Dynamic modelling of a wind catcher/tower turret for natural ventilation

Abstract

This paper discusses experimental and theoretical investigations and Computational Fluid Dynamics (CFD) modelling considerations to evaluate the performance of a square section wind catcher system connected to the top of a test room for the purpose of natural ventilation. The magnitude and distribution of pressure coefficients (C_p) around a wind catcher and the air flow into the test room were analysed. The modelling results indicated that air was supplied into the test room through the wind catcher's quadrants with positive external pressure coefficients and extracted out of the test room through quadrants with negative pressure coefficients. The air flow achieved through the wind catcher depends on the speed and direction of the wind. The results obtained using the explicit and AIDA implicit calculation procedures and CFX code correlate relatively well with the experimental results at lower wind speeds and with wind incidents at an angle of 08. Variation in the C_p and air flow results were observed particularly with a wind direction of 458. The explicit and implicit calculation procedures were found to be quick and easy to use in obtaining results whereas the wind tunnel tests were more expensive in terms of effort, cost and time. CFD codes are developing rapidly and are widely available especially with the decreasing prices of computer hardware. However, results obtained using CFD codes must be considered with care, particularly in the absence of empirical data. Practical application: There exist various modelling techniques for the investigation of the performance of natural systems such as wind catchers. These modelling techniques include simple calculation procedures, wind tunnel testing, salt bath, Computational Fluid Dynamics (CFD) and real building performance (POE studies). The calculation procedural models are simple to use, however, due to their simplicity they do not provide a full picture of the performance of the natural ventilation system and air movement inside rooms. Other models such as wind tunnels and CFD are more comprehensive but expensive and time consuming to use. Various commercial CFD models are available in the market today and not many of them are specifically designed for modelling of natural ventilation. Results obtained using CFD models should be considered with care specially in the absence of empirical data and if the results were obtained by novice users. Wind catchers are innovative techniques for the application of natural ventilation in buildings in temperate climates such as that of the UK. Their performance greatly depends on wind conditions. However, they should be designed as an integral part of the overall design of the HVAC system in a hybrid or mixed mode operation. The natural ventilation system of wind catchers should be exploited whenever possible, particularly in the hot summer months to reduce the energy and environmental cost of full operation of an air-conditioning system.