Oil film interferometry

Oil film interferometry relies on the principle that the rate at which oil thins on a surface is a function of the shear stress magnitude. There are various incarnations of oil film interferometry as discussed by Naughton and Sheplak (2000), although the method used at NASA Ames is Fringe Imaging Skin Friction Interferometry (FISF). The interference between the partially reflected light at the air-oil interface and the light reflected from the model surface will vary between constructive and destructive as the oil film thickness changes. This is observed as a series of light and dark bands or fringes, the spacing of which is proportional to the skin friction (Figure 7).




Figure 7: Interference fringes produced by Fizeau Interferometry: (a) Constructive interference produces bright bands, whereas (b) destructive interference produces dark bands (Naughton and Sheplak (2000))

The oil typically used is silicon oil and is applied in either square patches or drops to the surface. The surface should have a high index of refraction (ideally n=2), and therefore glass is ideal. For practical reasons, polished stainless steel or Mylar is often used. The light absorption of aluminium is too low to be used with PSP.

Illumination must be provided by a coherent light source, although the coherence length can be short (a few microns) due to the small thickness of the oil film. Light sources such as fluorescent bulbs, sodium lamps and black lights are often used. Lasers suffer from the expense of special optics and also the problem of specular reflection (speckle) near the surface. Imaging can be achieved using CCD cameras, preferably black and white. Calibration of the oil viscosity and its variation with temperature is necessary, although the measurement of the shear stress is absolute, requiring no additional reference measure. Mehta et al. (2000) state that using calibrated oil and an accurate measurement of the dynamic head and the incident light angle, it is possible to achieve accuracy in Cf better than ±5% and ±1% in vector direction.






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