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On the unsteady aerodynamics of stationary elliptic cylinders during organised wake condition Dikshit, Ajai Kumar


The aerodynamics of a set of two-dimensional, stationary elliptic cylinders with eccentricity of 0.44, 0.92 and 0.98 is studied experimentally during the organised wake condition (R=2x10⁴-10⁵). The results indicate the effect of eccentricity and angle of attack on mean and unsteady pressure coefficients, Strouhal number and wake geometry, the three important parameters in the aeroelastic instability study. In conjunction with the available literature, the information presented here attempts at providing better understanding of the bluff body aerodynamics in the region between the two extreme cases of circular cylinder (e=0) and flat plate (e=∞) . The cylinder eccentricity and attitude has considerable effect on vortex shedding frequency, with variation of the Strouhal number in the range 0.037-0.274. Although basing Strouhal number on projected dimension or wake width reduces its dependence on angle of attack, the use of transverse distance between the separation points provides, in addition, a smoother transition even at higher eccentricities and smaller angles of attack. The measurement of fluctuating pressure in the narrow band around the Strouhal frequency showed substantial dependence on the Reynolds number at zero angle of attack, particularly for the ellipses of high eccentricity Considerable phase difference may exist between the pressure signals, however, it has negligible effect on the unsteady lift. The thesis also presents analytical results on the location of shear layer separation as obtained using Görtler's series in conjunction with mean pressure profile. The analytical data compared favourably with the results of flow visualisation through Schlieren technique. The high speed movie of the near wake region gave preliminary information about the location of the first vortex and the oscillations of the separating shear layers. The rise and decay of the unsteady pressure in the vortex formation region appeared to substantiate the visual observations. In general, the wake geometry ratio varied around the Kármán stability value of 0.281. The correlation of spanwise results clearly emphasized the three dimensional character of the unsteady aerodynamics. In general, the spanwise variation of fluctuating pressure is enhanced with increase in angle of attack. The phase data suggest inclination of the vortex line at the model to be ≲11°. Increase in angle of attack results in improved alignment of the vortex line with the cylinder axis.

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