Open Access

ARTICLE

A Study on the Far Wake of Elliptic Cylinders

Subhankar Sen¹, Sanjay Mittal^{2, *}

1 Department of Mechanical Engineering, Indian Institute of Technology Kanpur, India.
2* Department of Aerospace Engineering, Indian Institute of Technology Kanpur, India.

Computer Modeling in Engineering & Sciences 2017, 113(1), 35-55. https://doi.org/10.3970/cmes.2017.113.033

Abstract

The evolution of far wake of stationary elliptic cylinders with angle of attack is investigated for the unsteady flow at Re = 200 using a stabilized finite- element method. The Reynolds number, Re, is based on the major axis of cylinder and free-stream speed. Cylinder aspect ratios, AR considered are 0.2, 0.5 and 0.8 while the angle of attack, α varies from 0◦ to 90◦. With increasing α, the Karman or primary shedding frequency of each cylinder decreases monotonically while the primary wavelength and time-averaged as well as r.m.s. drag increase monoton- ically. The time-averaged lift and moment coefficients initially increase with α and then decrease. The power spectrum analysis of unsteady treansverse velocity signal at various stations along the wake centerline is used to study the associated frequencies. The Karman or primary shedding frequency exists for each α. For the AR = 0.2 cylinder, the secondary and subsequently, low magnitude tertiary fre-quencies appear in the wake as α continues to increase from 30◦. In contrast to the decaying nature of primary frequency with α, secondary frequency exhibits a non- monotonic variation. The secondary frequency in general is not a sub-harmonic of the primary. This suggests that the secondary structures do not form due to merg- ing of primary vortices. With increasing α, the location of formation of secondary structures advances upstream. For a given orientation, the upstream advancement is more dominant with decreasing AR. Power spectrum analysis indicates that the wake state for confined flow is characterized by the primary frequency alone, i.e. the imposition of blockage suppresses the formation of secondary structures.

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