Visualization of ﬂow and vortex structure around a swimming
loach by dynamic stereoscopic PIV
Katsuya Nagayama Æ Toshimasa Tanaka Æ
Kazuhiro Tanaka Æ Hiroshi Hayami Æ
Received: 2 January 2007 / Revised: 23 August 2007 / Accepted: 2 December 2007 / Published online: 20 December 2007
Ó Springer-Verlag 2007
Abstract Loach has a unique swimming style of bending
the whole body and staying at the bottom of water. We
studied the three-dimensional ﬂow ﬁeld around and behind
the loach using stereoscopic-PIV. We captured ﬂow ﬁelds
in horizontal and vertical plane, and it seems loach leaves
vortex tube arches. From the analysis of body motion and
ﬂow ﬁeld, we propose ﬂow structure with vortex tube
arches connected along the loach body. After being
released, they are separated and ﬂow away and dissipate.
Living creatures in water have obtained a variety of moving
mechanisms by evolution, adapting for the environments.
Learning their optimized swimming mechanism is useful as
such information is used to design robots. Such research is
called aqua-bio-mechanism and a variety of moving
mechanisms have been studied (Azuma 1992; Bainbridge
1958; Bartol et al. 2002; Heinrich 1963; Kim et al. 1998;
Lauder et al. 2002; Lighthill 1970;Wu1971) and well
summarized (Sfakiotakis et al. 1999; Triantafyllou et al.
2000). Swimming kinematics and efﬁciency of carangiform
ﬁsh was modeled by slender body theory (Lighthill 1970).
Vortices generation and shedding due to body motion was
discussed in detail by Triantafyllou et al. (2000). Although
there is a huge literature on anguilliform and carangiform
swimmers, we believe that loach swimming using its whole
body is unique because it is anguilliform or subcarangiform
ﬁsh with caudal ﬁn and swims at the bottom of still water.
As loach motions are ﬂexible, we think it can be a model of
new type robots for example to inspect pipelines or human
body. On the other hand, dynamic PIV (Hayami et al.
2003a, b) is a strong tool to visualize the transient ﬂow ﬁeld,
and is expected to apply to bio-mechanism area.
We have already analyzed the basic two-dimensional
(2D) ﬂow ﬁeld about a loach swimming in shallow water
and found that loach leaves a vortex street like reverse
Karman vortices indicating propulsion (Nagayama and
Tanaka 2005a, b, 2006). But real ﬂow ﬁeld in nature is
three-dimensional (3D) ﬂow in deep water, so in this paper,
ﬂow around a moving loach in deep water will be shown
using stereoscopic PIV. We analyzed the ﬂow ﬁeld in
horizontal cross section and vertical cross section. Studying
the 3D ﬂow ﬁelds will help to clarify the mechanisms of
2 Experimental conditions
There are many kinds of ﬁsh with long body. Among them,
Black Kuhli loach, which is shown in Fig. 1, was chosen
here. Black Kuhli loach or Pangio Oblonga in scientiﬁc
name lives in India or South Asia. Its total length is about
60 mm, height 8 mm and width 6 mm.
Flow around a loach is visualized by particle image
velocimetry (PIV), and the experimental set-up is shown in
Fig. 2. Two cameras are used in stereoscopic PIV, and
three velocity components in 2D plane can be obtained.
The angle between two cameras is 30°. Experiment 1 is
This research article was submitted for the special issue on Animal
locomotion: The hydrodynamics of swimming (Vol. 43, No. 5).
K. Nagayama (&) Á T. Tanaka Á K. Tanaka
Kyushu Institute of Technology, Iizuka 820-8502, Japan
H. Hayami Á S. Aramaki
Kyushu University, Kasuga 816-8580, Japan
Exp Fluids (2008) 44:843–850