CASE STUDIES OF AERODYNAMIC INTERFERENCE AMONG CHIMNEYS

Abstract

The development of modern materials and construction techniques has resulted in the emergence of a new generation of structures that are often, to a degree unknown in the past, remarkably flexible, low in damping and light in weight. Such structures exhibit an increased susceptibility to the action of wind and earthquakes which are random in their character. Accordingly, it has become necessary to develop tools enabling the designer to estimate the effects of these forces with high degree of accuracy than was previously required. In the case of chimneys wind exerts a force not only along the direction of flow but also in the acrosswind direction. Analytical methods exist to predict the structural behaviour of an isolated structure. However, interference often occurs due to nearby structures of comparable dimensions, under which the behaviour becomes very complex and no analytical solution exists. As of now, the only way to evaluate the effect of interference is to resort to wind tunnel studies. In the present work wind tunnel studies were conducted to study the interference between chimneys. For this, two structures, one 83m high and other 272m high were chosen. The study has been carried out on the dynamic aeroelastic models of these prototypes in a boundary layer shear flow of (U/U.)=---(Z/Z0)0135 with a turbulent intensity of about 15% at the base. The results are calculated in terms of buffeting factors which is defined as BF — Maximum Response in Interference configuration Maximum Response in standalone configuration The results show that an increase of the order of 2 to 2.5 times of the isolated structure response can occur in the most adverse condition. Also, experiments show that for the alongwind direction the most adverse region is upto 3D distance distance in the alongwind and transverse directions where 'D' is the diameter of chimney at two-third's height. In the acrosswind direction this region varies from 2.5D to 7.5D centre to centre between the chimneys. In totality it can be said that the interference effect is substantially reduced in the interfering structures when they are placed beyond 7.5D, the distance being measured centre to centre of the structures.