 5%, the critical values were R e H = 7.9 × 10 4 and R e W = 3.0 × 10 4 . If the DR was enlarged to 10%, the R e H and R e W criteria for the concentration field were the same as the velocity fields. For the non-dimensional ventilation rate and re-entry ratio, the critical value for the R e independence was R e W = 1.4 × 10 4 for both the isolated buildings and the building arrays.

In time, dental health care has slowly expanded beyond emergency treatment to treat oral diseases. How to reduce the cross-transmission risk in dental surgery has raised much more attention. Considering the lack of consistency of fallow time (FT) in its necessity and duration, the highly sensitive laser light scattering method has been proposed to visualize the airborne lifetime and decay rate of suspended particles in the dental surgery environment. The FT is defined as when the number of suspended particles drops to the level that the next patient can safely enter after the aerosol-generating procedures (AGPs). The ultrasonic scaling was performed in the mock-up experimental dental clinic with 6 air changes per hour (ACH), and the instantaneous moments of the droplets were recorded by a high-speed camera. Without any mitigation measures, the estimated FT in the single dental surgery environment with 6 ACH was in the range of 27–35 min, significantly affecting the number of daily dental services. Despite the cooperation of high-volume evacuation (HVE [IO]) cannot eliminate the FT to zero minutes, the equipment could reduce the required FT by 3–11 min for the suspended particles reducing the baseline levels. Owing to the longer airborne lifetime of suspended particles, the relevant protection equipment, especially respiratory protection, is quite essential in dental surgery. The obtained results of this study will provide evidence to establish the revised FT in dental surgery guidelines and protect the health and wellbeing of urban dwellers.

In the setting of widespread severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) community transmission, reducing the exposure risk on dental professionals and the next patients is the key to reopening dental services in this pandemic environment. The study is motivated by the lack of understanding of the flow-field characteristics and droplet distribution during aerosol-generating procedures. The particle image velocimetry measurements with high temporal and spatial resolutions were performed under ultrasonic scaling in the mockup experimental dental clinic. Compared with other methods focusing on the settled droplet particles, the study focused on the visualization of suspended droplets. From the results of the velocity vector and trajectory map, the high-level contaminated area will be within 1 m from the oral cavity. The vortex structures were identified by the vorticity index. In the surface near the patient's head, a counterclockwise vortex would carry some droplets and contaminate this region. The small droplets circulated in the turbulence cloud and the droplet nuclei generated by dehydration are the two primary sources of suspended particles, which may cause airborne transmission in the dental clinic. About 65%–74% of the droplets in ultrasonic scaling were in the range of 50–180 μ m. The research will provide references to the development of the precaution measures to reduce the SARS-CoV-2 exposure risk of dental professionals.

Large eddy simulation (LES) is increasingly recognized as an important approach in the assessment of pedestrian level wind (PLW) conditions, while its numerical performance is sensitive to a few key parameter settings. This study aims to further identify the sensitivities to these main influencing parameters in modeling wind flow around a building array using LES. This is achieved by quantitatively comparing mean wind velocities from the LES predictions and those from a benchmark wind tunnel experiment. The parameters investigated are the mesh resolution, discretization time step and sampling period, vortices’ number of the inlet flow, the upstream distance of the computational domain, and the sub-grid scale (SGS) models. Results are quantified using four validation indices. Specifically, the correlation coefficient R between the predicted results using SGS model with the dynamic Smagorinsky-Lilly method and the experimental results is 0.89, which is higher than those between the experiment results and other three SGS models. Based on the sensitivity tests, it is recommended that the normalized discretization time step be set below 0.09, and that the normalized sampling period and vortices’ number be above 219 and 190, respectively, in the LES simulation of PLW flows around a building array.

Helmholtz resonator (HR) is one of the most basic acoustic models and is generally used to reduce low-frequency noise. However, it is only effective at its single resonance peak with narrow frequency band. In order to deal with low-frequency, broadband and hybrid noise in a ventilation ductwork system, a multiple HR arrays system is therefore proposed in this paper. Several tuned HRs mounted on the same cross-sectional area of the duct is considered as a transverse HR array. By distributing the transverse HR array periodically along the duct, the multiple HR arrays system is then proposed to eliminate hybrid noise in a ventilation ductwork system. The acoustic performance of the proposed multiple HR arrays system is analyzed theoretically and numerically. The transfer matrix method and the Bragg theory are used to investigate wave propagation through the duct. Owing to the coupling effects of the Bragg refection and HRs’ resonances, several broadband noise attenuation bands can be obtained. The theoretical predictions show good agreement with the three-dimensional Finite Element Method (FEM) simulation results. The present study provides a practical way in hybrid noise control application of the ventilation ductwork system and other research areas in respect of the HR.

Comfortable and healthy outdoor microclimates are beneficial to sustainable urban development. Based on a comprehensive comparison of some currently frequently used thermal comfort indices, including PMV, WBGT, PET, SET*, and UTCI, the differences among these indices are significant in dealing with the fundamental energy balance model, descriptive equations, and application boundary conditions. In order to validate these indices, a subjective questionnaire survey with field measurements was carried out on a university campus in Guangzhou in southern China. Results revealed strong linear relationships between operative temperature and mean radiant temperature (Tmrt), WBGT, PET, SET*, UTCI, as well as PMV. However, the relationships between these thermal comfort indices and the mean thermal sensation vote (MTSV) are not clear for a hot outdoor environment, especially when the operative temperature was above 34 °C. The ranges of the heat stress category and PMV need to be modified for the evaluation of hot outdoor environments.

In order to improve the effectiveness of a local exhaust ventilation system, an attempt is the changed hood geometry by adding a plate. The near fields of a simple exhaust and the annular slotted hoods with different slot widths are measured by a 2D-particle image velocimetry (PIV) system. The time-averaged velocity fields show that the slotted-hood designs enhance the capture velocities beyond half a diameter (0.5D) downstream the hood. The relative velocity increase coefficient (IC) along the hypothetical boundaries in the near fields was proposed to compared with the simple exhaust and determine the effect of slot width on the capture velocity enhancement, the IC for different slot widths is approximately 109%–130%. To predict the velocity at downstream of the hood for a given slot width and velocity, a unimodal-skewed distribution model was developed with a correlation coefficient of approximately 94%. Regarding turbulence statistics, the influence of the plate only involves the near hood region. The higher-order statistics also show no obvious differences between slotted hoods with different slot widths. In general, this study not only motivates us to design and improve the performance of LEV systems for forced advection problems by adding a baffle plate, but also provides benchmark flow data for a follow-up numerical simulation study.

Highlights • A general design framework is developed for improving low pedestrian level wind environment. • The improvement measures and evaluation criteria in current studies are reviewed and summarized. • A university campus is used as case study to demonstrate the design framework. • The outcomes of this paper can assist policy-makers to establish sustainable urban planning policies., An acceptable pedestrian level wind environment is essential to maintain an enjoyable outdoor space for city residents. Low wind velocity environment can lead to uncomfortable outdoor thermal experience in hot and humid summer, and it is unable to remove the pollutants out of city canyons. However, the average wind velocity at pedestrian level is significantly lowered by closely spaced tall buildings in modern megacities. To improve the low wind velocity environment at pedestrian level in high-density cities, a general framework and detailed guidelines are needed. This study is the first time to develop such a framework, and provide detailed guidelines for improving pedestrian level low wind velocity environment in high-density cities. Additionally, a detailed review and summarisation of evaluation criteria and improvement measures are presented in this paper, which provide additional options for urban planners. To investigate the performance of the framework, the Hong Kong Polytechnic University campus was utilised as a case study. Results showed that pedestrian level wind comfort was greatly improved with the developed framework. The outcomes of this study can assist city planners to improve the low wind velocity environment, and can help policy makers to establish sustainable urban planning policies.

High vehicular pollutants exposure to residents in near-road buildings raises special concerns in micro-scale urban science, as it causes severe health problems for those residents. This paper integrates a new parameter, i.e. personal intake fraction (IF_p), into computational fluid dynamics (CFD) simulations to investigate the flow and the resulted personal exposure of two-phase pollutants (CO and particular matters) in three-dimensional (3D) urban-like models. The impacts of street aspect ratios (building height/street width H/W = 0.5–1.5), viaduct settings, noise barriers and pollutant source locations were considered. 3D downward helical flows exist in the secondary streets perpendicular to the parallel approaching wind, which produces lateral pollutant transport across the interface between the main street and secondary streets. Therefore, the overall average IF_p ( ) of CO (∼0.23–0.59 ppm) in our current 3D urban models (H/W = 1) is much smaller than that in two-dimensional (2D) street canyon (∼3.25–5.21 ppm) models. Although narrower 2D street canyons usually present greater , our current 3D urban models do not show this monotone decreasing trend due to the complicated flow structures. of fine particles are always smaller than that of CO. Furthermore, if a single pollutant source is placed on the viaduct, become much smaller than that in cases with a single ground-level source. If the source location changes to the upstream secondary street, significantly decreases due to stronger local wind. Finally, of leeward-side cases usually exceeds that of windward-side cases by several times, but with viaduct settings, this leeward-to-windward ratio significantly decreases.

Optimized mounting positions for isolated vibratory machines in buildings require a minimum transmission of structure-borne sound power from the machine to the floor structure and relative stability of the isolated machine. Previous work by Mak and co-investigators has indicated the importance of using structure-borne sound power to assess vibration isolation and to select the best mounting positions by considering the structure-borne sound power transmission. This paper is a first attempt to utilize both the structure-borne sound power transmission and the rotational velocity (or the stability) of the machine to select the optimized mounting positions for an isolated vibratory machine. The results reveal that a vibratory machine should be symmetrically installed on diagonal lines of the receiving floor structure.

Lift-up buildings are advantageous in improving the wind circulation in a congested and compact city. However, the wind conditions in the void underneath a lift-up building, also known as the lift-up area, are vital for wind comfort of occupants of lift-up buildings. This study tested 28 lift-up buildings in a boundary layer wind tunnel to assess the influence of key design parameters; height and width of the main structure, and height, width, depth, and the shape of the central core on the wind conditions in the lift-up area. The results of the analyses show a significant influence of building height on the magnitude of wind speeds in the lift-up area while the width of the central core controls the area with low wind speeds. Tall buildings with short lift-up cores have small areas with acceptable pedestrian wind comfort, which can be increased by adopting corner modifications for the central core. The area of acceptable pedestrian wind comfort increases in oblique wind directions as the areas of high (>3.5 m s−1) and low (, Highlights • This study evaluated the influence of design of lift-up buildings on pedestrian wind comfort. • A total number of 28 lift-up buildings were tested in a boundary layer wind tunnel. • Dimensions of the building and the central core differently influenced pedestrian wind comfort. • A second-order, non-linear multivariable regression model was developed to predict pedestrian wind comfort.

This study examines the influence of building envelope features on interunit dispersion around multi-story buildings, when the presence of an upstream interfering building is also considered. Validated CFD methods in the steady-state RANS framework are employed. In general, the reentry ratios of pollutant from a source unit to adjacent units are mostly in the order of 0.1%, but there are still many cases being in the order of 1%. The influence of envelope features is dependent strongly on the interaction between local wind direction and envelope feature. In a downward dominated near-facade flow field, the presence of vertical envelope features forms dispersion channels to intensify the unidirectional spread. Horizontal envelope features help induce the dilution of pollutant to the main stream and weakens largely the vertical interunit dispersion. The large influences caused by the presence of envelope features extend the existing understanding of interunit dispersion based on flat-facade buildings.