Liquid crystal surface coatings of a few tens of microns thickness may be used to provide full field measurement of surface shear stresses in high - speed aerodynamic testing. In the figure above, obtained at M = 6.85, the high shear stress regions, such as those around the leading edge of the fin, are represented by blue. The low shear stress regions, such as immediately upstream of the fin are represented by red. For a full paper on this topic, click here.
The structure of liquid crystal material is such that the molecules in each layer are aligned in the same general direction (the director). This director rotates in each successive layer to give a helical structure. The pitch length of this helix falls within the wavelength of visible light such that liquid crystal coatings are highly optically active, scattering light selectively in any give direction. The pitch length changes with shear stress for unencapsulated crsytal mixtures making them useful as a shear stress sensor. However, the extreme sensitivity of the reflected colour to both illumination and viewing conditions means that the optical system requires careful calibration.
There are many different synthetic and naturally occurring mixtures available. Although different mixtures have different viscosities there is no correlation between the viscosity and the colour play ( that is the range of shear stresses corresponding to the transition from red through green to blue). I have found the BCN/165 type to be particularly useful in terms of giving a good colour range.
A good calibration rig is required, allowing the user to vary the illumination and viewing angles in both the horizontal and vertical planes. Shear stresses may be applied by using a carefully constructed rotational shear stress rig, as shown below.
The following slides shows the set - up for an experiment to examine the shear stress on an axisymmetric body in the transonic wind tunnel at City University in the UK. The angle of illumination was varied in the horizontal plane (as indicated in the sketch) in order to assess its effects on the measured colours.
Results from the rotational calibration rig show how the peak in the spectrum of reflected light shifts to lower wavelengths as the shear is increased (for fixed illumination and viewing conditions). At the same time the peak attenuates making the colour resolution less reliable at the highest shear stresses.
For a normal viewing direction, increasing the angle of the illumination from the normal in the direction of the shear stress, increases the range of colour play. It may be seen that the Hue varies almost linearly with shear stress initially but then becomes less sensitive at the higher stresses.
The changes in shear stress along the axisymmetric body are clearly shown in the following images, with the "wedge" shaped transition from laminar (red) to turbulent flow (green / blue).
This transition, for different Mach numbers, is shown more clearly by the variation of Hue with distance along the centre-line (X) from the model nose.
Taking two points, one in the laminar region (X/D=1.2) and the other in the turbulent region (X/D=3.5) and varying the Mach number, relationships may be approximated between the measured Hue and the shear stress from a boundary layer calculation.
This type of data may be modified taking into account the differences in the illumination angle along the model length.
Increasing the angle of illumination (bL) so that it lies more the direction of shear, increases the colour play, allowing resolution of hue and shear stress even at a lower Mach number of 0.39.
Acknowledgements: Prof N Toy, Mr D M Sykes, Dr Q H Hoang, Mr L Gaudet and MoD / DERA (now Qinetic)