Central Aerohydrodynamic Institute
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Shear Stress Sensitive Liquid Crystals

Boundary layer transition on civil aircraft wing model at V = 50 m/s and α = 30°
Boundary layer transition on civil aircraft wing model at V = 50 m/s and α = 30°

Visualization of the shear stress distribution on the aerodynamic models surface; determination of the laminar-turbulent boundary layer transition.

Liquid crystals (LC) method is an experimental method that uses the effect of selective reflection of the LC planar texture. The method allows obtaining the field of shear stress that is applied to the liquid crystal coating.

The model with the applied LC coating is illuminated with white light that is approximately perpendicular to the investigated surface. The color of the scattered light is registered up-flow. The other option is to illuminate up-flow and to register scattered light perpendicularly to the modal surface. The color changes from red to blue while the shear stress is increasing. The color does not depend on the coating thickness and is defined by the absolute value of the shear stress vector and the angle between shear stress vector and observation direction. Registration of the coating color at different azimuth angles allows defining the shear stress vector.

Technical Characteristics

  • Shear stress measuring range (τ = 1…20 Pa) depends on the LC coating composition used.
  • Operating temperature range is 15…45 °С.
  • Necessity of optical windows up-flow in re of the investigated surface.


LCs are reversible that allows investigating the several flow regimes in one run of wind tunnel. The models made of all materials may be measured.


LCs may be used in wind tunnels that operate in range from subsonic velocities to supersonic ones except the short-run wind tunnels (t&rt;200 ms). The model surface temperature limits also the application of method.