Detection of flow separation and reattachment using shear-sensitive
Abstract Coatings of pure chiral nematic liquid crystals are
known to change colour under different levels of surface
shear stress. In this study, the liquid crystal was used to
provide information about ﬂow separation and reattach-
ment on both a two-dimensional aerofoil and a delta wing.
The tests were carried out at a free-stream velocity of 28 m/s
and a number of incidence angles. The Reynolds numbers
based on the central chord length of the models were 200,000
and 270,000 for the aerofoil and delta wing models,
respectively. The study showed that locations of boundary
layer separation and reattachment can be identiﬁed from
spatial variations in the surface colour; the agreement
between the results and those obtained using surface oil ﬂow
was good. Issues relating to interpretation of the crystal
colour pattern and the limitation of this technique in
detection of ﬂow separation were also discussed.
Surface ﬂow visualization plays an important role in our
understanding of the physics of complex ﬂows. The sur-
face oil-ﬂow technique is one of the most commonly used
methods for visualizing ﬂow patterns close to solid bodies
exposed to airﬂow. The observed patterns not only provide
information about the ﬂow direction in the vicinity of the
surface but also indicate the location of boundary layer
separation and reattachment, if any. However, the results
from oil ﬂow tend to be affected by the presence of both
obstacles on the test surface and gravity when it is applied
on a curved model. In order to obtain more reliable results
in these circumstances, the author investigated alternative
ways of visualizing the surface ﬂow.
It is known that shear-sensitive liquid crystals can be
applied to a surface as a thin ﬁlm with a typical thickness
of 10 lm. The ﬁlm can respond to different levels of sur-
face shear stress by displaying different colours. Such a
thin coating is far less likely to be affected by gravity and
the presence of surface obstacles. Hence it appears to be a
suitable alternative technique for this application.
Liquid crystals were ﬁrst tested in wind tunnel experi-
ments more than 30 years ago (Klein and Margozzi 1969).
A renewed interest in their use in aerodynamics testing
began in the early 1990s when liquid crystals, which re-
spond to surface shear stress while remaining temperature
insensitive, became available. A certain amount of work
has already been carried out on the application of this type
of liquid crystals (Parmar 1991; Smith 1992; Toy et al.
1993; Reda and Wilder 1997; Reda et al. 1997, 1998). The
results show that shear-sensitive liquid crystals can be
used to detect ﬂow separation. However, to the best
knowledge of the author, detailed comparisons between
results from shear-sensitive liquid crystals and other
conventional techniques are not available in the literature.
In the work reported in this paper, both shear-sensitive
liquid crystals and oil ﬂow were used to visualize the ﬂow
around two commonly used aerodynamic shapes, a two-
dimensional aerofoil and a delta wing. The data obtained
from these two techniques about the location of ﬂow
separation and reattachment were examined and com-
pared. Issues relating to the interpretation of the crystal
colour pattern and the limitation of this technique in de-
tection of ﬂow separation were also discussed.
Shear-sensitive liquid crystal technique
Liquid crystals that are capable of displaying different
colours at different levels of shear are called chiral nematic
crystals. Their unique optical properties are attributed to
their particular molecular structures (Ireland et al. 1993).
The average orientation of the molecules in each molecular
layer can be deﬁned by a director. In chiral nematic
crystal, the director of each layer exhibits a small offset
relative to those of its adjacent layers. On a multilayer
scale, the director traces out a helix in space (Fig. 1).
This helical structure gives arise to an unusual optical
property called selective reﬂection. If white light is incident
to the crystal, most of the light will pass through, except
for light with a speciﬁc wavelength equal to the pitch of the
helix. For the crystal used here, the pitch length decreases
as the shear stress increases, thus the colour of the crystal
coating is typically characterized by a shift from rusty red
(corresponding to no shear) through the visible spectrum
to violet. Such a colour change is reversible, therefore
unlike the surface oil ﬂow, the same crystal coating can be
reused in several tests before its colour begins to fade.
Received: 17 May 2001 / Accepted: 10 January 2001
Published online: 16 April 2002
School of Engineering
University of Manchester, Manchester M13 9PL, UK
The author would like to thank S. O’Brient, C. Chew and P. Mason for
contributing to the data presented here. Comments from Dr. D.C.
Reda at NASA Ames Research Center, USA on the interpretation of
the liquid crystal colour information are also gratefully acknowledged.
Experiments in Fluids 32 (2002) 667–673 Ó Springer-Verlag 2002