Applications

PDPA Applications

1. Dispersion and Energy Transfer in a Two-Phase Shear Layer

One way to try and increase our understanding of turbulent two-phase flows is to study the interaction within fundamental, prototypical turbulent flowfields. The focus of this particular work was to try and understand the dispersion and turbulent interaction of a polydispersed distribution of water droplets with a turbulent shear layer. A shear layer is a common flow that occurs whenever two fluids streams of different velocity are brought together and allowed to mix. These flows are found in the near region of jets and separated flows behind blunt bodies, and are thus important to many engineering applications (such as spray combustion, atomization, materials processing, and chemical reactors).

Experimental measurement of conditional-averaged laser attenuation. The flow appears to move from right to left, with the high-speed, particle-laden fluid on bottom, and the initially stagnant, particle-free ambient air on top. The red color represents the greatest attenuation levels, traversing through the spectrum to the blue and black which represent an absence of particles

Experimentally, the flow is interesting becuase it exhibits a strong, coherent two-dimensional vortex structure which plays a dominant role in the evolution of the flow field. The above image sequence depicts the evolution of bulk spray concentration within a turbulent shear layer. The images were constructed using conditionally averaged laser attenuation measurements referenced to the subharmonic of the forced instability. Initially, two vortex cores can be seen forming at the spray boundary, which corresponds with the fundamental mode of the Kelvin-Hemholtz instability. During the succeeding images, the vortices grow, and then undergo a merging or vortex pairing event. The vortex pairing is an important event to characterize within the shear layer as it dictates the growth rate and other bulk features of the mixing region. In the above case, it can be seen that the pairing appears to homogenize the particles within the mixing region, but it should be pointed out that the attenuation measurements do not reflect the different evolution that occurs for each of the different size particles within the spray.

Group: Associate Professor Ken Kiger, Department of Mechanical Engineering, University of Maryland.