Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session E22: Instability: Wakes |
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Chair: Francois Gallaire, Ecole Polytechnique Federale de Lausanne Room: 2012 |
Sunday, November 23, 2014 4:45PM - 4:58PM |
E22.00001: Flow around a rectangular forebody with modified inlet conditions Renzo Trip, Jens H.M. Fransson The near wake behind a rectangular forebody with a smooth leading edge and a blunt trailing edge is investigated, whereby the boundary layer over the forebody is modified by means of wall suction. The laminar boundary layer subject to wall suction yields the asymptotic suction boundary layer (ASBL), whereas an initially turbulent boundary layer will start to relaminarize for high enough wall suction. Wall suction therefore provides the possibility to perform a comprehensive parametrical study. The wake characteristics, such as the base pressure and the shedding frequency, are related to the boundary layer thickness and shape. Time resolved, with respect to the vortex shedding frequency, planar Particle Image Velocimetry (PIV) measurements are performed to gain fundamental knowledge on the role of the topology of the recirculation region in this respect. The mean flow fields do also allow for a global stability and sensitivity analysis on the vortex shedding instability. [Preview Abstract] |
Sunday, November 23, 2014 4:58PM - 5:11PM |
E22.00002: On the destabilizing influence of surface tension in planar wakes Eyal Heifetz, Luca Biancofiore, Fran\c{c}ois Gallaire A counterintuitive destabilizing effect of the surface tension in planar immiscible wakes was observed by means of a linear global analysis (Tammisola et al., PoF, 2011) and Direct Numerical Simulations (Biancofiore et al., FDR, 2014), respectively. This destabilization can be interpreted by the presence of two different temporal unstable modes found when analyzing the local stability of an extracted velocity profile from the base flow. We approximate the wake velocity profile through a piecewise broken-line profile. We then explain the presence of these two temporal unstable modes using the Rossby wave (RW) perspective, which associates to each vorticity discontinuity an individual RW. The introduction of a finite amount of surface tension at the interface creates two capillary waves (CW) which travel with the same relative velocity but in opposite directions. The interaction of these four waves originates in two temporal unstable modes for both sinuous and varicose symmetries. Furthermore, we have captured the spatio temporal evolution of the interacting four-waves system by means of an impulse response analysis. The spreading of the wavepacket is significantly influenced by the coupling of the Rossby waves with the capillary waves, and is seen to favor absolute instability. [Preview Abstract] |
Sunday, November 23, 2014 5:11PM - 5:24PM |
E22.00003: Prediction of the hub vortex instability within wind turbine wakes and effects of the incoming wind and turbine aerodynamic characteristics Giacomo Valerio Iungo, Francesco Viola, Simone Camarri, Fernando Port\'e-Agel, Francois Gallaire Instability of the hub vortex, which is a vorticity structure present in wind turbine near-wake and mainly oriented along the streamwise direction, is predicted from wake velocity measurements. In this work, stability analysis is performed on wind tunnel velocity measurements acquired in the wake produced from a wind turbine model immersed in a uniform flow. Turbulence effects on wake dynamics are taken into account by modeling the Reynolds stresses through eddy-viscosity models, which are calibrated on the wind tunnel data. This formulation leads to the identification of one dominant mode associated with the hub vortex instability, which is characterized by a counter-winding single-helix mode. Moreover, this analysis also predicts accurately the frequency of the hub vortex instability observed experimentally. The hub vortex instability is also investigated by considering incoming wind fields with different turbulence characteristics, different turbine aerodynamic designs and operational regimes, which affect the morphology of the wake vorticity structures and their dynamics. The ultimate goal of this work consists in providing useful information for predicting wind turbine wake dynamics and their effects on downstream wake recovery, thus to maximize wind power harvesting. [Preview Abstract] |
Sunday, November 23, 2014 5:24PM - 5:37PM |
E22.00004: Stability of the laminar wake behind spinning axisymmetric bluff bodies: sensitivity and control Jose Ignacio Jimenez-Gonzalez, Carlos Martinez-Bazan, Wilfried Coenen, Carlos Manglano, Alejandro Sevilla We carry out direct and adjoint global stability analyses of the laminar wake behind several spinning axisymmetric bluff bodies, i.e. sphere, hemisphere, bullet-shaped bodies of ellipsoidal nose and spherical nose respectively; for moderate Reynolds numbers (Re$\le $450) and values of the spin parameter ($\Omega \le $1), defined as the ratio between the azimuthal velocity at the outer body surface and the free-stream velocity. Both the axisymmetric base flow computations and the assembling of the eigenvalue problems are tackled by means of the finite element solver FreeFEM$++$, computing finally the eigenmodes with an Arnoldi algorithm in Matlab. We show that spin acts as a stabilization mechanism for the wake behind bodies with a cylindrical trailing part, while it destabilizes the wake of the other geometries. The computation of the adjoint modes and the identification of the wavemaker allow us to discuss the nature of the different unstable modes found and understand the differences in the stabilizing or destabilizing effect of rotation due to the base flow modifications. The controllability of the unstable regimes by means of base bleed is also addressed. [Preview Abstract] |
Sunday, November 23, 2014 5:37PM - 5:50PM |
E22.00005: Wake transition and vortex street interaction in flows generated by traveling localized Lorentz forces in a shallow electrolyte layer Joel Roman, Sergio Cuevas We present an experimental and numerical study of the vortex street produced by a traveling localized Lorentz force, namely a magnetic obstacle, in a thin layer of electrolyte. The Lorentz force is generated by the interaction a localized magnetic field created by a small permanent magnet which travels with a uniform velocity underneath a rectangular container and a uniform D.C. current applied transversally to the motion of the magnet. We find that by increasing the Reynolds number (based on the velocity of the magnet) the wake generated by the magnetic obstacle presents a transition from the B\'enard-von K\'arm\'an (BvK) wake to the reversed BvK wake. In addition, we analyze the flow past a pair magnetic obstacles side-by-side in a thin layer of electrolyte by varying the separation between the magnets and the intensity of the applied current. The attention is focused in the interference of the wakes created by the magnetic obstacles. Numerical simulations based on a quasi-two dimensional numerical model present a satisfactory agreement with experimental results. [Preview Abstract] |
Sunday, November 23, 2014 5:50PM - 6:03PM |
E22.00006: Mean flow stability wave models for coherent structures in open shear flows: experimental assessment of potentials and limitations Kilian Oberleithner, Lothar Rukes, Oliver Paschereit, Julio Soria We report on a number of experimental and theoretical investigations of shear flow instabilities in jet flows. In these studies, linear stability analysis is employed to the time-averaged flow taken from experiments, contrasting the ``classic'' stability approach that is based on a stationary base flow. The eigenmodes of the time-averaged flow are considered as models for the nonlinearly saturated state of the instability waves. The accuracy of these models is validated through a detailed comparison with experiments. In this talk we outline the potential and limitation of these flow models for convectively and globally unstable jet flows. [Preview Abstract] |
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