Kunisato Seto

Nonlinear Vibration Properties of a Current-Controlled Attractive Type Maglev System

Because maglev system has very complicated dynamic properties due to nonlinearity of the magnetic force, it is a very interesting study subject for dynamics researchers. We, in this paper, focused our study interest on a current-controlled attractive type maglev system, and analyzed its nonlinear vibration properties by numerical simulation method. The current-controlled attractive type maglev system is a feedback system based on a PD-controller that adjusts the current of the electromagnet to stably levitate a body with constant gap. The ranges of the system’s feedback gains with which the system will be stable are determined by nonlinear system dynamics analysis. From the numerical simulation results, we found out that when the system is excited by a harmonic motion, the levitated body of the system shows many kinds of vibration phenomena with the variation of the system’s feedback gains in their available ranges. Especially, chaotic vibrations can also occur in this maglev system for some suitable system’s feedback gains.Copyright

Active noise control‐like performance of a spherical reflector

The principle of cancellation of tonal noise by using a spherical reflector, the designing method of an appropriate reflector for canceling jet screech, the advantage of a spherical compared to a flat reflector for canceling a tonal noise, and a prospective application for an aircraft to protect the fuselage from acoustic fatigue are discussed. The thrust loss due to the new technique was also evaluated directly with a vertical wind tunnel. The change in the aerodynamic structure was optically observed by using a schlieren apparatus along with a high‐speed video camera. [Work supported by grant of JSPS, Japan.]

Observations of aeroacoustic modifications of a supersonic jet by using a spherical reflector

Jet screech was eliminated by placing a spherical reflector at the nozzle exit of an underexpanded supersonic jet. The placement of the reflector minimized the sound pressure and this muted sound did not excite the unstable disturbance at the exit of nozzle and the loop of feedback mechanism disappeared, thus the generation of jet screech was cancelled. The proposed method was very effective in reducing overall sound pressure in the upstream region of the nozzle exit. Therefore, it could be a promising countermeasure against acoustic fatigue to protect the fuselage of an aircraft by incorporating it into a streamline form. The flow characteristic of the jet for the proposed system was observed by the Schlieren apparatus with a high speed video camera. A slit path was cut to the reflector for visualizing the jet properly. The shock structure of the jet was slightly modified and became more stable. [The research was supported by a grant from JSPS of the government of Japan.]

Control of supersonic jet noise with a spherical reflector

Jet screech is considered to be generated from a feedback cycle in an underexpanded supersonic jet. Experiments were carried out to control the screech tone by placing a spherical reflector at the nozzle exit. The reflector controlled the location of the sound image‐source and minimized the sound pressure at the nozzle exit. The weak sound could not excite the unstable disturbances at the exit and the feedback mechanism was cancelled and finally screech tone was eliminated. The new technique suppressed not only the screech tones but also the broadband noise and reduced the overall sound pressure of the jet. The performance of the proposed technique was checked with different sizes of reflectors by placing the reflectors at different upstream positions of the nozzle exit for different pressure ratios and the result was very good. [The research was supported by a grant from JSPS of the government of Japan.]

A Numerical Simulation of Controlling Chaotic Vibration of a Maglev System by OGY Method

In this paper, we first introduced the Model-based OGY method for controlling chaos of nonlinear continuous time dynamical system, and then applied this Model-Based OGY Method to a repulsive maglev system to control its chaotic vibrations. From the numerical simulation results, we found out that the Model-based OGY Method is successfully applied to controlling chaotic vibrations of the repulsive maglev system, and hence the effectivity of Model-based OGY Method is demonstrated.Copyright