Simplified Analytical Model for Across-Wind Response of Chimneys Around Critical Wind Velocity Regions

Abstract

There is an exponential growth in infrastructural facilities in countries like India. Power plant structures constitute a major portion of these structures. Over the last decade, a number of problems of tall chimneys of height ranging from 225 m to 280 m have been studied by CSIR-Structural Engineering Research Centre, a National laboratory under the Council of Scientific and Industrial Research, Government of India. Wind tunnel studies on aero-elastic models of chimneys under simulated atmospheric boundary layer conditions with and without rigid models of interfering power plant structures have been carried out. A number of chimneys were also proof checked for their design carried out by a number of industrial houses. The discussions with the designers have clearly revealed that they are most uncomfortable when they use code recommendations which are extremely complex to comprehend. Researchers and academicians would like to have the designer recommendations in line with the state of the art knowledge on the topic. One such area where possibly simplification to code recommendations would be extremely helpful is the evaluation of across wind response tall chimneys having circular cross section. A fixed circular cylinder in the path of a uniform oncoming flow forces it to separate and form two shear layers with building up of vortices. The vortices increase in strength, then gets separated forming a periodic vortex street down stream of the cylinder. The problem becomes more complex when there is turbulence in the incident flow. The problem is more complicated when the cylinder has flexibility which introduces fluid-structure interaction. When the oscillations of the cylinder become significant, the dynamics of the flow and the structure become dependant on the motion of the body itself. It is no wonder then that the resulting motion is extremely nonlinear. Wind tunnel testing on properly scaled models in simulated boundary layer flows is expected to provide the best estimates of dynamic response. However the inability to achieve equality of Reynolds number in the wind tunnel tests with those existing in full scale conditions had prompted Vickery (1984) to “doubt whether or not this is the best techniques for structures with circular cross sections”. He however suggested that in the case of more complex aerodynamic situations such as those associated with grouped chimneys, wind tunnel studies are better employed to explore the general behaviour for gaining better understanding. However Tamura (1990) observed that wind tunnel tests could be relied upon to predict the behaviour of the reinforced concrete chimneys”. He validated his observation by comparison of full scale tip acceleration of a RC chimney with those predicted from wind tunnel tests. Holmes (2001) compared the estimated maximum rms tip deflections of three structures with circular cross section using three different codal approaches and concluded clearly, the significant variations exist in estimated response. It is not as if the variations were either uniform overestimation or underestimation. Devdas Menon and Srinivasa Rao(1997) studied the uncertainties involved in codal recommendations for across wind load analysis of reinforced concrete The Eighth Asia-Pacific Conference on Wind Engineering, December 10–14, 2013, Chennai, India