Wind-Tunnel Simulation of Weakly and Moderately Stable Atmospheric Boundary Layers

Abstract

The simulation of horizontally homogeneous boundary layers that have characteristics of weakly and moderately stable atmospheric flow is investigated, where the wellestablished wind engineering practice of using 'flow generators' to provide a deep boundary layer is employed. Primary attention is given to the flow above the surface layer, in the absence of an overlying inversion, as assessed from first-and second-order moments of velocity and temperature. A uniform inlet temperature profile ahead of a deep layer, allowing initially neutral flow, results in the upper part of the boundary layer remaining neutral. A non-uniform inlet temperature profile is required but needs careful specification if odd characteristics are to be avoided, attributed to long-lasting effects inherent of stability, and to a reduced level of turbulent mixing. The first part of the wind-tunnel floor must not be cooled if turbulence quantities are to vary smoothly with height. Closely horizontally homogeneous flow is demonstrated, where profiles are comparable or closely comparable with atmospheric data in terms of local similarity and functions of normalized height. The ratio of boundary-layer height to surface Obukhov length, and the surface heat flux, are functions of the bulk Richardson number, independent of horizontal homogeneity. Surface heat flux rises to a maximum and then decreases.

Wind-Tunnel Simulation of Stable Atmospheric Boundary Layers with an Overlying Inversion

“…It is also generally advantageous in terms of instrumentation and the airflow model in question. This paper is an extension of Hancock and Hayden (2018).…”

“…A stable atmospheric boundary is a naturally occurring phenomenon, for example, arising after the cessation of daytime convective motions. In a previous paper, Hancock and Hayden (2018) investigated a technique for generating an artificially deepened boundary layer in a wind tunnel that had characteristics typical of weakly and moderately stable, horizontally-homogeneous atmospheric boundary layers, drawing on wind-engineering techniques employed extensively for studies in neutral flow. These well-established techniques use a system of tall flow generators and surface roughness in order to create profiles of mean velocity and turbulence quantities that have characteristics of the atmospheric boundary layer, where the term 'tall' is used to mean a height comparable with the boundary layer that is generated downstream (and in contrast to a transition trip that is much smaller).…”

“…Here, we use the term inversion to mean a temperature profile that is increasing with height above the top of the boundary layer, and created or 'imposed' in practice by means of a system of heaters in the upstream flow. Hancock and Hayden (2018) deliberately did not impose an inversion. However, they found that the temperature profile at the windtunnel working-section inlet must be non-uniform, i.e.…”

“…The influence of stability means that conditions set up in the initial part of the flow tend to be longer-lasting as stability increases. Further details, and a broader background to relevant wind-tunnel experiments, are given in Hancock and Hayden (2018); it is not necessary to repeat them here except to refer to Ohya and Uchida (2003) and Hancock and Pascheke (2014). Ohya and Uchida (2003) imposed a non-uniform inlet temperature profile in their experiment.…”

“…The overall objective is the extension of the simulation technique presented in Hancock and Hayden (2018) to the case where there is an overlying inversion, and such that the Reynolds stresses also decrease smoothly to a low turbulence level above the top of the boundary layer. As in that study, validation is based primarily on the local scaling frameworks of Nieuwstadt (1984) and Sorbjan (2010).…”