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 30´20´0.9 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.

The use of supply jet flows is the most common type of air distribution for general ventilation. Usually the supply flow rate is constant or slowly varying (VAV-systems) to cope with a varying load. A novel air distribution method, with the potential to reduce stagnation and to increase the ventilation efficiency, is to introduce rapid flow variations (pulsations). This paper reports on a fundamental study of this type of air distribution. The purpose of the study was to explore the effect of flow variations on stagnant zones and the levels of the turbulent kinetic energy and the relative turbulence intensity. A small scale room model is used that consists of an enclosure with a ventilation supply at the bottom and an extract at the top of the opposite wall. Water was used as an operating fluid and the model had a design which mainly generated a two-dimensional flow. The size of the model made it possible to investigate the two-dimensional velocity vector field using the Particle Image Velocimetry (PIV) method in regions corresponding to occupied regions. Further post processing was conducted from the resulting vector fields. The comparison between cases of constant inflow and pulsated inflow (flow variations with frequency of 0.5 Hz) was conducted for three domains: two belonging to the far-field occupied zone and one belonging to the near-field, downstream of the supply wall jet.

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 has 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.

A common room heating technique is to use a wall-mounted radiator without forced convection. The cold surrounding air passes adjacent to the warm surfaces of the radiator, gets heated, and the buoyancy difference gives this heated air a momentum to rise along the wall surface (as plume) and finally circulate and get mixed into the whole room. The properties of heated plumes are important for assessing the risk of soiling of the wall surfaces through particle deposition driven by thermophoresis and turbophoresis. It is important to identify where there is a transition from laminar to turbulent flow. With the objective to characterize the plume of heated air flow in the vicinity of wall surface, the airflow over the radiator is visualized and measured using the two-dimensional Particle Image Velocimetry (2D PIV) technique. The PIV technique yields two-dimensional vector fields of the flow. The resulted vector maps are size and peak validated and post processed using in house developed software to provide the average streamlines. In the near wall PIV measurements there are practical problems; generating a homogeneous global seeding that makes it possible to study both the plume and the surrounding entrainment region, and optical problems due to strong laser reflection from the wall surface which limits the investigation area. These issues are dealt with in the present study. In addition to visualization with PIV, visualization with a CMOS video camera was also conducted.

A common room-heating technique involves the use of a wall-mounted radiator without forced convection. The cold surrounding air passes adjacent to the warm surfaces of the radiator where it absorbs heat and gains momentum to rise along the wall surface and finally circulate in the entire room. Understanding the properties of heated airflows is important for several purposes. To understand the flow process it is important to identify where the transition from laminar to turbulent flow occurs and to quantify the turbulent fluctuations. With the objective to characterize the airflow in the vicinity of wall surfaces, the local climate over the radiator was visualized and measured using a two-dimensional particle image velocimetry technique. The PIV technique yields 2D vector fields of the flow. The resulting vector maps were properly validated and post-processed using in-house software to provide the average streamlines and other statistical information such as standard deviation, average velocity, and covariance of the entire vector field. The results show that, for a room with a typical heating power, the airflow over the radiator becomes agitated after an ordinate of N = 5 - 6.25 over the radiator upper level, in which N is the dimensionless length based on the thickness of the radiator. Practical problems encountered in near-wall PIV measurements include generating a homogeneous global seeding that makes it possible to study both plume and entraining region, as well as optical problems due to near-surface laser reflection that makes the measurement process more complicated.