Bulletin of the American Physical Society
2005 58th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 20–22, 2005; Chicago, IL
Session HQ: Jet Stability II |
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Chair: Ramkumar Parthasarathy, University of Oklahoma Room: Hilton Chicago Stevens 2 |
Monday, November 21, 2005 1:20PM - 1:33PM |
HQ.00001: Spatial stability and the onset of absolute instability of Batchelor vortex for high swirl numbers Luis Parras, Ramon Fernandez-Feria Batchelor's vortex has been commonly used in the past as a model for aircraft trailing vortices. Using a temporal stability analysis, Fabre and Jacquin [\em J. Fluid Mech. \bf 500, \rm 239 (2004)] have recently found new viscous unstable modes for the high swirl numbers of interest in actual large-aircraft vortices. We look here for these unstable viscous modes occurring at large swirl numbers ($q>1.5$), and large Reynolds numbers ($Re>10^3$), using a spatial stability analysis, thus characterizing the frequencies at which these modes become convectively unstable for different values of $q$, of $Re$, and for different intensities of the uniform axial flow. We consider both jet-like and wake-like Bartchelor's vortices, and are able to reach values of $Re$ as high as $10^8$. We also characterize the onset of absolute instability of these unstable viscous modes for large $q$. [Preview Abstract] |
Monday, November 21, 2005 1:33PM - 1:46PM |
HQ.00002: Global modes in hot jets and their acoustic far field Lutz Lesshafft, Patrick Huerre, Pierre Sagaut, Marc Terracol The self-excited, highly synchronized formation of ring vortices in absolutely unstable hot jets is studied in direct numerical simulations. Oscillations in the hydrodynamic near field and in the acoustic far field are computed from the Navier-Stokes equations, resolved on the same computational domain. Experimental near field measurements are reproduced to very good accuracy. The numerically observed vortex shedding frequencies are then compared to the linear instability properties of the unperturbed flow state. It is found that over a range of parameters the onset of self-excited oscillations can be attributed to the presence of absolute instability in sufficiently hot jets, in full agreement with predictions drawn from the theory of nonlinear global modes. In cases where the initial jet shear layer is highly receptive to external forcing, feedback from aeroacoustic disturbances generated downstream may influence the frequency selection process. A highly directive acoustic far field is radiated by nonlinear global modes, concentrated in the direction of the jet axis. [Preview Abstract] |
Monday, November 21, 2005 1:46PM - 1:59PM |
HQ.00003: The absolute and convective transition in subsonic variable-density axisymmetric wakes Philippe Meliga, Denis Sipp, Laurent Jacquin, Jean-Marc Chomaz Aerodynamic buffet occurs in transonic afterbody flows due to unsteadiness of the separated flow in the vicinity of the rear region. As a step to understand the mechanisms triggering this phenomenon, we analyse the local stability properties of compressible variable-density axisymmetric wakes. The stability problem is solved by use of a collocation method providing a complete set of eigenvalues and associated eigenfunctions. The impulse response of the flow is then investigated by locating saddle points in the complex wavenumber plane and Briggs' criterion is used to determine the nature of local instabilities. Regions of convective and absolute instabilities are identified for two distinct helical modes of azimuthal wavenumber m=1, and absolute-convective transition is studied as a function of the wake momentum thickness, the ratios of centerline to ambient fluid density and velocity, and the Mach number in the ambient fluid. [Preview Abstract] |
Monday, November 21, 2005 1:59PM - 2:12PM |
HQ.00004: Linear Stability of a Compressible Coaxial Jet with Continuous Velocity and Temperature Profiles Dominique Perrault-Joncas, Sherwin Maslowe The stability of compressible coaxial jets is pertinent to the jet noise problem for turbofan engines. Because the bypass stream mixes with both the exhaust and the ambient air, multiple inflection points occur in the velocity and temperature profiles. In accordance with Rayleigh's theorem for axisymmetric incompressible shear flows, it turns out that there are three possible modes, only two of which are unstable. We have computed the properties of these modes when the exhaust flow is at Mach one with profiles corresponding to the experiments reported by Papamoschou (AIAA-2003-1059). The variation with the diameter ratio and other parameters will be shown. An integration contour indented in the complex plane was required to deal with wavenumbers for which modes are slightly amplified or neutral. It was found that the less amplified mode is important in that there is instability for a much greater range of axial wavenumber. [Preview Abstract] |
Monday, November 21, 2005 2:12PM - 2:25PM |
HQ.00005: Buoyancy Effects on the Instability of Low-Density Gas Jets Ramkumar Parthasarathy, Kasyap Pasumarthi A low-density gas jet injected into a high-density ambient gas is known to exhibit self-excited global oscillations for certain conditions. The primary objective of the proposed research is to study buoyancy effects on the origin and nature of this flow instability. Linear stability analysis was used for this purpose. The flow was assumed to be non-parallel. Viscous and diffusive effects were ignored. The effects of the inhomogeneous shear layer and the Froude number (signifying the effects of gravity) on the temporal and spatio-temporal results were delineated. A decrease in the density ratio (ratio of the density of the jet to the density of the ambient gas) resulted in an increase in the temporal amplification rate of the disturbances. The temporal growth rate of the disturbances increased as the Froude number was reduced. The absolute instability characteristics of the jet indicated positive absolute temporal growth rates at all Froude numbers and different axial locations. As buoyancy was removed, the previously existing absolute instability disappeared at all locations establishing buoyancy as the primary instability mechanism in self-excited low-density jets. [Preview Abstract] |
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