Vortex-Boundary Interactions
Experimental Fluid Dynamics Laboratory
Department of Mechanical Engineering
Southern
Projects:
· Vortex Rings Impinging on a Thin Screen
· Vortical and Turbulent Flow through a Permeable Matrix
· Vortex Rings Impinging on an Inclined Surface
Vortex Rings Impinging on a Thin Screen
A vortex ring impinging on a solid surface normal to the axis of the ring is a classical problem illustrating a variety of interesting vortex dynamics including generation of secondary vorticity, unsteady separation, vortex rebound, etc. A problem intermediate between a vortex ring propagating in clear fluid and impinging on a solid surface is a vortex ring impinging on a thin permeable screen. As the screen allows exchange of fluid across the interface, the vortex dynamics are altered in unique ways. This project investigated the dynamics of vortex rings impinging on thin screens using planar laser induced fluorescence (PLIF) and digital particle image velocimetry (DPIV) for screens with a range of open area ratios (f). The screens were simple rectangular arrays of thin rods (like screen-door screens).
PLIF of a vortex ring interacting with a f = 0.58 screen is shown in the images below. The trajectories of the primary (VR1), secondary (VR2), tertiary (VR3), and transmitted (VRT) vortices are indicated by the colored lines. The new flow behavior resulting from the permeability of the interface include the formation of a transmitted vortex ring, a switch in the relative positions of the secondary and tertiary vortices (which is a direct result of fluid exchange across the screen), and the upstream propagation of the secondary vortex. The transmitted vortex ring propagates at a much slower velocity, indicative of the energy dissipated by the interaction with the screen. The orderly nature of the flow during the interaction process is noteworthy. In the case of a solid boundary, the flow breaks down and the vortices disintegrate rapidly following the interaction with the boundary.
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DPIV was used to quantitatively assess the kinetic energy dissipated by the vortex ring interaction with the screen for screens with several different f. The difference in the kinetic energy between the initial vortex ring and the downstream (transmitted) flow (DE) was computed from the DPIV data assuming axisymmetry and is plotted below for several cases. The sharp drop in DE as f increases indicates the strong sensitivity of the flow to f. Even very open screens effected substantial energy dissipation. This behavior may be useful for tailoring flow conditioning devices or developing passive flow control strategies for highly turbulent flows.
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Personnel: Christian Naaktgeboren
Collaborator: José Lage (SMU)
Conference Proceedings:
C. Naaktgeboren,
P.S. Krueger, and J.L. Lage, “Experimental Investigation of Vortex Ring Interaction
with a Permeable Flat Surface,” 3rd International Conference on Applications of
Porous Media, May, 2006,
C. Naaktgeboren, A.B. Olcay, P.S. Krueger, and J.L.
Lage, “Vortex Ring Interaction with a Permeable Flat Surface,” 58th Annual Meeting of the
Division of Fluid Dynamics, November, 2005, Chicago, IL.
C. Naaktgeboren, A.B. Olcay, P.S. Krueger, “Vortex
Rings Impinging on Solid and Porous Boundaries,” (Video Entry) Gallery of Fluid Motion, 58th Annual Meeting of the
Division of Fluid Dynamics, November, 2005, Chicago, IL.
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Acknowledgement: This material is based upon work supported by the National Science Foundation under Grant No. 0347958. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.
Vortical and Turbulent Flow through a Permeable Matrix
Flows through permeable media (such as filters and particle beds or arrays of elements like screens or forested trees) are a challenge to study and model because of the complex geometry (making numerical computation difficult) and opacity of most materials (making optical flow measurements impossible). This project aims to tackle (or at least dent) both problems by a combined numerical/experimental study using an array of permeable screens as a model permeable media. The experimental portion of the investigation is studying vortex rings and steady jet flows interacting with the permeable matrix in order to understand the interaction of large scale vortical structures and turbulence with highly permeable media. The screens are transparent and the working fluid is refractive-index matched with the screens to allow digital particle image velocimetry (DPIV) measurements of the flow field within the matrix. The flow field data will be used for verifying and tuning closure models for simulating turbulence in porous media. The net outcome of this combined numerical/experimental effort will be a validated closure model that captures the key physics of flow through complex, permeable media without requiring fine-scale resolution of the solid-boundary geometry. Such results will be useful for understanding and modeling a wide variety of practical flows including pollutant dispersion in urban landscapes, microbursts from thunderstorm activity over dense vegetation, unsteady combustion in or near porous materials, pulsatile jet-drying of textiles, and pulsed jet agitation of clothing for trace contaminant sampling.
Schematics of the screen design and frame to hold the permeable matrix are
shown below. The screens can be removed
from the matrix and interchanged with screens of different open area ratios or
different rod diameters. By leaving
slots in the frame empty, the spacing between screens can be changed. Currently DPIV measurements are being made
for vortex rings interacting with different screen configurations.
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Isometric view of a sample screen. |
Isometric view of the assembled matrix. |
Personnel:
Collaborator (Numerical Component): José Lage (SMU)
Conference Proceedings:
M.N. Musta and P.S. Krueger, “Vortex
Ring Interaction with Multiple Permeable Screens,” 61st annual meeting of the American Physical Society
Division of Fluid Dynamics, November, 2008,
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Acknowledgement: This material is based upon work supported by the National Science Foundation under Grant No. 0652046. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.
Vortex Rings Impinging on an Inclined Surface
This project will consider
multiple vortex rings impinging on an inclined surface as a model for the
interaction of coherent structures in turbulent boundary layers. Fully 3D volumetric measurements of the flow
evolution will be made using the TSI V3VTM
system purchased as part of an NSF Major Research Instrumentation (MRI) grant.
A preliminary result for a vortex
ring impinging on a plate inclined at approximately 30° relative to the
initial axis of the ring is shown below.
The figure shows iso-contours of the z-component of vorticity. The plate (not shown) is at the back (left)
of the measurement volume. The ring has
tilted somewhat as it approached the plate (so that the ring axis is more
aligned with the direction normal to the plate) and secondary vorticity
generated by flow separation as the ring approached the plate has begun to wrap
around the primary vortex in the bottom of the image.
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Z-component of vorticity for an
axisymmetric vortex ring impinging on a plate inclined at approximately 30° to
the initial axis of the ring. |
Personnel: Lauren Couch
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Acknowledgement: This material is based upon work supported by the National Science Foundation under Grant No. 0821420. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.
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