Coherent vortical structures in numerical simulations of buoyant plumes from wildland fires

2005 ◽  
Vol 14 (1) ◽  
pp. 61 ◽  
Author(s):  
Philip Cunningham ◽  
Scott L. Goodrick ◽  
M. Yousuff Hussaini ◽  
Rodman R. Linn
Keyword(s):  
Three Dimensional ◽  
Vortex Pair ◽  
Moderate Intensity ◽  
Heat Sources ◽  
Wildland Fires ◽  
Buoyant Plumes ◽  
Atmospheric Flow ◽  
Vortical Structures ◽  
Elliptical Ring ◽  
Counter Rotating Vortex Pair

The structure and dynamics of buoyant plumes arising from surface-based heat sources in a vertically sheared ambient atmospheric flow are examined via simulations of a three-dimensional, compressible numerical model. Simple circular heat sources and asymmetric elliptical ring heat sources that are representative of wildland fires of moderate intensity are considered. Several different coherent vortical structures that dominate the plume structure and evolution are evident in the simulations, and these structures correspond well with those observed in plumes from wildland fires. For the circular source, these structures include: (i) a counter-rotating vortex pair aligned with the plume trajectory that is associated with a bifurcation of the plume, (ii) transverse shear-layer vortices on the upstream face of the plume, and (iii) vertically oriented wake vortices that form periodically with alternating sign on either side of the downstream edge of the plume base. For the elliptical ring source, a streamwise counter-rotating vortex pair is apparent on each flank, and a transverse horizontal vortex is observed above the head of the source. In all simulations the plume cross section is represented poorly by a self-similar Gaussian distribution.

scholarly journals On the formation of the counter-rotating vortex pair in transverse jets

2001 ◽  
Vol 446 ◽  
pp. 347-373 ◽  
Author(s):  
L. CORTELEZZI ◽  
A. R. KARAGOZIAN
Keyword(s):  
Flow Field ◽  
Computational Study ◽  
Near Field ◽  
Three Dimensional ◽  
Vortex Pair ◽  
Transverse Jets ◽  
Vortical Structures ◽  
Jet In Crossflow ◽  
Dynamical Generation ◽  
Counter Rotating Vortex Pair

Among the important physical phenomena associated with the jet in crossflow is the formation and evolution of vortical structures in the flow field, in particular the counter-rotating vortex pair (CVP) associated with the jet cross-section. The present computational study focuses on the mechanisms for the dynamical generation and evolution of these vortical structures. Transient numerical simulations of the flow field are performed using three-dimensional vortex elements. Vortex ring rollup, interactions, tilting, and folding are observed in the near field, consistent with the ideas described in the experimental work of Kelso, Lim & Perry (1996), for example. The time-averaged effect of these jet shear layer vortices, even over a single period of their evolution, is seen to result in initiation of the CVP. Further insight into the topology of the flow field, the formation of wake vortices, the entrainment of crossflow, and the effect of upstream boundary layer thickness is also provided in this study.

Formation of a Counter-Rotating Vortex Pair in a Plume in a Cross-Flow

2010 ◽  
Author(s):  
Masahiko Shinohara
Keyword(s):  
Vortex Ring ◽  
Three Dimensional ◽  
Cross Flow ◽  
Vortex Pair ◽  
Boussinesq Approximation ◽  
Streamwise Vorticity ◽  
Three Dimensional Flow ◽  
Flow Feature ◽  
Heated Area ◽  
Counter Rotating Vortex Pair

Numerical simulations are performed to study the formation of a counter-rotating vortex pair (CVP), a dominant flow feature in plumes inclined in a cross-flow. The unsteady three-dimensional flow fields are calculated by a finite difference method using the Boussinesq approximation. A plume rises from an isothermally heated square surface facing upward in air. Calculations show that the CVP originates not from horizontal spanwise vorticity in the velocity boundary layer on the bottom wall around the heated area, but from horizontal streamwise vorticity just above each side of the heated area. When the cross-flow begins after a plume forms a vortex ring in the cap above the heated area in a still environment, the vortex ring does not form a CVP. However, as the cap and the stem of the plume move downwind, a rotation about the streamwise axis appears just above each side edge of the heated area and grows into the CVP. We discuss the effect of entrainment into the stem and cap on the formation of the streamwise rotation that causes the CVP.

scholarly journals Three-dimensional instabilities and transient growth of a counter-rotating vortex pair

2009 ◽  
Vol 21 (9) ◽  
pp. 094102 ◽  
Author(s):  
Claire Donnadieu ◽  
Sabine Ortiz ◽  
Jean-Marc Chomaz ◽  
Paul Billant
Keyword(s):  
Three Dimensional ◽  
Vortex Pair ◽  
Transient Growth ◽  
Counter Rotating Vortex Pair

Experimental study of the instability of unequal-strength counter-rotating vortex pairs

2003 ◽  
Vol 474 ◽  
pp. 35-84 ◽  
Author(s):  
J. M. ORTEGA ◽  
R. L. BRISTOL ◽  
Ö. SAVAŞ
Keyword(s):  
Three Dimensional ◽  
Subsequent Development ◽  
Vortex Pair ◽  
Tip Vortex ◽  
Two Dimensional ◽  
Experimental Conditions ◽  
Vortex Pairs ◽  
Circulation Ratio ◽  
Counter Rotating Vortex Pair ◽  
Wing Tip

A rapidly growing instability is observed to develop between unequal-strength counter- rotating vortex pairs. The vortex pairs are generated in a towing tank in the wakes of wings with outboard triangular flaps. The vortices from the wing tip and the inboard tip of the flap form the counter-rotating vortex pair on each side of the wing. The flow fields are studied using flow visualization and particle image velocimetry. Both chord- based and circulation-based Reynolds numbers are of O(105). The circulation strength ratios of the flap- to tip-vortex pairs range from −0.4 to −0.7. The initial sinuous stage of the instability of the weaker flap vortex has a wavelength of order one wing span and becomes observable in about 15 wing spans downstream of the wing. The nearly straight vortex filaments first form loops around the stronger wing-tip vortices. The loops soon detach and form rings and move in the wake under self-induction. These vortex rings can move to the other side of the wake. The subsequent development of the instability makes the nearly quasi-steady and two-dimensional wakes unsteady and three-dimensional over a distance of 50 to 100 wing spans. A rectangular wing is also used to generate the classical wake vortex pair with the circulation ratio of −1.0, which serves as a reference flow. This counter-rotating vortex pair, under similar experimental conditions, takes over 200 spans to develop visible deformations. Velocity, vorticity and enstrophy measurements in a fixed plane, in conjuction with the flow observations, are used to quantify the behaviour of the vortex pairs. The vortices in a pair initially orbit around their vorticity centroid, which takes the pair out of the path of the wing. Once the three-dimensional interactions develop, two-dimensional kinetic energy and enstrophy drop, and enstrophy dispersion radius increases sharply. This rapid transformation of the wake into a highly three-dimensional one offers a possible way of alleviating the hazard posed by the vortex wake of transport aircraft.

scholarly journals Contributions of the wall boundary layer to the formation of the counter-rotating vortex pair in transverse jets

2011 ◽  
Vol 676 ◽  
pp. 461-490 ◽  
Author(s):  
FABRICE SCHLEGEL ◽  
DAEHYUN WEE ◽  
YOUSSEF M. MARZOUK ◽  
AHMED F. GHONIEM
Keyword(s):  
Boundary Layer ◽  
Boundary Condition ◽  
Near Field ◽  
Vortex Pair ◽  
Transverse Jets ◽  
Vortical Structures ◽  
Wall Boundary Layer ◽  
Wall Boundary ◽  
Jet Trajectory ◽  
Counter Rotating Vortex Pair

Using high-resolution 3-D vortex simulations, this study seeks a mechanistic understanding of vorticity dynamics in transverse jets at a finite Reynolds number. A full no-slip boundary condition, rigorously formulated in terms of vorticity generation along the channel wall, captures unsteady interactions between the wall boundary layer and the jet – in particular, the separation of the wall boundary layer and its transport into the interior. For comparison, we also implement a reduced boundary condition that suppresses the separation of the wall boundary layer away from the jet nozzle. By contrasting results obtained with these two boundary conditions, we characterize near-field vortical structures formed as the wall boundary layer separates on the backside of the jet. Using various Eulerian and Lagrangian diagnostics, it is demonstrated that several near-wall vortical structures are formed as the wall boundary layer separates. The counter-rotating vortex pair, manifested by the presence of vortices aligned with the jet trajectory, is initiated closer to the jet exit. Moreover tornado-like wall-normal vortices originate from the separation of spanwise vorticity in the wall boundary layer at the side of the jet and from the entrainment of streamwise wall vortices in the recirculation zone on the lee side. These tornado-like vortices are absent in the case where separation is suppressed. Tornado-like vortices merge with counter-rotating vorticity originating in the jet shear layer, significantly increasing wall-normal circulation and causing deeper jet penetration into the crossflow stream.

scholarly journals Formation of surface trailing counter-rotating vortex pairs downstream of a sonic jet in a supersonic cross-flow

2018 ◽  
Vol 850 ◽  
pp. 551-583 ◽  
Author(s):  
Mingbo Sun ◽  
Zhiwei Hu
Keyword(s):  
Separation Bubble ◽  
Three Dimensional ◽  
Cross Flow ◽  
Vortex Generator ◽  
Vortex Pair ◽  
Formation Mechanisms ◽  
Wall Jet ◽  
Recirculating Flow ◽  
Sonic Jet ◽  
Counter Rotating Vortex Pair

Direct numerical simulations were conducted to uncover physical aspects of a transverse sonic jet injected into a supersonic cross-flow at a Mach number of 2.7. Simulations were carried out for two different jet-to-cross-flow momentum flux ratios ($J$) of 2.3 and 5.5. It is identified that collision shock waves behind the jet induce a herringbone separation bubble in the near-wall jet wake and a reattachment valley is formed and embayed by the herringbone recirculation zone. The recirculating flow in the jet leeward separation bubble forms a primary trailing counter-rotating vortex pair (TCVP) close to the wall surface. Analysis on streamlines passing the separation region shows that the wing of the herringbone separation bubble serves as a micro-ramp vortex generator and streamlines acquire angular momentum downstream to form a secondary surface TCVP in the reattachment valley. Herringbone separation wings disappear in the far field due to the cross-interaction of lateral supersonic flow and the expansion flow in the reattachment valley, which also leads to the vanishing of the secondary TCVP. A three-dimensional schematic of surface trailing wakes is presented and explains the formation mechanisms of the surface TCVPs.

On steady and pulsed low-blowing-ratio transverse jets

2013 ◽  
Vol 714 ◽  
pp. 393-433 ◽  
Author(s):  
G. Bidan ◽  
D. E. Nikitopoulos
Keyword(s):  
Numerical Study ◽  
Cross Flow ◽  
Vortex Pair ◽  
Leading Edge ◽  
Mie Scattering ◽  
Formation Mechanisms ◽  
Transverse Jets ◽  
Vortical Structures ◽  
Blowing Ratio ◽  
Counter Rotating Vortex Pair

AbstractThe present experimental and numerical study focuses on the vortical structures encountered in steady and pulsed low-blowing-ratio transverse jets ($0. 150\leq \mathit{BR}\leq 4. 2$), a configuration hardly discussed in the literature. Under unforced conditions at low blowing ratio, a stable leading-edge shear-layer rollup is identified inside the jet pipe. As the blowing ratio is increased, the destabilization and evolution of this structure sheds light on the formation mechanisms of the well-known transverse jet vortical system. A discussion on the nature of the counter-rotating vortex pair in low-blowing-ratio transverse jets is also provided. Under forced conditions, the experimental observations support and extend numerical results of previous fully modulated jet studies. Large-eddy simulation results provide scaling parameters for the classification of starting vortices for partly modulated jets, as well as information on their three-dimensional dynamics. The counter-rotating vortex pair initiation is observed and detailed in both Mie scattering visualizations and simulations. The observations support a mechanism based on stretching of the starting vortical structures because of inviscid induction and partial leapfrogging. Two modes of cross-flow ingestion inside the jet pipe are described as the pulsed jet cycles from high to low values of blowing ratio.

Noncanonical Short Hole Jets-in-Crossflow for Turbine Film Cooling

2005 ◽  
Vol 73 (3) ◽  
pp. 474-482 ◽  
Author(s):  
Michael W. Plesniak
Keyword(s):  
Film Cooling ◽  
Cross Flow ◽  
Vortex Pair ◽  
Structural Features ◽  
Global Features ◽  
Vortical Structures ◽  
Jet In Crossflow ◽  
Jet Trajectory ◽  
Image Velocimetry ◽  
Counter Rotating Vortex Pair

This paper presents a review of research done over the past several years at Purdue on non-canonical jets-in-crossflow. It is a retrospective and an integrative compilation of results previously reported as well as some new ones. The emphasis is on jets emanating from “short” holes, with length-diameter ratios of one or less. A canonical jet-in-crossflow configuration is one in which a fully developed jet issues from a long pipe fed by a large plenum, into a semi-infinite cross flow. The configuration presented here is noncanonical in the sense that jet issues from a short hole and thus the flow is unable to “adjust” to the hole, unlike the case of a long hole in which fully developed pipe flow can be attained. This is motivated by gas turbine film cooling applications. Experimental results acquired with particle image velocimetry will primarily be presented, with some complementary information gained from RANS simulations of the flow. Many different aspects of the problem have been investigated, and in this paper the focus will be on structural features within the hole and in the developing jet and crossflow interaction. A significant result is that the in-hole vortical structures, depending on their sense of rotation, tend to augment or weaken the primary counter-rotating vortex pair. This impacts global features such as jet trajectory and spreading.

Cooperative elliptic instability of a vortex pair

1998 ◽  
Vol 360 ◽  
pp. 85-119 ◽  
Author(s):  
T. LEWEKE ◽  
C. H. K. WILLIAMSON
Keyword(s):  
Three Dimensional ◽  
Finite Size ◽  
Phase Relationship ◽  
Vortex Pair ◽  
Matching Condition ◽  
Vortical Structures ◽  
Long Wavelength ◽  
Dimensional Instability ◽  
First Time

In this paper, we investigate the three-dimensional instability of a counter-rotating vortex pair to short waves, which are of the order of the vortex core size, and less than the inter-vortex spacing. Our experiments involve detailed visualizations and velocimetry to reveal the spatial structure of the instability for a vortex pair, which is generated underwater by two rotating plates. We discover, in this work, a symmetry-breaking phase relationship between the two vortices, which we show to be consistent with a kinematic matching condition for the disturbances evolving on each vortex. In this sense, the instabilities in each vortex evolve in a coupled, or ‘cooperative’, manner. Further results demonstrate that this instability is a manifestation of an elliptic instability of the vortex cores, which is here identified clearly for the first time in a real open flow. We establish a relationship between elliptic instability and other theoretical instability studies involving Kelvin modes. In particular, we note that the perturbation shape near the vortex centres is unaffected by the finite size of the cores. We find that the long-term evolution of the flow involves the inception of secondary transverse vortex pairs, which develop near the leading stagnation point of the pair. The interaction of these short-wavelength structures with the long-wavelength Crow instability is studied, and we observe significant modifications in the longevity of large vortical structures.

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