Large eddy simulation of flow past a circular cylinder of low aspect ratio ( $AR=1$ and $3$ ), spanning subcritical, critical and supercritical regimes, is carried out for $2\times 10^3 \le Re \le 4\times 10^5$ . The end walls restrict three-dimensionality of the flow. The critical $Re$ for the onset of the critical regime is significantly lower for small aspect ratio cylinders. The evolution of secondary vortex (SV), laminar separation bubble (LSB) and the related transition of boundary layer with $Re$ is investigated. The plateau in the surface pressure due to LSB is modified by the presence of SV. Proper orthogonal decomposition of surface pressure reveals that although the vortex shedding mode is most dominant throughout the $Re$ regime studied, significant energy of the flow lies in a symmetric mode that corresponds to expansion–contraction of the vortex formation region and is responsible for bursts of weak vortex shedding. A triple decomposition of the time signals comprising of contributions from shear layer vortices, von Kármán vortex shedding and low frequency modulation due to the symmetric mode of flow is proposed. A moving average, with appropriate size of window, is utilized to estimate the component due to vortex shedding. It is used to assess the variation, with $Re$ , of strength of vortex shedding as well as its coherence along the span. Weakening of vortex shedding in the high subcritical and critical regime is followed by its rejuvenation in the supercritical regime. Its spanwise correlation is high in the subcritical regime, decreases in the critical regime and improves again in the supercritical regime.
The flow past a circular cylinder at a low Reynolds number (40 ≤ Re ≤ 180) is investigated. A stabilized finite element method is utilized to solve the incompressible flow equations in two-dimensions. The critical Re for the onset of vortex shedding (Rec) is estimated to be 46.985. The variation of time-averaged coefficient of drag (C¯D) with Re is found to be non-monotonic for Re > Rec. Unlike for the steady flow, the pressure component of C¯D increases with an increase in Re in a short range of Re for Re > Rec. This increase is due to a significant rise in the peak suction, near the shoulder of the cylinder, of the time-averaged flow, with Re. Several definitions of vortex formation length (Lf), proposed in the past, are reviewed and compared. A new definition, based on the fluctuation in the local kinetic energy of the flow, is proposed. The variation of Lf with Re is compared with Lw, the separation bubble length. Lf is found to be significantly larger than Lw for Re close to Rec. The difference between the two lengths decreases with an increase in Re. The meaning of Lf, in terms of flow physics, is explored. It is found that the vortices form in the near wake, even for Re close to Rec. They become stronger as they convect downstream and gain full strength at a location Lf downstream of the cylinder, beyond which they begin to decay.
The effect of height of a trip and its location on the transition of boundary layer on a cylinder is studied using large eddy simulations for [Formula: see text]. The Reynolds number, Re, is based on the free stream speed and diameter of the cylinder ( D). Two modes of transition are observed: (a) natural, for a relatively small trip of height [Formula: see text], via formation of a laminar separation bubble (LSB) and (b) direct, for a large trip of height [Formula: see text], wherein the formation of LSB is bypassed and the trip disturbs the flow enough to cause separation of the boundary layer and its subsequent turbulent reattachment. Transition delays the final separation leading to a very significant reduction in drag, often referred to as drag crisis. The delay is more for natural as compared to direct transition. Consequently, the drag at the end of crisis is lower for natural transition. The 1.0% trip at [Formula: see text] leads to a more delayed flow separation than one at [Formula: see text] from the front stagnation point. The drag crisis takes place in two stages for a cylinder with trip. During each of the two stages, asymmetric transition on the two sides results in generation of circulation and lift force. The effect of trip is felt even by the non-trip side. The cylinder experiences a relatively large “reverse lift” during the second stage of drag crisis. While natural transition is accompanied by intermittency of LSB, direct transition is associated with intermittency in laminar vs turbulent attachment of the flow following its separation at the trip.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.