A Wind-Tunnel Artificially-Thickened Simulated Weakly Unstable Atmospheric Boundary Layer

Abstract

A wind-tunnel simulation of an atmospheric boundary layer, artificially thickened as is often used in neutral flow wind-loading studies, has been investigated for weakly unstable stratification, including the effect of an overlying inversion. Rather than using a uniform inlet temperature profile, the inlet profile was adjusted iteratively by using measured downstream profiles. It was found that three cycles are sufficient for there to be no significant further change in profiles of temperature and other quantities. Development to nearly horizontally-homogeneous flow took a longer distance than in the neutral case because the simulated layer was deeper and therefore the length scales larger. Comparisons show first-order and second-order moments quantities are substantially larger than given by 'standard forms' in the mixed layer but are close in the surface layer. Modified functions, obtained by matching one to the other, are suggested that amount to an interpolation in the mixed layer between the strongly unstable and the weakly unstable cases.

A Wind-Tunnel Simulation of the Wake of a Large Wind Turbine in a Weakly Unstable Boundary Layer

“…Note, "! is the difference in temperature between that at height h and that at the surface, and U 0 is the velocity at height h. Zhang et al (2013) observed a more rapid reduction in the maximum velocity deficit and higher wake turbulence, although the differences were not as large as in the present investigation, owing to the difference in Richardson number. They only give measurements in a vertical plane (aligned with the turbine hub axis), so there is no information about the width of the wake, for example.…”

“…of Zhang et al (2013) was at Ri b = -0.13, whereas in the present study it was -0.34, where Ri b = g h !" /("U 0 2 ).…”

“…Details are mostly as given in Hancock et al (2013) and, for the model turbine, in Hancock and Pascheke (2014b), but are repeated here for completeness. In terms of simulating a wind flow in a boundary-layer tunnel, two obvious similarity parameters are the ratio of the turbine rotor diameter to the Obukhov length, D/L 0 , and the ratio h/L 0 .…”

“…The baseline boundary layers are a neutral boundary layer and a weakly convective boundary layer, details of which are given in Hancock et al (2013). In summary, the neutral boundary layer was based on ESDU (2001,2002) guidelines, adjusted to a reference speed of 10 m s -1 at a height of 10 m, for a typical offshore sea-surface roughness, and a full-to-model scale ratio of 300:1.…”

Modelling the wind energy resource in complex terrain and atmospheres. Numerical simulation and wind tunnel investigation of non-neutral forest canopy flow

“…Additional experiments were then conducted in which a surface and free stream stratification were introduced to develop a stable boundary layer. Whilst it is now possible to conduct unstable experiments in the EnFlo facility (Hancock et al, 2013), this option was not available at the time of experimentation.…”