Energy budget analysis of aeroelastic limit-cycle oscillations

In this work limit-cycle oscillations (LCOs) caused by aerodynamic non-linearitieswere analysed using fluid-structure interaction (FSI) simulations of a two-degree-of-freedomairfoil system. A comparison with sinusoidal forced motion oscillation simulations at the fundamental frequency was performed. The results show that using the first harmonic component for the forced motion oscillations is a good approach to describe the limit-cycle oscillations considered in this work. Following the work of G. Dietz, G. Schewe and H. Mai (2004), Journal of Fluids and Structures, Vol. 19, an energy budget analysis of the limit-cycle oscillations was performed. Power analysis of the LCO reveals that the power of the lift and moment show non-linear behaviour with increasing amplitude. The linearised equivalent power components that would occur in case of flutter were computed. These show that the defect in the power of the lift in the non-linear case is caused by the increase of the phase of the lift with oscillation amplitude, which is the result of the unsteady shock wave motions on both upper and lower surface of the airfoil. The power of the moment also shows a defect, which is much smaller than in case of the lift. This defect is caused by variations in both the magnitude and the phase of the moment with oscillation amplitude

Prediction of aeroelastic limit-cycle oscillations based on harmonic forced motion oscillations

Aeroelastic limit-cycle oscillations due to aerodynamic non-linearities are usually investigated using coupled fluid-structure interaction simulations in the time domain. These simulations can become computationally expensive, especially if the global bifurcation behaviour is of interest. To reduce the computational effort of parameter studies, an extension of the well-known p-k method is developed. In this frequency domain method, the aerodynamic forces and moments are now not only dependent on the frequency, but also on the oscillation amplitudes and the phase angle between the degrees of freedom. The first harmonic components of the aerodynamic forces are interpolated in the parameter space applying response surface modelling. The limit-cycle oscillation amplitude and mode shape are then found iteratively. The amplitude-dependent p-k method is verified and validated using an analytical testcase; the two-degree-of-freedom van der Pol oscillator. Excellent agreement with the time domain solution and the analytical solution is obtained. The amplitude-dependent p-k method is then applied to a two-degree-of-freedom airfoil system where the aerodynamic forces are computed from Euler simulations. Fluid-structure interaction simulations are performed for validation of the method. Both methods show good agreement in the bifurcation behaviour of the limit-cycle oscillation amplitude and mode shape

Prediction of Aeroelastic Limit-Cycle Oscillations Based on Harmonic Forced-Motion Oscillations

Aerodynamic

Inhalation of nitric oxide prevents ischemic brain damage in experimental stroke by selective dilatation of collateral arterioles

RATIONALE: Stroke is the third most common cause of death in industrialized countries. The main therapeutic target is the ischemic penumbra, potentially salvageable brain tissue that dies within the first few hours after blood flow cessation. Hence, strategies to keep the penumbra alive until reperfusion occurs are needed. OBJECTIVE:: To study the effect of inhaled nitric oxide on cerebral vessels and cerebral perfusion under physiological conditions and in different models of cerebral ischemia. METHODS AND RESULTS:: This experimental study demonstrates that inhaled nitric oxide (applied in 30% oxygen/70% air mixture) leads to the formation of nitric oxide carriers in blood that distribute throughout the body. This was ascertained by in vivo microscopy in adult mice. Although under normal conditions inhaled nitric oxide does not affect cerebral blood flow, after experimental cerebral ischemia induced by transient middle cerebral artery occlusion it selectively dilates arterioles in the ischemic penumbra, thereby increasing collateral blood flow and significantly reducing ischemic brain damage. This translates into significantly improved neurological outcome. These findings were validated in independent laboratories using two different mouse models of cerebral ischemia and in a clinically relevant large animal model of stroke. CONCLUSIONS:: Inhaled nitric oxide thus may provide a completely novel strategy to improve penumbral blood flow and neuronal survival in stroke or other ischemic conditions