The following paper deals with the visualization of flow around multiple inline circular cylinders. The conclusions from the visualizations are noted. Paper will serve has the base for further numerical investigations on the topic.

1. 1
Proceedings of the 40th
National Conference on Fluid Mechanics and Fluid Power
December 12-14, 2012, NIT, Hamirpur, Himachal Pradesh, India
Paper ID
FMFP2013 138
Experimental flow visualization for flow around multiple in-line circular cylinders at
low Reynolds number
Amey Vasulkar
Department of Mechanical Engineering,
College of Engineering Pune – 411005
ameyvasulkar@gmail.com
Tanmay Srivastav
Department of Mechanical Engineering,
College of Engineering Pune – 411005
tanmay.sri.93@gmail.com
C. M. Sewatkar*
Department of Mechanical Engineering,
College of Engineering Pune – 411005
cms.mech@coep.ac.in
ABSTRACT
Literature reports synchronous, quasi-periodic and
chaotic flow regime for in-line square cylinders.
The present work is an attempt to identify these
flow regimes for circular cylinders using flow
visualization technique. A water tunnel has been
constructed for this purpose with an arrangement of
dye visualization for flow around four in-line
circular cylinders. The experiments are carried out
for 1.0 ≤ s/d ≤ 6.0 at low Reynolds number. The
synchronous flow is noticed at lower spacing and it
assumes chaotic behavior with increase in the
spacing for a given Reynolds number. These results
may serve as a benchmark results for future
numerical simulations.
Keywords In-line cylinder, wake interference
INTRODUCTION
This paper intends to understand the phenomena of
wake interference and its effects on flow regimes
when flow occurs around multiple circular cylinders
placed in-line with flow. When large numbers of
cylinders are present in the flow, not only do they
shed vortices individually, but these vortices are
interfered by the downstream cylinders which
receives non uniform flow. This wake interference
leads to a complex behavior of the flow, the
understanding of which is sufficiently challenging
and poorly understood. Such a study is relevant for
many practical engineering applications such as
flow around turbine farms, closely spaced
buildings, offshore structures and pipelines,
structures of bridges, nuclear power plant steam
generator, heat exchangers, thermo-acoustic re-
generators, suspension bridges, towers, mast and
wires, turning vanes in duct elbows, multicolored
airfoils, flow around large building blocks,
combined movement of vehicles along the
highways, etc. However, despite the widespread
applications of flow around large number of bluff
bodies the investigation of such a flow started
rather late in comparison with investigations into
single and two bluff bodies.
Most of the literature on flow around two circular
cylinders reported about the negative drag
experienced by the second cylinder and the flow
regimes for two cylinders system. Whether the
similar observations hold good for large number of
cylinders has not been sufficiently explored. We
have arbitrarily chosen four circular cylinders,
which is representative of a sufficiently large size.
It is however expected that the flow around the last
few cylinders will become independent of the
cylinder number when a large number of cylinders
are actually present in the flow domain.
The present study is undertaken with a view to: (i)
documenting the flow patterns that the flow can
exhibit, (ii) comparing the results for multiple
cylinders with those for two tandem circular
cylinders, and (iii) throwing light on flow
phenomena when the cylinders are arranged in
tandem versus a side-by-side configuration. The
relevant non dimensional governing parameters for
such flows are spacing (s/d) and Reynolds number

2. 2
(Re = U0d/ν), where s is the surface-to-surface
distance between cylinders, d is size of the cylinder,
U0 is the uniform inlet velocity, and ν is the
kinematic viscosity of the fluid.
LITERATURE REVIEW
The flow across in-line cylinders (Fig. 1a) is a
simple configuration but is complex in terms of
flow phenomenon, since the flow reaching the
downstream cylinder is unsteady. The major focus
of earlier work was to understand the forces
experienced by the cylinders. Among these, Mittal
et al. (1997) argued that the qualitative nature of the
flow depends strongly on the arrangement of
cylinders and Reynolds number. Carmo et al.
(2010) studied the secondary instabilities that occur
in the early stages of the transition in the wake of
the flow around two in-line circular cylinders. Xu
and Zhou (2004) in their experimental work argued
that the near wake generated by the upstream
cylinder interferes and influences the vortex
shedding from downstream cylinder. The two
vortex shedding processes therefore influence each
other, resulting in the lock-on phenomenon. Effect
of spacing and the shear parameter at Re = 100 for
two in-line cylinders has been studied by
Lankadasu and Vengadesan (2008). It is argued that
the Strouhal number decreases with increasing
shear parameter and there are more than one
shedding frequency at high shear parameters and
spacing (s/d). To verify this phenomenon for large
number of in-line cylinders is an interesting
problem.
The typical regimes reported for flow around two
cylinders are single bluff body, shear layer
reattachment, and synchronization of vortex
shedding from the cylinders. Wang et al. (2010)
revisited these regimes in their experimental study
to investigate the dependence of Strouhal number
on spacing and to explore the flow characteristics
relevant to critical spacing and hysteretic mode of
transitions at critical spacing. Mittal, Kumar &
Raghuvanshi (1997) reported that when the flow
becomes unsteady, the downstream cylinder, which
lies in the wake of the upstream cylinder,
experiences relatively large unsteady forces that
may lead to wake-induced flutter. Carmo,
Meneghini & Sherwin (2010a) proposed that if the
separation is less than the drag inversion spacing,
the downstream cylinder has a stabilizing effect on
the flow. For such cases, the three-dimensional
structures appeared later in terms of Reynolds
number than for the flow around an isolated
cylinder. On the other hand, if the separation is
greater than the drag inversion spacing, the initial
stages of the transition in the wake occurred in a
similar way to that of an isolated cylinder. Their
comprehensive study shows the different ways in
which the stages of the transition scenario respond
to flow interference. A comprehensive and recent
review of literature on two cylinders is available in
Sumner (2010).
There are relatively few studies with multiple (>2)
in-line cylinders in the flow. Harichandan & Roy
(2010) obtained steady wake patterns at Re D 100
and sparse von K´arm´an Vortex Street at Re D
200, with three in-line circular cylinders. The
unsteady forces experienced by the cylinders are
also documented in their numerical study; the
forces become more severe at small spacing
(s/d=1.0). When the longitudinal gap is increased
(s=d D 4:0), there was no flow separation or
reattachment of the shear layer from the upstream
cylinder to the immediate downstream cylinder;
instead, von K´arm´an vortex streets were observed
between the cylinders. However, the vortex
shedding from the downstream cylinder was highly
disturbed by the impingement of the upstream
vortex streets emerging from the uppermost and
middle cylinders. A gap vortex shedding
mechanism was proposed by Hetz, Dhaubhadel &
Telionis (1991), who experimentally studied the
flow around five in-line circular cylinders (known
as a pentad). For s/d = 0.1 to 0.8 with Re = 1× 104
to 5 × 105
, it was conjectured that such a gap vortex
shedding mechanism is present in the flow across a
bank of cylinders and could be excited if driven by
structural vibrations. Liang, Papadakis & Luo
(2009) studied the flow around six in-line circular
cylinders for 1.1≤ s/d ≤ 3:0 at Re = 100. They
proposed that an increase in the spacing makes the
flow more asymmetric and induces vortex shedding
starting from the last cylinder and proceeding
upstream. Further, the force statistics are
maximized in the spacing region of s/d = 2.0 – 2.6

3. 3
U0
s
d
and then drops drastically at s/d = 3.0 for the last
three cylinders.
Sewatkar et al. (2012) presented the flow regimes
obtained in the case of six in-line of square
cylinders as a function of spacing and Reynolds
number. These flow regimes are namely
synchronous, quasi-periodic-I, quasi-periodic-II and
chaotic. It is argued that the flow is synchronous at
lower spacing and assumes chaotic behavior at
larger spacing through quasi-periodic behavior. It is
also argued that the flow is mostly quasi-periodic-II
in nature for which the time periods for vortex
shedding and wake interference frequencies are
variable. It is further reported that the Reynolds
number also strongly affects these flow regimes.
The flow is synchronous at low Reynolds number
while it is chaotic for high Reynolds number. This
flow regime transition is exactly as a function of
spacing and Reynolds number is exactly opposite in
the case of flow across a row of cylinders. The
results of flow regime transition can also be tested
for in-line circular cylinders which may help to
establish the basic flow patterns for flow across
large number of circular cylinders. The present
work explores the flow regime transitions as a
function of spacing with the help of flow
visualization.
PHYSICAL DESCRIPTION AND
EXPERIMENTAL DETAILS OF THE
PROBLEMS
The physical problem considered here is flow
across four circular cylinders arranged in-line as
shown in figure 1(a). All cylinders are of identical
size with the same distance between two
consecutive cylinders. The leading cylinder with
side d, which is also the characteristic length scale,
is exposed to a constant and uniform velocity, U0.
The experiments are conducted using a closed-
circuit water channel made of a 1.5 cm thick acrylic
sheet of outer dimensions 120 cm x 40 cm x 40 cm;
see figure 1(b) for a schematic of the experimental
setup. Water is pumped into the channel using a 6 l
s-1
capacity pump which produced a flow velocity
of the order of 0.8–4 cm s-1
. The dye is injected at
the upstream location of first cylinder. The dye was
injected after the steady state of the flow is
established. Olympus Make 14.5 M Pixel video
camera was used for capturing the videos. The
frame at a particular time instant is used here for the
purpose of understanding and describing the flow
behavior. The actual videos shall be shared at the
time of presentation during the conference.
Magnetic flow meter is used for the measurement of
flow. Initially the experiments were carried out for
flow across single cylinder to ensure that the
essential phenomenon of vortex shedding occurs
behind the cylinder. This helped to establish
confidence in taking up further flow visualization
studies for multiple cylinders.
(a)
(b)
Figure 1 Schematic outline of (a) flow
around four in line circular cylinders (b)
experimental set-up

4. 4
RESULTS AND DISCUSSION
The brief results for the problem considered are
presented in this Section. Mainly the streak lines
obtained through flow visualization are presented
for different spacing at Re = 180. This is the upper
limit of Reynolds number beyond which flow
assumes three dimensional natures and becomes
turbulent (Saha et al., 2002). The effect of spacing
on flow behavior is explained finally in the form of
discussion.
s/d = 1.0
The streak lines obtained by flow visualization for
s/d = 1.0 are presented in figure 2. It is noticed that
a single wake is formed behind the cylinders at this
spacing, as if the flow is occurring over a single
cylinder. Although formation of shear layer begins
at the first cylinder itself, the vortex shedding
occurs essentially behind the last cylinder. Notice
the flow interference for such a low spacing. The
development of wake behind the first cylinders is
immediately interfered by its neighboring
downstream cylinder such that a single wide wake
is formed and the downstream cylinders are
enveloped in this wake. The wake is wavy in nature
which may lead to the development of sinusoidal
lift on all the cylinders. This has been reported by
Sewatkar et al. (2012) for in-line square cylinders.
Liang et al. (2009), noticed a pair of standing
vortices is noticed in the gap regions at comparable
values of spacing and Reynolds number (s/d = 1.1
and Re = 100). In the present work we notice that
the wake in its formative stage behind first cylinder
is interfered by the second cylinder. Since the gap is
very small the flow assumes the motion such that
the shear layer from first cylinder attaches at the top
or bottom of second cylinder. The magnitude of
velocity in the shear layers is comparatively small
hence the rate vorticity generation behind second
and further downstream cylinders is small. The
vorticity formed at the top and bottom of all the
cylinders is compounded together and shed behind
the last cylinder. Thus a single wake is formed in
which all the downstream cylinders are enveloped.
This makes the flow synchronous.
Figure 2 Streak lines obtained using dye
flow around four in line circular cylinders
at s/d = 1.0 at Re = 185
2.0 ≤ s/d ≤ 4.0
The streak lines obtained by flow visualization for
s/d = 2.0, 3.0 and 4.0 are presented through figures
3 to 5. It is noticed that the vortex formed behind
the first cylinder grows in the first gap. The
presence of the second cylinder however interferes
with this growing vortex. Similar interference
phenomenon applies to other cylinders also. The
interference alters the velocity and pressure field
inside the wake and this may lead to variation in the
forces experienced by the cylinders. It is argued by
Sewatkar et al. (2009) that the interaction of
vortices behind the cylinders also alters the forces
by cylinders. In the in-line situation there is
interference as noticed in the flow visualization.
Understanding of the wake interference is thus
important to understand the nature of forces. It is
argued here that that in the range of spacing
mentioned in this section the nature of forces may
be quasi-periodic as discussed by Kumar et al.
(2008), Sewatkar el al. (2009), Sewatkar et al.
(2012).
Figure 3 Streak lines obtained using dye
flow around four in line circular cylinders at
s/d = 2.0 at Re = 185

5. 5
Figure 4 Streak lines obtained using dye flow
around four in line circular cylinders at s/d = 3.0
at Re = 185
Figure 5 Streak lines obtained using dye
flow around four in line circular cylinders at
s/d = 4.0 at Re = 185
s/d = 5.0
The streak lines obtained from flow visualization
for s/d = 5.0 are shown in figure 6. Notice that the
flow domain has become very large along the
streamwise direction, making it difficult to capture
the flow. The vortices are shed in the gap between
the cylinders and hit the downstream cylinder
(figure 4). The vortices from different downstream
cylinders coalesce together. The location of vortices
does not show any relationship among themselves
as they move downstream, and the flow appears
chaotic. It is interesting to note that for flow around
in-line cylinders the chaos occurs are large spacing;
however for vertical row of cylinders the chaos
occurs at small spacing. In the former case it is due
to wake interference while in latter case it is due to
wake interaction
DISCUSSION ON WAKE INTERFERENCE
Owing to the small gap between the cylinders
(figure 2) the convection in the gap is negligible as
compared to diffusion. This is also attributed to the
fact that the local Reynolds number is small due to
the small local length scale for low spacing. Thus,
the vorticity generated in the gap is diffused away
without vortex shedding. When the downstream
cylinder is placed very close to the upstream
cylinder, the wake structure of the upstream
cylinder is not altered substantially, because
immediately behind the upstream cylinder the
streamwise velocity is either close to zero or
negative. This is similar to the single-cylinder case,
where obviously there is no interference. Thus, the
wake formed by closely spaced cylinders is similar
to that of a single cylinder.
At larger spacing, the vortex is shed in the gap
between the cylinders. The downstream cylinder
interferes with the wake, which may lead to the
pressure variations which are relatively more
severe. The nature of the wake does not remain
structured, owing to the increased lateral movement
of the fluid in and out of the gap (figures 3 and 4).
This lateral movement increases with an increase in
the spacing, leading to either a quasi-periodic or
chaotic regime. The transition of flow regimes
should indeed cause the nature of forces on the
cylinders to change, as already noted Sewatkar et al.
(2012).
Figure 5 Streak lines obtained using dye
flow around four in line circular cylinders at
s/d = 5.0 at Re = 185
CONCLUSION
The flow across four identical circular cylinders
placed one behind the other (in-line or tandem
configuration) has been studied in this work

6. 6
experimentally using flow visualization. The
experiments are carried out for 1.0 ≤ s/d ≤ 5.0 at
Reynolds number of 185. The flow visualization
suggests that the flow behavior for in-line cylinders
is strongly affected by the spacing between the
cylinders. At low spacing the flow is synchronous
in nature while at larger spacing the flow is chaotic.
The transition from synchronous to chaotic occurs
through the quasi-periodic behavior of flow. The
flow regimes obtained in the present work are
compared with the definitions and characteristics
definitions for these flow regimes reported by
Kumar et al. (2008), Sewatkar et al. (2009) and
Sewatkar et al. (2012). The variation in flow
regimes may be attributed to the phenomenon of
wake interference in which the incoming flow for
downstream cylinders consists of vortices shed
from its upstream counterpart. These results may
serve as a benchmark results for future numerical
simulations. The videos shall be shared at the time
of presentation.
NOMENCLATURE
d diameter of a cylinder
Re Reynolds number (U0d/)
s spacing between surfaces of adjacent
cylinders
s/d Spacing ratio between the cylinders
U0 free stream velocity at inlet of domain
 Kinematic viscosity
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