Wind tunnel observation of flow and diffusion under stable stratification

Abstract

Neutral and four different levels of stable stratification (weak to strongly stable i.e. Ri δ = 0.048−0.248 ) were created in the stratified wind tunnel at NIES. The effects of stable stratification on the spread of a plume from a small, circular, ground level release were examined by flow visualization, trace gas concentration measurement, and flow field velocity and turbulence measurements. As stable stratification increased, flow field visualization showed that the turbulence was increasingly suppressed, especially near the surface, and the vertical plume spread was also inhibited with increasing stability. The normalized velocity power exponent, m, increased from 0.17 to 1.04 (for neutral to strong stability), and the u and w components of turbulent intensity were suppressed. The vertical concentration power exponent, p (from C ~ exp (−z p) ), varied from 1.3 for the neutral case, to 3.0 for the strong stable case. The observed values of p showed reasonably good agreement with the values of p estimated from the half-empirical diffusion equation. The vertical plume spread, σ z , vs distance downwind decreased with increasing stability, appearing as logical extensions of the Pasquill-Gifford chart. However, the horizontal plume spread, σ y , vs distance at first decreased with increasing stability then began to increase as the stability increased from moderate to strongly stable, approaching the neutral case curve. This behavior is caused by the suppression of vertical motion at the stronger stable stratifications which leads to increased plume meandering and plume collapse.