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 M18: Vortex Dynamics: Applications |
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Chair: Kamran Moseni, University of Florida Room: 2004 |
Tuesday, November 25, 2014 8:00AM - 8:13AM |
M18.00001: Vortex shedding from cylinders with circular fins Jeff McClure, Serhiy Yarusevych The flow development around uniform cylinders with circular fins is investigated experimentally. Finned cylinders with a diameter ratio ($D/d)$ of two, a fin thickness ratio ($t/d)$ of 0.027, and a range of fin pitch ratios 0.083 $\le c/d \le $ 1.0 are studied at a Reynolds number (\textit{Re}$_{D})$ of 3150, which pertains to the separated shear layer transition regime. All experiments are performed in a water flume facility using time-resolved, two-component, planar Particle Image Velocimetry measurements in spanwise and transverse planes, as well as Laser Doppler Velocimetry. The independent PIV measurements in two different planes capture spatio-temporal development of the main vortical structures in the cylinder wake. A comparative analysis of the results obtained for uniform and finned cylinder models is performed to investigate the effect of circular fins and their spacing on turbulent wake development. The experimental data is used to characterize the near-wake development, vortex formation, and the evolution of coherent structures. The results show that vortex shedding characteristics exhibit strong dependence on the fin pitch ratio and, for a given pitch ratio, differ significantly from those observed for a uniform cylinder of equivalent diameter. [Preview Abstract] |
Tuesday, November 25, 2014 8:13AM - 8:26AM |
M18.00002: Dynamics of Isolated Tip Vortex Cavitation Pepijn Pennings, Johan Bosschers, Tom Van Terwisga Performance of ship propellers and comfort levels in the surroundings are limited by various forms of cavitation. Amongst these forms tip vortex cavitation is one of the first appearing forms and is expected to be mainly responsible for the emission of broadband pressure fluctuations typically occurring between the 4th to the 7th blade passing frequency (approx. 40-70 Hz). These radiated pressure pulses are likely to excite parts of the hull structure resulting in a design compromise between efficiency and comfort. Insight is needed in the mechanism of acoustic emission from the oscillations by a tip vortex cavity. In the current experimental study the tip vortex cavity from a blade with an elliptic planform and sections based on $NACA$ $66_2-415$ with meanline $a=0.8$ is observed using high speed shadowgraphy in combination with blade force and acoustic measurements. An analytic model describing three main cavity deformation modes is verified and used to explain the origin of a cavity eigenfrequency or ``vortex singing'' phenomenon observed by Maines and Arndt (1997) on the tip vortex cavity originating from the same blade. As no hydrodynamic sound originating from the tip vortex cavity was observed it is posed that a tip flow instability is essential for ``vortex singing.'' [Preview Abstract] |
Tuesday, November 25, 2014 8:26AM - 8:39AM |
M18.00003: Circulation based analysis of an axisymmetric, deformable, jetting-cavity body Michael Krieg, Kamran Mohseni Here a methodology for calculating pressure distribution internal to a generic, deformable, axisymmetric, body with an internal cavity region is presented. The pressure distribution is derived by integrating the momentum equation along the axis of symmetry, and then along the cavity boundaries where the velocities are prescribed. Unknown velocity integrals are extracted from the total circulation of characteristic regions, and basic modeling is provided to relate the circulation in these regions to deformation parameters. From the pressure distribution, the total instantaneous jetting force is calculated along with total work required to drive the fluid motion. A prototype jet actuator was designed and tested to determine the circulation in and around the device as well as the actual forces. The total instantaneous forces acting on the actuator are observed to be well modeled by the pressure analysis during both expulsion and refilling phases of the jetting cycle. The functional dependence of total forcing on both jet velocity and acceleration is presented. It was observed that for all phases of the jetting cycle total required work is lower for impulsive velocity programs with fast accelerations than sinusoidal velocity programs with smoother gradual accelerations. Sinusoidal programs result in a peak in pressure (force) at the same instant when the manipulator driving fluid motion is at its maximum velocity; for the impulsive programs these peaks are out of phase and overall energy consumption is reduced. [Preview Abstract] |
Tuesday, November 25, 2014 8:39AM - 8:52AM |
M18.00004: Wake dynamics behind a harbor seal vibrissa: a comparative view by PIV measurements Yingzheng Liu, Shaofei Wang, Hanping Chen A comprehensive study was performed of wake dynamics behind a scaled-up model of harbor seal vibrissa, and the baseline configurations of circular cylinder, wavy cylinder and the elliptical cylinder were provided for comparison. A low-speed water channel and wind tunnel were employed for the model tests at the Reynolds number 102 $\sim$ 104 based on diameter of the cylinder. A load cell and Particle Image Velocimetry were synchronized to measure the fluctuating lift/drag forces and the instantaneous flow field, respectively. By means of the comparative study, the unique three-dimensional wake characteristics in response to contour variations of the harbor seal vibrissa was elucidated through the Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD) analyses of the measured flow field, demonstrating the ability of the vibrissa to suppress the vortex-induced vibration. [Preview Abstract] |
Tuesday, November 25, 2014 8:52AM - 9:05AM |
M18.00005: Harbor seal whiskers synchronize with upstream wake over a range of distances Heather Beem, Michael Triantafyllou Harbor seal whiskers have been shown to exhibit unique vibration properties as they encounter vortex wakes [1]. Seals may use this information to detect hydrodynamic trails left by fish prey. A scaled model, which captures the undulatory morphology of the harbor seal whisker and is designed to freely vibrate, is tested here to explore these properties in more detail. This model is towed downstream of a larger cylinder, which generates a vortex wake. Effects of downstream distance, lateral distance, and diameter ratio between the two objects are explored. Frequency measurements are collected simultaneously through use of a pressure sensor placed in the wake. Cross-correlation of the whisker motion and cylinder wake pressure provides evidence that frequency synchronization holds for a range of separation distances and wake generator sizes. \\[4pt] [1] Beem, H., Triantafyllou, M. (2013). ``Harbor seal whiskers synchronize with frequency of upstream wake,'' Bulletin of the American Physical Society 58. [Preview Abstract] |
Tuesday, November 25, 2014 9:05AM - 9:18AM |
M18.00006: Rotor Vortex Wake in Close Proximity of Walls in Hover Mehmet Fatih Konus, Omer Savas Expanding flight envelopes of rotorcraft raise concerns about their behavior in very close proximity of walls or corners where the separation between the wall and the rotor disk can almost vanish. A series of experiments are conducted in a water tank to study the hover behavior of the wake of a 25-cm diameter three-bladed rotor at 8 rev/s. Particle image velocimetry, strain gage force balance measurements and flow visualization are employed. The vortex wake, which is axisymmetric on the average in an unbounded surrounding, is distorted increasingly with decreasing separation from a wall or corner. The vortex wake bends toward the wall and into the corner. The individual helical filaments off the rotor tips are distorted and closely follow the wall. Intermittent reversed vortical flow regions appear upstream of the rotor disk in the proximity of the wall. The mean streamlines indicate that the wake is bent toward the wall or into the corner. The component of the thrust vector along the axis of the rotor decreases. These observation suggest that the thrust vector progressively deviates from the geometric axis of the rotor. [Preview Abstract] |
Tuesday, November 25, 2014 9:18AM - 9:31AM |
M18.00007: High Re wall-modeled LES of aircraft wake vortices in ground effect Olivier Thiry, Gregoire Winckelmans, Matthieu Duponcheel We have been able to perform wall-resolved LES, using a fourth order code, to simulate (aircraft) wake vortices interacting with the ground, also with cross or head winds, up to Reynolds numbers of the order of $Re=\Gamma/\nu= 2 \times 10^4$. The present work aims at providing higher Re simulations, and also simulations with rough walls (e.g., grass), through the use of LES with near wall modeling. Various types of models are compared: point-wise and averaged algebraic models, and two-layers models. When using averaged models, the averaging methodology is of importance, since there is essentially no homogeneous direction in the case of wake vortices in ground effects. Uni- and multi-directional averaging strategies, with and without additional time averaging will be considered. When two-layer models are used, a RANS sub-layer will be compared to a simpler approach based on simplified turbulent boundary layer equations. The approaches are first validated on simpler flows, channel flow or wake flow, for which reference wall-resolved LES or DNS results are available. [Preview Abstract] |
Tuesday, November 25, 2014 9:31AM - 9:44AM |
M18.00008: Effect of Vorticity Amplification on Flow Separation from Landing Gear Wheels Graham Feltham, Philip McCarthy, Alis Ekmekci The flow near the stagnation point of landing gear wheels has been previously shown to support a mechanism for inbound streams of weak vorticity to collect, growth, and amplify into large-scale discrete vortex structures. The current experimental study is an extension to investigate the effects of these vortex structures on the separation characteristics of the flow around the outboard sides of the wheels. Experiments were performed in a water channel with qualitative understanding of the flow topology achieved by employing the hydrogen bubble visualization technique and quantitative measurements performed using Particle Image Velocimetry (PIV). The upstream vorticity source is a platinum wire (d = 100 $\mu$m) placed 30 mm upstream of the model wheels. The Reynolds number based on wire diameter is 21 and based on wheel diameter (D = 152 mm) is 32,500. The inbound pair of vorticity streams impinged at the wheel surface where maximum vortex growth and amplification occurs as identified by previous experiments. The growth and shedding of the resulting vortical structures is shown to alter the shape and size of the separation bubbles on the outboard sides of the wheels. A vortex identification and tracking method is applied to map the growth and movement of the observed structures. [Preview Abstract] |
Tuesday, November 25, 2014 9:44AM - 9:57AM |
M18.00009: Vorticity amplification near the stagnation point of landing gear wheels: effect of the orientation of the impinging vorticity Mingyao Gu, Graham Feltham, Alis Ekmekci When oncoming streams of weak vorticity aligned with the axle axis of a two-wheel landing gear impinge near the forward stagnation point of the wheels, a mechanism for vorticity collection, growth, amplification into discrete large-scale vortices, and shedding was formerly shown to exist. In the current study, the impinging vorticity streams are perpendicular to the axle axis, i.e. in a vertical orientation as opposed to the horizontal orientation before. Experiments are conducted in a recirculating water channel using hydrogen bubble visualization and particle image velocimetry at a Reynolds number of 32,500 (based on the wheel diameter). As with the horizontal orientation, vorticity collection and amplification are observed, but the large-scale vortices thus formed are stretched around the wheel circumference in contrast to being stretched around the wheel sides, as observed for the horizontal orientation. This flow behavior varies with the impingement location of the vorticity streams across the wheel width. Maximum vorticity amplification occurs at a critical impingement location and drastically alters the flow separation along the wheel circumference. In addition, the instantaneous vortical structures are identified and tracked using a Galilean-invariant criterion. [Preview Abstract] |
Tuesday, November 25, 2014 9:57AM - 10:10AM |
M18.00010: Development and validation of a 2-D compressible vortex particle-mesh method Philippe Parmentier, Gregoire Winckelmans, Philippe Chatelain A compressible hybrid Vortex Particle-Mesh (VPM) method is being developed to study unsteady and fully compressible flows, either confined or unconfined. The particles are advected by the local velocity field and carry the vorticity, dilatation, density and enthalpy fields. They also change volume so as to conserve mass. The velocity is expressed into solenoidal and irrotational components using the Helmholtz decomposition. In the present approach, a Fourier-based method is used to efficiently solve the corresponding Poisson problems; it can handle bounded and unbounded problems. The underlying grid is also used to perform the spatial differential operations (except the lagrangian advection) as well as the redistribution of particles and the particle-mesh operations. The no-slip condition is enforced at solid walls while a nonreflecting boundary condition is used at the far field boundaries. The methodology is validated on prototypical unbounded vortical flows and on a driven cavity flow. [Preview Abstract] |
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