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Numerical simulations are performed to study the effect of freestream turbulence on small-amplitude limit-cycle oscillations of an airfoil at transitionalReynolds numbers.Aone-degree-of-freedomaeroelastic modelwas coupledwith the National Research Council Canada in-house computational-fluid-dynamics code INSflow to perform unsteady Reynolds-averaged Navier-Stokes simulations for flows around a rigid NACA 0012 airfoil in free-to-rotate conditions. Without coupling a transitionmodel, unsteadyReynolds-averagedNavier-Stokes computations basedonthe commonly used shear-stress-transport turbulencemodel could not capture the limit-cycle oscillations. Thiswas expected because it had been previously shown that the limit-cycle oscillations were fed by negative aerodynamic damping due to laminar boundary-layer separation. A correlation-based transition model was then implemented in the code and applied to investigate the turbulence effects. The computed results confirmed qualitatively the experimental observations of the turbulence effects on the limit-cycle oscillations by varying the values of the turbulence intensity and length scale. It was observed that the limit-cycle oscillationsweremore sensitive to the turbulence intensity than to the latter. In general, the freestream turbulence suppressed the limit-cycle oscillations. The pitch amplitude became smaller when the freestream turbulence intensity was increased, which was qualitatively in good agreement with experimental results. The limit-cycle oscillations could not be sustainedwhen the input freestreamturbulence level reached around 1.25%.An examination of the unsteady flowfield revealed that the freestream turbulence affected the location of the separation points and suppressed the laminar-turbulent transition process.

This study focuses on the thermal-diffusive oscillations of the full spectrum of co-flow laminar flames-from premixed to non-premixed through partially premixed flames for Lewis numbers sufficiently greater than unity. A premixedness parameter is identified using the analytical solution to the mixing field downstream of a splitter plate. Different premixedness levels ranging from zero (fully non-premixed) to unity (fully premixed) are then used as Dirichlet data for inlet concentrations of the reactants to the flame. The initial value problem is numerically solved by adopting a thermo-diffusive model and single-step finite rate chemical reaction and using higher-order compact schemes. A thumb-shaped region in the Damk�hler number - premixedness parameter space is identified as corresponding to limit cycle oscillations of the flame. This fills the gap between what is reported for the pulsating instability of non-premixed edge flames recently and fully premixed flames earlier. The thumb-shaped region enlarges with increase in the Lewis number in the range of 1.5-1.7, with marked increase in the oscillatory amplitude of the total heat release. A subcritical Hopf bifurcation is seen to occur at the boundary of this region in the above parametric space. The mechanism of oscillations is studied: hysteresis is found between the upstream and downstream propagation of the flame due to the thermal-diffusive imbalance. The oscillations are plotted in the phase space of the domain-integrated mass and heat diffusive fluxes of and to the reactants. The area of the limit cycle peaks at a particular Damk�hler number and premixedness parameter and drops to zero on both sides in this parametric space. � 2012 The Combustion Institute.

Digital control of power converters has been an area of considerable research interest in recent times. One of the problems which arises in these systems is that of the limit cycle oscillations that occur due to quantization in the feedback loop. This paper investigates the limit cycle oscillations that occur in the digitally controlled version of the buck converter with a proportional-integral controller. The amplitude and frequency of the oscillations that may occur on two duty cycle levels are investigated and related to the controller gain parameters. The analysis shows that it is not possible to guarantee that limit cycle oscillations on two levels will not occur simply by adjusting the gain parameters, and yields a condition which will prevent oscillations on two levels from occurring.

The teetering tail rotor of the Canadian CH-149 Cormorant helicopter fleet experienced limit cycle oscillation (LCO) phenomenon in certain adverse flight conditions in which the transient high amplitude vibration resulted in significant increase of dynamic strain to the tail rotor structures. This could contribute to pre-mature damage of the composite half hub among other causes. Therefore it is required to evaluate the integrity of the tail rotor structure using realistic aeroelastic parameters of the tail rotor in the rotating frame. This paper presents a novel center frequency scaling factor theory and related parameter identification methodologies in order to estimate and track the variation of the critical aeroelastic parameters in the rotating frame during LCO events based on vibration information measured exclusively in the stationary frame. This information would enable update of the tail rotor stability diagram in order to confidently evaluate the impact of LCO events to the structural integrity of the Cormorant tail rotor structures. Based on the developed procedures and identified parameters, recommendations are provided for data analysis and techniques to improve the fidelity of results from aeroelastic simulation analysis for the Cormorant tail rotor structures without the need to install sensors in the rotating frame. © 2012 AIAA., 53rd AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics and Materials Conference, 23 April 2012 through 26 April 2012, Honolulu, HI