Passive scalar dispersion in slightly stable boundary layers

Non-neutral stratified flow have a huge role in mixing and dispersing pollutant plumes in atmospheric flows. Despite this, laboratory experiments investigating the effects of thermal stratification on scalar dispersion have been so far extremely rare. To bridge this gap, wind tunnel experiments have been performed in stable boundary layers developing over a rough wall. An L-shaped source was placed in a fully-developed turbulent boundary layer with varying Richardson number. Simultaneous measurements of concentration, temperature and velocity were performed. A full characterisation of the flow field has been performed as a preliminary task, analyzing single point velocity statistics, focusing on second turbulent kinetic energy production and dissipation phenomena. Expected characteristics of turbulence suppression by means of stratification have been observed. Moreover, length scales and time scales with have been computed using an Eulerian approach. A good agreement has been observed with the boundary layers simulated for this work and other boundary layers in previous studies. Subsequently, we analysed concentration field as induced by releases emitted by two source sizes of different size and placed in flows with different stratification strengths (as well as the reference neutral case). The statistical moments of the concentration measurements were analysed up to the fourth order. A comparison between probability density function of concentration and the gamma distribution has shown a good agreement.Moreover, turbulent fluxes and turbulent diffusion coefficients have been computed by means of standard gradient diffusion hypothesis. Based on these data we could evaluate the dependence of the turbulent Schmidt number and of characteristic mixing time scales on different stability conditions.