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Previous studies have shown that low-frequency vortex oscillations occur around a hemisphere–cylinder body at different angles of attack, but the underlying mechanism is still unclear. In this study, we examine the origin of the vortex oscillation using numerical simulations and global linear stability analysis. The vortex oscillation is reproduced using numerical simulations, and the oscillatory modes are computed through dynamic mode decomposition (DMD). We obtain the base flow through a selective frequency damping method, which exhibits a pair of steady leeward vortices over the body. The four unstable modes are computed using a modified Arnoldi iteration. The antisymmetric mode with a Strouhal number of 0.105 is discovered to be responsible for the alternate oscillation of the vortex pair, and the mode with a Strouhal number of 0.220 corresponds to the in-phase vortex oscillation. Their frequencies have good agreement with the modes of DMD. The other two unstable modes with higher frequencies, one antisymmetric and one symmetric, are harmonic frequencies of the above two modes. The study conclusively verifies that the vortex oscillation over a hemisphere–cylinder body originates from a global flow instability. [ABSTRACT FROM AUTHOR]

An experimental study about the tip-disturbance effects on the asymmetric vortex breakdown of a chined forebody was conducted using a wing-body model. The vortices of the chined forebody could produce an asymmetric breakdown at high angles of attack when adding an artificial disturbance onto the nose tip. The forebody vortex with asymmetric breakdown would further interact with the vortices of the strake and wing, consequently inducing significant differential loads on two sides of the wing. When the tip disturbance was located on the leeward side and approached the nose tip, the disturbance had a more prominent effect on the asymmetric breakdown. Moreover, the asymmetry level of the vortex breakdown would increase with increasing geometric sizes of the tip disturbance. However, when a splitter plate was placed in the leeward symmetry plane of the model, the asymmetry of the vortex breakdown would disappear, even adding a tip disturbance.

Previous studies have revealed that vortex oscillations exist around slender bodies at low Reynolds numbers where the boundary layers undergo laminar separation. This investigation aims to extend the study to higher Reynolds numbers where the boundary layers exhibit turbulent separation. A hemisphere–cylinder body with a fineness ratio of 24.5 was numerically simulated using detached eddy simulation at angles of attack (AOAs) of 30°–80° and was analyzed using dynamic mode decomposition (DMD). The fineness ratio is the ratio of length to diameter of the cylinder. The Reynolds number based on the cylinder diameter is fixed at Re = 3.0 × 106. The results indicate that, at AOA < 45°, the downstream wake vortices around the slender body exhibit weak oscillations in phase, corresponding to symmetric modes, which is much different from the cases with laminar separation in the previous studies. At AOA > 45°, the vortex flow over the slender body is divided into two parts: forebody vortex oscillations with lower frequencies and shedding of afterbody vortices with higher frequencies. The vortex oscillations produce greater sectional side-force than the vortex shedding, and the associated flow structures are similar to the laminar case, although the separation points in this case are greatly delayed due to turbulent separation. The DMD results at a typical AOA of 50° show that the leading oscillatory mode is antisymmetric, corresponding to alternate vortex oscillation over the forebody; apparent interactions exist between the vortex oscillation and vortex shedding. The vortex shedding region moves forward toward the nose with increasing AOAs. In addition, at the AOAs of 50°–80°, the non-dimensional frequencies for the vortex shedding can be approximately collapsed into a linear relationship with respect to axial location of the afterbody cylinder if the crossflow velocities normal to the cylinder are employed to normalize the frequencies. The vortex-oscillation frequencies, however, are independent of the crossflow velocities, and no suitable scale was found to collapse the data at present. [ABSTRACT FROM AUTHOR]

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 (Rec), only weak horizontal oscillations (antisymmetric modes) were observed downstream of the vortex pairs at low frequencies and a small amplitude. At Re > Rec, 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. [ABSTRACT FROM AUTHOR]