Wang W.  

Oil Film Interferometry technique for Skin Friction Measurement

The oil-film interferometric technique has been used to measure skin friction for the last 30 years in one form or another. The technique has relative ease of implementation and the solid theoretical underpinning which could provide spatial qualitative as well as quantitative information without interfering with the flow.
In this project, the oil-film interferometry (OFI) skin friction technique is described and applied to subsonic and supersonic flows in the tunnels at CAAA (China Academy of Aerospace Aerodynamics). The details for applying the technique are discussed. Results are shown for tests that illustrate the oil-film's good ability to measure the skin friction.

Details of Setup
1 Test Surface   The first step is to prepare the test surface so that it is optically smooth and partially reflective. The interference is a result of the reflected light from the test surface interfering with the light from the air–oil interface (Fig. 2). Investigation and experiments contrastapproves that, the easiest approach is to apply thin sheets of Mylar with black pigment and adhesive backing to the test surface. The self-adhesive back makes it easy to apply, and it strongly adheres to the surface. The combination of Mylar (index of refraction of 1.67) and black pigment embedded in the Mylar provides a partially reflective surface that reflects light with about the same intensity as does the air–oil interface.
2 Oil Procedure  Silicon oil is used as the oil due to its good clarity and vaporizes not readily. Silicon oil in viscosities of 5~100,000 cS is provided on the market. The choice of viscosity is based on tunnel run time, dynamic pressure, and desired displacement between adjacent interference patterns (or commonly called fringes) at the end of the test. The oil patches (line segments, drops, smudges, etc.) should be spaced far enough apart so that one oil-flow patch does not significantly run into another patch.
3 Light Disposal  The oil should be illuminated with as nearly a monochromatic light source as possible. Low-pressure sodium bulbs are used, which emit at a single wavelength, (actually a closely spaced doublet),  =589 nm and  =589.6 nm.
The oil film interferometry requires an expansive (extended) light source, i.e., light emitted from a wide range of positions. To overcome this problem, in our experiments ,the entire wind tunnel top and side walls have been used as a reflector (with walls painted white), with the camera positioned on the overhead window’s (small non-painted) viewing hole.

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