Viscid Oil Method. Visualization of Surface Streamlines and Shear Stress
A new method of surface flow visualization is developed at TsAGI. The essence of new method is to register some minor shift of oil film applied on the tested object surface and to reproduce numerically the complete surface flow pattern.
Some optically contrast (luminescent) hard particles are added to the oil film, and at least two images of particle distribution on the tested model surface at some time interval are recorded by digital-camera. These images are processed by cross-correlation analysis (similar to analysis used in PIV method) to obtain the particles movement vectors.
Direction of particles shift is tangential in re of surface streamlines that allows calculating them and making visible the surface streamline pattern. Shear stress value can be determined from magnitude of particles shift if the oil layer thickness and its viscosity are known.
Due to the high accuracy of correlation method even small shift of oil (just for several pixels) is sufficient to visualize flow. As a result, several flow regimes can be investigated using one preparation procedure of model. Since both of the analyzed images are acquired at the same flow parameters, the obtained flow pattern corresponds to these parameters and does not depend on the prehistory of the flow, including the wind tunnel run start process.
The oil viscosity may vary in wide range and is possible to be matched to the friction stress presupposed.
Visualization of surface streamlines and boundary layer transition. Measuring the shear stress distribution is possible in future.
- The method allows finding the surface streamlines and shear stress distribution.
- The oil film thickness is 20…50 μm.
- The investigated surface area is limited by digital camera resolution and UV pulsed illuminator power. Currently, there is a technical possibility to research surface areas of up to 1 m.
- Several flow regimes can be tested within one wind tunnel run.
- Method can be used as an effective technique to validate CFD methods.
Research of full-scale range of velocities i.e. from subsonic velocities to hypersonic ones. The restriction is valid only on the model surface temperature (< 200 °C).