9.4 Skin Friction Measurement
215
where μ, is the dynamic viscosity of the oil, τ, the skin friction, and x, the distance
from the tip of the wedge. The main issue with this technique lies in the fact that
the oil film shears accordingly with the transient behaviour of the wind tunnel, from
start-up till test speed and making difficult to identity initial time corresponding to
a particular speed measurement. This introduces challenges in locating the apex of
the oil film.
These challenges could be addressed by measuring the thickness of the oil wedge
at two closely spaced points, which makes it possible to determine the effective
“time” and “initial” time parameters, hence the skin friction. The method can be
extended to three-dimensional flows if the direction of the skin friction vector at the
wall is known.
9.4.5 Liquid Crystals Thermography
The skin friction can be determined using liquid crystal thermography, a combination
of optically active organic compounds with the property of reflecting light at a particular wavelength in response to stimuli such as temperature, pressure, shear stress,
magnetic and electric fields. For wind tunnel applications, it is possible to retain
only one stimulus by suppressing the response to others. Thus, there are compounds
that respond only to shear stresses under certain circumstances (or temperature for
heat transfer measurements, see below). In practice, the model is covered with a
thin film of the substance and illuminated by a white light source, the liquid crystal
film then producing an image in the visible range which is recorded by a camera.
The technique has several advantages: its very high sensitivity, its reversible nature
and the ability to provide information on an entire surface. Thus, the liquid crystals
are used to identify the laminar to turbulent boundary-layer transition region, the
transition process being visualised by a colour change. Quantitative measurement of
the skin friction is in principle possible by determining the colour changes under the
action of the shear stress. An image analysis by digital image processing allows the
determination of the wavelength λ of the reflected light. The local value of the skin
friction is then deduced from a calibration giving the wall shear stress τ as a function
of λ.
9.4.6 Adjustment Based on the Logarithmic Law
of Turbulent Boundary Layers
At sufficiently high Reynolds numbers, the turbulent boundary layer adopts an equilibrium state characterised by a velocity distribution satisfying the so-called welldefined law of the wall for turbulent boundary layers shown in Fig. 9.23. Starting
215
where μ, is the dynamic viscosity of the oil, τ, the skin friction, and x, the distance
from the tip of the wedge. The main issue with this technique lies in the fact that
the oil film shears accordingly with the transient behaviour of the wind tunnel, from
start-up till test speed and making difficult to identity initial time corresponding to
a particular speed measurement. This introduces challenges in locating the apex of
the oil film.
These challenges could be addressed by measuring the thickness of the oil wedge
at two closely spaced points, which makes it possible to determine the effective
“time” and “initial” time parameters, hence the skin friction. The method can be
extended to three-dimensional flows if the direction of the skin friction vector at the
wall is known.
9.4.5 Liquid Crystals Thermography
The skin friction can be determined using liquid crystal thermography, a combination
of optically active organic compounds with the property of reflecting light at a particular wavelength in response to stimuli such as temperature, pressure, shear stress,
magnetic and electric fields. For wind tunnel applications, it is possible to retain
only one stimulus by suppressing the response to others. Thus, there are compounds
that respond only to shear stresses under certain circumstances (or temperature for
heat transfer measurements, see below). In practice, the model is covered with a
thin film of the substance and illuminated by a white light source, the liquid crystal
film then producing an image in the visible range which is recorded by a camera.
The technique has several advantages: its very high sensitivity, its reversible nature
and the ability to provide information on an entire surface. Thus, the liquid crystals
are used to identify the laminar to turbulent boundary-layer transition region, the
transition process being visualised by a colour change. Quantitative measurement of
the skin friction is in principle possible by determining the colour changes under the
action of the shear stress. An image analysis by digital image processing allows the
determination of the wavelength λ of the reflected light. The local value of the skin
friction is then deduced from a calibration giving the wall shear stress τ as a function
of λ.
9.4.6 Adjustment Based on the Logarithmic Law
of Turbulent Boundary Layers
At sufficiently high Reynolds numbers, the turbulent boundary layer adopts an equilibrium state characterised by a velocity distribution satisfying the so-called welldefined law of the wall for turbulent boundary layers shown in Fig. 9.23. Starting