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A nonlinear harmonic disturbance observer for nonlinear systems subject to harmonics is designed and stability of the proposed observer is established using passivity approach. A systematic procedure to choose the nonlinear gain function in the observer is proposed. The proposed nonlinear disturbance observer can be integrated with a linear/nonlinear controller to improve its disturbance attenuation ability for nonlinear systems under harmonics. [DOI: 10.1115/1.1543551] Keywords: Nonlinear Systems, Disturbance Attenuation, Harmonic Disturbances, Passivity, Stability

In this paper, a new technique for shaping the line current and reducing the total harmonic distortion in a three-phase bridge rectifier, feeding a capacitive load, is presented. Using the topology of a three-level three-switch pulsewidth modulated (PWM) rectifier, a new control technique has been implemented. The main objective here is to minimize the total harmonic distortion (THD) of the line current under different load conditions (20%-100% of full load). A review of the problems inherent in a bridge rectifier feeding a capacitive load and the possible solutions are first presented. Subsequently, the analysis of the new circuit and the control technique used are described, along with simulation results. Finally, the experimental results on a 1.6-kW prototype are presented. Index Terms - Modulation technique, power quality improvement, uncontrolled three-phase rectifier.

Laminar-turbulent transition mechanisms induced by a harmonic point source disturbance in a flat-plate boundary layer with adverse pressure gradient are investigated by fourth-order accurate spatial direct numerical simulation based on the complete three-dimensional Navier--Stokes equations for incompressible flow. The disturbance is introduced into the two-dimensional base flow by time-periodic simultaneous blowing and suction within a circular spot at the wall to quietly mimic the momentum input by an active loudspeaker below a hole in the surface in respective experiments. Thus a wave train consisting of pure Tollmien--Schlichting waves of a single frequency and a large number of obliqueness angles is stimulated, and its downstream evolution in both physical and spectral space is investigated. A breakdown scenario dominated entirely by oblique modes is observed that shows a span-wise peak/valley amplitude splitting with the valley plane at the centerline of the wave train. Dominant vorticity structures develop off centerline, and a clear-cut M-shaped structure is formed in final stages related to the wall shear being a footprint of a first pair of [lambda]-vortices in the flow.

An experiment conducted to help students understand sound quality determined by harmonic content is discussed. A flexible piece of laboratory hose filled with lead, a funnel, and clamps are needed, in which the hose is clamped and plucked and the students measure the velocity of the wave created.