PASSIVE SCALAR DISPERSION IN A TURBULENT MIXING LAYER

Abstract

Experimental and numerical studies of spatially developed turbulent mixing layers with passive scalar concentrations was performed. In the experiment, a mixing layer was created by an S-shaped splitter plate in a wind tunnel, with a velocity ratio of 2:1. A concentration field was realized by injecting incense smoke into the high-speed side of the mixing layer. Simultaneous measurements of the velocity, vorticity and concentration fields were performed.

A 12-sensor hot-wire probe was used to measure the velocity field and its gradients, while the concentration field was recorded by taking digital pictures of the laser-illuminated smoke. The statistics of the velocity and vorticity fields agree well with previous research. By synchronizing the velocity and concentration measurements, concentration fluxes were determined. Octant analysis was performed on the flux data to explore the scalar transport processes. Conditional planar average of flow properties was also performed to determine their spatial distribution with respect to the large-scale vortices.

A large-eddy simulation, designed to match the experimental conditions, was performed to provide three-dimensional pictures of the mixing layer. A new approach to effectively specify the inflow boundary condition was proposed. Passive particles were released and tracked to simulate the scalar concentration field. Numerical interpolation schemes were examined for performing the particle tracking tasks. The simulation statistically supported the experimental result while providing insight about the flow topology, from which scalar transport models by the rib vortices and roller vortices were proposed and examined.