A report describes direct numerical simulations of single- and two-phase, temporally developing transitional mixing layers at Reynolds numbers (based on the initial vorticity thickness and mean velocity difference) from 200 to 600. As many as 300 × 332 ×180 grid points were used to discretize the gas phase. As many as 5.7 ×106 individual evaporating droplets of various sizes, present in liquid-to-gas mass ratios between 0 and 0.5, were tracked in a Lagrangian reference frame. The gas phase was described by the Navier-Stokes equations for a compressible fluid, augmented by species-transport equations and by the energy equation.
This work was done by Josette Bellan and Richard Miller of Caltech for NASA's Jet Propulsion Laboratory. To obtain a copy of the report, "Evolution of Single-Phase and Droplet Laden Transitional Mixing Layers," access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Physical Sciences category. NPO-20705
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Simulations of Evolving Transitional Mixing Layers
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Overview
The document presents a technical report on direct numerical simulations of evolving transitional mixing layers, focusing on both single-phase and two-phase flows. Conducted by researchers Josette Bellan and Richard Miller at the Jet Propulsion Laboratory (JPL) for NASA, the study explores the dynamics of turbulent flows laden with evaporating droplets, which are relevant in various aerospace applications.
The simulations are characterized by high Reynolds numbers, ranging from 200 to 600, based on the initial vorticity thickness and mean velocity difference. The researchers utilized a significant computational effort, employing up to 300 x 332 x 180 grid points to discretize the gas phase and tracking as many as 5.7 million individual evaporating droplets with polydisperse size distributions. The mass loading ratios of the droplets varied between 0 and 0.5, allowing for a comprehensive analysis of the interactions between the gas and liquid phases.
The governing equations for the gas phase were derived in a compressible form, incorporating mass, momentum, and energy exchange between the gas and the dispersed liquid phase. The numerical methods employed included eighth-order accurate central finite differencing for spatial derivatives and fourth-order explicit Runge-Kutta integration for time advancement. This sophisticated approach enabled the researchers to capture the complete transition to small-scale turbulence, with momentum-thickness Reynolds numbers reaching as high as approximately 1,400 during the second pairing of spanwise vortices.
The findings of the study extend previous work on turbulent flows by providing insights into the evolution of the mixing transition and the computational challenges associated with simulating such complex interactions. The report emphasizes the importance of efficient parallel computing techniques, particularly using the Message Passing Interface (MPI), to handle the large-scale simulations required for this research.
Overall, this document contributes significantly to the understanding of droplet-laden turbulent flows and the transition to turbulence, highlighting the intricate dynamics involved in mixing layers. The research has implications for various fields, including aerospace engineering, where understanding fluid dynamics is crucial for the design and optimization of propulsion systems and other technologies. The work underscores the collaborative efforts between JPL and NASA in advancing scientific knowledge and computational capabilities in fluid dynamics.
