Liquid Crystal for studying Aerodynamic Shear Stress

It is well known that Liquid Crystals (LC) will exhibit color change when subjected to external stimuli, for example, changing color due to temperature change. However, they may also exhibit color change when subjected to aerodynamic shear stress within both laminar and turbulent boundary layers. ESI has undertaken an analysis of a number of different formulations of liquid crystals including Chiral-Nematic and Cholesteric ones as well as a mixture of both, and subjected them to both a mechanical shearing mechanism as well as a number of different aerodynamic conditions ranging from low Sub-sonic to near Hypersonic conditions at numbers of practical interest.

Typically, the Chiral-Nematic LC’s have relatively low viscosity and are easily applied but may wear more rapidly than the higher viscosity Cholesteric counterparts. The table shows the full range of viscosities covered by these LC’s.

shear stress sensitive liquid crystals

Figure 1: Test Rig for Mechanical Calibration of Liquid Crystals

Figure 2 provides a calibration chart of Hue (color) against Shear Stress for the above set of LC’s obtained with the rotational shear stress rig. The importance of lighting angle cannot be overstated. 

ESI has also been instrumental in determining the use of Liquid Crystals for a range of aerodynamic situations, as shown in the following Table, with calibration of the LC’s undertaken under both mechanical and aerodynamic conditions.

shear stress sensitive liquid crystals

Initial studies using a specifically designed rotational shear stress rig (Figure 1), has highlighted the importance of illuminating the crystals, using an appropriate light source and illumination angle to the direction of shearing motion, as well as the importance of ensuring the camera is also set at the optimum viewing angle for the proper color enhancement.

shear stress sensitive liquid crystals

Figure 2: Hue-Shear Stress Calibration for Various Illumination Angles

For example, a 25 mm cylinder with an ogive nose section was tested at a range of Mach numbers from 0.37 to 1.00 in a transonic wind tunnel, Figure 3.

shear stress sensitive liquid crystals

Figure 3: Effect of Hue Along the Axis of the Body

for Different Mach Numbers

shear stress sensitive liquid crystals

Figure 4: Variation of Hue with Shear Stress

Figure 4 shows the change in Hue with Shear Stress along the axis of the body taken just past the ‘shoulder’ of the model for this Mach number range. Taking into account the change in incident lighting angle, the data from the different runs tend to collapse upon a single calibration curve.

Aerodynamic models have also been investigated at a freestream Mach number of 2.00, in a supersonic wind tunnel. In one case a half model of a cone/cylinder combination, Figure 5, was used and the effect of Hue of the liquid crystals around the nose and in the wake of the half model is shown, with the normalized Shear Stress levels (Figure 6). 

shear stress sensitive liquid crystals

Figure 5: Variation of Hue around the nose and the wake of a half model at 2.00

shear stress sensitive liquid crystals

Figure 6: Normalized Shear Stress Results

Obtained from the Half Model

The second model tested at Mach 2.00 was that of a three dimensional cavity that was set at different angles to the freestream flow. The dimension of the cavity were 123.8 mm long, 10.5 mm wide and 5.3 mm deep and was constructed into a turntable so that the cavity could be rotated and set at various angles to the oncoming boundary layer, Figures 7 and 8.

shear stress sensitive liquid crystals

Figure 7: Effect of Liquid Crystals Within and Around the

Cavity Set at 75 Degrees to the Flow

shear stress sensitive liquid crystals

Figure 8: Effect of Liquid Crystals Within and Around the

Cavity Set at 88 Degrees to the Flow

A complete analysis of the distribution of the Hue within the cavity for a range of flow angles is shown in Figure 9 where it can be observed that the Hue (and therefore the Shear Stress) is basically uniform throughout the floor of the cavity when set at 0 degrees to the flow direction. However, as the cavity is rotated towards 90 degrees, there is a very marked change in the distribution especially close to the downstream end of the cavity.

On closer inspection of the cavity at high angles of attack to the freestream flow a number of vortices and wake patterns can be observed. Figure 14 is a typical distribution of the liquid crystals within and around the downstream end of the cavity with Figure 15 depicting the internal vortices that are being produced by the flow regime as well as the wake patterns that are, not only occurring due to the body geometry, but also due to the internal vortices being ejected into the external flow.

shear stress sensitive liquid crystals
shear stress sensitive liquid crystals

Figure 9: Distribution of the Hue (Shear Stress) throughout

the cavity as it is rotated to the flow direction

ESI has performed liquid crystal tests in a Mach 3 facility, Figure 10, to determine the type of crystal that would be the most effective at this high velocity. In this case the Cholesteric crystals with their higher viscosities provided the better response since the Chiral Nematic ones were eroded before the full Mach number was reached.

For these tests, LC’s were applied to a circular flat plate within the boundary layer of the Mach 3 wind tunnel and illuminated and viewed through a very small window, Figure 11.

Figure 10: Camera and lighting position for the Mach 3.0 facility

On starting the wind tunnel the color change from red through to blue is quite dramatic over the short duration as the flow transitions  from M = 0 to M = 3.0, Figure 11. Further tests with holding the Mach number at M = 3.0, but varying the static pressure between 100, 310 and 500 psi shows the sensitivity of these crystals, Figure 11.

shear stress sensitive liquid crystals

Figure 11: Color Change of the Liquid Crystals as the

Flow Increases from M=0 (Red) to M=3 (Blue)

Further investigations have been undertaken to determine whether LC’s could be used at higher Mach numbers. In this case a flat plate model with a small rib protuberance was installed in a Mach 6.85 wind tunnel capable of operating at five different stagnation pressure (Modes). Figure 12 shows the results of the color changes along the plate (chordwise) for the different Modes, with Figure 13 providing the distribution of Hue against chordwise position.

shear stress sensitive liquid crystals

Figure 12: Liquid Crystal color change along the flat plate for the different Modes (Mach 6.85)

A computed shear stress distribution along the plate was undertaken for each of the five Modes and this was plotted against the Hue, Figure 14, and shows how the LC’s have responded to each of the five settings. 

shear stress sensitive liquid crystals

Figure 15: Surface Shear Stress Patterns Around the Nose of the Rib as Depicted by Liquid Crystals

shear stress sensitive liquid crystals

Figure13: Distribution of the Hue Along the Plate for

the Different Modes (Mach 6.85)

shear stress sensitive liquid crystals

Figure 14: Shear Stress Levels Along the Flat Plate

for the Different  Modes at Mach 6.85

Figure 15 shows the complex shear stress pattern, as depicted by the Liquid Crystals, around the nose of the small rib, providing evidence of the stagnation point, wake and boundary layers associated with this protuberance.