Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session D12: Vortex Dynamics and Vortex Flows II |
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Chair: Lorenz Sigurdson, University of Alberta Room: 336 |
Sunday, November 24, 2013 2:15PM - 2:28PM |
D12.00001: Vortex rings impinging on porous boundaries Stuart Dalziel, Anna Mujal Colilles Vortex rings and their collisions with simple, rigid boundaries have long been studied, both in their own right and as a prototype for turbulent interactions with boundaries. Over the last few years this paradigm has been extended to the impact of vortex rings on a bed of particles. Of principal interest here has been the resuspension/erosion of the particle layer. While in some parameter ranges the boundary may still appear to the vortex ring as though it is a simple rigid solid, the reality is that even if the particles do not move the boundary will be porous. Through a series of experiments, this paper explores some aspects of how the interaction between a vortex ring and a boundary is modified when the boundary is porous. The study is fundamental, and while motivated initially by the impact of a ring on sediment layers, the interaction of vortical structures and turbulence with porous boundaries has much broader applications. [Preview Abstract] |
Sunday, November 24, 2013 2:28PM - 2:41PM |
D12.00002: ABSTRACT WITHDRAWN |
Sunday, November 24, 2013 2:41PM - 2:54PM |
D12.00003: Simulation of the Initial 3-D Instability of an Impacting Drop Vortex Ring Lorenz Sigurdson, Justin Wiwchar, Jens Walther Computational vortex particle method simulations of a perturbed vortex ring are performed to recreate and understand the instability seen in impacting water drop experiments. Three fundamentally different initial vorticity distributions are used to attempt to trigger a Widnall instability, a Rayleigh centrifugal instability, or a vortex breakdown-type instability. Simulations which simply have a perturbed solitary ring result in an instability similar to that seen experimentally. Waviness of the core which would be expected from a Widnall instability is not visible. Adding an opposite-signed secondary vortex ring or an image vortex ring to the initial conditions, to trigger a Rayleigh or breakdown respectively, does not appear to significantly change the instability from what is seen with a solitary ring. This suggests that a Rayleigh or vortex breakdown-type instability are not likely at work, though tests are not conclusive. Perhaps the opposite-signed secondary vortex was not strong enough or placed appropriately. Elliptical streamlines , as expected, are visible in the core of the solitary ring at early times. Support from the Canadian Natural Sciences and Engineering Research Council grant 41747 is gratefully acknowledged. [Preview Abstract] |
Sunday, November 24, 2013 2:54PM - 3:07PM |
D12.00004: Amplification of Vorticity Near the Stagnation Point of Landing Gear Wheels Graham Feltham, Alis Ekmekci In this experimental investigation, a stream of steady weak vorticity impinging near the stagnation point of a landing gear wheel is shown to grow and amplify into large-scale vortices that coherently shed from the point of generation. To produce the upstream vorticity, a platinum wire of 100 micron diameter, similar to that used in hydrogen bubble visualization technique, is placed upstream of the wheel model. Experiments are conducted in a recirculating water channel. The wheel diameter is D = 152 mm. The Reynolds number based on the wire diameter is 21 and based on the wheel diameter is 32,500. Qualitative understanding of the vorticity amplification and eventual vortex shedding near the stagnation region of the wheel is achieved by employing the hydrogen bubble visualization technique while quantitative insight is collected using Particle Image Velocimetry (PIV). The size and frequency of the shed vortices are found to depend on the wheel geometry as well as the magnitude and impingement point of the inbound vorticity. [Preview Abstract] |
Sunday, November 24, 2013 3:07PM - 3:20PM |
D12.00005: Large eddy simulation of a vortex ring impinging on a bump Xi-Yun Lu, Heng Ren Large eddy simulation of a vortex ring impacting a three-dimensional bump has been carried out for different bump heights and vortex core thicknesses related to thin and thick vortex rings. Various fundamental mechanisms dictating the flow behaviors, including the dynamics and instability of vortex ring, the evolution of vortical structures, and the flow transition from laminar to turbulent state, have been studied systematically. Based on the analysis of the evolution of vortical structures, the formations of loop-like vortices wrapping around the primary and secondary vortex rings and the hair-pin vortices due to the severe distortion of the secondary ring are investigated. The circulation of the vortex ring reasonably elucidates some typical phases of flow evolution. Further, the analysis of turbulent kinetic energy reveals the transition from laminar to turbulent state. [Preview Abstract] |
Sunday, November 24, 2013 3:20PM - 3:33PM |
D12.00006: Interaction of a Vortex Ring Parallel to a Plane Wall Mitchell Albrecht, Douglas Bohl In this work, Laser Induced Fluorescence (LIF) is used to investigate the motion of a vortex ring parallel to a plane wall. When the wall is more than 1.75 generator diameters (D$_{\mathrm{gen}})$ away from the center of the generator, there is no observed effect on the path of the vortex ring. When the wall is closer, the vortex ring initially convects parallel to the wall and then turns towards the wall. The location at which the ring begins to turn towards the wall is a function of the wall location. This motion is consistent with inviscid theory. For moderate distance (1.75 D$_{\mathrm{gen}}$ to 0.75 D$_{\mathrm{gen}})$ both legs of the vortex ring break up before interacting with the wall. When the wall is very close to the vortex ring (\textless 0.75 D$_{\mathrm{gen}})$, the leg of the vortex ring closest to the wall first moves towards, then bounces and moves away from the wall. Meanwhile, the leg farthest from the wall continues towards the wall and interacts, forming boundary layer and new shed structures. This process is qualitatively similar to the interaction of a vortex ring normal to a plane wall. [Preview Abstract] |
Sunday, November 24, 2013 3:33PM - 3:46PM |
D12.00007: Numerical study of vorticity-enhanced heat transfer Xiaolin Wang, Silas Alben Vortices produced by vibrated reeds and flapping foils can improve heat transfer efficiency in electronic hardware. Vortices enhance forced convection by boundary layer separation and thermal mixing in the bulk flow. In this work, we modeled and simulated the fluid flow and temperature in a 2-D channel flow with vortices injected at the upstream boundary. We classified four types of vortex streets depending on the Reynolds number and vortices' strengths and spacings, and studied the different vortex dynamics in each situation. We then used Lagrangian coherent structures (LCS) to study the effect of the vortices on mixing and determined how the Nusselt number and Coefficients of performance vary with flow parameters and Peclet numbers. [Preview Abstract] |
Sunday, November 24, 2013 3:46PM - 3:59PM |
D12.00008: Controlling vortex breakdown in swirling pipe flows: experiments and simulations David J.C. Dennis, Christophe Seraudie, Robert J. Poole A laminar, incompressible, viscous pipe flow with a controllable wall swirl has been studied both numerically and experimentally across a Reynolds number range of 2 to 30. 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. Flow visualisations show that at a certain swirl ratio, which can be different for growing and decaying swirl at the same Reynolds number, vortex breakdown occurs. The variation of this critical swirl ratio with Reynolds number is explored and good agreement is found between the experimental and numerical methods. At high Re the critical swirl ratio tends to a constant value, whereas at low Re the product of the Reynolds number and the square of the swirl ratio tends to a constant value in agreement with an existing analytical solution. For decaying swirl the vortex breakdown manifests itself on the pipe axis, whereas for growing swirl it forms near the pipe wall. The vortex flow formed at critical conditions is found to increase radially and axially with increasing Reynolds number and swirl ratio. [Preview Abstract] |
Sunday, November 24, 2013 3:59PM - 4:12PM |
D12.00009: Flow instability and vortex street in eccentric annular channels George Choueiri, Stavros Tavoularis Flow development in an eccentric annular channel with a diameter ratio of 0.5 has been investigated using flow visualization, two-component laser Doppler velocimetry and planar and stereoscopic particle image velocimetry. The eccentricity $e$ was varied between 0.3 and 0.9 and the Reynolds number was $1000 \leq \mathrm{Re} \leq 18000$. For sufficiently large $e$ and $\mathrm{Re}$, large differences developed between the velocity in the gap region and the one in the rest of the channel; these were accompanied by flow instability and the generation of a quasi-periodic vortex street, which manifested itself by strong cross-flows across the gap and an increase in axial velocity in the gap region, but also affected the flow in the entire channel. The vortex strength was highest for $e\approx 0.7$ and the Strouhal number of the cross-flow oscillations (based on bulk velocity and core diameter) increased with increasing $\mathrm{Re}$, reaching an asymptote near 0.12 for $\mathrm{Re}\geq 10000$. [Preview Abstract] |
Sunday, November 24, 2013 4:12PM - 4:25PM |
D12.00010: Stability Analysis of the Vortex Rope Formed in Draft Tubes Girish Kumar Rajan, John Cimbala Studies on draft tube surge have shown that there are undesirable effects in the form of violent pressure fluctuations caused by a helical vortex (often called the vortex rope), formed in the draft tube due to a shear layer produced by a central stalled region with lesser axial velocities, and the swirling main-flow. The vortex rope is formed when hydroturbines operate away from the best efficiency point, and affects the efficiency of the turbine severely. Thus, in order to reduce these undesired effects of the vortex rope, a proper understanding of its structure and stability is necessary. This project, which is in progress, involves a numerical investigation of the vortex rope and its elimination, and a mathematical analysis that could possibly throw some light on the stability of the rope. Several cases have been simulated in ANSYS-FLUENT with the draft tube geometry obtained from the FLINDT project. It is then possible to obtain the vortex rope parameters as functions of the discharge coefficient. In addition, the simulations are also expected to provide information on the mean velocity field in the draft tube. These relations might also be of some help in the stability analysis, which should identify the modes that are unstable. [Preview Abstract] |
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