| 1. |
- 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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| 2. |
- 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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| 3. |
- 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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| 4. |
- 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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| 5. |
- 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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| 6. |
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| 7. |
- Buccolieri, Riccardo, et al.
(författare)
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Drag force rose representing the interaction between urban geometries and wind
- 2021
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Ingår i: 15th ROOMVENT (Roomvent 2020) virtual conference. - 9788894612301 ; , s. 85-88
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Konferensbidrag (refereegranskat)abstract
- The drag force generated by aligned arrays of cubes of different packing density and exposed to different wind directions in a wind tunnel is discussed. Results allowed to build a drag force rose which shows that the drag force increases with increasing packing density till λp = 0.25 for any wind direction. It is also shown that, independent of the packing density, the drag force increases with increases deviation of WD from the perpendicularity.
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| 8. |
- Carrilho da Graça, Guilherme, et al.
(författare)
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Pumping ventilation of corner and single sided rooms with two openings
- 2021
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Ingår i: Building and Environment. - : Elsevier. - 0360-1323 .- 1873-684X. ; 205
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Tidskriftsartikel (refereegranskat)abstract
- Corner rooms with two or more open windows in perpendicular facades can be naturally ventilated in cross-ventilation or pumping ventilation. These two airflow regimes also occur in rooms with two openings in the same façade, in the form of single sided pumping or cross sided ventilation. This paper presents an experimental and numerical simulation study of the scale and occurrence of these two flow regimes for rooms in a rectangular building exposed to wind. Flow visualization and tracer gas measurement of effective airflow were performed in an atmospheric boundary layer wind tunnel using a rectangular model of a three-story building (1/20 scale) with a ventilated middle floor. Experimental results show that pumping ventilation occurs when the wind is perpendicular to the façade (single sided rooms) or aligned with the building corner (corner rooms). In addition to these two perfectly aligned wind directions, pumping also occurs for a range of incoming wind angles: ±19° for single sided; and ±9° for corner rooms. As a result, for isolated rectangular buildings that have, at least, one single sided and two corner rooms in each facade, pumping ventilation can potentially occur in two or more rooms for 62 % of incoming wind directions. To investigate the transition between steady cross-ventilation and unsteady pumping ventilation, three-dimensional computational fluid dynamics large eddy simulations were performed to obtain wind generated pressures in the ventilation openings. Results show that the transition from cross-ventilation to pumping occurs when the steady pressure becomes smaller than the unsteady component. These results are used to develop a pressure based simplified model for corner ventilation that can predict effective airflow from external wind generated pressures with an average error below 10.2 %. © 2021 Elsevier Ltd
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| 9. |
- 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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| 10. |
- Cehlin, Mathias, Doktor, 1974-, et al.
(författare)
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Towards benchmarking of urban air quality based on homogenous surface emission
- 2023
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Ingår i: Results in Engineering (RINENG). - : Elsevier. - 2590-1230. ; 20
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Tidskriftsartikel (refereegranskat)abstract
- Here, it is presented a possible methodology and experimental model for benchmarking of air quality in cities. The concept behind the methodology is that a city’s inherent structure affects the potential for contaminant removal due to the resistance it poses to inflow. The approach is based on homogenous emission across the street surface network, representing a worst-case situation. Different levels of complexity can be used for benchmarking, making it valuable for evaluating different layouts. Additionally, an urban ventilation index suitable for these kinds of experimental studies has been suggested.
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