Bulletin of the American Physical Society
2006 59th Annual Meeting of the APS Division of Fluid Dynamics
Sunday–Tuesday, November 19–21, 2006; Tampa Bay, Florida
Session KI: Vortex Dynamics and 3D Vortex Flows VI |
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Chair: Thomas Corke, University of Notre Dame Room: Tampa Marriott Waterside Hotel and Marina Meeting Room 5 |
Monday, November 20, 2006 5:15PM - 5:28PM |
KI.00001: A two--dimensional model problem to explain the evolution of a jet in crossflow Krishnan Mahesh, Suman Muppidi A two-dimensional model problem is used to study the evolution of the cross-section of a transverse jet and the counter-rotating vortex pair (CVP). The solution to the model problem shows deformation of the jet similar to that observed in a transverse jet, and also yields a CVP. These phenomena are explained in terms of the acceleration the jet experiences in the direction of the crossflow, and the pressure field around the jet. The initial stages of the jet's evolution are at constant acceleration while the later stages are at constant velocity. The effects of Reynolds number and velocity ratio on the evolution of the jet are used to explain the dependence of CVP formation on velocity ratio as observed experimentally. [Preview Abstract] |
Monday, November 20, 2006 5:28PM - 5:41PM |
KI.00002: Interaction of a Crossflowing Jet with a Downstream Fin on a Full-Scale Flight Vehicle Configuration Steven Beresh Stereoscopic particle image velocimetry data have been acquired for studying jet/fin interaction created by exhaust plumes from spin rockets on a full-scale model of a finned body of revolution. Crossplane measurements just upstream of the leading edge of the fin root display the counter-rotating vortex pair that dominates the far-field of the interaction and the remnant of the horseshoe vortex near the vehicle surface. Velocity fields measured over a range of flow conditions and model orientations show that the vortex of negative sign is always closer to the fins than its positive counterpart and does not greatly change location as flow parameters are altered. The circulation of this vortex correlates with a reduction in the simultaneously-measured vehicle roll torque. Further correlations are hindered by untreatable bias errors in the velocimetry. Instead, a model of the vortex structure derived from the velocimetry data reveals that the angle of attack induced upon the fins by the counter-rotating vortex pair correlates with the roll torque loss. Similar correlations suggest that in level flight this effect is dominant, but at angle of attack the horseshoe vortex on the windward side has an additional influence. [Preview Abstract] |
Monday, November 20, 2006 5:41PM - 5:54PM |
KI.00003: Reynolds Number Dependence of Vortex Ring Formation by Transient Jet Ejection Paul S. Krueger Vortex ring formation by the sudden ejection of a jet from tube and orifice openings is investigated numerically for jet Reynolds number (\textit{Re}) in the range 10 -- 2000 and jet slug length-to-diameter ratios ($L$/$D)$ in the range 0.5 -- 6.0. This \textit{Re} range brackets nearly inviscid behavior (vortex sheet roll-up) at the high end and highly diffusive behavior at the low end. The present investigation is motivated by how the enhanced role of viscosity at low \textit{Re} affects the development and properties of the resulting vortex rings. The results for \textit{Re} = 2000 show classical behavior, namely, compact vortex rings at low $L$/$D$ and a leading vortex ring followed by a trailing jet for $L$/$D$ sufficiently high. As \textit{Re} decreases below 100, viscous diffusion leads to rapid radial growth of the vortex ring trajectories, and rapid decay of total circulation and kinetic energy. For all \textit{Re}, the ratio of the impulse obtained during jet ejection to that from a steady, uniform jet of the same duration increases with $L$/$D$ until a trailing jet appears. The maximum impulse ratio achieved increases as \textit{Re} decreases for the tube configuration, but the opposite trend is observed for the orifice configuration. [Preview Abstract] |
Monday, November 20, 2006 5:54PM - 6:07PM |
KI.00004: Computational Assessment of Orifice Geometry for Ring Vortex Generation John Maryott, Joel Peltier, Eric Paterson, Michael Krane, Arnold Fontaine This talk will present results of a computational assessment of how orifice geometry influences ring vortex formation. The long-term goal is to maximize vortex ring circulation and coherence at high Reynolds number. The computations are performed using acuSolve, a 2nd-order accurate (time {\&} space), finite-element flow solver from acuSim Corporation of Mountain View, CA. Orifice concepts include tangential suction and blowing, as well as divergence angle. Vortex formation behavior for each nozzle configuration is compared to a canonical case of a flow pulse from a straight, sharp-edged nozzle, where the pulse in each case has the same L/D and waveform shape. [Preview Abstract] |
Monday, November 20, 2006 6:07PM - 6:20PM |
KI.00005: Thrust Optimization in Pulsatile Vortex Generators in Liquid Medium Mike Krieg, Torin Clark, Kamran Mohseni Vortex rings are coherent structures effective at transporting momentum, circulation and energy across long distances through a fluid medium. An array of periodic vortex rings can be created by a series of pulsatile jets. Similar jet propulsion is the primary method of movement for Cephalopod such as squid. Inspired by the propulsion of squid and jellyfish we have designed and built vortex generators for propulsion and low speed maneuvering of small underwater vehicles. The vortex generator consists of a cavity with a moving diaphragm on one side and an exit orifice on the other side. The diaphragm or a plunger is activated by an electric motor. As a result, the amplitude, frequency, and profile of the actuated diaphragm are easily controlled. This investigation is focused on identifying the parameters that control the thrust generation in this mechanism and its optimization. A sensitive load cell is employed to directly measure thrust generation while these parameters are varied. It is found that the formation number, actuation frequency, and plunger profile are among the most relevant parameters that control thrust generation. [Preview Abstract] |
Monday, November 20, 2006 6:20PM - 6:33PM |
KI.00006: Simplified Wake Model of a Flapping Wing Thomas Apker, Thomas Corke A vortex wake model consisting of two parts, a strong leading-edge vortex that is shed during the flapping cycle and a continuously attached vortex line determined by quasi-steady lifting line theory was developed. The leading edge vortex is essentially an expression of the Magnus effect, while the strength of the wing-tip vortex is determined by unsteady lifting line theory. Combined, these produce the ``ladder vortex" pattern seen downstream of root-flapping wings with fixed span, such as insects and most man-made flapping wing vehicles. A small flapping wing experimental setup in still air was used to provide experimental comparison to the model. Measurements include flow visualization and velocity obtained using a stereo PIV system. The flapping mechanism was mounted on a two-component force balance to obtain time-resolved lift and thrust. Data were ensemble averaged with the flapping phase cycle and used to calculate vorticity. These were then reconstructed to show the space-time development of vorticity shed from the wing during the flapping motion to compare to the model predictions. [Preview Abstract] |
Monday, November 20, 2006 6:33PM - 6:46PM |
KI.00007: Symmetry properties of the reverse B\'enard-von K\'arm\'an vortex street produced by a flapping foil Ramiro Godoy-Diana, Jean-Luc Aider, Jose-Eduardo Wesfreid We study experimentally the vortex streets produced by a high-aspect-ratio pitching foil placed in a hydrodynamic tunnel. Particle image velocimetry (PIV) measurements give access to the spatio-temporal characteristics of the vorticity field in the wake and allow for a calculation of the spatial distribution of velocity fluctuations as well as an indirect estimate of the forces on the foil. A parametric study in terms of the non-dimensional frequency (the Strouhal number) and amplitude of the flapping motion allows to identify: 1) the transition from the well-known B\'enard-von K\'arm\'an (BvK) wake to the reverse BvK vortex street that characterizes propulsive wakes, and 2) the symmetry breaking of this reverse BvK wake. The latter constitutes a spontaneous mechanism giving rise to a mean vortex-induced lift force on the flapping foil. [Preview Abstract] |
Monday, November 20, 2006 6:46PM - 6:59PM |
KI.00008: Lagrangian Coherent Structures in Vortex Shedding Behind a Two Dimensional Airfoil Blake Cardwell, Kamran Mohseni The processes of turbulent mixing and vortex shedding in the wake region of airfoils greatly influence the dynamics and performance of many engineering devices, including airplanes and turbines. Understanding these processes is the key to improving device performance and efficiency. Lagrangian Coherent Structures (LCS) are employed in this study in order to predict and control the wake structure and vortex shedding over an airfoil. Lagrangian Coherent Structures provide insight into the location and behavior of separation points, fluid transport mechanisms, and are responsible for fluid mixing. This presentation will cover the fundamentals of Lagrangian Coherent Structures and how they have been applied to vortex shedding behind a 2D airfoil. Detailed animations of LCS behavior will be provided, including the movement of passive fluid tracers to illustrate the effect of LCS on fluid mixing. Additionally, a discussion of the physical behavior of these structures, and potential application for flow control will be presented. [Preview Abstract] |
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