Vortex dislocations in wake-type flow induced by spanwise disturbances

Abstract

Vortex dislocations in wake-type flow induced by three types of spanwise disturbances superimposed on an upstream velocity profile are investigated by direct numerical simulations. Three distinct modes of vortex dislocations and flow transitions have been found. A local spanwise exponential decay disturbance leads to the appearance of a twisted chainlike mode of vortex dislocation. A stepped spanwise disturbance causes a streamwise periodic spotlike mode of vortex dislocation. A spanwise sinusoidal wavy disturbance with a moderate waviness causes a strong unsteadiness of wake behavior. This unsteadiness starts with a systematic periodic mode of vortex dislocation in the spanwise direction followed by the spanwise vortex shedding suppressed completely with increased time and the near wake becoming a steady shear flow. Characteristics of these modes of vortex dislocation and complex vortex linkages over the dislocation, as well as the corresponding dynamic processes related to the appearance of dislocations, are described by examining the variations of vortex lines and vorticity distribution. The nature of the vortex dislocation is demonstrated by the substantial vorticity modification of the spanwise vortex from the original spanwise direction to streamwise and vertical directions, accompanied by the appearance of noticeable vortex branching and complex vortex linking, all of which are produced at the locations with the biggest phase difference or with a frequency discontinuity between shedding cells. The effect of vortex dislocation on flow transition, either to an unsteady irregular vortex flow or suppression of the Kármán vortex shedding making the wake flow steady state, is analyzed. Distinct similarities are found in the mechanism and main flow phenomena between the present numerical results obtained in wake-type flows and the experimental-numerical results of cylinder wakes reported in previous studies.

Cylinder-wall interference effects on finite-length wavy cylinders at subcritical Reynolds number flows

“…Intuitively, the extents to which wake structures and behaviour are altered by these cylinders depend heavily not only upon parameters which dictates non-wavy cylinder flows such as Reynolds number, aspect-ratio, end-wall effects, free-stream turbulence level and surface roughness, but also wavelength and wave amplitude as well. Better understanding of how these nodes and saddles affect the canonical flow-past-circular-cylinder scenario has important implications in terms of flow stability analysis [Ling and Zhao (2009), Garbaruk and Crouch (2011)], bluff-body drag reductions and vortex-induced vibrations [Bearman and Owen (1998), Darekar and Sherwin (2001), Lam et al (2004aLam et al ( , 2004b]. Therefore, it should not be surprising that a large number of the above-mentioned studies tried to establish the critical relationships between the resultant wake behaviour, pressure distributions and lift/drag levels.…”

Mechanism Responsible for the Complete Suppression of Kármán Vortex in Flows Past a Wavy Square-Section Cylinder

“…The wake profile at 𝑥 = 2.783, remarkably different from the classical wake profile in the non-wavy cylinder, is very similar to that used in the study of the wake-type flow. [9] The complete suppression of vortex shedding is also observed by introducing the disturbance on the upstream velocity profile. The vertical vorticity as a unique additional vorticity is directly resulted from such disturbance.…”

Spanwise vortex dislocation in the wake of segmented blunt trailing edge

“…Darekar and Sherwin (2001) have reported, through their numerical study of the flow over a square cylinder with a wavy stagnation face, a complete suppression of the Karman vortex shedding and about 16% reduction in the total drag with sufficiently steep waviness. Ling and Zhao (2009) introduced spanwise sinusoidal wavy disturbance in a two dimensional periodic wake flow and found that the Karman vortex shedding ceased to exist for a moderate waviness. In both these numerical investigations, the evolution of the streamwise and transverse vorticities was observed in the region of inflection points of the spanwise wavy disturbance thereby weakening the spanwise vorticity of the Karman vortex and causing its dislocation.…”