Graduate Seminar in Fluid Mechanics

September 21, 2018 @ 4:00 pm – 5:00 pm
132 Gilman Hall

4:10-4:35 p.m. Presentation

“Characterization of the Flow Induced by Pitching and Heaving Hydrofoil Controlled by a Novel Electromagnetic Suspension System”

Presented by JIBU JOSE (Adviser: Prof. Katz)

Experimental studies of aeroelastic flutter use pitching and heaving foils to characterize the complex flow response to coupled bending and torsion. Unlike spring-mass or cam systems generating restoring forces and moments used in literature, we designed a novel electromagnetic apparatus with controlled heave and pitch of a hydrofoil suspended in a water tunnel, to study this phenomenon.  It can provide electromagnetic restoring forces constraining the motion of the foil (passive) or prescribe time-varying heaving and pitching (active). The springs have been replaced with custom designed linear motors generating vertical forces up to 400N to control heave and oil cooled rotary motors to control the pitch angle with up to 2Nm torque, allowing a translational range of ±15 mm and pitch angle of ±18o. Being an electronic device, it can generate linear or non‐linear restoring forces in response to the displacement, velocity, and acceleration of the modified NACA 0015 acrylic hydrofoil. Signal provided by accelerometers installed in the endplates monitor real-time pitch and heave of the foil and act as feedback for the LabVIEW based control system. The foil is mounted in the test section of water tunnel section of JHU refractive index matched facility, facilitating unobstructed optical access enabling simultaneous acceleration and 3D time resolved velocity measurements, which could be used to compute the motion profile and the pressure distribution over the hydrofoil.

4:35-5:00 p.m. Presentation

“Modelling and Prediction of Large Amplitude Flow-Induced Pitching Oscillations of an Airfoil”

Presented by KARTHIK MENON (Adviser: Prof. Mittal)

In this study, we perform a computational study of large amplitude limit-cycle pitching oscillations of a rigid airfoil at low Reynolds number. A sharp-interface immersed boundary method is used to simulate incompressible flow, and is two-way coupled with a 2-degree-of-freedom linear structural model to model the dynamics of this system. We explore the effect of varying spring stiffness and equilibrium angle of attack, and analyze the resulting amplitude and frequency of flutter. We show distinct regimes of flutter and vortex shedding, and provide some insight into the mechanism driving the dynamics in these regimes. A fluid dynamic time-scale governing the vortex shedding and bifurcation to large amplitude flutter is proposed based on this physical mechanism. Further, we analyze the effect of varying the location of the elastic axis, and introduce maps of energy extraction as a way to understand the subsequent dynamics. Finally, we show using a simple “linearized” model that this tool also enables us to analyze, predict, as well as control, the transient response of the system.

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