Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session D10: Instability: Jets, Wakes and Shear Layers II: Wakes I |
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Chair: Alis Ekmekci, University of Toronto Room: 25C |
Sunday, November 18, 2012 2:15PM - 2:28PM |
D10.00001: The response of the wake past a bullet-shaped body to axisymmetric ZNMF forcing at high Reynolds numbers Georgios Rigas, Aimee Morgans, Jonathan Morrison It has been generally acknowledged that wakes of axisymmetric bodies are dominated by the shedding of large-scale coherent structures of azimuthal wavenumbers $m=\pm1$. In the present study, the effect of harmonic axisymmetric forcing ($m=0$) on the vortex shedding mode is investigated experimentally for $Re=2 \cdot 10^5$. The shedding mode was found to be insensitive to low levels of axisymmetric excitation, whereas for moderate levels a strong parametric subharmonic (2:1) resonance occurs among them. We show that the predictions of a model based on two coupled Stuart-Landau equations describing the evolution of the vortex shedding mode, nonlinearly coupled with the forced axisymmetric mode, are in good agreement with the experimental findings. The model parameters are identified based on experimental data. [Preview Abstract] |
Sunday, November 18, 2012 2:28PM - 2:41PM |
D10.00002: Modification of mean wake flow behind very slender axially symmetric bodies by nonlinear convectively unstable helical modes J.T.C. Liu, Kiseok Lee Recent experiments of Asai, et al. (2011) confirm earlier experiments of Sato {\&} Okada (1966), Peterson {\&} Hama (1976) that, for sufficiently slender axially symmetric bodies placed in a stream parallel to the axes, only convectively unstable modes exist. This is exploited theoretically (and computationally) by imposition of the most unstable helical modes to modify the otherwise round laminar wake flow. The local linear region is first considered theoretically to obtain the group velocities as a function of the streamwise distance, and which compared well with existing measurements. This information in used as convection velocity in a time-dependent nonlinear computation, as suggested by Spalart {\&} Yang (1987) in the boundary layer case. The Reynolds stress modification of the developing laminar mean wake flow is assessed. The round wake is modified into an elliptic-like cross section for equal amplitudes for the n=+1,-1 modes; the consequences of unequal upstream amplitudes, such as a would be found in a slight axis misalignment, are presented. Accompanied are the energy transfer mechanisms between the mean flow and the modal content and that between the modes. [Preview Abstract] |
Sunday, November 18, 2012 2:41PM - 2:54PM |
D10.00003: Transitions to chaos in the wake of an axisymmetric bluff body Yannick Bury, Thierry Jardin This work aims at understanding the dynamical process that leads to the onset of chaos in the flow past a blunt-based axisymmetric bluff body. On the basis of direct numerical simulations, conducted for Reynolds numbers ranging from 100 to 900, we show that the flow undergoes multiple transitions, successively giving rise to the SS, RSPa, RSPb, RSPc and RSB wake states. In particular, the RSPc state, revealed in this work via long-term computations, is characterized by intermittent vortex stretching denoting the onset of chaos and the potential occurence of a third instability that superimposes to the first and second instability associated with state RSPa and RSPb respectively. Interestingly, the reflectional symmetry that characterizes the RSP states is still retained. Hence, chaos is triggered before the symmetry breaking and the occurence of the RSB state. [Preview Abstract] |
Sunday, November 18, 2012 2:54PM - 3:07PM |
D10.00004: The turbulent wake of a submarine model at varying pitch and yaw angle Anand Ashok, Alexander Smits Experiments are reported to examine the effects of pitch and yaw angle on the mean flow and turbulence in the wake of an axisymmetric submarine model (DARPA SUBOFF model). Measurements in the wake were performed at a Reynolds number based on the length of 2.4 $\times $ 10$^{6}$. Mean velocity and three-component turbulence measurements were performed using Pitot probes and cross wires in the span-wise plane at three different downstream positions: 5, 7.5 and 10 diameters downstream of the trailing edge. The pitch and yaw angles were in the range 0 and $\pm $10\r{ }. Work supported by ONR Grant N00014-09-1-0263. [Preview Abstract] |
Sunday, November 18, 2012 3:07PM - 3:20PM |
D10.00005: On the Control of Flow past a Circular Cylinder: Use of a Singe Spanwise Protrusion Alis Ekmekci, Tayfun Aydin, Antrix Joshi An experimental investigation is conducted to evaluate the effects of a single spanwise protrusion on the flow past a circular cylinder. Consideration is given to a range of Reynolds numbers from 5,000 to 30,000 and three different protrusions of circular cross-section that vary in diameter from 2.9{\%} to 5.9{\%} of the cylinder diameter. Varying the angular location of the protrusion on the cylinder surface, critical locations are investigated via hot-film anemometry and hydrogen-bubble visualization. For all the Reynolds numbers and protrusion sizes studied, two angular locations are shown to be the most critical. These angles depend on the wire size and Reynolds number. Protrusion at the first critical angle results in significant attenuation in the spectral amplitude of velocity fluctuations; whereas, at the second critical angle, it leads to amplification. Long-time records of hydrogen-bubble images show, at the first critical angle, recurrent appearance of periods with no detectable Karman vortex shedding and short periods during which regular Karman shedding resumes. Time traces of velocity fluctuations, obtained from hot-film measurements, also depict irregularities at this critical location. [Preview Abstract] |
Sunday, November 18, 2012 3:20PM - 3:33PM |
D10.00006: POD Analysis of the Wake Behind a Foamed and a Finned Cylinder Morteza Khashehchi, Kamel Hooman, Thomas Roesgen, Andrew Ooi Particle Image Velocimetry (PIV) has been carried out to investigate the wake region behind a foamed and a finned cylinder. The experiments are conducted for a wide range of Reynolds numbers from $1000$ to $10000$. Two dimensional results of planar PIV reveal the important aspects of the local flow features of the circular finned and foamed cylinders. These include turbulent boundary layer development over the surface and a delayed separation of the flow resulting in a smaller wake size in each case. The application of Proper Orthogonal Decomposition (POD) to the PIV velocity fields of the two cylinder types is also discussed. The POD computed for the measured velocity fields for all cases shows that the first two spatial modes are contained most of the kinetic energy of the flow irrespective to the cylinder type. These two modes are also responsible for the large-scale coherence of the fluctuations. For three different cylinder types, the first four eigenmodes of the flow field were calculated and their structures were analyzed. The first four eigenmodes reveal the details about the global mean flow structure, with the large-scale structure being mainly related to the most energetic flow motion. [Preview Abstract] |
Sunday, November 18, 2012 3:33PM - 3:46PM |
D10.00007: Experimental sensitivity analysis of the global mode frequency of cylindrical bodies with blunt trailing edges at large Re Olivier Cadot, Mathieu Grandemange, Vladimir Parezanovic The global mode frequency modification of a square and a ``D'' shape cylinders due to the insertion in the flow of a small local and steady disturbance is investigated experimentally at Re=20000. Sensitivity maps are built by measuring the global mode frequency of the flow for many positions of the disturbance around the cylinder. Sensitive regions of either large or low frequencies are identified. It is shown that their spatial structures become independent on the size of the disturbance if this size is smaller to any boundary layers in the flow. In that case, the frequency changes are mostly interpreted by the ability of the disturbance to modify the mean flow through local vorticity injection or change in the local turbulent properties. Theoretical predictions of these sensitive regions from Meliga et al. will be presented in another abstract. [Preview Abstract] |
Sunday, November 18, 2012 3:46PM - 3:59PM |
D10.00008: Experimental investigation of flow instabilities behind a cube Lukasz Klotz, Sophie Goujon-Durand, Jos\'e Eduardo Wesfreid The wake behind a cube has been experimentally investigated in a water tunnel using LIF visualization and PIV method. Measurements were carried out for a moderate Reynolds numbers, ranging up to 400. The basic flow shows four pairs of trailing vortex. Subsequent regimes were detected, with regular and instationnary instabilities. Values of onsets of instability have been determined and the nonlinear evolution of the perturbations, discussed in the framework of Landau models. The streamwise vorticity, obtained from PIV measurements, has been decomposed into azimuthally Fourier modes. The obtained bifurcation branches show symmetry breaking, corresponding to each regime obtained. The experimental results we present are in good agreement with a previous numerical simulation. The observed sequence of transitions, for flow instabilities behind a cube, are compared with those reported for flow behind a sphere and disks. [Preview Abstract] |
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