Graduate Seminar in Fluid Mechanics

November 13, 2020 @ 4:00 pm – 5:00 pm

Join on-line via Zoom:
Friday, November 13, 2020
4:00 p.m. – 5:00 p.m. (EST)

“The Dynamics of Inertial Particles in a Under-Expanded Sonic Jet”

(Adviser: Prof. Rui Ni)

Particle-laden flows are ubiquitous in nature and in many engineering applications, covering a large range of spatial and temporal scales from the formation of planetary disks to the supersonic plume-surface interaction during Lunar and planetary landing. As particles traverse a series of expansion and compression waves in the compressible flow regime, they could potentially significantly modulate the flow characteristics as the particle mass loading increases. In this study, a high-speed under-expanded sonic jet is seeded with monodisperse solid particles, with mean diameters of 29.5 μm, 41.5 μm, and 98 μm. A range of mass loadings were used, and particles were tracked using a ultra-high-speed system to obtain their Lagrangian particle trajectories, velocities, and accelerations. The results acquired from this unique system provide a set of new high-fidelity experimental data for studying high-speed particle-laden flows and may reveal new insights into the physics of inertial particles traveling through compressible media. Earlier results of this work are compared with numerical simulations performed at the University of Michigan, which show good agreement between the two studies.

This work was supported by a NASA Space Technology Graduate Research Opportunity.

“Experimental Investigation of the 3D Flow Structure around a Pair of Cubes Immersed in the Inner Part of a Turbulent Channel Flow”

Presented by JIAN GAO
(Adviser: Prof. Joseph Katz)

The origin and evolution of the three-dimensional flow structures around a pair of roughness cubes embedded in the inner part of a turbulent channel flow (Reτ=2300) are measured using microscopic dual-view tomographic holography. The cubes’ height, a=1 mm, corresponds to 91 wall units or 3.9% of half channel height. They are aligned in the spanwise direction and separated by a, 1.5a, and 2.5a. This study focuses on the mean flow structure, and the data resolution allows detailed characterization of the open separated regions upstream, along the sides, on top, and behind the cubes, as well as measurements of wall shear stress from velocity gradients. The flow features a horseshoe vortex, a vortical canopy engulfing each cube, a near wake arch-like vortex, and multiple interacting streamwise vortices. Most of the boundary layer vorticity is entrained into the horseshoe vortex. The canopy, consisting of wall-normal vorticity to the sides, and spanwise vorticity on top of the cube, originates from the front surface. The streamwise vortices originate from realignment of the other components along the corners of the front surface. Merging of streamwise structures around and behind each cube causes formation of a large streamwise vortex rotating in the same direction as the inner horseshoe leg, with remnants of the outer leg under it. This merging occurs earlier and the entire flow structure becomes more asymmetric with decreasing spacing. Peaks and minima in the distributions of the bottom wall shear stress are associated with the formation of and interactions among the near-wall vortices.

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