Impact of buoyancy on vortex ring development
in the near ﬁeld
Received: 14 October 2007 / Revised: 13 August 2009 / Accepted: 25 September 2009 / Published online: 13 October 2009
Ó Springer-Verlag 2009
Abstract The development of a buoyant vortex ring in
the near ﬁeld was examined experimentally, and the ﬁnd-
ings were compared with those of a non-buoyant ring with
a similar Reynolds number. The experiments were per-
formed in a water tank, and the vortices were produced by
a cylindrical tube of aspect ratio 2. Laser sheet ﬂow visu-
alization and PIV measurements were carried out. In the
near ﬁeld, the initial column of the buoyant ﬂuid breaks
down due to the presence of Rayleigh–Taylor instability at
the buoyant ﬂuid interface. Subsequently, a large diameter
vortex ring with a large spreading rate, compared with the
non-buoyant ring, emerges. The celerity of buoyant vortex
continued to decrease throughout the range examined, in
contrast to the constant celerity of the non-buoyant ring.
The vorticity in the core of buoyant and non-buoyant
vortex rings is symmetric and has a Gaussian distribution.
However, the buoyant vortex ring evolves into a thin core
ring, whereas the non-buoyant ring becomes a thick core
ring shortly after the ring formation. This difference is
brought on by the rapid entrainment and the signiﬁcant
growth of the buoyant ring following the breakup of the
The formation and subsequent evolution of vortex rings
having a density equal to the ambient ﬂuid has been studied
extensively in the past. Shariff and Leonard (1992) and
Lim and Nickels (1995) provide comprehensive reviews of
the dynamics of homogeneous vortex rings in quiescent,
unstratiﬁed surroundings. Vortex rings in the laboratory are
typically generated by forcing a slug of ﬂuid from a tube or
oriﬁce. The vortex sheet on the ﬂuid slug rolls up into a
vortex core. The majority of past work has focused on
vortex rings with stroke ratios of about one. Gharib et al.
(1998) established that vortex generators with stroke ratios
greater than about four produce a vortex ring with a dis-
connected trailing jet. Application of optical velocimetry
techniques has allowed detailed examination of the struc-
ture of vortex rings, speciﬁcally within the core (Max-
worthy 1977; Didden 1979; Glezer and Coles 1990).
Weigand and Gharib (1997) have showed that vorticity in
the core of laminar vortex rings has a Gaussian distribution.
The structure of vorticity within the vortex plays an
important role in the ﬂow evolution, and has been used to
categorize vortex rings into two limiting cases of thin and
thick core rings. Vorticity is conﬁned to a small diameter
region, compared to the ring diameter, in thin core rings.
On the other hand, vorticity is present throughout a volume
with a diameter comparable to that of the ring for thick
core rings, similar to a Hill’s vortex (Saffman 1970, 1992).
Buoyant vortex rings form a different class of vortices
where the density difference between the ring and ambient
ﬂuid profoundly alters the ﬂow dynamics (Turner 1957).
Vorticity in the core of buoyant vortex rings consists of two
components: that associated with the vortex sheet rolled up
into the core and the baroclinic vorticity due to density
gradients. The circulation of a buoyant vortex can then be
D. Bond Á H. Johari
Mechanical Engineering Department,
Worcester Polytechnic Institute, Worcester,
MA 01609, USA
H. Johari (&)
California State University, Northridge, CA, USA
Exp Fluids (2010) 48:737–745