Optimal Vortex Ring Formation: A Study of Vortex Ring Formation in Starting and Pulsed Jets (Keynote)

2003 ◽  
Author(s):  
Morteza Gharib
Keyword(s):  
Vortex Ring ◽  
Basic Element ◽  
Ring Formation ◽  
Nozzle Diameter ◽  
Vortex Rings ◽  
Jet Flows ◽  
Ambient Fluid ◽  
Pulsed Jets ◽  
Vortex Ring Formation ◽  
Maximization Principle

Pulsatile jet flows are found in many industrially relevant fluid mechanical problems. A common feature of these flows is that they are fundamentally a series of fluid pulses. This aspect of pulsatile jets implies vortex rings are a basic element of the resulting flow. The significance of this observation is based in part on the tendency of vortex rings to entrain ambient fluid during their formation, but more so on the recent discovery of the phenomenon of vortex ring pinch off. This phenomenon was characterized for starting jets (individual pulses) showing that for pulses sufficiently long with respect to the nozzle diameter (i.e., sufficiently large L/D), the vortex ring stops forming and pinches off from the generating jet. This represents a maximization principle for vortex ring formation and suggests that any effects associated with vortex ring formation in pulsatile jets (e.g., enhanced entrainment), might be able to be optimized by properly selecting the L/D for each pulse.

scholarly journals The role of optimal vortex formation in biological fluid transport

2005 ◽  
Vol 272 (1572) ◽  
pp. 1557-1560 ◽  
Author(s):  
John O Dabiri ◽  
Morteza Gharib
Keyword(s):  
Vortex Ring ◽  
Fluid Transport ◽  
Biological Fluid ◽  
Ring Formation ◽  
Vortex Rings ◽  
Jet Flows ◽  
Vortex Ring Formation ◽  
Biological Studies ◽  
Macro Scale ◽  
Animal Phyla

Animal phyla that require macro-scale fluid transport for functioning have repeatedly and often independently converged on the use of jet flows. During flow initiation these jets form fluid vortex rings, which facilitate mass transfer by stationary pumps (e.g. cardiac chambers) and momentum transfer by mobile systems (e.g. jet-propelled swimmers). Previous research has shown that vortex rings generated in the laboratory can be optimized for efficiency or thrust, based on the jet length-to-diameter ratio ( L / D ), with peak performance occurring at 3.5< L / D <4.5. Attempts to determine if biological jets achieve this optimization have been inconclusive, due to the inability to properly account for the diversity of jet kinematics found across animal phyla. We combine laboratory experiments, in situ observations and a framework that reduces the kinematics to a single parameter in order to quantitatively show that individual animal kinematics can be tuned in correlation with optimal vortex ring formation. This new approach identifies simple rules for effective fluid transport, facilitates comparative biological studies of jet flows across animal phyla irrespective of their specific functions and can be extended to unify theories of optimal jet-based and flapping-based vortex ring formation.

Circulation Generation and Vortex Ring Formation by Conic Nozzles

2009 ◽  
Vol 131 (9) ◽  
Author(s):  
Moshe Rosenfeld ◽  
Kakani Katija ◽  
John O. Dabiri
Keyword(s):  
Vortex Ring ◽  
Vortex Generator ◽  
Ring Formation ◽  
Vortex Rings ◽  
Two Dimensional ◽  
One Dimensional ◽  
Dimensional Flow ◽  
Vortex Ring Formation ◽  
Exit Nozzle ◽  
Two Dimensional Flow

Vortex rings are one of the fundamental flow structures in nature. In this paper, the generation of circulation and vortex rings by a vortex generator with a static converging conic nozzle exit is studied numerically. Conic nozzles can manipulate circulation and other flow invariants by accelerating the flow, increasing the Reynolds number, and by establishing a two-dimensional flow at the exit. The increase in the circulation efflux is accompanied by an increase in the vortex circulation. A novel normalization method is suggested to differentiate between two contributions to the circulation generation: a one-dimensional slug-type flow contribution and an inherently two-dimensional flow contribution. The one-dimensional contribution to the circulation increases with the square of the centerline exit velocity, while the two-dimensional contribution increases linearly with the decrease in the exit diameter. The two-dimensional flow contribution to the circulation production is not limited to the impulsive initiation of the flow only (as in straight tube vortex generators), but it persists during the entire ejection. The two-dimensional contribution can reach as much as 44% of the total circulation (in the case of an orifice). The present study offers evidences on the importance of the vortex generator geometry, and in particular, the exit configuration on the emerging flow, circulation generation, and vortex ring formation. It is shown that both total and vortex ring circulations can be controlled to some extent by the shape of the exit nozzle.

Vortex Ring Formation in Wall-Bounded Domains

2010 ◽  
Author(s):  
Kelley C. Stewart ◽  
Pavlos P. Vlachos
Keyword(s):  
Vortex Ring ◽  
Propagation Velocity ◽  
Cylindrical Tube ◽  
Ring Formation ◽  
Vortex Rings ◽  
Frequency Noise ◽  
Bounded Domains ◽  
Piston Cylinder ◽  
Vortex Ring Formation ◽  
Wall Interaction

Vortex ring formation and propagation have been studied extensively in quiescent semi-infinite volumes. However, very little is known about the dynamics of vortex-ring formation in wall-bounded domains where vortex wall interaction will affect both the vortex ring pinch-off and propagation velocity. This study addresses this limitation and studies vortex formation in radially confined domains to analyze the effect of vortex-ring wall interaction on the formation and propagation of the vortex ring. Vortex rings were produced using a pneumatically driven piston cylinder arrangement and were ejected into a long cylindrical tube parallel to the piston cylinder arrangement which defined the confined downstream domain. Two different domains were studied with diameters twice and four times the size of the piston cylinder. A semi-infinite unbounded volume with no downstream cylinder was also investigated for comparison. The piston stroke-to-diameter ratio (L/D0) for the studied vortex rings was varied between 0.75 and 3 with corresponding Reynolds numbers, based on circulation, of approximately 500 to 8,000. Velocity field measurements were performed using planar Time Resolved Digital Particle Image Velocimetry (TRDPIV). The TRDPIV data were processed using an in-house developed cross-correlation PIV algorithm and post processed using Proper Orthogonal Decomposition to remove high frequency noise. The propagation velocity and vorticity were investigated and vortex identification was used to track the changing size, location, and circulation of the vortices. The combination of these parameters was used to investigate the effects of wall interaction on vortex ring formation and propagation.

A Unified Energy Feature of Vortex Rings for Identifying the Pinch-Off Mechanism

2017 ◽  
Vol 140 (1) ◽  
Author(s):  
Yang Xiang ◽  
Hong Liu ◽  
Suyang Qin
Keyword(s):  
Vortex Ring ◽  
Critical Value ◽  
Ring Formation ◽  
Experimental Results ◽  
Vortex Rings ◽  
Piston Cylinder ◽  
Dimensionless Energy ◽  
Vortex Ring Formation ◽  
Energy Feature ◽  
Off Time

Owing to the limiting effect of energy, vortex rings cannot grow indefinitely and thus pinch off. In this paper, experiments on the vortex rings generated using a piston-cylinder apparatus are conducted so as to investigate the pinch-off mechanisms and identify the limiting effect of energy. Both theoretical and experimental results show that the generated vortex rings share a unified energy feature, regardless of whether they are pinched-off or not. Moreover, the unified energy feature is quantitatively described by a dimensionless energy number γ, defined as γ=(E/I2Γωmax) and exhibiting a critical value γring = 0.14 ± 0.01 for the generated vortex rings. This unified energy feature reflects the limiting effect of energy and specifies the target of vortex ring formation. Furthermore, based on the tendency of γ during vortex ring formation, criteria for determining the two timescales, i.e., pinch-off time and separation time, which correspond to the onset and end of pinch-off process, respectively, are suggested.

Vortex Ring Formation on Drop Coalescence With Underlying Liquid

2013 ◽  
Author(s):  
Bahni Ray ◽  
Gautam Biswas ◽  
Ashutosh Sharma
Keyword(s):  
Numerical Simulations ◽  
Vortex Ring ◽  
Level Set ◽  
Flat Surface ◽  
Volume Of Fluid ◽  
Ring Formation ◽  
Vortex Rings ◽  
Drop Coalescence ◽  
Vortex Ring Formation ◽  
To Come

Numerical simulations using coupled level-set and volume-of-fluid (CLSVOF) method has been carried out to capture the vortex ring when a drop coalesces on a pool of liquid. A study has been done for the formation and motion of vortex rings generated when drops of liquid are allowed to come into contact at zero velocity with a quiescent flat surface of the same liquid.

Vortex Rings in Bio-Inspired and Biological Jet Propulsion

2008 ◽  
Vol 58 ◽  
pp. 237-246 ◽  
Author(s):  
Paul S. Krueger ◽  
Ali A. Moslemi ◽  
J. Tyler Nichols ◽  
Ian K. Bartol ◽  
William J. Stewart
Keyword(s):  
Vortex Ring ◽  
Swimming Performance ◽  
Vortex Rings ◽  
Short Pulses ◽  
Pulsed Jets ◽  
Pulsed Jet ◽  
Vortex Ring Formation ◽  
Speed Advantage ◽  
Vehicle Motion

Pulsed-jets are commonly used for aquatic propulsion, such as squid and jellyfish locomotion. The sudden ejection of a jet with each pulse engenders the formation of a vortex ring through the roll-up of the jet shear layer. If the pulse is too long, the vortex ring will stop forming and the remainder of the pulse is ejected as a trailing jet. Recent results from mechanical pulsedjets have demonstrated that vortex rings lead to thrust augmentation through the acceleration of additional ambient fluid. This benefit is most pronounced for short pulses without trailing jets. Simulating vehicle motion by introducing background co-flow surrounding the jet has shown that vortex ring formation can be interrupted, but only if the co-flow is sufficiently fast. Recent in situ measurements on squid have captured vortical flows similar to those observed in the laboratory, suggesting thrust augmentation may play a role in their swimming performance. Likewise, recent measurements with a mechanical self-propelled pulsed-jet vehicle (“robosquid”) have shown a cruise-speed advantage obtained by pulsing.

scholarly journals A model for vortex ring formation in a starting buoyant plume

2000 ◽  
Vol 416 ◽  
pp. 173-185 ◽  
Author(s):  
MICHAEL SHUSSER ◽  
MORTEZA GHARIB
Keyword(s):  
Vortex Ring ◽  
Time Scale ◽  
Ring Formation ◽  
Vortex Rings ◽  
Uniform Density ◽  
Dimensionless Time ◽  
Buoyant Plume ◽  
Piston Cylinder ◽  
Vortex Ring Formation ◽  
Formation Number

Vortex ring formation in a starting axisymmetric buoyant plume is considered. A model describing the process is proposed and a physical explanation based on the Kelvin–Benjamin variational principle for steady vortex rings is provided. It is shown that Lundgren et al.'s (1992) time scale, the ratio of the velocity of a buoyant plume after it has travelled one diameter to its diameter, is equivalent to the time scale (formation time) proposed by Gharib et al. (1998) for uniform-density vortex rings generated with a piston/cylinder arrangement. It is also shown that, similarly to piston-generated vortex rings (Gharib et al. 1998), the buoyant vortex ring pinches off from the plume when the latter can no longer provide the energy required for steady vortex ring existence. The dimensionless time of the pinch-off (the formation number) can be reasonably well predicted by assuming that at pinch-of the vortex ring propagation velocity exceeds the plume velocity. The predictions of the model are compared with available experimental results.

scholarly journals Investigation of vortex ring formation with account of generator piston motion

2017 ◽  
Vol 208 ◽  
pp. 012045 ◽  
Author(s):  
I V Khramtsov ◽  
VV Palchikovskiy ◽  
A A Siner ◽  
Yu V Bersenev
Keyword(s):  
Vortex Ring ◽  
Ring Formation ◽  
Piston Motion ◽  
Vortex Ring Formation
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