Contaminant dispersion from an elevated time-dependent source

摘要

The release of waste materials from chemical plants or industries into a river or the atmosphere over a period of time is a common phenomenon. So the dispersion of contaminants for a time-dependent release is an important problem in controlling pollution. The present paper examines the streamwise dispersion of passive contaminant released from a time periodic source in a fully developed turbulent flow. A finite difference implicit method has been used to solve the unsteady convective diffusion equation employing a combined scheme of central and 4-point upwind differences. It is shown how the mixing of concentration of solute is influenced by the logarithmic velocity, the eddy diffusivities and the frequency of oscillatory release. The behaviour of iso-concentration lines in the vertical plane along the centreline due to the combined effect of velocity and turbulent diffusion has been examined for low and high frequencies of injection. The problem has been studied for time periodic continuous line and point sources. The concentration profiles for the steady elevated sources agree well with the existing experimental data. It has been observed that for low frequency, the iso-concentration lines become elongated without any oscillation in concentration in the longitudinal direction, whereas for higher frequency, the contours show a decaying oscillatory nature in concentration distribution along the downstream direction. The essential differences between dispersion from injection heights near the boundary and those away from the boundary are explained in terms of the relative importance of convection and eddy diffusion.