Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 55, Number 16
Sunday–Tuesday, November 21–23, 2010; Long Beach, California
Session GE: Instability: Jets and Wakes I |
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Chair: Alis Ekmekci, University of Toronto Room: Long Beach Convention Center 102C |
Monday, November 22, 2010 8:00AM - 8:13AM |
GE.00001: Bifurcation and stability analysis of a jet in crossflow at low velocity ratios Milo\v{s} Ilak, Philipp Schlatter, Shervin Bagheri, Dan Henningson We study an incompressible jet in crossflow at low values of the jet-to-crossflow velocity ratio R using Direct Numerical Simulation and linear stability and sensitivity analysis. A Hopf bifurcation is found to occur slightly above R=0.75, and the frequency of the limit cycle oscillation is the frequency of the unstable linear global eigenmode. We find that the frequency of the limit cycle is the same as that of the most unstable eigenmode even for higher values of R, for which multiple unstable modes are found from the linear stability analysis. Using the leading adjoint global eigenmode near the first bifurcation point, we identify the location of the ``wavemaker,'' i.e., the region in the flow most sensitive to localized feedback. This region, interpreted as the core of the first instability, is found to be in the shear layer that surrounds the counter-rotating vortex pair which is the dominating feature of the dynamics of the jet. Furthermore, we perform a sensitivity analysis of the most unstable eigenvalue near the bifurcation point using the first direct-adjoint eigenmode pair. [Preview Abstract] |
Monday, November 22, 2010 8:13AM - 8:26AM |
GE.00002: Unsteady Simulation of an ASME Venturi Flow in a Cross Flow Jeremy Bonifacio, Hamid Rahai Unsteady numerical simulations of an ASME venturi flow into a cross flow were performed. The velocity ratios between the venturi flow and the free stream were 25, 50, and 75{\%}. Two cases of the venturi with and without a tube extension have been investigated. The tube extension length was approximately 4D (here D is the inner diameter of the venturi's outlet), connecting the venturi to the bottom surface of the numerical wind tunnel. A finite volume approach with the Wilcox K-$\omega $ turbulence model were used. Results that include contours of the mean velocity, velocity vector, turbulent kinetic energy, pressure and vortices within the venturi as well as downstream in the interaction region indicate that when the venturi is flushed with the surface, there is evidence of flow separation within the venturi, near the outlet. However, when the tube extension was added, the pressure recovery was sustained and flow separation within the venturi was not present and the characteristics of the flow in the interaction region were similar to the corresponding characteristics of a pipe jet in a cross flow. [Preview Abstract] |
Monday, November 22, 2010 8:26AM - 8:39AM |
GE.00003: Low Density Transverse Jet Shear Layer Instabilities and their Control Daniel Getsinger, Cory Hendrickson, Aaron Sung, Owen Smith, Ann Karagozian This work describes an experimental characterization of the instabilities forming in the near-field of the variable density transverse jet shear layer. Jets composed of mixtures of helium and nitrogen are injected from a converging nozzle mounted flush with an injection wall, issuing into an air crossflow. The jet-to-crossflow density ratio $S$ is varied between 1.00 and 0.14 by changing the proportions of nitrogen and helium. Jet-to-crossflow momentum flux ratios $J$ are varied in the range $\infty>J\geq2$ at each value of $S$. The results of hotwire anemometry measurements in the jet shear layer indicate that a transition from convective to absolute instability has dependence on the jet-to-crossflow momentum flux ratio $J$ and separate dependence upon the density ratio $S$. This transition, in a similar fashion to that examined in the equidensity transverse jet\footnote{Davitian, et al., {\bf JFM}, to appear, 2010} and in other types of shear flows, is characterized by several clear spectral features, including sharp spectral peaks, resistance to low level acoustic forcing for the globally unstable (self-excited) case, and broadband oscillations with high receptivity to applied forcing for the convectively unstable case. The presence of crossflow is observed to alter the global instability transition classically observed in low density free jets, allowing it to occur at higher density ratios $S$ as $J$ is reduced. [Preview Abstract] |
Monday, November 22, 2010 8:39AM - 8:52AM |
GE.00004: Sensitivity analysis of low-density reacting jets Gary Chandler, Joseph Nichols, Matthew Juniper, Peter Schmid A Low-Mach-number formulation of the Navier--Stokes equations is used to simulate an axisymmetric low-density jet diffusion flame that exits into stationary surroundings through a hole in a flat solid wall. A lifted flame that is marginally-stable in a hydrodynamic sense is considered. The equations are linearized about a steady solution of the nonlinear system and a corresponding set of adjoint equations is formed. Direct-linear and adjoint global modes are found with direct numerical simulation (DNS) and provide a map of the most sensitive locations to external forcing and external heating. Acoustic excitation is modeled as an external force in the momentum equation and a map of the most sensitive locations of the flame to acoustic excitation is given. The most sensitive locations to force feedback and to heat and drag from a hot-wire are then analyzed. Force feedback can occur from the placement of a sensor-actuator in the flow or can be considered as a mechanism for global instability. The lifted flame is particularly sensitive to outside disturbances and acoustic forcing in the non-reacting zone. [Preview Abstract] |
Monday, November 22, 2010 8:52AM - 9:05AM |
GE.00005: Global modes of compressible subsonic jets Xavier Garnaud, Lutz Lesshafft, Peter J. Schmid, Patrick Huerre Global instability modes are computed for spatially developing jets at high subsonic Mach number. Both isothermal and hot configurations are considered. The jet exits a cylindrical nozzle, which is included in the numerical domain. Particular attention is directed to the aero-acoustic features of the jet, and the acoustic far-field is resolved as part of the global mode. Accurate resolution of sound propagation requires large computational domains, as well as high-order discretization schemes, which is numerically challenging with existing techniques, in particular in terms of memory requirements. We present a novel method for the computation of direct and adjoint eigenmodes of the global instability problem. Temporal filtering, applied to a time-stepping approach, allows to extract user-selected modes at significantly lower computational cost than common matrix-based techniques. [Preview Abstract] |
Monday, November 22, 2010 9:05AM - 9:18AM |
GE.00006: Analysis of time-resolved tomographic PIV data of a transitional jet Peter Schmid, Daniele Violato, Fulvio Scarano Time-resolved tomographic particle image velocimetry (TR-TOMO-PIV) data of a transitional water jet at a Reynolds number of Re = 1500 have been obtained capturing all relevant spatial and temporal scales. These flow fields have then been processed by the dynamic mode decomposition (DMD) which extracts frequencies and associated coherent structures that constitute a significant part of the overall dynamics. Three data sets, covering the primary instability near the nozzle, the rise of secondary features further downstream and the breakdown into turbulence, have been analyzed, and frequency distributions and principal flow structures will be presented. Besides a temporal analysis of the data, a spatial analysis (in the axial direction) will be performed and compared to the findings from the temporal framework. [Preview Abstract] |
Monday, November 22, 2010 9:18AM - 9:31AM |
GE.00007: Swirling Turbulent Jet Structure Characterization Using Proper Orthogonal Decomposition of Flow Visualization Images Jonathan Naughton, Richard Semaan Planar Laser Scattering (PLS) was used to acquire instantaneous cross-sectional images of a swirling jet. The jet was seeded with small oil droplets whereas the ambient air was unseeded. Light from a laser sheet passing through the jet was scattered from the droplets and imaged using a camera. Proper Orthogonal Decomposition (POD) was applied to $\sim$1000 images to determine the energy containing structure in the flow. A POD implementation was used that took advantage of axisymmetry to assure structure consistent with the flow. The results indicate that jets with a degree of swirl that exceeded a certain threshold exhibit an increased importance of the second azimuthal mode in the near-field as compared to a non-swirling jet. At distances further downstream, the mode two dominance decreases and mode one has an increased importance. Reconstructions of the swirling jet using the dominant modes provides evidence that the swirling jet contains very different turbulent structure in the near field as compared to non- swirling jets. The findings of this work are consistent with recent experimental and theoretical studies and provide guidance for future studies characterizing the structure responsible for swirling jet's unique behavior. [Preview Abstract] |
Monday, November 22, 2010 9:31AM - 9:44AM |
GE.00008: Self excited oscillations in swirling jets: Stability analysis and empirical mode construction Kilian Oberleithner, Moritz Sieber, Christian Navid Nayeri, Christian Oliver Paschereit, Christoph Petz, Hans Christian Hege, Bernd Noack, Israel Wygnanski Swirling jets undergoing vortex breakdown are known to be dominated by strong harmonic oscillations. Our experiments suggest the existence of a self-excited global mode having a single dominant frequency. The wave-maker of this oscillatory mode is found to be located in the jet center causing the swirling jet to precess. The oscillations trigger a convectively unstable co-rotating counter-winding helical structure that is located at the periphery of the recirculation zone. The resulting time-periodic 3D velocity field is predicted theoretically by employing linear stability analysis. It compares remarkably well to empirical 3D-modes that were constructed from uncorrelated 2D snapshots of PIV data, using proper orthogonal decomposition (POD). Stability analysis is further employed to detect regions of absolute instability and to derive the temporal growth-rate of the global mode. Results are compared to time-resolved measurements of the transient growth of the global mode. [Preview Abstract] |
Monday, November 22, 2010 9:44AM - 9:57AM |
GE.00009: Acoustic-gravity waves generated by wake flows Christophe Millet, St\'ephane Le Diz\`es The wavy wall analogy framework is used to obtain a model problem for the acoustic-gravity wave field generated by a three-dimensional wave packet, that may be seen as a model for wake flow instabilities. In this study, we use asymptotic methods to analyse the manner in which the pressure field structure changes, and more specifically, we estimate the properties of acoustic and gravity waves in terms of saddle-point contributions. The saddle-points are computed from the general dispersion relation that we deduce from a compressible model with earth rotation and non-Boussinesq effects. Particular attention is paid to the far-field limit for which a single saddle-point contribution enables the description of both acoustic and gravity waves, also depending on the streamwise phase velocity of the wave packet. The transition from low-frequency acoustic waves (or infrasounds) to gravity waves can be treated in the same way as the acoustic radiation of supersonic two-dimensional shear layers. [Preview Abstract] |
Monday, November 22, 2010 9:57AM - 10:10AM |
GE.00010: Evolution of the turbulent/non-turbulent interface of an axisymmetric turbulent jet M. Khashehchi, A. Ooi, I. Marusic, J. Soria Measurements of a turbulent round air jet, using Particle Image Velocimetry (PIV), were made in order to investigate the dynamics and transport processes at the continuous and well-defined bounding interface between the turbulent and non-turbulent regions (T/NT) of the flow. The jet Reynolds number was \textit{Re}$_{D}=3000$ and the measurements were made between $0$ and $50$ nozzle diameters from the nozzle exit. A velocity thresholding technique was used and found to compare well against available results obtained using similar detection criterion reported in the literature. The evolution of the coherent turbulent structures at the interface is considered as the jet evolves from the nozzle, as indicated by the conditionally averaged streamwise velocity, azimuthal vorticity, turbulent intensity and Reynolds shear stress across the interface. A clear change in behaviour is noted going from the near-field region, $x/d=0$ to $8$, to the far field (self-similar) region. These will be described and discussed. [Preview Abstract] |
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