Global and local spreading rate estimation in forced plumes

Abstract

A new methodology is proposed to study turbulent axisymmetric plume behaviour based on the analysis of dye forced plume images. The methodology is based on the plume geometrical information deduced from the frames and allows us to obtain measurements in all the axial and radial extensions of the plume (vertical and radial profiles) and its use in a sequence of consecutive frames allows the analysis of the time evolution of the vertical and radial measurements.The time evolution of the vertical and radial measurements allows us to study the spreading rate coefficient in detail. We define the concept of the local spreading rate (βlocal) and its maximum value frequently used in the studies reported in the literature and named here the classical spreading rate (βclassic).As a case study, two experiments were carried out, with two values of Atwood number (0.001 and 0.01) and a fixed source distance Ho(2 cm). The results confirm the variability of the spreading rate with distance from the source (axial evolution) and show a complex time evolution depending on the z-coordinate. In addition, an asymptotic trend was observed but the asymptotic behaviour was not a constant value also depending on the z-coordinate.This methodological approach has the advantage of being noninvasive and easily exportable and requires only the recording of fluid flows (for example, volcanic plumes). Generally, it is not possible to directly control and/or measure a real fluid flow (a geophysical or pollution flow) and the only thing that can be done is its recording and analysis it, which our method allows.