A wind tunnel evaluation of methods for estimating surface roughness length at industrial facilities

Abstract

This paper discusses three objective methods for estimating surface roughness length based on the physical dimensions of structures or obstructions at a refinery (or other industrial sites of interest). The three methods are referred to as the Lettau method, simplified Counihan method, and Counihan method. These three methods were evaluated using five wind tunnel databases. The databases consisted of scale models of three refineries and two uniform roughness configurations. Velocity profiles were measured in the wind tunnel over these refinery models and roughness configurations, and were subsequently analyzed to estimate the surface roughness, z 0. Seven different methods were used to estimate surface roughness from the velocity profiles and a wide range of z 0 estimates was obtained from these methods. Only two of the methods were deemed adequate for estimating surface roughness length for situations with large roughness elements and where a change of roughness has occurred. These two methods were selected to represent ‘true’ estimates of the surface roughness length for the modeled refineries and roughness configurations. A statistical evaluation of the predicted (Lettau, simplified Counihan and Counihan) and observed surface roughness lengths was then carried out using a statistical analysis program developed by the American Petroleum Institute (API). The results of the evaluation showed that the Lettau method provides a good estimate (within a factor of 0.5–1.5 at the 95% confidence interval) of surface roughness length and one that is better than the other methods tested.