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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.

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.

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.

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.

Unavoidable discontinuities due to in-duct elements in a ventilation ductwork system result in the generation of localized turbulence. Some of the turbulence energy is converted into noise. The flow-generated noise problem caused by in-duct elements is due to the complicated acoustic and turbulent interactions of multiple in-duct flow noise sources. Prediction of the flow-generated noise at the design stage is important in engineering, since it is almost impossible to solve the problem after the installation of a ventilation ductwork system. The measurement and computational fluid dynamics simulation can provide an accurate prediction but are expensive in terms of resources. There is an urgent need for developing a practical prediction method for the flow-generated noise in air ducts. Current design guides only provide a prediction method for aerodynamic sound produced by a single in-duct element in the ventilation system, which is usually not in accord with actual systems. An interaction factor β m is therefore proposed to take account of the interaction effects of multiple in-duct elements. Experimental measurements were conducted to verify the proposed prediction results. The predicted results show an acceptable agreement with the measurements. The proposed method provides engineers a practical generalized technique for predicting noise produced by multiple in-duct elements.

Many CFD studies have investigated the influence of computational parameters on the predicted concentration distribution of pollutants around isolated buildings, but such studies for building arrays are still lacking. This study systematically evaluated the influence of four computational and two physical parameters on pollutant dispersion in building arrays, including turbulence models, grid resolution, discretization of time step size Δ t , length of sampling period, aspect ratio of the arrays, and release rate of tracer gas. Throughout these evaluations, a set of published wind tunnel experimental data was used to validate the CFD models. For concentration simulations, the Large Eddy Simulation (LES) model gave the most accurate results but still had limitations in areas near the source, whereas the Detached Eddy Simulation (DES) and the Reynolds Averaged Navier-Strokes (RANS) RNG k − e models underperformed in some areas. The results of the LES and DES simulations varied with changes in Δ t ∗ and sampling length until Δ t ∗ was less than 0.24 and the sampling length was higher than 2400 Δ t ∗ for LES and 1200 Δ t ∗ for DES. A larger aspect ratio did not necessarily result in a higher concentration field than a smaller ratio. An increase in the tracer gas release rate did not change the general dispersion characteristics, but it still affected the concentration distribution in the areas near the source and resulted in a larger polluted area. The findings of this study are intended to contribute to improvements in the quality of CFD simulations of pollutant dispersion in building arrays.

Pedestrian level wind environment is affected by stagnated airflow in high-density cities. This study provides an understanding of the effects of building height and porosity size on pedestrian level wind comfort. The computational fluid dynamics (CFD) technique is utilized to reproduce wind flow around buildings at pedestrian level, and new wind comfort criteria for a low wind environment are adopted to evaluate wind comfort. More specifically, the Steady Reynolds Averaged Navier–Stokes (RANS) renormalization group (RNG) k–e turbulence model is employed in this study, and the accuracy of the simulation results are assured by validation against the wind tunnel test data. The influence of different building heights and porosity sizes on wind comfort around an isolated building and a group of buildings are subsequently examined. It is shown that an increase building height could improve wind comfort inside the site boundary for both the isolated building and group of buildings. Furthermore, the wind comfort benefits increased when porosity is on the first floor compared to when it is on the second floor. Moreover, larger porosity size generally results in better wind comfort than smaller porosity size. From a practical view point, this study provides information for city planners and architects to use in the improvement of pedestrian level wind comfort, without losing land use efficacy.

This paper focuses on the hybrid noise control in a duct using a periodic dual Helmholtz resonator (HR) array. A dual HR which consists of two HRs connected in series (neck-cavity-neck-cavity) leads to two resonance frequencies. A dual HR is analogous to a two degrees of freedom mechanical system. Based on the lumped model, the resonance frequencies and transmission loss of a dual HR have been derived. However, a dual HR is only effective at its resonance peaks with relative narrow bands. Aiming at broader noise attenuation bands for hybrid noise control at low frequencies, a periodic dual HR array is proposed in this paper. The wave propagation in a duct mounted with a periodic dual HR array is investigated theoretically and numerically. The Bragg wave theory and transfer matrix method are developed to conduct the investigation. The predicted theoretical results fit well with the Finite Element Method. Owing to the coupling of Bragg reflection and dual HR’s resonances, a periodic dual array can provide much broader noise attenuation bands at the designed resonance frequencies of the dual HR. The proposed periodic dual HR array can be used in practical engineering work to reduce hybrid noise at low frequencies.

In this paper, speech intelligibility in 9 classrooms of a middle school and 11 classrooms of a university in Hong Kong was investigated. The subjective speech intelligibility tests were conducted with students aged from 12 to 21 in these classrooms. Besides, objective acoustical measurements were performed in each listening position and testing conditions in each classroom. The relationship between subjective speech intelligibility scores and speech transmission index (STI) was discussed based on regression models. The effects of different age groups on the speech intelligibility were compared. The results show that speech intelligibility scores increase with STI value for all age groups. The speech intelligibility scores increase as the age increases under the same STI condition. The differences between age groups are decreased with the increase of STI values. English speech intelligibility scores in Hong Kong are always lower compared with native language researches under the same values of STI. Better STI values and better acoustical environment are needed because English is not the native language for students in Hong Kong but the official educational language.

Quality and efficiency of computational fluid dynamics (CFD) simulation of pedestrian level wind environment in a complex urban area are often compromised by many influencing factors, particularly mesh quality. This paper first proposes a systematic and efficient mesh generation method and then performs detailed sensitivity analysis of some important computational parameters. The geometrically complex Hong Kong Polytechnic University (HKPolyU) campus is taken as a case study. Based on the high-quality mesh system, the influences of three important computational parameters, namely, turbulence model, near-wall mesh density and computational domain size, on the CFD predicted results of pedestrian level wind environment are quantitatively evaluated. Validation of CFD models is conducted against wind tunnel experimental data, where a good agreement is achieved. It is found that the proposed mesh generation method can effectively provide a high-quality and high-resolution structural grid for CFD simulation of wind environment in a complex urban area.

People spend most of their lives in buildings and unavoidably perceive the sound in their surroundings. The understanding of the human perceptual dimensions of sound is important for obtaining the occupant-oriented decision-making in future building designs. This paper presented a systematic review of the studies analyzing the human perceptual dimensions of sound. Studies that used of principal component analysis (PCA) or factor analysis (FA) for the semantic differential method (SDM) applications of the subjective measurements of the human perceptions of sound were identified in the study selection process and included in the quantitative analysis. Forty-five eligible studies covered a wide range of sounds in the general indoor environment, including machines, building facilities, human voices, human activities, transportations, and the urban environment. The meta-analysis of the factor analysis integrated the data from the 39 individual studies and generated the evidence-based results of the review. Three major perceptual dimensions of sound were found to be ”Evaluation,” ”Potency,” and ”Activity” which referred to the human general judgment, the sensitivity to the magnitude, and the sensation of the temporal and spectral compositions of the perceived sound respectively. It implied that not only the energy level but also the energy distribution of the stimulations in the environment affects our perceptions. The review also provided insights into the selection of the suitable perceptions, the suggestions on the SDM applications, and the acoustics index development for the quantification of the psychological impacts of sound on the occupants in the indoor and outdoor environment. This research provides directions for future psychoacoustics studies to analyze the correlations between the objective stimulations and the human perceptions.

People's outdoor thermal sensation varies from that indoors. The highly asymmetric solar radiation and transient wind environment are the main causes. The University of California-Berkeley developed a multi-nodal human body thermal regulation model (the UCB model) to predict human thermal sensation and comfort in asymmetric and transient indoor environments. However, few studies compared its predictions with the survey responses outdoors. In this study, subjects' thermal sensations outdoors were surveyed and compared with the UCB model predictions. Meteorological parameters were monitored using a microclimate station, and over a thousand human subjects were surveyed. Results point out that subjects were highly sensitive to the changes in wind speed, especially under low-radiation conditions. However, the UCB model failed to predict such a high sensitivity. Besides, subjects had a higher tolerance to high air temperatures in outdoor environments when the solar radiation was acceptable, but the UCB model over-predicted the TSV (thermal sensation vote) in such conditions. Both the on-site results and the predictions by UCB model showed that subjects were more sensitive to wind speed in hotter environments while they were least sensitive to solar radiation in neutral thermal conditions. This study helps to reveal the potential of a multi-nodal thermal regulation model to address the asymmetric and transient features of outdoor environments and indicates the need to further refine the model for better quantitative prediction of outdoor thermal sensations.

The lift-up building design has been demonstrated to provide favorable wind comfort, but there is a lack of investigation on optimum wind comfort condition. This study coupled computational fluid dynamics (CFD) technique and response surface methodology (RSM) to determine the most desirable wind comfort around an isolated building with lift-up design. A multi-variable optimization method is proposed to determine optimum wind comfort and the corresponding lift-up design variables, namely, lift-up height ( H L ), core aspect ratio ( A R ) and core number ( N ). To better illustrate wind comfort around the building, the wind comfort in the lift-up area and the podium area are investigated separately. The Detached Eddy Simulation (DES) approach is employed throughout the whole CFD simulation process. The quality and goodness of the established RSM models are examined by analysis of variance and genetic algorithm is applied to generate optimal design solution. The generated results illustrate good performance of the established RSM model. Results show that the optimum wind comfort is obtained when H L is 8 m, A R is 10%, and N is 6. The lift-up core aspect ratio is subsequently found to have greatest effect on wind comfort among the three design variables in both the lift-up area and the podium area. In addition, the proposed method is applicable to other similar environmental design conditions and the outcomes of study can also be of great value in the improvement of wind comfort in compact urban cities.

The noise attenuation capacity of a HR defined as the integral of transmission loss in the frequency domain is first derived.The theoretical formula of noise attenuation capacity shows that it is only determined by the geometries of the neck and the cross-sectional area of the duct.The theoretical formula is validated by FEM simulation using COMSOL Multiphysics.The noise attenuation capacity is a significant supplement to theoretical studies and engineering applications of HRs. Helmholtz resonator (HR) is one of the most basic acoustic models and has been widely used in engineering applications due to its simple, tunable and durable characteristics. The transmission loss index is mainly used to evaluate the acoustic transmission performance. Based on the transmission loss index, this paper proposes the noise attenuation capacity index as one of the key parameters to evaluate the noise attenuation performance of a HR. The noise attenuation capacity is defined as the integral of transmission loss in the frequency domain. The theoretical formula of a HR's noise attenuation capacity is first derived in this study. It indicates that the noise attenuation capacity of a HR is only related to geometries of the neck and duct's cross-sectional area. The cavity volume has no effects on its noise attenuation capacity. The proposed theoretical formula of a HR's noise attenuation capacity is validated by Finite Element Method (FEM) simulation using commercial software (COMSOL Multiphysics). The proposed noise attenuation capacity of a HR should therefore be considered as one of the main acoustic characteristics of a HR. It is hoped that the present study could provide a stepping stone for the investigation of the HR's or other silencers noise attenuation capacity and potential applications in all research areas in respect of the HR.

This paper presents a theoretical study of an inertial actuator connected to an accelerometer by a local feedback loop for active vibration control on a floating raft. On the criterion of the minimum power transmission from the vibratory machines to the flexible foundation in the floating raft, the best mounting positions for the inertial actuator on the intermediate mass of the floating raft are investigated. Simulation results indicate that the best mounting positions for the inertial actuator vary with frequency. To control time-varying excitations of vibratory machines on a floating raft effectively, an automatic control system based on real-time measurement of a cost function and automatically searching the best mounting position of the inertial actuator is proposed. To the best of our knowledge, it is the first time that an automatic control system is proposed to move an actuator automatically for controlling a time-varying excitation.

Buildings are physical obstacles that diminish the benefits of wind by creating turbulence and reducing airflow velocities in urban settings. Particularly in the inner garden or amenities situated at the pedestrian level in the central space of a group of buildings, the efficiency of pollutant dispersion determines air quality and outdoor health. In this study, the effects of incident wind angles on wind velocity and pollutant distribution inside the central space of two building arrays arranged in ‘L’ and ‘U’ shapes were examined using computational fluid dynamics simulations. The ‘L’-shaped array performed better than the ‘U’-shaped array by forming a smaller low-wind-velocity (LWV) zone in the central space of the buildings (34.9%–76.11 % of the total space). The ‘L’-shaped array performed best at an incident wind angle of 225°, in which the LWV zone covered only 34.52 % of the central space. A 90° incident angle produced the largest LWV zone (81.80 %) for the ‘U’-shaped array. The ‘L’-shaped array generally distributed pollutants better than the ‘U’-shaped array. However, the ‘U’-shaped array with a 180° wind angle had a smaller high pollutant concentration area than the ‘L’-shaped array with a wind angle of 225° (Kc > 218.5 covering only 0.37 %). The worst vertical dispersion corresponded to a 135° wind angle for the ‘L’-shaped array, which recorded the highest Kc at the mid-level of the building. In conclusion, the appropriate selection of building array designs and their orientations allows for the most effective use of wind flow to enhance ventilation and pollutant dispersion.

This paper presents a theoretical study of the acoustic performance of different Helmholtz resonator (HR) array configurations. A dual HR consisting of two HRs connected in series (neck-cavity-neck-cavity) could be considered as a serial HR array. Two HRs mounted on the same cross-section of the duct constitute a parallel HR array. A lined HR array is composed of two HRs installed on the longitudinal direction of the duct. Since HR is reactive silencer without energy consumption, the energy storage capacity ( C TL ) of HR arrays could be defined as the area under transmission loss curve. The transfer matrix method is developed to conduct the investigation. The predicted theoretical results fit well with the Finite Element Method (FEM) simulation results. The results indicate that the installation methods have significant effects on transmission loss curves. However, the C TL of the dual HR equals the C TL of each single component HR mounted on the duct in despite of the dual HR having two HRs. The C TL of the parallel HR array is equivalent to the C TL of the lined HR array, which is twice the C TL of the dual HR. The C TL should therefore be considered as one of the main acoustic characteristics of a HR or an array and be taken into consideration in noise control optimization and HR design.

With the development of the urban city, increasing attention has been paid to outdoor thermal comfort. In this paper, an analysis of the sensitivities of different factors, including the personal factors and physical parameters of the thermal environment was conducted. The results showed that there was a strong linear relationship between the Physiological Equivalent Temperature (PET) and operation temperature. When the operation temperature was lower than 32 °C, the effect of air velocity on the PET was positive. However, the effects of other factors, including relative humidity, clothing insulation, and metabolic rate, on the PET were insignificant. An exponential relationship was found between the UTCI and the operation temperature. The effect of air velocity change on the UTCI became weaker and weaker with the increase of operation temperature. Compared with the PET, the linear relationship between the UTCI and relative humidity was clearer. A field survey of thermal comfort carried out in Guangzhou University was used for the validation of the thermal comfort models. It was observed that the clothing insulation requirement increased with the decrease of air temperature in autumn. The variations in metabolic rate were also obvious, from 1met to 3.8 met. More than 70% of the people had been walking before they arrived at the survey locations. In addition, there were some differences in the neutral PET and UTCI temperature between the metabolic rates of 1.0–2.0 met and of 2.6 met. Meanwhile, models of MTSV against the PET and UTCI under different metabolic rates were established.

This paper presented a theoretical study of the vibration control of a floating raft system using periodic structures. The band gap properties of the periodic structures, the power flow and the power transmissibility of the floating raft system were investigated by using the transfer matrix method. To minimize the power flow through periodic structures in a floating raft system, the geometrical parameters of the periodic structures were optimized by using a genetic algorithm. The numerical results demonstrated that the optimum periodic structure can provide broader stop band regions. The stop band regions of the optimum periodic structure contained all the harmonic frequencies of the force excitation in the floating raft system. The numerical results validated that the proposed optimization approach is sufficiently capable for the design of periodic structures. The proposed optimization approach has potential use for the development of vibration and shock isolation systems such as floating raft systems.

In this paper, an assessment model based on multi-layer fuzzy comprehensive evaluation method (FCE) of classroom acoustical environment is proposed. The model classifies five major factors affecting overall assessment model into several subsets alternatives. The weightings of these main criteria and alternatives were collected through questionnaires among students based on analytic hierarchy process methodology (AHP). An evaluation score was calculated from the proposed model with the weightings generated from AHP method. In this paper, classrooms in the Hong Kong Polytechnic University were used to develop the assessment model. The result shows that the evaluation score of PolyU classrooms is about 87.2, which refers to “Good” evaluation set. It indicates that classrooms in PolyU needs to be improved. The weightings generated from AHP method can be considered for the importance of each alternatives. The assessment model can provide proper recommendation to universities for acoustic treatment so as to increase the acoustic quality of the educational environment.