Turbulent Wakes in the Atmospheric Boundary Layer

Summary

The wakes behind moving bodies and the associated ‘fluid resistance’ were studied intensively by fluid dynamicists from about 1920 onwards. Many of their studies were stimulated by the rapidly evolving field of aeronautics. Detailed studies of flow past cylinders and other bodies led to some understanding of flow separation and the characteristics of the ‘near wake’, while boundary-layer approximations, conservation laws and linearisation of the perturbation momentum equations led to ‘far wake’ solutions for laminar and, with appropriate closure hypotheses, for turbulent wakes.

There are additional complications when we study the flow around and behind surface-mounted obstacles, including hills, imbedded within a pre-existing boundary layer, and much of the theoretical work on these flows is relatively recent. Wind engineers have a special interest in the detailed flow around buildings, especially at ground level where high winds can cause considerable discomfort or even danger to pedestrians. Wind-tunnel studies have been the usual technique for investigating these problems. Wind engineers and agricultural meteorologists have both been interested in windbreaks or shelterbelts and much of what we know about wakes in the atmospheric boundary layer stems from their studies. In the last ten years there has been renewed interest in wind energy and several studies of the wakes behind wind turbines.

This review will attempt to summarise results on wakes and drag for surface-mounted obstacles and hills., As for free wakes, theoretical results are somewhat easier to obtain, and rather more general, for the far wake region, well downstream of the obstacle itself. Momentum budget considerations lead to some interesting conjectures. Close to the obstacle, the detailed shape becomes important and quantitative results become rather ‘obstacle-specific’, and generally complex.