Chinsuk Hong

Active control of resiliently-mounted beams with a moment pair actuator

Various strategies are considered for the active control of a flexible beam, mounted on springs at either end. As the spring stiffness is decreased from an infinite value to zero, the boundary conditions of the beam change from being simply supported to being free–free, and a rich variety of behaviour is observed. Feedforward control of either the beams kinetic energy or radiated sound power using a moment pair actuator is considered first. It is found that for small spring stiffness minimization of radiated sound power leads to significant increases in overall beam vibration. The stability and performance of feedback controllers is then considered. It is found that using a moment pair actuator and a velocity sensor the feedback controller is only conditionally stable, particularly when the actuator and the sensor are close to the ends of the beam and the supporting stiffness is small. The reductions in sound and vibration are small under these conditions.

Local feedback control of light honeycomb panels

This paper summarizes theoretical and experimental work on the feedback control of sound radiation from honeycomb panels using piezoceramic actuators. It is motivated by the problem of sound transmission in aircraft, specifically the active control of trim panels. Trim panels are generally honeycomb structures designed to meet the design requirement of low weight and high stiffness. They are resiliently mounted to the fuselage for the passive reduction of noise transmission. Local coupling of the closely spaced sensor and actuator was observed experimentally and modeled using a single degree of freedom system. The effect of the local coupling was to roll off the response between the actuator and sensor at high frequencies, so that a feedback control system can have high gain margins. Unfortunately, only relatively poor global performance is then achieved because of localization of reduction around the actuator. This localization prompts the investigation of a multichannel active control system. Globalized reduction was predicted using a model of 12-channel direct velocity feedback control. The multichannel system, however, does not appear to yield a significant improvement in the performance because of decreased gain margin.

Transitional wall pressure fluctuations on axisymmetric bodies

This paper reports the characteristics of wall pressure fluctuations in the transition region of the flow over axisymmetric bodies. Measurements are conducted in a low noise wind tunnel to obtain the boundary-layer flow field, the local static pressures, and the wall pressure fluctuations using hot wires, micromanometers and flush-mounted microphones. The spatial and the temporal developments of Tollmien-Schlichting (T-S) waves were first observed in the time domain. The wall pressure fluctuations in the transitional boundary-layer flows were of intermittent pulses and intensified as they were convected downstream at typically 63% of the upstream velocity. The Wigner-Ville distributions are then obtained to examine the energy evolution jointly in time and in frequency. The center frequency of the T-S wave is decreased with increases of the boundary-layer thickness and the bandwidth of the energy distribution is broadened as the local Reynolds number is increased. Finally the nondimensional spectra of the transitional wall pressure fluctuations scaled on the outer variables was obtained and it was then found that the characteristic frequency of the T-S wave was related to the outer variables as omegadelta( *)/U(infinity) approximately 0.2. The peak level of the pressure fluctuations during late transition at the characteristic frequency is about 10 dB higher than that of the fully developed flow.

Design of Feedforward Controller with Predicted Structural Velocity using Modal Expansion Method

Jeong-Mo Ku1, Min-Jung Sim1, Weui-Bong Jeong1, Chinsuk Hong2, Byung-Kyu Jung3 1 Department of Mechanical Engineering, Pusan National University Jang-Jeon Dong, Busan, Korea ustaryou@pusan.ac.kr; cldbzl5281@daum.net; wbjeong@pusan.ac.kr 2Department of Mechanical Engineering, Ulsan College Mu-Geo Dong, Ulsan, Korea cshong@uc.ac.kr 3Agency for Defense Development Jin-Hae Gu, Changwon, Korea bkjung@add.re.kr

Wave characteristics of a cylinder with periodic ribs

This paper deals with wave characteristics of a cylinder with periodic ribs. The equation of motion of the stiffened cylinder is first derived using a receptance coupling method. The dispersion diagrams of the stiffened cylinder are then obtained in order to figure out the effects of the ribs on the wave propagation. Due to the effect of the ribs, the dispersion curves are found to be repeated along the axis of the wavenumber with the repetition period of 2π/d, where d is the rib distance. Also, dispersion curves are found to show a pass and stop band of the waves. The stop bands appeared at the wavenumber of half of the wavenumber periodicity. The stop band becomes wider as the increase of the circumferential order. The waves in the pass bands are propagating well through the ribs without decay. In contrast, the waves in the stop bands are not propagating, but decaying the magnitudes. The decay of the responses in the stop band increases as the circumferential order increase. The change of the rib stiffness causes the cut-on frequency to change and the modal order to jumble. The change of the rib stiffness also leads to generate a wave whose phase velocity is positive, while group velocity is negative.