Zbigniew Popiolek

Integrating CFD and building simulation

Abstract To provide practitioners with the means to tackle problems related to poor indoor environments, building simulation and computational fluid dynamics can usefully be integrated within a single computational framework. This paper describes the outcomes from a research project sponsored by the European Commission, which furthered the CFD modelling aspects of the ESP-r system. The paper summarises the form of the CFD model, describes the method used to integrate the thermal and flow domains and reports the outcome from an empirical validation exercise.

Accuracy Limitations for Low-Velocity Measurements and Draft Assessment in Rooms

The measurement of air temperature, mean air speed, and turbulence intensity is required in order to assess air distribution and draft discomfort in ventilated rooms. The measurements are also used for validation of computational fluid dynamics (CFD) predictions. The uncertainty of the measurements must be known in order to perform reliable assessment and validation. At present, a low-velocity thermal anemometer (LVTA) with an omnidirectional (spherical) sensor is most often used in practice for measuring air speed due to its low price and easy and convenient operation. The accuracy of the speed measurement and of the draft risk determination can be affected by numerous factors, for example, directional sensitivity of the velocity sensor, natural convection flow generated by the heated velocity sensor, dynamic response of the anemometer, calibration of the anemometer, velocity and temperature gradients in the flow of measurement, etc. The impact of these factors can be minimized substantially by improvement of the anemometer design, proper use of the instrument during measurement, and correction of the measured data. However, the extent to which the measuring accuracy can be improved is limited. In this paper, the combined impact of error sources on the accuracy of mean speed, standard deviation of speed, and turbulence intensity that may occur during measurements with LVTAs is analyzed. The minimum uncertainty that is realistically achievable in practice is identified. The requirements for low-velocity anemometers prescribed in the present standards are critically reviewed and revised. New requirements that will decrease the uncertainty of low-velocity measurements are suggested for inclusion in future ventilation standards. The uncertainty in determination of draft discomfort is defined. Thus, the definition of realistic requirements in thermal comfort standards as well as validation of CFD predictions is made possible.

Exposure of health care workers and occupants to coughed airborne pathogens in a double-bed hospital patient room with overhead mixing ventilation

The exposure of a doctor and a second patient was studied in a simulated two-bed hospital isolation room. The room was ventilated at three air change rates (3 h−1, 6 h−1, and 12 h−1) by mixing air distribution keeping at 22°C (71.6°F). The effect of the distance between the doctor and the coughing person, the posture of the coughing patient (lying sideways facing the doctor or on back), and the position of the doctor (facing the coughing patient or standing sideways) was examined with respect to exposure to coughed air. A thermal manikin with realistic body shape and surface temperature distribution was used to resemble the doctor. A coughing patient (equipped with cough generator) lying in one bed and another patient in the second bed were simulated by two heated dummies with simplified geometry. The cough consisted of 100% CO2. The peak cough time was 4 s, when the doctor was closest to the sick patients bed, and more than doubled for the exposed patient. The level of exposure (peak concentration level) depended strongly on the positioning and distance of the doctor from the infected patient and posture of the coughing patient. Peak concentration level varied widely from 194 to 10,228 ppm. Ventilation rates of 12 h−1 (recommended by present hospital standards) resulted in background velocities exceeding 0.5 m/s (98.43 fpm), suggesting elevated risk from draught discomfort.

Impact of geometry of a sedentary occupant simulator on the generated thermal plume: Experimental investigation

The characteristics of the thermal plume generated by a sitting person were experimentally studied using four human body simulators with different complexities of geometry but equal surface area and heat generation: a vertical cylinder, a rectangular box, a dummy, and a thermal manikin. The experiments were performed in a climate chamber with an air temperature and mean radiant temperature of 23°C (73.4°F) and an upward airflow with a velocity of less than 0.05 m/s (0.164 ft/s). Distributions of air temperature excess and air speed were measured in the thermal plume 0.70 m (2.30 ft) above each of the simulators. The results show that the thermal plume generated by the dummy comprising head, torso, and legs has a similar shape of the cross-section and integral characteristics to the manikin plume; therefore, the dummy can successfully be used as a simulator of a sitting person. Simple shaped models of a sitting human without a clear indication of legs, such as a cylinder and a rectangular box, are not recommended for use. The plume of the cylinder is symmetrical and concentrated, while the plume above the rectangular box is sensitive to the surroundings and can have two maximums of air speed distribution.

Assessment of uncertainty in measurements with low velocity thermal anemometers

Abstract The important error sources associated with measurements using low velocity thermal anemometers incorporating an omnidirectional velocity sensor (LVTA) are identified and quantified. The impact of natural convection, directional sensitivity and dynamic response of the anemometer are modelled. The developed models, together with a database of instantaneous velocity records obtained by means of a Laser Doppler Anemometer are used to estimate the uncertainty of mean velocity and standard deviation of velocity measurements by LVTA. The uncertainty is due to the separate and the combined impact of natural convection and directional sensitivity of omnidirectional velocity sensors as well as the dynamic response of the anemometers. The total uncertainty due to all error sources is estimated. Comparison of measurements performed with a LVTA and a 3-D Laser Doppler Anemometer served to test the method developed and showed good agreement. The results of this study make it possible to improve the quality of validation of CFD predictions of room air movement based on measurement results. The results of this study allow for the determination of realistic requirements in future standards regarding the characteristics of LVTA and indices for the prediction of human response based on indoor velocity measurements. The method developed can be used by manufacturers of LVTA devices to optimize the LVTA design.