Experiments on self-sustained oscillations of leeward vortices over a hemisphere cylinder

Recent numerical studies have indicated the existence of a new type of vortex unsteadiness around a hemisphere cylinder from low to high angles of attack, characterized by large-scale alternate oscillations of leeward-vortex pairs. This investigation conclusively confirms the existence of vortex oscillations in experiment by directly measuring oscillatory vortex structures and explores their origin and evolution with increasing Reynolds number (Re = 957–6780, Re = U_∞D/ν, where U_∞ is freestream velocity, D is the diameter of the body, and ν is the coefficient of kinematic viscosity). The results indicate that the Reynolds number strongly influences the stability of the vortex pairs and oscillations of the vortices. As Re is less than a critical Re (Re_c), only weak horizontal oscillations (antisymmetric modes) were observed downstream of the vortex pairs at low frequencies and a small amplitude. At Re > Re_c, stronger vortex oscillations were observed with a mean dimensionless frequency of St = 0.11 (St = f D/U_∞), where the amplitude of the oscillations increased with the value of Re. In this case, the oscillations of the vortex pairs consisted of antisymmetric and symmetric modes, where the antisymmetric modes were dominant and corresponded to alternate oscillations of the vortex pairs, and the symmetric modes were much weaker and corresponded to in-phase oscillations of the vortex pairs. The estimated wavelengths imply that the vortex oscillations originated in long-wave instabilities. However, the behaviors of these instabilities were significantly different from those of Crow-type long-wave instabilities in which symmetric modes that are insensitive to the Reynolds number are dominant.