Influence of a roughness length error on vertical wind speed extrapolation for 2D ideal hills using an OpenFOAM^® RANS simulation

Technion – Israel Institute of Technology, Faculty of Aerospace Engineering, 32000 Haifa, Israel

^a e-mail: rosen@aerodyne.technion.ac.il
^b e-mail: rshenkar@tx.technion.ac.il

Abstract

In modelling wind flow over a flat and complex terrain, the choice of roughness-length-distribution is critical for accurate wind speed predications. This choice is often made based on the Davenport scale [1]. A satellite or aerial image of the location is used [2–4] and subjective or objective measures are used for translating the image into roughness length - z₀ - based on the Davenport scale. The choice of a Davenport table value is generally regarded as within an error of plus or minus one table value, leading to a ±6% error in wind speed predictions for a flat terrain in neutral conditions [5]. In the paper this error is studied for a non-flat terrain, using a series of CFD simulations for 2D hills of various combinations of aspect ratio and steepness. Results show that the wind speed prediction error as a result of a “wrong” parameterization decreases with hill steepness until separation occurs. As a result of separation the error increases slightly, and then decreases again with a further increase in hill steepness.