| 1. |
- Sattari, Amir, 1980-, et al.
(författare)
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PIV Visualisation study in a two-dimensional room model with rapid time varying ventilation flow rates
- 2011
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Ingår i: ROOMVENT, The 12th International Conference on Air Distribution in Rooms. - : Tapir Academic Press. - 9788251928120
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Konferensbidrag (refereegranskat)abstract
- Optimal control of inlet jet flows is of wide applicative interest in order to enhance mixing and reduce stagnation in a ventilated room. The general approach in mechanical ventilation is to use a constant flow rate forced convection system providing the ventilation air. This type of ventilation may cause several problems such as draught, stagnation at certain occupied locations, and subsequently low ventilation efficiencies. An alternative to increase the ventilation quality that has been of interest in this study is to introduce flow variations, which is considered as a potential to reduce stagnation and increase efficiency of the ventilation. The study was conducted as a model experiment in a small-scale, two-dimensional (2-D) room model with dimensions 30200.9 cm3 with water as operating fluid. The size of the model made it possible to investigate the 2-D velocity vector field within the entire room using Particle Image Velocimetry (PIV) method and further consequent dynamical and statistical analyses have been done from the resulted PIV vector fields. The comparison between cases of constant flow rate and flow variations have been conducted for the cases of two set of base flow rates and for each one, the cases of constant flow rate and flow variations with frequencies of 0.3, 0.4 and 0.5 Hz, is considered. In this investigation we show that the calm region, with a large stagnation zone, without pulsating inflow condition becomes more active in the sense that the stagnation points are moved and that the small-scale structures are grown for increasing pulsation frequency.
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| 2. |
- Sattari, Amir, 1980-, et al.
(författare)
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Plaster finishes in historical buildings – Measurements of surface structure, roughness parameters and air flow characteristics
- 2013
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Ingår i: Plaster finishes in historical buildings. - 9788888307268 ; , s. 69-75
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Konferensbidrag (refereegranskat)abstract
- Soiling of surfaces in historical buildings by deposition of particles is a common problem. Minimizing soiling is an important goal for conservation of structures and objects. The surfaces give rise to an interference with the air motions along the surfaces. Properties of surfaces may therefore influence the particle deposition. It is well known that with increasing roughness of the surfaces the particle deposition rate increases. The properties of surfaces in historical buildings are not well documented. We have investigated samples of surfaces finished by wood float finish, steel float finish and brushed finish. As a reference we have used an MDF board. The geometrical properties of the surfaces have been documented by using the stripe projection method. The resistance to airflow along the surface and the turbulence generated by the surfaces have been investigated by recording the boundary layer flow over the surfaces in a special flow rig. The work reported is part of a project where the process of soiling is studied both in laboratory and in field studies. The air velocity adjacent to the surfaces will be recorded with both PIV (Particle Image Velocimetry) and hot-wire technique. The temperature gradient close to the walls will be recorded with cold-wire technique.
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| 3. |
- Buccoliere, Riccardo, et al.
(författare)
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City breathability and its link to pollutant concentration distribution within urban-like geometries
- 2010
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Ingår i: Atmospheric Environment. - : Elsevier. - 1352-2310 .- 1873-2844. ; 44:15, s. 1894-1903
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Tidskriftsartikel (refereegranskat)abstract
- This paper is devoted to the study of pollutant concentration distribution within urban-like geometries. By applying efficiency concepts originally developed for indoor environments, the term ventilation is used as a measure of city “breathability”. It can be applied to analyse pollutant removal within a city in operational contexts. This implies the evaluation of the bulk flow balance over the city and of the mean age of air. The influence of building packing density on flow and pollutant removal is, therefore, evaluated using those quantities. Idealized cities of regular cubical buildings were created with packing density ranging from 6.25% to 69% to represent configurations from urban sprawl to compact cities. The relative simplicity of these arrangements allowed us to apply the Computational Fluid Dynamics (CFD) flow and dispersion simulations using the standard k– turbulence model. Results show that city breathability within the urban canopy layer is strongly dependent from the building packing density. At the lower packing densities, the city responds to the wind as an agglomeration of obstacles, at larger densities (from about 44%) the city itself responds as a single obstacle. With the exception of the lowest packing density, airflow enters the array through lateral sides and leaves throughout the street top and flow out downstream. The air entering through lateral sides increases with increasing packing density.At the street top of the windward side of compact building configurations, a large upward flow is observed. This vertical transport reduces over short distance to turn into a downward flow further downstream of the building array. These findings suggest a practical way of identifying city breathability. Even though the application of these results to real scenarios require further analyses the paper illustrates a practical framework to be adopted in the assessment of the optimum neighbourhood building layout to minimize pollution levels.
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| 4. |
- Cehlin, Mathias, et al.
(författare)
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Measurements of Air Temperatures Close to a Low-Velocity Diffuser in Displacement Ventilation Using Infrared Camera
- 2002
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Ingår i: Energy and Buildings. - : Elsevier. - 0378-7788 .- 1872-6178. ; 34, s. 687-698
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Tidskriftsartikel (refereegranskat)abstract
- The near zone of supply air diffusers is very critical for the indoor climate. Complaints of draft are often associated with low-velocity diffusers in displacement ventilation because the air is discharged directly into the occupied zone. Today, the knowledge of the near zone of these air supply diffusers is insufficient, causing an increased need for better measuring methods and representation of the occupied zone. A whole-field measuring technique has been developed by the authors for visualization of air temperatures and airflow patterns over a large cross-section. In this particular whole-field method, air temperatures are measured with an infrared camera and a measuring screen placed in the airflow. The technique is applicable to most laboratory and field test environments. It offers several advantages over traditional techniques; for example, it can record real-time images within large areas and capture transient events. The purpose of this study was to conduct a parameter and error analysis of the proposed whole-field measuring method applied to a flow from a low-velocity diffuser in displacement ventilation. A model of the energy balance, for a solid measuring screen, was used for analyzing the influence of different parameters on the accuracy of the method. The analysis was performed with respect to the convective heat transfer coefficient, emissivity, screen temperature and surrounding surface temperatures. Theoretically, the temperature difference between the screen and the ambient air was found to be 0.2–2.4 °C for the specific delimitation in the investigation. However, after applying correction the maximum uncertainty of the predicted air temperature was found to vary between 0.62 and 0.98 °C, due to uncertainties in estimating parameters used in the correction. The maximum uncertainty can be reduced to a great extent by estimating the convective heat transfer coefficient more accurately and using a screen with rather low emissivity.
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| 5. |
- Kabanshi, Alan, et al.
(författare)
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Occupants’ perception of air movements and air quality in a simulated classroom with an intermittent air supply system
- 2019
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Ingår i: Indoor + Built Environment. - : Sage Publications. - 1420-326X .- 1423-0070. ; 28:1, s. 63-76
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Tidskriftsartikel (refereegranskat)abstract
- The study reported herein builds on occupant response to an intermittent air jet strategy (IAJS), which creates periodic airflow and non-isothermal conditions in the occupied zone. Previous research has highlighted the benefits of IAJS on thermal climate and supports energy saving potential in view of human thermal perception of the indoor environment. In this study, the goal was to explore occupant acceptability of air movements and perceived indoor air quality, and to determine a way of assessing acceptable air movement conditions under IAJS. Thirty-six participants were exposed to twelve conditions: three room air temperatures (nominal: 22.5, 25.5 and 28.5 oC), each with varied air speeds (nominal: <0.15 m/s under mixing ventilation (MV), and 0.4, 0.6 and 0.8 m/s under IAJS) measured at the breathing height (1.1 m). The results show that participants preferred low air movements at lower temperatures and high air movements at higher temperatures. A model to predict percentage satisfied with intermittent air movements was developed, and predicts that about 87% of the occupants within a thermal sensation range of slightly cool (-0.5) to slightly warm (+0.5), in compliance with ASHRAE standard 55, will find intermittent air movements acceptable between 23.7 oC and 29.1 oC within a velocity range of 0.4 – 0.8 m/s. IAJS also improved participants’ perception of air quality in conditions deemed poor under MV. The findings support the potential of IAJS as a primary ventilation system in high occupant spaces such as classrooms.
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| 6. |
- Liu, Wei, Assistant Professor, 1987-, et al.
(författare)
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The effects of room length on jet momentum flux
- 2022
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Ingår i: 16th ROOMVENT Conference, ROOMVENT 2022. - : EDP Sciences. ; , s. 01001-01001
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Konferensbidrag (refereegranskat)abstract
- As to mixing ventilation in indoor environments, the turbulent jet plays a major role in driving the air movement, contaminant transport, and heat transfer. The main characteristic of a turbulent jet is its momentum flux. By entrainment of air, the flow of a jet increases and may enhance the flooding of contaminant. In investing the jet's momentum flux, it is generally regarded that the supply jet collides with the opposing wall and the jet is transformed into a wall jet. However, this is not always true if a jet is not sufficiently strong, or the length of a room is large. Therefore, this study adopted computational fluid dynamics (CFD) to investigate the supply jet development and its momentum flux by varying the room length. Initially, the width of the air supply inlet was the same with that of the room. By defining n as the ratio of room length and height, when n = 3, there is a horizontal a vortex which is the normal behaviour. When the room length increased further, the supply jet was unable to collide with the opposing wall. This investigation got two vertical vortices at the room end which is new. The two new vertical vortices were most pronounced for n = 5. It is possible that increasing the length of the room introduces a gradual transition towards a flow in a rectangular duct. This flow is probably very much governed by the side walls. Therefore, this study reduced the width of the air supply inlet by half and maintained the same flow rate. However, a single vertical vortex was identified at the room end for n = 5. In both scenarios, the supply jet may create new vortices that would enhance the flooding of contaminants.
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| 7. |
- Albuquerque, Daniel, et al.
(författare)
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LES simulation of oscillating natural ventilation driven by vortex shedding in isolated buildings
- 2020
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Ingår i: Proceedings of Building Simulation 2019: 16th Conference of IBPSA. - : IBPSA. - 9781775052012 ; , s. 644-649
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Konferensbidrag (refereegranskat)abstract
- A recently published study presented a new type of natural ventilation (NV) flow, named pumping ventilation. The oscilatory mechanism of vortex shedding that occurs at the wake region of an isolated building drives this new type of ventilation in rooms with two (or more) openings facing the leeward or windward side of an isolated building. This paper presents a validated Large Eddy Simulation (LES) study of oscillating/pumping NV in an isolated building using three different separations (s') between its two windows. LES is validated using an experimental database from measurements performed at the University of Gavle boundary layer wind tunnel (WT). The measurements use a cubic model with 0.45m side representing a three-story building at a 1/20 scale that allows the use of bottom-hung windows. LES results show a good agreement with the measured non-dimensional ventilation rates. A dimensionless analysis shows the dominant frequencies of the pumping flow, are close to the Strouhal frequency.
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| 8. |
- Albuquerque, Daniel P., et al.
(författare)
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Experimental and numerical investigation of pumping ventilation on the leeward side of a cubic building
- 2020
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Ingår i: Building and Environment. - : Elsevier. - 0360-1323 .- 1873-684X. ; 179
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Tidskriftsartikel (refereegranskat)abstract
- Unstable interaction between shear layers that form in the wake of an isolated building exposed to wind can drive natural pumping ventilation in windward and leeward facing rooms with two or more horizontally separated openings. This paper presents an experimental and numerical study of pumping ventilation in a three-story cubic building with two leeward openings in its middle floor. Reduced-scaled measurements were performed in the University of Gävle atmospheric-boundary-layer wind tunnel. The ventilation mechanism was investigated using smoke visualization, hot wire anemometry and particle image velocimetry. Effective ventilation rates were obtained using a tracer gas decay method. Experimental results confirmed that pumping ventilation is a 3D oscillatory unstable phenomenon with periodic behavior over several oscillation cycles. Measured flowrates show a linear relation between the effective ventilation rate and window separation. The numerical simulations used two turbulence modeling approaches: unsteady Reynolds-averaged Navier-Stokes (URANS) and large eddy simulation (LES). Both URANS and LES could predict vortex shedding frequency with an error below 5%. LES showed a good agreement with the measured ventilation rates, with an error below 10%, while URANS underestimated ventilation rates by at least 40%. The ventilation efficiency, obtained by LES, ranged between 0.60 and 0.75 (for the case with larger window separation). The results show that LES may be a suitable simulation approach for pumping ventilation. In contrast, URANS cannot simulate pumping ventilation.
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| 9. |
- Andersson, Harald, 1987-, et al.
(författare)
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FAST-AIR: Fast analytic systems for tracer-gas assessment in indoor research: Development and testing of CO2 tracer-gas system.
- 2024
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Konferensbidrag (refereegranskat)abstract
- The time constant of ventilation of rooms in buildings is between 15 minutes (in office spaces) to 2 hours (in residential buildings). Currently, most of the tracer gas system analyzers on the market have a minute-based time constant and depending on the channels a cycle of sampling and analysis may take up to 6 minutes, E.g., 6 channel system. Essentially, only mean values are recorded with most present tracer gas analyzers. This is a hindrance for detailed temporal analysis of conditions in the room and consequently is does not capture the resolution of the influence of the internal flow on air and contaminant distribution. The current paper presents work on the development and testing of a fast response CO2 tracer-gas system with a time constant of 1 second. In contrast to the present analyzers, not only the mean values but also the whole statistical distribution of variables can be recorded, and pulse responses can be analyzed. This makes the system viable for measurement and analysis of not only spatial but also temporal distribution of contaminants. For example, recirculating airflow in the room generated by flooding of ventilation air is possible to be measured and thus making it easy to extend the analyses of the process of ventilation far beyond the possibilities with current systems.
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| 10. |
- Antoniou, Nestoras, et al.
(författare)
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CFD and wind-tunnel analysis of outdoor ventilation in a real compact heterogeneous urban area : evaluation using “air delay”
- 2017
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Ingår i: Building and Environment. - : Elsevier BV. - 0360-1323 .- 1873-684X. ; 126, s. 355-372
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Tidskriftsartikel (refereegranskat)abstract
- Outdoor urban ventilation in a real complex urban area is investigated by introducing a new ventilation indicator – the "air delay". Computational Fluid Dynamics (CFD) simulations are performed using the 3D steady Reynolds-Averaged Navier-Stokes (RANS) and Large Eddy Simulation (LES) approaches. The up-to-date literature shows the lack of detailed evaluations of the two approaches for real compact urban areas. This study further presents a systematic evaluation of steady RANS and LES for the assessment of the ventilation conditions in a dense district in Nicosia, Cyprus. The ventilation conditions within the urban area are investigated by calculating the distribution of the age of air. To better assess the outdoor ventilation, a new indicator, the "air delay" is introduced as the difference between the local mean age of air at an urban area and that in an empty domain with the same computational settings, allowing the comparison of the results in different parts of the domain, without impact of the boundary conditions. CFD results are validated using wind-tunnel measurements of mean wind speed and turbulence intensity performed for the same urban area. The results show that LES can accurately predict the mean wind speed and turbulence intensity with the average deviations of about 6% and 14%, respectively, from the wind-tunnel measurements while for the steady RANS, these are 8% and 31%, respectively. The steady RANS simulations overestimate the local mean air delay. The deviation between the two approaches is 52% at pedestrian level (2 m).
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