Numerical approach to determination of equivalent aerodynamic roughness of Industrial chimneys

Abstract

In building practices, smokestacks are commonly sheathed with smooth metal plates. The wind load acting on such sheathed smokestacks can be calculated according to applicable standards. However, requirements for sheathing smokestacks with corrugated metal have been recently increasing due to dilatation of the metal plate. For these cases, the calculation standards do not define parameters for determining the sheathing roughness that affects the load size. The calculation standards take into account only air flow around a cylinder with a coarse (rough) surface. In the case of a greater unevenness of sheathing of the flown around body, the standard takes into account only the roughness height, in this case, the metal corrugation height regardless of its type and shape. This fact may result in a large increase in frontal wind resistance.

One of the options to determine the load acting on the flown around smokestack sheathed with metal other than conventional smooth metal is a numerical solution. Creation of a corrugated sheathing mesh is demanding on the number of cells in the calculation area, and therefore, this approach is unrealistic for solution on desktop PCs at the present. The purpose of this numerical study is to determine an appropriate substitute, equivalent aerodynamic roughness, and thus the possibility of modelling in a simplified calculation area using the "wall function". A properly determined equivalent aerodynamic roughness contributes to the correct definition of the drag coefficient value that defines the size of the wind load acting on the flown around object (in Fluent drag coefficient).

The presented thesis simulates airflow around a realistic smokestack of a circular section for two different types of sheathing - corrugated and trapezoidal plates. The task is dealt with using the finite volume method using CFD codes in Ansys Fluent software. The numerical solution results are evaluated and compared with standard regulations.