Packet structure of surface eddies in the atmospheric boundary layer

摘要

A smoke visualization experiment has been performed in the first 3 m of neutral and unstable atmospheric boundary layers at very large Reynolds number (Re theta > 10~6). Under neutral atmospheric conditions mean wind profiles agree well with those in the canonical flat plate zero-pressure-gradient turbulent boundary layer. The experiment was designed to minimize the temperature difference between the passive marker (smoke) and the air to ensure that any observed structures were due to vortical, rather than buoyant, motions. Images acquired in the streamwisewall-normal plane using a planar laser light-sheet are strikingly similar to those observed in laboratory experiments at low to moderate Reynolds numbers. They reveal large-scale ramp-like structures with downstream inclination of 3 deg-35 deg . This inclination is interpreted as the hairpin packet growth angle following the hairpin vortex packet model of Adrian, Meinhart, and Tomkins. The distribution of this characteristic angle agrees with the results of experiments at far lower Reynolds numbers, suggesting a similarity in structures among low, moderate, and high Reynolds number boundary layers at vastly different scales. These results indicate that the hairpin vortex packet model extends over a large range of scales. The effect of vertical heat transport in an unstable atmosphere on wall structures is investigated in terms of the hairpin vortex packet model.