Fluid Entrainment by Vortex Rings
Experimental Fluid Dynamics Laboratory
Department of Mechanical Engineering
Southern
Entrainment During Vortex Ring Formation
Vortex rings (commonly known as smoke rings) can be formed by sudden ejection of fluid from a tube or orifice (see picture of vortex ring generator below). During ejection, the jet rolls up into a toroidal spiral (ring) and in the process, entrains some of the ambient fluid into the spiral. This process is easily visualized by dying the fluid in the tube with a fluorescent dye and illuminating it with a laser sheet (known as planar laser induced fluorescence or PLIF) to obtain a cross-section of the flow as shown below. Likewise, the velocity field measured using digital particle image velocimetry (DPIV) can be used to obtain the Finite-Time Lyapunov Exponent field, ridges of which define Lagrangian Coherent Structures (LCS) and identify the boundary of the ejected fluid in a manner similar to PLIF (see below). DPIV together with LCS allows the ejected fluid domain to be identified simultaneously with the velocity field. Both PLIF and DPIV/LCS can be used to track the entrainment of ambient fluid during vortex ring formation. LCS provides more information, however, including identification of the ambient fluid that will be entrained into the ring before the jet is initiated and the ability to track the momentum obtained by the entrained fluid separate from the ejected fluid.
Understanding the entrainment process is important for enhancing or optimizing fluid mixing through jet pulsation or modeling thrust production by pulsed jets (see the pulsed-jet micropropulsion project). The process depends very strongly on the piston stroke-to-jet diameter ratio (L/D) and the functional form of the jet velocity variation with time (velocity program). Results show that short L/D pulses with rapid initiation and termination provide the greatest net entrainment of ambient fluid. Similarly, short L/D pulses tend to have a higher fraction of their net impulse associated with ambient fluid.
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Piston-Cylinder Vortex Ring Generator |
PLIF of vortex ring formation. Notice the black spirals indicating ambient fluid entrainment. |
Finite-Time Lyapunov Exponent field obtained from backward time integration of DPIV measurements. Ridges identify LCS and correspond to the ejected fluid boundary (compare with PLIF). |
Personnel: Ali Olcay
Publications:
A.B. Olcay, and P.S. Krueger, “Measurement of Ambient Fluid Entrainment During Vortex Ring Formation,” Experiments in Fluids, pp. 235 – 247, Vol. 44, 2008.
A.B. Olcay, and P.S. Krueger, “Momentum evolution of ejected and entrained fluid during laminar vortex ring formation,” Theoretical and Computational Fluid Dynamics (in review).
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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.
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