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“Starting Flow Through a Tube with a Flexible Exit”
Presented by SUBHAMOY GUPTA
(Adviser: Prof. Katz)
Vortex rings, associated with unsteady starting jets, have been found to be the essential flow feature in many fluid-dynamical systems in nature. To understand the effect of passive flexibility on the generation and evolution of vortex rings, an experimental setup with a piston-cylinder mechanism in a water tank was developed. A flexible skirting with minimal resistance was added to the exit of the cylinder to mimic the passive flexibility. The flow visualization was done using fluorescein dye and the flow field was quantitatively measured with the help of Particle Image Velocimetry (PIV). The structural dynamics is seen to be dependent on the type of flow pulse (instantaneously started, accelerating etc.) and the length of the skirting. PIV measurements indicate a reduction in circulation of the vortex ring and the flow field for flows with flexible exit, compared to non-flexible cases. The study suggests that the presence of passive flexibility in the vortex generator can significantly change the properties of a starting vortex ring.
“Structural Uncertainty in LES Linear Eddy Viscosity Models”
Presented by YIFAN DU
(Adviser: Prof. Zaki)
Whether the structural form of a large-eddy simulation (LES) model can enable accurate predictions of statistical moments of the resolved scales is examined. In order to quantify these structural uncertainties, we seek the optimal model coefficients which achieve the most accurate predictions of available statistical observations. A cost functional is defined as the discrepancy between the model predictions and the data, which is minimized using an ensemble variational approach in order to determine the optimal control vector. The procedure is repeated for difference choices of the observations, and the results highlight the competing objectives that the structure of linear eddy viscosity models cannot fully satisfy.