Turbulence structure and similarity in the separated flow above a low building in the atmospheric boundary layer

Separated and reattaching flows over sharp-leading-edge bluff bodies are important to investigate in order to improve our understanding of practical flows such as the case of low-rise buildings in the atmospheric boundary layer. In this study, Particle Image Velocimetry measurements of the separated-reattaching flows over the roof surface of a low-rise building model were taken for six different turbulent boundary layer conditions. The results were analyzed to understand how the incident turbulence affects the flow field of the separation bubbles above the low-rise building roof. The mean flow field above the roof-surface was found to be approximately similar across the six terrain conditions using the mean reattachment length in the streamwise direction and the maximum mean thickness of the separated shear layer in the vertical direction. However, the turbulence stresses are not similar which is attributed to high levels of initial turbulence kinetic energy in the separated shear layer. This leads to fundamental differences in the initial development of the separated flow when compared to flows with lower turbulence in the incident stream. The results indicate that the Kelvin-Helmholtz instability may be altered, or perhaps even suppressed, in the initial flow development region. This leads to substantially different turbulence statistics and characteristics within the separated shear layers

Effects of turbulence on the mean pressure field in the separated-reattaching flow above a low-rise building

[[abstract]]The effects of upstream turbulence in the atmospheric boundary layer flow on the mean surface pressure distribution within the separated flow above a typical low-rise building roof are investigated experimentally. Time-averaged Navier-Stokes equations are used to evaluate the pressure gradients from planar particle image velocimetry data. The pressure fields are reconstructed by integrating the pressure gradients using an analytic interpolation approach. This reconstruction approach is validated by successfully matching the reconstructed pressure to Bernoulli's equation along a streamline far from the body and with pressure measurements on the surface of the body. Through this process, the mean pressure field can be directly explained from the mean velocity and turbulence fields near the roof. For high turbulence intensity levels, the maximum suction coefficient on the roof surface was found to be increased. Such increased magnitudes are directly related to the reduced size of mean separation bubble in higher turbulence, more rapid variation of the velocity magnitude near the leading edge, and enhanced variation of the turbulence stresses. On the other hand, a higher rate of surface pressure recovery is found in the leeward portion of the separation bubble, which is mainly due to the more rapid variation of the turbulence stresses.[[notice]]補正完