The relationship between mean and instantaneous structure in turbulent passive scalar plumes

Abstract

Laboratory investigations of a turbulent scalar plume are performed to investigate the relationship between instantaneous scalar structure and the resulting mean scalar statistics. A planar laser-induced fluorescence technique is used to image two-dimensional instantaneous spatial plume structure at various locations and in three orthogonal planes. Long image sequences are used to calculate time-averaged scalar statistics (concentration mean, variance and intermittency), and the relationship between these statistics and the observed instantaneous scalar structure is discussed. We present both snapshots and animations of instantaneous scalar structure at various locations within the boundary layer. As with all boundary layer phenomena, the structural variation is greatest in the vertical direction (normal to the bed). The existence of a persistent, relatively uniform layer of dye within the viscous sublayer is identified. In this layer, instantaneous concentrations are moderate, but the persistence of the dye produces a relatively high mean concentration. Above this layer, stronger fluctuations and higher peak concentrations are present, but lower values of the intermittency produce lower mean concentrations. It is argued that a combination of three time-averaged statistics (mean, variance and intermittency) is required to deduce meaningful information about the nature of the instantaneous scalar structure. This article was chosen from selected Proceedings of the Second International Symposium on Turbulence and Shear Flow Phenomena (KTH-Stockholm, 27-29 June 2001) ed E Lindborg, A Johansson, J Eaton, J Humphrey, N Kasagi, M Leschziner and M Sommerfeld.