This numerical study investigates a circular cylinder placed in a uniform stream and moving along a slender figure-8-path, using a 2D computational method based on the finite difference method. The effects of in-line amplitude of oscillation and of frequency ratio are investigated. Computations for varying amplitude values were carried out at Re = 150, 200 and 250 for a clockwise orbit (in the upper loop). Time-mean and rms values of force coefficients yielded smooth curves and tended to increase with amplitude.
The effect of frequency ratio was investigated at Re = 200, 250 and 300 in the lock-in domain for both clockwise (CW) and anticlockwise (ACW) orientation. Results differ substantially depending on the direction of orientation. Mechanical energy transfer was always positive in ACW direction, which may lead to vortex-induced vibration, and always negative for CW orientation. The time-mean of drag was much lower for CW over the whole frequency ratio domain investigated. For the CW orbit vortex switches were found at specific frequency ratios at Re = 250 and 300. Limit cycle curves for the CW orbit before and after a jump were symmetric, mirror images, and quite complex, while vorticity contours were close to symmetry. These results indicate the possibility of symmetry-breaking bifurcation.
Authors:Dragan Dimitrovski, Dragana Radosavljević, Miloje Rajović, and Rade Stoiljković
It is shown that Sturm theorems, formulated in the 1830’s (, ,  and ) and valid for second order linear homogeneous differential equation L(y)≡y″+a(x)y′+b(x)y=0, could as well be formulated for the class of nonhomogeneous linear differential equations L(y)=f(x). Criteria for the existence of oscillatory solutions of nonhomogeneous equations, as well as more exact locations of the zeros are given.
Mathematical modeling of toluene oxidation under unsteady state catalyst created by forced oscillations of concentration of
toluene in the reactor inlet was carried out. It is shown that, under periodic reactor operation, the benzaldehyde yield was
up to several times higher than the steady state value. This was explained by the participation of different oxygen species
in the partial and total oxidation on the catalyst surface.
Because of similar pathophysiologic changes, oleic acid (OA)-induced pulmonary edema has been well established as an experimental model of certain types of ARDS. Data in the literature indicate changes mostly in global pulmonary mechanical parameters (lung resistance and compliance) during permeability-type edema. Therefore, we designed this study (1) to separate the OA-induced mechanical responses into airway and parenchymal components, and (2) to examine the relationship between the mechanical parameters and the degree of edema. Anaesthetized, paralyzed, mechanically ventilated rats were given iv. OA in doses of 0 (C n=9), 0.05 (OA0.05 n=8), 0.1 (OA0.1 n=10) and 0.3 (OA0.3 n=5) ml/kg. Respiratory system impedance was measured with a wave-tube low-frequency forced oscillation technique, and a model fitting was used to estimate airway (Raw) and lung tissue parameters (G, parenchymal damping; H, elastance). Pulmonary edema was quantified by gravimetric analysis (WW/DW, wet-to-dry weight ratio). In the OAL0.05 group, transient, but significant increase in Raw, only slight increase in H, and no response in G was observed. Different responses were obtained in OA0.1: significant Raw, G, and H values in survivors; rapid and significantly higher responses in all three parameters in non-survivors. Extremely large parameter values were measured in OA0.3. We found that OA caused dose-related increases in WW, DW and WW/DW. Highly significant correlations were found between the degree of edema and G or H, but not Raw. This study demonstrates that low dose of OA had only transient lung mechanical effects; however, it resulted in mild edema. The higher dose elicited significant airway and tissue changes (smaller responses in survivors than in non-survivors), and severe edema. The strong correlation between lung tissue parameters and the degree of edema suggests that the OA-induced acute lung injury is manifested primarily in the alterations in parenchymal mechanics.