Degree

Doctor of Philosophy (PhD)

Department

Engineering Science

Document Type

Dissertation

Abstract

This research investigates shock-induced aerodynamic breakup (aerobreakup) of fluids with different stress-strain behaviors. The central hypothesis is that shear stress from highspeed gas flow manifests differently depending on a fluid’s strain response, influencing droplet breakup morphology, fragment size distribution, and drag-induced acceleration. To test this, experiments and simulations were performed on μm–mm sized droplets of Newtonian (water) and shear-thinning fluids (nanofluids or xanthan gum). Two experimental setups were used: an open-ended shock tube and a closed conventional shock tube. In the openended setup, a normal shock impacts a stationary droplet held by an acoustic levitator. Droplet breakup and gas dynamics are visualized using high-speed back-illuminated and schlieren imaging. Breakup of water and nanofluids (TiO2 and Al2O3 dispersions) at shock speeds near Mach 1 showed droplet flattening, liquid sheet formation, and atomization. The resulting mist expanded under the high-speed gas jet. Nanofluids exhibited delayed breakup due to higher viscosity and Ohnesorge number. Axisymmetric flow simulations captured gas dynamics and initial deformation. To study higher Mach number effects, a custom-built closed shock tube was used, reaching Mach 2.13 to 3.03 using helium and a vacuum-driven section. Droplets were generated via column breakup and imaged with high-speed cameras. Increasing Mach number led to transitions from bag to multi-bag to shear stripping and catastrophic breakup modes, consistent with higher Weber number. Quantitative image analysis showed increasing droplet centroid velocities with Mach number. Water droplets accelerated faster due to early misting, while xanthan gum accelerated more slowly due to viscoelastic ligament formation. Nanofluids broke up more rapidly at higher Mach numbers, likely due to lower surface tension. Deformation metrics showed increasing vertical elongation and reduced horizontal flattening with Mach number. Breakup initiation and total breakup times were modeled using the Wert and Hsiang models based on Weber and Ohnesorge numbers. These closed shock tube results confirm that fluid rheology and surface tension significantly influence breakup behavior and post-breakup droplet dynamics under highspeed aerodynamic loading.

Date

7-23-2025

Committee Chair

Shyam Menon

DOI

10.31390/gradschool_dissertations.6865

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