2 results
Vortex-induced vibrations of a cylinder in planar shear flow
- Simon Gsell, Rémi Bourguet, Marianna Braza
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- Journal:
- Journal of Fluid Mechanics / Volume 825 / 25 August 2017
- Published online by Cambridge University Press:
- 20 July 2017, pp. 353-384
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The system composed of a circular cylinder, either fixed or elastically mounted, and immersed in a current linearly sheared in the cross-flow direction, is investigated via numerical simulations. The impact of the shear and associated symmetry breaking are explored over wide ranges of values of the shear parameter (non-dimensional inflow velocity gradient, $\unicode[STIX]{x1D6FD}\in [0,0.4]$) and reduced velocity (inverse of the non-dimensional natural frequency of the oscillator, $U^{\ast }\in [2,14]$), at Reynolds number $Re=100$; $\unicode[STIX]{x1D6FD}$, $U^{\ast }$ and $Re$ are based on the inflow velocity at the centre of the body and on its diameter. In the absence of large-amplitude vibrations and in the fixed body case, three successive regimes are identified. Two unsteady flow regimes develop for $\unicode[STIX]{x1D6FD}\in [0,0.2]$ (regime L) and $\unicode[STIX]{x1D6FD}\in [0.2,0.3]$ (regime H). They differ by the relative influence of the shear, which is found to be limited in regime L. In contrast, the shear leads to a major reconfiguration of the wake (e.g. asymmetric pattern, lower vortex shedding frequency, synchronized oscillation of the saddle point) and a substantial alteration of the fluid forcing in regime H. A steady flow regime (S), characterized by a triangular wake pattern, is uncovered for $\unicode[STIX]{x1D6FD}>0.3$. Free vibrations of large amplitudes arise in a region of the parameter space that encompasses the entire range of $\unicode[STIX]{x1D6FD}$ and a range of $U^{\ast }$ that widens as $\unicode[STIX]{x1D6FD}$ increases; therefore vibrations appear beyond the limit of steady flow in the fixed body case ($\unicode[STIX]{x1D6FD}=0.3$). Three distinct regimes of the flow–structure system are encountered in this region. In all regimes, body motion and flow unsteadiness are synchronized (lock-in condition). For $\unicode[STIX]{x1D6FD}\in [0,0.2]$, in regime VL, the system behaviour remains close to that observed in uniform current. The main impact of the shear concerns the amplification of the in-line response and the transition from figure-eight to ellipsoidal orbits. For $\unicode[STIX]{x1D6FD}\in [0.2,0.4]$, the system exhibits two well-defined regimes: VH1 and VH2 in the lower and higher ranges of $U^{\ast }$, respectively. Even if the wake patterns, close to the asymmetric pattern observed in regime H, are comparable in both regimes, the properties of the vibrations and fluid forces clearly depart. The responses differ by their spectral contents, i.e. sinusoidal versus multi-harmonic, and their amplitudes are much larger in regime VH1, where the in-line responses reach $2$ diameters ($0.03$ diameters in uniform flow) and the cross-flow responses $1.3$ diameters. Aperiodic, intermittent oscillations are found to occur in the transition region between regimes VH1 and VH2; it appears that wake–body synchronization persists in this case.
Physical analysis of the transition to turbulence in the wake of a circular cylinder by three-dimensional Navier–Stokes simulation
- HÉLÈNE PERSILLON, MARIANNA BRAZA
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- Journal:
- Journal of Fluid Mechanics / Volume 365 / 25 June 1998
- Published online by Cambridge University Press:
- 25 June 1998, pp. 23-88
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The transition to turbulence of the flow around a circular cylinder is studied by a three-dimensional numerical simulation of the Navier–Stokes equations system in the Reynolds number range 100–300. The numerical method is second-order accurate in space and time and Neumann boundary conditions are used at the two boundaries in the spanwise direction; non-reflecting boundary conditions are specified for the outlet downstream boundary. This study predicts the frequency modulation and the formation of a discontinuity region delimited by two frequency steps within the present Reynolds number range. These features are related to the birth of streamwise vorticity and to the kinetic energy distribution in the near wake. The development of the mean dynamic quantities, the Reynolds stress correlations and the variation of their maximum values are provided in this region, where the similarity laws do not hold. The spatial evolution of the von Kármán mode and of its spectral amplitude are quantified and the variation laws of the maximum spectral amplitude and of its location as a function of Reynolds number are established. The critical Reynolds number for the appearance of the first discontinuity in the present flow system is evaluated by the fully nonlinear approach.