Dynamics of a buoyant plume in linearly stratified environment using simultaneous PIV PLIF
Dynamics of a buoyant plume in linearly stratified environment using simultaneous PIV PLIF measurement Harish N. Mirajkar 1 and Sridhar Balasubramanian 1,2 Department of Mechanical Engineering, IDP in Climate Studies, Indian Institute of Technology Bombay, India harishnmirajkar@gmail.com Abstract The presence of stratified layer in atmosphere and ocean leads to buoyant vertical motions, commonly referred to as plumes. It is important to study the mixing dynamics of a plume at a local scale in order to model their evolution and growth. Such a characterization requires measuring the velocity and density of the mixing fluids simultaneously. Here, we present the results of a buoyant plume propagating in a linearly stratified medium with a density difference of 0.5 thus yielding a buoyancy frequency of N =0.15 s 1 To understand the plume behaviour, statistics such as centerline and axial velocity along varying downstream locations, turbulent kinetic energy, Reynolds stress, and buoyancy flux were measured. The centerline velocity was found to decrease with increase in height. The Reynolds stress and buoyancy flux profiles showed the presence of a unstable layer and the mixing associated within that layer. Introduction Buoyant jets and plumes are commonly seen in both environmental and industrial ap plications. They occur whenever a source of buoyancy creates a rising motion of fluid upwards and away from the source. A buoyant jet is an initial momentum dominated flow that has a non zero density difference between medium and the surrounding ambient fluid, and thus at some distance becomes buoyancy dominated. Some common occur rences of buoyant jet and plume are in hydrothermal vents/oil spills in ocean, rising of ash plumes from the volcanic eruptions, ocean overflows and wastewater discharge into ocean. A variety of factors may influence the behavior of buoyant jets and plumes in field applications, including cross flows of winds, stratification of ambient medium, and boundary interactions. While numerous studies on characterization of jets interacting in homogeneous environment exists, only a few focus on plume in a linearly stratified environment. A buoyant plume that interacts with a stably stratified environment will behave differently compared to a uniform environment. In stratified environments, mix ing near the source of the plume will cause it to become denser, where the density of the surrounding fluid linearly decreases upwards. Eventually, the buoyancy force acting on the jet will change signs and become negative. The momentum will eventually diminish, where the jet succumbs to a finite height and spreads out horizontally, causing the plume to become two dimensional. Pioneer research on this topic were first done by Morton et al. (1956) to measure the maximum height, Z m for the buoyant plume in the stratified flow using the following flow parameters: the local buoyancy flux (B 0 the local momentum flux (M 0 and the buoyancy frequency (N defined as N (g/r b (r/z) 1/2 where r b is the bottom density of the stratified tank and r/z is the vertical density gradient. Following the seminal work of Morton et al, other researcher have also measured the plume dynamics in the stratified environment [see for e.g.Bloomfield and Kerr (1998) Webster et al. (2001) VIII th Int. Symp. on Stratified Flows, San Diego, USA, Aug. 29 Sept. 1, 2016