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 G33: Vortex Dynamics: General |
Hide Abstracts |
Chair: Geoffrey Spedding, University of Southern California Room: 2022 |
Monday, November 24, 2014 8:00AM - 8:13AM |
G33.00001: A numerical study of the laminar necklace vortex system and its effect on the wake for a circular cylinder Gokhan Kirkil, George Constantinescu Large Eddy Simulation is used to investigate the structure of the laminar horseshoe vortex (HV) system and the dynamics of the necklace vortices as they fold around the base of a circular cylinder mounted on the flat bed of an open channel for Reynolds numbers defined with the cylinder diameter, D, smaller than 4,460. The study concentrates on the analysis of the structure of the HV system in the periodic breakaway sub-regime which is characterized by the formation of three main necklace vortices. For the relatively shallow flow conditions considered in this study (H/D 1, H is the channel depth), at times, the disturbances induced by the legs of the necklace vortices do not allow the SSLs on the two sides of the cylinder to interact in a way that allows the vorticity redistribution mechanism to lead to the formation of a new wake roller. As a result, the shedding of large scale rollers in the turbulent wake is suppressed for relatively large periods of time. Simulation results show that the wake structure changes randomly between time intervals when large-scale rollers are forming and are convected in the wake (von Karman regime), and time intervals when the rollers do not form. [Preview Abstract] |
Monday, November 24, 2014 8:13AM - 8:26AM |
G33.00002: A Generic Mechanism for Splitting and Shedding of Vortices and Recirculation Regions Kevin Cassel, Michael Boghosian Vortex shedding is a common feature in many high-Reynolds number internal and external flows. While several mechanisms have been put forth to explain this important phenomenon in specific settings, a general framework that unifies these mechanisms and applies in a broad class of flows has not been forthcoming. A surprisingly simple minimal flow unit is identified in the present study and shown to apply in a wide variety of settings in which vortices or recirculation regions are found to split and shed in the vicinity of smooth surfaces. There are two necessary conditions for this mechanism: 1) a region of low momentum fluid (as found at the center of a slowly moving vortex or recirculation region), and 2) a pressure or body force having a particular structure acting on the region of low momentum. While the impetuous for the pressure or body force may vary, its action on the vortex or recirculation region is generic. The basic framework is illustrated and causality established through calculation of several simple model problems, and computational results for some canonical flows, such as shedding behind a circular cylinder and flow over a forward-facing step, are used to illustrate how the generic mechanism can be identified in real flows. [Preview Abstract] |
(Author Not Attending)
|
G33.00003: Wake states and forces associated with a cylinder rolling down an incline under gravity Farah Yasmina Houdroge, Mark Thompson, Kerry Hourigan, Thomas Leweke The flow around a cylinder rolling along a wall at a constant velocity was recently investigated by Stewart et al. (JFM, 643, 648, 2010). They showed that the wake structure varies greatly as the Reynolds number was increased, and that the presence of the wall as well as the imposed motion of the body have a strong influence on the dominant wake structure and the wake transitions when the body is placed in free stream. In this work, attention is given to the flow dynamics and the fluid forces associated with a cylinder rolling down an incline under the influence of gravity. Increasing the inclination angle or the Reynolds number is shown to destabilize the wake flow. For a body close to neutrally buoyancy, the formation and shedding of vortices in its wake result in fluctuating forces and a final kinematic state in which the body's velocity is not constant. The non-dimensionalization of the main equations allows us to determine the essential parameters that govern the problem's dynamics. Furthermore, through numerical simulations we analyse in more detail the time-dependant fluid forces and the different structures of the wake in order to gain a better understanding of the physical mechanisms behind the motions of the fluid and the body. [Preview Abstract] |
Monday, November 24, 2014 8:39AM - 8:52AM |
G33.00004: Shear-thinning effects on vortex breakdown in swirling pipe flows: experiments and simulations David Dennis, Tom Petit, Deacon Thompson, Robert Poole Laminar pipe flow with a controllable wall swirl has been studied both numerically and experimentally to explore the behaviour of inelastic shear-thinning fluids. The pipe consists of two smoothly joined sections that can be rotated independently about the same axis. The circumstances of flow entering a stationary pipe from a rotating pipe (decaying swirl) and flow entering a rotating pipe from a stationary pipe (growing swirl) have been investigated. A numerical parametric study using a simple power law model is conducted and reveals the axial length of the recirculation region is increased for shear-thinning fluids and decreased for shear-thickening (in comparison to the Newtonian reference). The critical swirl ratio required to induce the breakdown at a range of Reynolds numbers and extent of shear-thinning is investigated and a method of scaling is presented that collapses all the data for all fluids (shear-thickening, Newtonian and shear-thinning) onto a single universal curve. Experimental visualisations using an aqueous solution of Xantham Gum (shear-thinning) confirm the conclusions drawn from the numerical results. [Preview Abstract] |
Monday, November 24, 2014 8:52AM - 9:05AM |
G33.00005: Wall-separation and vortex-breakdown zones in a solid-body rotation flow in a rotating pipe Zvi Rusak, Shixiao Wang The axisymmetric dynamics of perturbations on a solid-body rotation flow with a uniform axial velocity in a rotating, finite-length circular pipe is studied via global analysis methods and numerical simulations. We first describe the bifurcation diagram of steady-state solutions of the flow problem as a function of the swirl ratio $\omega$. We prove that the base columnar flow is a unique steady-state solution when $\omega$ is below a critical level, $\omega_1$. This state is asymptotically stable and a global attractor of the flow dynamics. However, when $\omega>\omega_1$, we reveal, in addition to the base columnar flow, the co-existence of states that describe swirling flows around either centerline stagnant breakdown zones or wall pseudo-stagnant zones. The base columnar flow is a min-max point of the energy functional that governs the problem while the swirling flows with wall-separation and breakdown zones are global and local minimizer states and attractors of the flow dynamics. We also find additional min-max states that are transient attractors of the flow dynamics. The wall-separation states have same chance to appear as that of the breakdown states and there is no hysteresis loop between these states. [Preview Abstract] |
Monday, November 24, 2014 9:05AM - 9:18AM |
G33.00006: The effect of aspect ratio on vortex rings within the wake of impulsively-started flat plates John Fernando, David Rival Vortex pinch-off has been the focus of many studies since it was first observed for vortices produced via piston-cylinder arrangements. Minimal work has been performed on other vortex generation methods. The current study investigates vortex rings behind impulsively-started circular, square, and elliptical flat plates. Preliminary force and PIV measurements show temporal/spatial similarities between vortex growth in the wake of the circular and square plates. Forces and vortex evolution are also shown to be strongly coupled; the presence of stable wake vortex rings results in a reduction of plate drag. For all three plates, pinch-off is initiated by the formation of a positive pressure gradient on the leeward side of the plate, which terminates mass transport to the vortex. It is hypothesized that an increase in aspect ratio (AR) from unity results in isolated vortex lines with non-uniform vorticity along the leading edges. Strong spanwise velocity gradients and stretching near the plate tips facilities vortex detachment. Results from experiments on rectangular plates with varying ARs are discussed and the effect of stretching and tilting in the tip region is investigated. [Preview Abstract] |
Monday, November 24, 2014 9:18AM - 9:31AM |
G33.00007: Vortex Formation Behind an Inclined 2-Dimensional Thin Flat Plate Meraj Mohebi, David H. Wood, Robert J. Martinuzzi Stereo Particle Image Velocimetry was used to measure the turbulent wake of a 2D flat plate inclined relative to a uniform stream as a heuristic model for airfoils and wind turbine blades at high incidence. Phase Averaging was performed to study the vortex dynamics and relate these to the force characteristics. Below 90$^{\circ}$, immediately behind the plate, rounder and more organized trailing edge vortices form which possess higher circulation and are associated with higher Reynolds stresses than the counter-rotating, weaker and elongated leading edge vortices. The quasi-periodically shed vortices on the sides of the wake decay in strength at different rates to reach a circulation ratio of -1 within a distance less than 5 chords downstream of the plate for all angles. This equalization of vortex strength is related to an increase in turbulence diffusion, due to mostly-incoherent 3-dimensionality which progressively increases as the inclination angle is reduced, and convective transfer of vorticity between counter-rotating vortices. The wake experiences a sudden change in vortex formation mechanism at around 40$^{\circ}$. At this angle, the frequency analysis on the signals of a pair of micro-pressure transducers in the wake also shows a discontinuity in the trends. [Preview Abstract] |
Monday, November 24, 2014 9:31AM - 9:44AM |
G33.00008: Experimental study of periodic flow effects on spanwise vortex Cruz Daniel Garcia Molina, Erick Javier Lopez Sanchez, Gerardo Ruiz Chavarria, Abraham Medina Ovando We present an experimental study about the spanwise vortex produced in a flow going out of a channel in shallow waters. This vortex travels in front of the dipole. The velocity field measurement was done using the PIV technique, and DPIVsoft (https://www.irphe.fr/$\sim$meunier/) was used for data processing. In this case the flow has a periodic forcing to simulate ocean tides. The experiment was conducted in a channel with variable width and the measurements were made using three different values of the aspect ratio width-depth. We present results of the position, circulation of this spanwise vortex and the flow inversion effect. The change of flow direction modify the intensity of the vortex, but it does not destroy it. The vertical components of the velocity field contributes particle transport. [Preview Abstract] |
Monday, November 24, 2014 9:44AM - 9:57AM |
G33.00009: Flow Interference between a Square (Upstream) and a Circular Cylinder: Flow Pattern Identification Nithin S. Kumar, Ajith Kumar R, Jayalakshmi Mohan In this paper, flow interference between an upstream square cylinder and a circular cylinder of equal size is studied in tandem arrangement. The main objective of this invesigation is to identify the possible flow patterns at different spacing ratios, L/B where L is the centre-to-centre distance between the cylinders and B is the characteristic dimension of the bodies. All the experiments are conducted in a water channel and the test Reynolds number is 2100 (based on B). L/B is varied from 1.0 to 5.0. The flow visualization experiments are videographed and then analyzed frame-by-frame to capture the finer details of the flow patterns. Flow over single square and circular cylinders is analyzed first. Then, flow interference between two square cylinders is investigated. Subsequently, flow over a square-circular configuration is investigated. No such systematic studies are reported so far. Different flow patterns are observed for the square-circular configuration. Additionally, the time of persistence of each flow pattern have been recorded over a sufficiently long period of time to see the most dominant flow pattern. The schedule of occurrence of flow patterns have also been studied during this investigation. This study bears considerable practical relevance in the context of possible interference effects occurring in engineering structures such as buildings, bridges etc. [Preview Abstract] |
Monday, November 24, 2014 9:57AM - 10:10AM |
G33.00010: Flow Interference between a Circular (Upstream) and a Square Cylinder: Flow Pattern Identification Jayalakshmi Mohan, Ajith Kumar R, Nithin S Kumar In this paper, flow interference between an upstream circular cylinder and a square cylinder of equal size is studied in tandem arrangement. The main objective of this invesigation is to identify the possible flow patterns at different spacing ratios, L/B where L is the centre-to-centre distance between the cylinders and B is the characteristic dimension of the bodies. All the experiments are conducted in a water channel and the test Reynolds number is 2100 (based on B). L/B is varied from 1.0 to 5.0. The flow visualization experiments are videographed and then analyzed frame-by-frame to capture the finer details of the flow patterns. Flow over single square and circular cylinders is analyzed first. Then, flow interference between two circular cylinders is investigated. Subsequently, flow over a circular-square configuration is investigated. No such studies are reported so far. Different flow patterns are observed for the circular-square configuration. Additionally, the time of persistence of each flow pattern have been recorded over a sufficiently long period of time to see the most dominant flow pattern. The schedule of occurrence of flow patterns have also been studied during this investigation. This study is very much relevant in the context of possible interference effects occuring in engineering structures such as buildings, heat exchanger tubes etc. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700