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  • 1.
    Blomqvist, Claes
    et al.
    KTH, Superseded Departments, Civil and Architectural Engineering.
    Sandberg, Mats
    Högskolan i Gävle, Avdelningen för inomhusmiljö.
    A Note on Air Movements through Horizontal Openings in Buildings2002In: The 8th International Conference on Air Distribution in Rooms: Inividual Controlled Environment, 2002Conference paper (Refereed)
  • 2.
    Blomqvist, Claes
    et al.
    KTH, Superseded Departments, Civil and Architectural Engineering.
    Sandberg, Mats
    Högskolan i Gävle.
    Air Movements through Horizontal Openings in Buildings – A Model Study2004In: The International Journal of Ventilation, ISSN 1473-3315, E-ISSN 2044-4044, Vol. 3, no 1, p. 1-10Article in journal (Refereed)
    Abstract [en]

    A building contains a number of large openings like doors and staircases. When the temperature of the spaces connected by these openings differs, the difference in density will cause air movements through them. Horizontal air movements through vertical openings in buildings like doors and windows are well investigated while studies of air movements through horizontal openings like stairwells are less frequent and therefore this work is focusing on this case.

    The paper reports on an experimental study of the possibility of using buoyancy forces to distribute air and heat through horizontal openings. The experiments have been carried out in a scale model with water as the operating fluid.

    The result of the study shows that the flow rate through a horizontal opening is roughly half of the flow rate through a vertical opening for the same conditions, probably caused by the more complex flow pattern in the horizontal opening. A staircase below the horizontal opening will guide the flow somewhat and will cause a small increase of the fluid exchange through the opening.

  • 3.
    Blomqvist, Claes
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering.
    Sandberg, Mats
    Högskolan i Gävle, Avdelningen för inomhusmiljö.
    Conversion of electric heating in buildings An unconventional alternative2008In: Energy and Buildings, ISSN 0378-7788, E-ISSN 1872-6178, Vol. 40, no 12, p. 2188-2195Article in journal (Refereed)
    Abstract [en]

    To decrease the electric energy used for heating buildings it has become desirable to convert direct electrical heating to other heat sources. This paper reports on a study of the possibility of using an unconventional method for conversion to avoid installing an expensive hydronic system. The conversion method combines the ventilation and heating systems and uses air instead of water for distribution of heat within the building, taking advantage of thermal forces and the special properties of gravity currents. Full-scale tests have been carried out in a test apartment inside a laboratory hall where the conditions could be controlled. Temperatures and efficiency of ventilation have been measured to ensure that the demands with respect to thermal climate and air exchange were fulfilled. The results show that it is possible to use the method for heating and ventilation when converting the heating system, but further work has to be done to develop a detailed solution that works in practice. (C) 2008 Elsevier B.V. All rights reserved.

  • 4.
    Blomqvist, Claes
    et al.
    KTH, Superseded Departments, Civil and Architectural Engineering.
    Sandberg, Mats
    KTH, Superseded Departments, Civil and Architectural Engineering.
    Measurements and Control of Air Movements within a Building1997In: 18th AIVC Conference: Ventilation and Cooling, 1997, p. 427-436Conference paper (Other academic)
    Abstract [en]

    There are a number of methods available concerning with distribution of air in buildings. Within control research, one can find new control algorithms which have not yet been used in practice. These new algorithms open the possibility of developing and implementing of new demand controlled ventilation systems.

    In a building the internal air motions are due both to differences in temperature and pressure differences caused by the ventilation system. Therefore, one fundamental question is to what extent it s possible to control the air motions within a building using fan powered ventilation in combination with temperature control.

    The aims of this paper is to report on measurements done to examine the influence of temperature differences between rooms on the air exchange through open doors in a building and to explore the use of modern control technique to minimise the temperature difference.

    The result of the measurements shows that even very small (0.1-0.2°C) temperature differences between rooms cause bi-directional air flows in the doorways of a magnitude that exceed the flow rates caused by the mechanical ventilation system. Therefore it is necessary to control the temperatures in the rooms to make it possible for the ventilation system to distribute the air to those parts of the building where it is needed.

  • 5.
    Blomqvist, Claes
    et al.
    KTH, Superseded Departments, Civil and Architectural Engineering.
    Sandberg, Mats
    Högskolan i Gävle, Avdelningen för inomhusmiljö.
    Spread of Gravity Currents within a Multi-Room Building2000In: The 7th International Conference on Air Distribution in Rooms: Ventilation for Health and Sustainable Environment, 2000Conference paper (Refereed)
  • 6.
    Blomqvist, Claes
    et al.
    KTH, Superseded Departments, Civil and Architectural Engineering.
    Sandberg, Mats
    KTH, Superseded Departments, Civil and Architectural Engineering.
    To what extent can one with mechanical ventilation control the air motions within a building1996In: ROOMVENT'96: 5th International Conference on Air Distribution in Rooms, 1996, p. 265-272Conference paper (Other academic)
    Abstract [en]

    There are a number of methods available concerning with air distribution in buildings. Within control research, one can find new control algorithms which have not been used in practice yet. These new algorithms open the possibility of developing and implementing of new demand controlled ventilation systems.

    In a building the internal air motions are due both to differences in temperature and due to pressure differences induced by the ventilation system. Therefore, one fundamental question is to what extent one can with fan powered ventilation control the air motions within a building.

    The aims of this paper is to report on development of methods to study the air motions in a multi room residence apartment using various combinations of exhaust and supply air management. The experimental work includes measurements of air flow rates in door openings in both directions and use of various tracer gas methods to determine the supply air flow to each room, and identifying flow paths.

    In an accompanying paper (Björsell 1996) is reported on the results from a simulation of the performance of different control algorithms.

  • 7.
    Blomqvist, Claes
    et al.
    KTH, Superseded Departments, Civil and Architectural Engineering.
    Sandberg, Mats
    Högskolan i Gävle, Avdelningen för inomhusmiljö.
    Transition from Bi-directional to Unidirectional Flow in a Doorway1998In: ROOMVENT '98 : proceedings: 6th International Conference on Air Distribution in Rooms, 1998, p. 539-546Conference paper (Refereed)
    Abstract [en]

    The air flow in a doorway is governed by density difference caused by temperature difference and pressure difference caused by mechanical ventilation. Tests have been carried out in a unique indoor test house where the room to room to temperature difference could be controlled very accurately with a new control system. In addition to these tests some tests were carried out in a scale model with water as the operating fluid. Two main criteria of unidirectional flow in a doorway have been explored:

     

    1a.     The recorded mean velocity is unidirectional

    1b.     The neutral height is equal to the height of the door

    2.       Unidirectional flow in the sense that

     

    there is no transfer of contaminant from one room to another. To explore condition one the velocity profile in the doorway have been recorded by transversing the door opening. Condition two has been explored by using tracer gas technique.

  • 8. Brinkworth, B. J.
    et al.
    Sandberg, Mats
    KTH Research School University of Gävle, Department of Indoor Environment.
    Design procedure for cooling ducts to minimise efficiency loss due to temperature rise in PV arrays2006In: Solar Energy, ISSN 0038-092X, E-ISSN 1471-1257, Vol. 80, no 1, p. 89-103Article in journal (Refereed)
    Abstract [en]

    The principal variable to be fixed in the design of a PV cooling duct is its depth, and hence the hydraulic diameter of its cross-section D. Analysis of the flow and heat transfer in the duct under still-air (buoyant flow) conditions, when the temperature rise is greatest, is validated by measurements on a full-scale test rig. It is shown that there is an optimum value of this design variable, such that for an array of length L the minimum temperature occurs when the ratio L/D is about 20. The optimum value is not affected much by other quantities, including the slope of the array. In practical situations, the flow is obstructed by devices across the duct inlet and outlet to exclude insects, birds and rain, and by structural support members crossing the duct interior. It is shown that the latter are no cause for concern, since the effect of the reduction in the flow-rate due to their presence is more than offset by an increase in heat transfer through additional turbulent mixing. It is also shown that array temperatures are strongly reduced by wind effects, which increase both the heat lost from the front surface of the array and by enhancement of the flow in the duct. Though the trends are clear, limitations are encountered in the present state of knowledge in both areas.

  • 9. Buccolieri, R.
    et al.
    Sandberg, Mats G.
    Högskolan i Gävle, Sweden .
    Study of the effects of building density and overall shape of a city on pollutant dispersion by combination of wind tunnel experiments and CFD simulations2008In: Hrvatski Meteoroloski Casopis, ISSN 1330-0083, Vol. 43 PART 2, p. 651-655Article in journal (Refereed)
    Abstract [en]

    Despite the improvement made in controlling local air pollution, urban areas are undergoing increasing environmental pressures and poor air quality is one of the major concerns. Recently, much attention has focused on the relationship between urban form and sustainability. There are indications that the density and the overall shape of cities can have implications on street level ventilation and the "compact city" is by many regarded as the most sustainable urban form. In this framework, this paper is devoted to the study of flow and pollutant dispersion from a ground level line source at pedestrian level within different urban configurations. The urban-like configurations vary from the scenario of an urban sprawl to the opposite scenario of a compact city. Wind tunnel experiments and CFD simulations are performed to evaluate pollutant concentrations in each of the idealized city structures. The overall aim is that of assessing and clarifying the effect of city density on atmospheric flow patterns and pollutant dispersion.

  • 10. Buccolieri, R.
    et al.
    Sandberg, Mats G.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering.
    Di Sabatino, S.
    An application of ventilation efficiency concepts to the analysis of building density effects on urban flow and pollutant concentration2011In: International Journal of Environment and Pollution, ISSN 0957-4352, Vol. 47, no 1-4, p. 248-256Article in journal (Refereed)
    Abstract [en]

    This paper is devoted to the study of flow and pollutant dispersion within different urban configurations by means of wind tunnel experiments and Computational Fluid Dynamics (CFD) simulations. The influence of the building packing density was evaluated in terms of ventilation efficiency. We found that air entering the array through the lateral sides and that leaving through the street roofs increased and the lateral spread of the pollutant released from a ground level line source decreased with increasing packing density. Ventilation efficiency concepts developed for indoor environments appear a promising tool for evaluating the urban air quality as well.

  • 11. Buccolieri, R.
    et al.
    Sartoretto, F.
    Giacometti, A.
    Di Sabatino, S.
    Leo, L. S.
    Pulvirenti, B.
    Sandberg, Mats
    Department of Technology and Built Environment, KTH Research School, University of Gävle, Gävle, Sweden.
    Wigö, Hans
    Department of Technology and Built Environment, KTH Research School, University of Gävle, Gävle, Sweden.
    Flow and pollutant dispersion within the canal grande channel in venice (Italy) via CFD techniques2010In: HARMO 2010 - Proceedings of the 13th International Conference on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes, 2010, p. 760-764Conference paper (Refereed)
    Abstract [en]

    The present paper is aimed at the analysis of flow and pollutant dispersion in a portion of the Canal Grande (Grand Canal) in Venice (Italy) by means of both Computational Fluid Dynamics (CFD) FLUENT simulations and wind tunnel experiments performed at the University of Gävle (Sweden). For this application, Canal Grande can be viewed as a sort of street canyon where the bottom surface is water and bus boat emissions are the major source of pollution. Numerical investigations were made to assess the effect of the water surface on air flow and pollutant concentrations in the atmosphere. One of the challenges has been to deal with the interface between two immiscible fluids which requires ad-hoc treatment of the wall in terms of the numerical scheme adopted and the grid definition which needs to be much finer than in typical numerical airflow simulations in urban street canyons. Preliminary results have shown that the presence of water at the bottom of the street canyon modifies airflow and turbulence structure with direct consequences on concentration distribution within the domain.

  • 12. Buccolieri, Riccardo
    et al.
    Sandberg, Mats
    KTH Research School University of Gävle, Department of Indoor Environment.
    Di Sabatino, Silvana
    City breathability and its link to pollutant concentration distribution within urban-like geometries2010In: Atmospheric Environment, ISSN 1352-2310, E-ISSN 1873-2844, Vol. 44, no 15, p. 1894-1903Article in journal (Refereed)
    Abstract [en]

    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-epsilon 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.

  • 13. Elvsen, Per-Ake
    et al.
    Sandberg, Mats
    Högskolan i Gävle.
    Buoyant jet in a ventilated room: Velocity field, temperature field and airflow patterns analysed with three different whole-field methods2009In: Building and Environment, ISSN 0360-1323, E-ISSN 1873-684X, Vol. 44, no 1, p. 137-145Article in journal (Refereed)
    Abstract [en]

    The instantaneous velocity field and temperature field were measured and file airflow patterns visualised close to a diffuser for displacement ventilation. Since the low-velocity diffuser was located above the floor and the inlet air temperature was below the room temperature, the flow was governed by both momentum and buoyancy forces. The data were recorded with whole-field measuring techniques, particle streak velocimetry (PSV), particle image velocimetry (PIV) and infrared thermography (IR), in conjunction with a low thermal mass screen. The environment is very complex, supply of buoyant air with a commercial supply terminal with 20 nozzles pointing in different directions. which makes it difficult to use point-measuring techniques or computational fluid dynamics (CFD). The aim was twofold: (a) to explore what kind of information can be derived Front whole-field measurement techniques in this context and (b) to investigate the trajectory of the flow discharged into the room and the entrainment of the flow.

  • 14.
    Fallenius, Bengt E. G.
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Sandberg, Mats
    Sattari, Amir
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Experimental study on the effect of pulsating inflowto a closed volume2011Report (Other academic)
  • 15.
    Fallenius, Bengt E. G.
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Sattari, A.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Sandberg, Mats
    University of Gävle.
    Experimental study on the effect of pulsating inflow to an enclosure for improved mixing2013In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 44, p. 108-119Article in journal (Refereed)
    Abstract [en]

    Optimal control of inlet jet flows is of broad interest for enhanced mixing in ventilated rooms. The general approach in mechanical ventilation is forced convection by means of a constant flow rate supply. However, this type of ventilation may cause several problems such as draught and appearance of stagnation zones, which reduces the ventilation efficiency. A potential way to improve the ventilation quality is to apply a pulsating inflow, which has been hypothesised to reduce the stagnation zones due to enhanced mixing. The present study aims at testing this hypothesis, experimentally, in a small-scale two-dimensional water model using Particle Image Velocimetry with an in-house vortex detection program. We are able to show that for an increase in pulsation frequency or alternatively in the flow rate the stagnation zones are reduced in size and the distribution of vortices becomes more homogeneous over the considered domain. The number of vortices (all scales) increases by a factor of four and the swirl-strength by about 50% simply by turning on the inflow pulsation. Furthermore, the vortices are well balanced in terms of their rotational direction, which is validated by the symmetric Probability Density Functions of vortex circulation (Γ) around Γ= 0. There are two dominating vortex length scales in the flow, namely 0.6 and 0.8 inlet diameters and the spectrum of vortex diameters become broader by turning on the inflow pulsation. We conclude that the positive effect for enhanced mixing by increasing the flow rate can equally be accomplished by applying a pulsating inflow.

  • 16.
    Fredriksson, Jan
    et al.
    KTH Research School University of Gävle, Department of Indoor Environment.
    Sandberg, Mats
    KTH Research School University of Gävle, Department of Indoor Environment.
    The effect of false ceiling on the cooling capacity of passive chilled beams2009In: Building and Environment, ISSN 0360-1323, E-ISSN 1873-684X, Vol. 44, no 7, p. 1426-1430Article in journal (Refereed)
    Abstract [en]

    Passive chilled beams are often used to provide cooling or additional cooling when the ventilation system cannot cope with the whole cooling load. The advantage of passive cooling is that it is a silent cooling. Often the chilled beams are installed above a false ceiling and thereby the room is subdivided into two compartments. From the chilled beam a plume is generated. Make-up air (return air) needs to flow into the upper compartment to substitute the airflow generated by the chilled beam. Therefore openings for this purpose are installed in the false ceiling. Small openings constitute a resistance to the flow and the locations of the openings affect the flow pattern. The overall performance was studied in a mock-up of a real office by changing both the size and position of the openings for the make-up air. A uniform heating source was arranged by covering the floor with a heating foil. The best location and size of the openings were explored by both recording the heat absorbed by the beam and the temperature in the room. Minimum temperature attained in the room is the signature of the most efficient cooling. To achieve efficient cooling with a uniform floor-based heating source, two conditions must be fulfilled: a) the return opening area must be at least equal to the horizontal area of the chilled beam: b) the return air openings must be located at the perimeter of the room. In general we can expect conditions a) and b) to be applicable irrespective of type of heat, but for point sources we could achieve the best cooling by placing the return air opening above the heat source.

  • 17.
    Fredriksson, Jan
    et al.
    KTH Research School University of Gävle, Department of Indoor Environment.
    Sandberg, Mats
    KTH Research School University of Gävle, Department of Indoor Environment.
    The effect of false ceilings on the performance of passive chilled beams2007In: IAQVEC 2007 Proceedings - 6th International Conference on Indoor Air Quality, Ventilation and Energy Conservation in Buildings: Sustainable Built Environment, 2007, p. 77-83Conference paper (Refereed)
    Abstract [en]

    Sweden is a country with a severe winter climate. Nevertheless, cooling during working hours is often required all year around in offices. Frequently passive chilled beams are used for providing additional cooling. Often the chilled beams are installed above a false ceiling and thereby the room is subdivided into two compartments. From the chilled beam a plume is generated. Mock up air (return air) is required to flow into the upper compartment to substitute the airflow generated by the chilled beam. Therefore purpose provided openings are installed in the false ceiling. The openings constitute a resistance to the flow and the locations of the openings affect the flow pattern. The overall performance was studied in a mock up of a real office and it was found that the cooling capacity was affected by the size and location to the openings. To achieve efficient cooling two conditions shall be fulfilled: a) The return opening area shall at least be equal to the horizontal area of the chilled beam b) The return air openings shall be located at the perimeter of the room.

  • 18. Hang, Jian
    et al.
    Li, Yuguo
    Sandberg, Mats
    KTH Research School, Laboratory of Ventilation and Air Quality, University of Gävle.
    Experimental and numerical studies of flows through and within high-rise building arrays and their link to ventilation strategy2011In: Journal of Wind Engineering and Industrial Aerodynamics, ISSN 0167-6105, E-ISSN 1872-8197, Vol. 99, no 10, p. 1036-1055Article in journal (Refereed)
    Abstract [en]

    Urban ventilation implies that wind from rural areas may supply relatively clean air into urban canopies and distribute rural air within them to help air exchange and pollutant dilution. This paper experimentally and numerically studied such flows through high-rise square building arrays as the approaching rural wind is parallel to the main streets. The street aspect ratio (building height/street width, H/W) is from 2 to 5.3 and the building area (or packing) density (lambda(p)) is 0.25 or 0.4. Wind speed is found to decrease quickly through high-rise building arrays. For neighbourhood-scale building arrays (1-2 km at full scale), the velocity may stop decreasing near leeward street entries due to vertical downward mixing induced by the wake. Strong shear layer exists near canopy roof levels producing three-dimensional (3D) vortexes in the secondary streets and considerable air exchanges across the boundaries with their surroundings. Building height variations may destroy or deviate 3D canyon vortexes and induced downward mean flow in front of taller buildings and upward flow behind taller buildings. With a power-law approaching wind profile, taller building arrays capture more rural air and experience a stronger wind within the urban canopy if the total street length is effectively limited. Wider streets (or smaller lambda(p)), and suitable arrangements of building height variations may be good choices to improve the ventilation in high-rise urban areas.

  • 19. Hang, Jian
    et al.
    Li, Yuguo
    Sandberg, Mats
    Claesson, Leif
    Wind conditions and ventilation in high-rise long street models2010In: Building and Environment, ISSN 0360-1323, E-ISSN 1873-684X, Vol. 45, no 6, p. 1353-1365Article in journal (Refereed)
    Abstract [en]

    We regarded high-rise cities as obstacles and channels to wind. We first studied wind conditions and ventilations in idealized high-rise long street models experimentally and numerically with a constant street width (W = 30 mm), variable street heights (H = 2 W, 2.5W, 3W, 4W), variable street lengths (L = 47.4W, 79W. 333W, 667W) and a parallel approaching wind. The flow rates penetrating into windward entries are a little larger than the reference flow rate in the far upstream free flow through the same area with windward entries in all models. The stream-wise velocity decreases along the street as some air leaves upwardly across street roofs. Near the leeward entry, there is a downward flow which brings some air into the street and results in an accelerating process. In the neighborhood scale long streets (L = 47.4W and 79W), wind in taller streets is stronger and the ventilation is better than a lower one. For the city scale long streets (L = 333W and 667W), a constant flow region exists where the vertical velocity is zero and the stream-wise velocity remains constant. In such regions, turbulent fluctuations across the street roof are more important to air exchange than vertical mean flows. In a taller street, the process to establish the constant flow conditions is longer and the normalized balanced horizontal flow rate is smaller than those in a lower street. In the city scale long streets, the turbulence exchange rate can be 5-10 times greater than the mean flow rate.

  • 20. Hang, Jian
    et al.
    Sandberg, Mats
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology.
    Li, Yuguo
    Age of air and air exchange efficiency in idealized city models2009In: Building and Environment, ISSN 0360-1323, E-ISSN 1873-684X, Vol. 44, no 8, p. 1714-1723Article in journal (Refereed)
    Abstract [en]

    Wind can provide relevantly clean external (rural) air into urban street network, i.e. city ventilation. The local mean age of air denotes the time it takes for the external air to reach a location after entering the urban canopy layer. The air exchange efficiency denotes the efficiency of flushing the street network with external air. However, difficulties exist in calculating the local mean age of air in a city due to open boundaries. The traditional experimental homogeneous emission method is adapted here in a CFD method to predict the urban local age of air and analyze the air exchange efficiency for city ventilation. Three simple city models are considered, including a round city model, a square city model and a long rectangular city with one main street parallel to the approaching wind or with two crossing streets. The difference in the city shape results in significant difference in the local mean age of air. In the round city of one narrow street, two inflows through street openings converge close to the city centre and exits through the street roof, so the air close to the city centre is relatively old and the air exchange efficiency is low (30%). For a round city with two crossing streets, a slightly non-parallel wind to the main street generates younger air and the higher air exchange efficiency in the city.

  • 21. Hang, Jian
    et al.
    Sandberg, Mats
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology.
    Li, Yuguo
    Effect of urban morphology on wind condition in idealized city models2009In: Atmospheric Environment, ISSN 1352-2310, E-ISSN 1873-2844, Vol. 43, no 4, p. 869-878Article in journal (Refereed)
    Abstract [en]

    Wind conditions in urban environments are important for a number Of reasons. They can serve to out of the urban environment and to moderate urban microclimatic conditions if transport air Pollutants satisfactory, yet can compromise pedestrian comfort and safety if not. We aim to study experimentally and numerically the effects of urban morphology (e.g., overall city form (skyline), street orientation, and street configuration) on wind conditions in cities. This report considers our initial investigations of two idealized city forms that are coincidentally similar to ancient Roman cities that were organized On One OF two primary streets - a main north-south street, the cardus maximus, and a secondary east-west street, the decumanus maximus - and contained within a well-defined perimeter. We first consider round and square city models with one main street set parallel to the approaching wind and a secondary street producing an intersection at city centre. Not Surprisingly, wind conditions in the two city models are dissimilar due to their shape differences. We then consider a long rectangular city model with a fully developed steady flow region along the main street. If the main street of the round city model is narrow, the parallel approaching wind cannot blow through the entire street and a penetrating inflow exists at the leeward opening. For the Found city model with two crossing streets, a slightly non-parallel wind to the main Street generates a stronger wind level in the entire street volume, Crown.

  • 22. Hang, Jian
    et al.
    Sandberg, Mats
    KTH Research School University of Gävle, Department of Indoor Environment.
    Li, Yuguo
    Claesson, Leif
    (KTH Research School University of Gävle, Department of Indoor Environment.
    Flow mechanisms and flow capacity in idealized long-street city models2010In: Building and Environment, ISSN 0360-1323, E-ISSN 1873-684X, Vol. 45, no 4, p. 1042-1053Article in journal (Refereed)
    Abstract [en]

    It is an open question whether a street network of a city has a certain flow capacity characterizing the flow that can pass through the street network. It s our hypothesis that at least the simple street network has a certain flow capacity. With the purpose of exploring this we studied numerically and experimentally the flow capacity in some idealized long-street models continuously lined with buildings and exposed to a parallel approaching wind. The height of all the models is the same (H = 69 mm). Three groups of models were studied: models with the same uniform street width (W=H) but different lengths (L = 21.7H, 43.5H, 72.5H); models with the same length (L = 43.5H) but different uniform width (W = H, 2H. 4H); and models with a change of width at half distance, L/2. In the last of the three cases, the width of the upstream half was always the same (W1 = H), but there was a wider (W2 = 1.25H, 1.5H, 2H) or narrower (W2 = 0.75H, 0.5H) downstream half. We normalized flow rates by a reference flow rate equal to the flow rate through an opening far upstream with the same area as the windward entry. The normalized flow rate through the windward entry was about 1.0 in all cases. For a sufficiently long-street models, a flow balance is established, creating a fully developed region with a constant horizontal flow (flow capacity) and zero vertical mean velocity. The street length does not affect the flow capacity but as expected the width of the street affects the flow capacity.

  • 23. Hang, Jian
    et al.
    Sandberg, Mats
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology.
    Li, Yuguo
    Claesson, Leif
    Pollutant dispersion in idealized city models with different urban morphologies2009In: Atmospheric Environment, ISSN 1352-2310, E-ISSN 1873-2844, Vol. 43, no 38, p. 6011-6025Article in journal (Refereed)
    Abstract [en]

    The mechanism of pollutant dispersion in idealized city models is investigated numerically by the introduction of a uniformly distributed pollutant source at street pedestrian level. We first study three short city forms with a single main street or two crossing streets, characterized by street length/street height ratios of L/H = 6 or 7 and a street height/street width ratio of H/W = 1, including a sharp-edged round city model, a smooth-edged round city model, and a sharp-edged square city model. For short city models with a single street and a parallel approaching wind, pollutant dilution mainly depends on the horizontal flow rate which decreases along the street. This decreasing rate is smallest for the smooth-edged round city model, which results in the lowest street concentrations. For city models with two crossing streets and the approaching wind parallel to the main street. the differences in overall city form result in different dispersion processes. For a sharp-edged round city model with two crossing streets, an approaching wind slightly non-parallel to the main street generates a lower pollutant concentration in the entire street volume. We also studied a sharp-edged round city model with one narrow street (L/H = 6: H/W = 6.7), finding that the uniformly distributed pollutants are transported from two street entries to the city centre, and are then removed out across the street roof. In contrast to the short city models we studied a single-street sharp-edged long rectangular city model (L/H = 21.7; H/W = 1) in which the horizontal flow rate remained nearly constant in a region far from the two entries. Within this region the turbulence across the street roof contributed more to the pollutant removal than vertical mean flows.

  • 24. Heiselberg, P.
    et al.
    Sandberg, Mats G.
    KTH Research School, University of Gävle.
    Evaluation of discharge coefficients for window openings in wind driven natural ventilation2006In: The International Journal of Ventilation, ISSN 1473-3315, E-ISSN 2044-4044, Vol. 5, no 1, p. 43-52Article in journal (Refereed)
    Abstract [en]

    This paper describes the classical approach for calculation of wind driven airflow through large openings in buildings and discusses the fulfilment of the limiting assumptions. It is demonstrated that the limiting assumptions are not fulfilled for large openings in buildings for cross ventilation, and therefore, the classical approach is not appropriate for prediction of airflow through large openings in buildings in the cross ventilation case. Using the approach for real openings and estimating the discharge coefficient for window openings has also not been very successful. The discharge coefficient cannot be regarded as a constant and it is very difficult to estimate correct values resulting in less accuracy of prediction of natural ventilation.

  • 25. Kobayashi, T.
    et al.
    Sagara, K.
    Yamanaka, T.
    Kotani, H.
    Sandberg, Mats
    KTH Research School University of Gävle, Department of Indoor Environment.
    Stream tube analysis of cross-ventilated simple-shaped model and detached house2007In: IAQVEC 2007 Proceedings - 6th International Conference on Indoor Air Quality, Ventilation and Energy Conservation in Buildings: Sustainable Built Environment, 2007, p. 139-146Conference paper (Refereed)
    Abstract [en]

    Conventional method to predict ventilation rate induced by wind is based on the orifice equation associated with the discharge coefficient and wind pressure coefficient. In the cross-ventilation phenomena, however, this method has a problem due to the difficulty to predict resistance of the building related with total pressure loss. In this paper, therefore, the stream tube caught by the inlet opening is analyzed to investigate the pressure loss due to the transformation (and possibly convergence and divergence) of the stream tube. Two types of model, simple-shaped rectangular model and detached house model, were analyzed with three cases of porosity by using CFD prediction. Flow fields inside and outside of the model are to be compared between two types of model. Besides, based on the stream tube analysis, cross-sectional area and average pressure inside the stream tube will be shown and compared between two types of model. For the detached house model, finally, static pressure inside the stream tube will be compared with that on the floor.

  • 26.
    Kobayashi, Tomohiro
    et al.
    University of Gävle, KTH Research School.
    Sagara, K.
    Yamanaka, T.
    Kotani, H.
    Sandberg, Mats G.
    University of Gävle, KTH Research School.
    Wind driven flow through openings: Analysis of the stream tube2006In: The International Journal of Ventilation, ISSN 1473-3315, E-ISSN 2044-4044, Vol. 4, no 4, p. 323-336Article in journal (Refereed)
    Abstract [en]

    Wind approaching a building provided with openings on the windward and leeward sides has a choice, either it flows through the openings or flows around and above the building. This choice gives rise to a dominant stream tube containing the fluid flowing through the openings. In this paper the stream tube is analysed based on wind tunnel measurements and CFD simulation. A house model with dimensions 120 mm (Width)× 120 mm (Height)× 180 mm (Length) was provided with rectangular openings of equal size located opposite each other. The end walls were thin giving rise to a sharp edged opening. The size of the openings expressed as the porosity (opening area divided by the façade area) was 1.3 %, 5.2 %, 11.6 %, 20.7 % and 46.5 %. In the wind tunnel, velocity including velocity fluctuations and pressure were measured along the centre line through the openings. In the CFD prediction it was possible to visualize the stream tube by the method of "flying particles". This made it possible to explore the change in shape of the stream tube and to calculate the cross-sections of the stream tube at different positions and to know the total pressure distribution within the stream tube cross section. Finally, the discharge coefficient based on stream tube analysis was compared to that from a conventional chamber method.

  • 27. Kobayashi, Tomohiro
    et al.
    Sagara, Kazunobu
    Yamanaka, Toshio
    Kotani, Hisashi
    Sandberg, Mats
    KTH Research School, University of Gävle.
    Power transportation inside stream tube of cross-ventilated simple shaped model and pitched roof house2009In: Building and Environment, ISSN 0360-1323, E-ISSN 1873-684X, Vol. 44, no 7, p. 1440-1451Article in journal (Refereed)
    Abstract [en]

    The ultimate goal of this work is to establish a prediction method based on Power Balance Model for prediction of flow rate through cross-ventilated building. For the establishment of Power Balance Model, the lost power across stream tube sections must be determined in advance. However, the loss of power in the stream tube was not well studied by other researchers but this concerned critical step forms the focus of the present CFD study in which transported power in stream tubes formed at two selected models: (i) a suspended rectangular model, and (ii) a pitched roof single-storey house model standing on a flat ground surface, was documented. For the development of a new method applying to predict the lost power, decrease of the transported power across the stream tubes through both types of models will finally be shown in this paper.

  • 28. Kobayashi, Tomohiro
    et al.
    Sagara, Kazunobu
    Yamanaka, Toshio
    Kotani, Hisashi
    Takeda, Shogo
    Sandberg, Mats
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Energy and Furnace Technology.
    Stream Tube based Analysis of Problems in Prediction of Cross-Ventilation Rate2009In: The International Journal of Ventilation, ISSN 1473-3315, E-ISSN 2044-4044, Vol. 7, no 4, p. 321-334Article in journal (Refereed)
    Abstract [en]

    The airflow rate of a building ventilated by wind is usually predicted by using the wind pressure coefficients obtained for a sealed building and discharge coefficients based on measuring the airflow characteristics through an opening in a sealed chamber (chamber method). This can result in the underestimation of wind driven flow through large openings located on opposite sides of a room. In this paper, the discharge coefficient, based on the chamber method, and the actual condition of cross-ventilation are calculated and compared with each other by means of stream tube analysis. The driving pressure based on wind pressure coefficients obtained from a sealed building are also compared with those based on pressures inside the stream tube of the actual flow field representing a porous rather than sealed building. A building model of dimensions 120 mm (width)x120 mm (height)x180 mm (length) was used for the analyses. The size of openings, expressed as the porosity (opening area divided by facade area), was 11.6 %, 20.7 % and 46.5 %. These models were analyzed by CFD simulation and the stream tubes caught by the opening were determined. From the analysis the errors in discharge coefficient and wind pressure coefficient were identified. Finally, the flow rate based on these discharge coefficients and driving pressures were calculated and compared. It is shown that the effect of the underestimation of the discharge coefficients by the chamber method is significant for all cases of porosity studied in this paper. Moreover, it is shown that the use of wind pressure coefficients is not appropriate for the case of extremely large openings.

  • 29. Li, Biao
    et al.
    Luo, Zhiwen
    Sandberg, Mats
    Högskolan i Gävle, Sweden.
    Liu, Jing
    Revisiting the 'Venturi effect' in passage ventilation between two non-parallel buildings2015In: Building and Environment, ISSN 0360-1323, E-ISSN 1873-684X, Vol. 94, p. 714-722Article in journal (Refereed)
    Abstract [en]

    A recent study conducted by Blocken et al. (Numerical study on the existence of the Venturi effect in passages between perpendicular buildings. Journal of Engineering Mechanics, 2008, 134: 1021-1028) challenged the popular view of the existence of the 'Venturi effect' in building passages as the wind is exposed to an open boundary. The present research extends the work of Blocken et al. (2008a) into a more general setup with the building orientation varying from 0 degrees to 180 degrees using CFD simulations. Our results reveal that the passage flow is mainly determined by the combination of corner streams. It is also shown that converging passages have a higher wind-blocking effect compared to diverging passages, explained by a lower wind speed and higher drag coefficient. Fluxes on the top plane of the passage volume reverse from outflow to inflow in the cases of alpha = 135 degrees, 150 degrees and 165 degrees. A simple mathematical expression to explain the relationship between the flux ratio and the geometric parameters has been developed to aid wind design in an urban neighborhood. In addition, a converging passage with a = 15 is recommended for urban wind design in cold and temperate climates since the passage flow changes smoothly and a relatively lower wind speed is expected compared with that where there are no buildings. While for the high-density urban area in (sub)tropical climates such as Hong Kong where there is a desire for more wind, a diverging passage with alpha = 150 degrees is a better choice to promote ventilation at the pedestrian level.

  • 30. Lim, E.
    et al.
    Ito, K.
    Sandberg, Mats
    KTH.
    New ventilation index for evaluating imperfect mixing conditions - Analysis of Net Escape Velocity based on RANS approach2013In: Building and Environment, ISSN 0360-1323, E-ISSN 1873-684X, Vol. 61, p. 45-56Article in journal (Refereed)
    Abstract [en]

    The concentration of contaminant in a room is not always uniformly distributed and hence it is important to evaluate the ventilation efficiency at various points or domains in the room to optimize and reduce the ventilation rate and the air-conditioning load of the room. Various ventilation indices have been developed to evaluate the ventilation efficiency of a point or a domain based on the contaminant concentration, for example, the age of air, the Scale for Ventilation Efficiency series, Visitation Frequency and Purging Flow Rate.This paper presents a new concept of ventilation index, Net Escape Velocity (hereafter NEV), as an index for ventilation efficiency in an indoor environment. NEV represents the effective velocity at which the contaminant is transported/diluted from a target point. The objectives of the present work are to clarify the definition and concept of NEV on the basis of CFD simulation and to investigate the calculation methods of NEV. NEV is defined by contaminant concentration, convective flux and diffusion flux at a point. Using NEV normalized by the convection velocity at a target point, we can obtain information of the turbulent diffusion effect for removal/dilution contaminant and of the direction of diffusion flux which is the same or not with convective flux. It can be said that NEV is an index of ventilation efficiency that can evaluate the ventilation performance at a point and enable understanding of the forming structure of a contaminant concentration at a point.

  • 31. Lim, E.
    et al.
    Ito, K.
    Sandberg, Mats
    KTH Research School University of Gävle, Sweden.
    Performance evaluation of contaminant removal and air quality control for local ventilation systems using the ventilation index Net Escape Velocity2014In: Building and Environment, ISSN 0360-1323, E-ISSN 1873-684X, Vol. 79, p. 78-89Article in journal (Refereed)
    Abstract [en]

    A concept of ventilation efficiency, Net Escape Velocity (NEV), developed by authors presents the net and integrated velocity of contaminant transport by convection and diffusion effect at a point within the room. The NEV is the effective ventilation rate with a velocity scale determining the contaminant concentration at a target point and can be expressed by vector and scalar quantities. It is the most important characteristic of NEV concept. An expanded concept of NEV (NEV*), under an assumption that the inflow flux of a contaminant on the control volume is a contaminant generation, was proposed. We believe that the NEV and NEV* distributions can provide helpful information for ventilation design to control contaminants.The purpose of this study was to demonstrate the advantage and contribution of NEV* to current ventilation design procedure by using numerical analysis. Here, it was evaluated by the NEV* that the contaminant removal performances of local ventilation systems which are a kitchen exhaust hood in a kitchen environment, a push-pull hood in an industrial environment and an adsorptive building material in a test chamber. The distributions of the NEV* as vector quantities under the different flow and diffusion fields were analyzed to investigate contaminant leakage across the hypothetical boundaries of the control target domain of the local ventilation hood and to investigate the contaminant concentration reduction performance of the adsorptive building materials.

  • 32.
    Lundström, Hans
    et al.
    KTH Research School University of Gävle, Department of Indoor Environment.
    Sandberg, Mats
    KTH Research School University of Gävle, Department of Indoor Environment.
    Mosfegh, Bahram
    Temperature dependence of convective heat transfer from fine wires in air: A comprehensive experimental investigation with application to temperature compensation in hot-wire anemometry2007In: Experimental Thermal and Fluid Science, ISSN 0894-1777, E-ISSN 1879-2286, Vol. 32, no 2, p. 649-657Article in journal (Refereed)
    Abstract [en]

    Heat transfer from a fine wire to air has been experimentally investigated. High accuracy measurements, where both the air temperature and wire temperature have been varied systematically and independently have made it possible to map the behavior of the heat transfer process for different velocities, air temperatures, and wire temperatures. Based on these results a compensation method is proposed which makes it possible to temperature compensate hot wires of large aspect ratio and at low Reynolds numbers for anemometry measurements with velocity calibration only at one air temperature.

  • 33.
    Sandberg, Mats
    KTH Research School University of Gävle, Department of Indoor Environment.
    Whole-field measuring methods in ventilated rooms2007In: HVAC & R RESEARCH, ISSN 1078-9669, E-ISSN 1938-5587, Vol. 13, no 6, p. 951-970Article in journal (Refereed)
    Abstract [en]

    The occupied zone of a ventilated room constitutes a large volume, and the temperature, velocity, and concentration cannot be mapped simultaneously over a whole field with standard point (local) measuring techniques. The situation is improving due to the introduction of whole-field measurement techniques. Whole-field techniques are relatively new and under development; therefore, this paper describes the principles behind the techniques and their pros and cons. All techniques are optical techniques in the sense that pictures are taken by an array of sensors responsive to different wavelengths of light depending on the application. The velocity components can be recorded with particle image velocimetry (PIV) or particle streak velocimetry (PSV). Both methods are based on adding tracer particles, which ideally follow the air motions; a digital camera records their displacement during a specified time interval. Recording the displacement corresponds to the Lagrangian formulation of fluid mechanics. With PIV, the displacement of groups of particles is recorded, in contrast to the PSV method, where the displacement of a single particle is recorded. PIV can provide high-resolution information over small regions, whereas the PSV method can cope with large areas. The two-dimensional concentration distributions of certain contaminants can be obtained by using tomography. The attenuation of light along a bundle of lines (rays) directed in a certain direction (view) and crossing the region with the contaminant is recorded. The measurements are repeated for several other angles. Based on the one-dimensional information in the form of attenuation of light along rays running in different directions, the two-dimensional concentration can be reconstructed by a reconstruction algorithm. The temperature distribution can be recorded and visualized with an infrared camera and a measuring screen that ideally attains the room air temperature. New development of the classical Schlieren technique has made it possible to visualize the air temperature distribution over large fields.

  • 34. Spazzini, P. G.
    et al.
    Todde, Valentino
    KTH Research School, University of Gavle, HiG, Department of Indoor Environment, 80176 Gavle.
    Sandberg, Mats
    KTH Research School University of Gävle, Department of Indoor Environment.
    Low-speed measurements using a hot-wire anemometer2007In: 14th International Flow Measurement Conference 2007, FLOMEKO 2007, 2007, p. 149-158Conference paper (Refereed)
    Abstract [en]

    In the present work, a method for calibrating hot-wire anemometers for low-speed measurements is described. After a brief recall of the working principle of the instrument, the problems connected with the measurement of low-speed flows are described. A method for overcoming these problems is proposed, and the special features of the specific calibration are discussed. Two different mathematical treatments of the calibration data are analyzed and compared. Finally, some test measurements are presented, showing that the results are in line with the expectations.

  • 35.
    Todde, Valentino
    et al.
    Högskolan i Gävle, KTHs-forskarskola för inomhusmiljö.
    Spazzini, Pier Giorgio
    Sandberg, Mats
    Högskolan i Gävle, KTHs-forskarskola för inomhusmiljö.
    Experimental analysis of low-Reynolds number free jets2009In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 47, no 2, p. 279-294Article in journal (Refereed)
    Abstract [en]

    The present paper analyzes the features of a low-Reynolds number free submerged jet with special regard to statistical quantities on the jet centerline. Measurements in an environment with very low disturbances allowed to observe details of turbulence and higher-order moments. Some peculiar features of the measured (natural) jet are shown to be in correspondence to observations referring to forced higher-Reynolds number jets. In particular, it is shown that, at low Reynolds numbers, the initial region of the jet is dominated by well-defined vortices in the shear layer. This result is substantiated by both the statistical moments and the spectral analysis. The presence of two distinct regimes is evidenced and discussed from a physical standpoint, also in relation to the mathematical analysis of the jet structure from the bibliography.

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