Electromagnetic Actuator

The arrangement of the cylinder in the test section and the apparatus used for achieving forced in-plane oscillations of the cylinder are shown in Figure 3.20. A dual channel power oscillator with two electromagnetic drive units was used to generate controlled movement of the cylinder. The actuator assembly was procured from Spanktronics. The electromagnetic actuator consists of the yoke, magnet and pole tip, and the moving coil assembly. It works by virtue of the interaction between the magnetic field and an oscillating current flowing in the coil of the moving assembly. Under such circumstances, a force is generated at right angles to the line of flux and the conductor carrying the current. This force is proportional to the product of instantaneous current and the magnetic flux density. The useful frequency range of the actuator is equal to 1-200 Hz. The maximum amplitude is 1.5 mm, corresponding to 50% of the cylinder size.
The cylinder was mounted horizontally and fixed on the two electromagnetic drive units on each side of the test cell. Proper care was taken to ensure that no leakage occurred at the junctions of the test cell and the cylinder. During experiments, the shedding frequency of a stationary cylinder was first measured using a hotwire. The cylinder was subsequently excited at various harmonics around the shedding frequency in the streamwise direction. The amplitude of excitation was set by the voltage input to the electromagnetic actuator. Both actuators operated from a single power source and thus ensuring identical phase. In an open loop arrangement, the activation signal was generated from a built-in signal generator in the power oscillator. With feedback, the activation signal was generated from the hotwire output with suitable amplication and phase inversion to the drive unit. Here, the feedback signal was low pass-ltered and sent to the digital-to-analog converter. The amplitude of the cylinder displacement was measured from magnified images of the cylinder oscillation using the CCD camera.