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Instabilities in the oscillatory flow of a complex fluid
M Torralba1, A A Castrejón-Pita, G Hernández
1Departament ECM, Facultat de Física, Universitat de Barcelona, Avenida Diagonal 647, E-08028 Barcelona, Catalonia, Spain.
Summary
This study investigates fluid dynamics in a vertical tube under oscillatory pressure. Complex fluids develop instabilities like vortices, unlike Newtonian fluids, due to their unique rheology.
Area of Science:
- Fluid Dynamics
- Rheology
- Nonlinear Systems
Background:
- Understanding fluid behavior under oscillatory flow is crucial.
- Complex fluids exhibit non-Newtonian properties affecting their dynamics.
- Previous work identified parallel shear flow in complex fluids at low amplitudes.
Purpose of the Study:
- To experimentally study fluid dynamics in a vertical tube with oscillatory pressure gradients.
- To compare the behavior of Newtonian and viscoelastic shear-thinning fluids.
- To investigate the onset and evolution of flow instabilities in complex fluids.
Main Methods:
- Experimental investigation of fluid flow in a vertical tube.
- Utilized Particle Image Velocimetry (PIV) for 2D velocity field measurements.
- Tested a range of driving amplitudes (0.8–2.5 mm) and frequencies (2.0–11.5 Hz).
Main Results:
- Newtonian fluids maintained a stable laminar flow across all tested conditions.
- Viscoelastic fluids showed instabilities at higher driving amplitudes, forming symmetric vortices.
- Further increases in driving amplitude led to vortex structure instability and complex, asymmetric flow patterns.
- Inertial effects were negligible (Reynolds number < 10⁻¹), highlighting the role of fluid rheology.
Conclusions:
- The complex rheology of viscoelastic fluids drives shear-induced instabilities in parallel shear flow.
- The studied system serves as a model for nonlinear instabilities in complex fluids.
- Flow behavior is highly dependent on driving amplitude and fluid viscoelasticity.
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