Measurement of Fluid Friction Acting on Flat Plate

There are several types of air resistance acting on an aircraft. One of them, frictional resistance, accounts for half of the air resistance, and thus, measurement of the frictional resistance plays an important role in engineering practice. Oil film interferometry is one of the methods for measuring the frictional resistance. Oil dropped on an object is thinly stretched by the frictional resistance. Interference fringes are generated when monochromatic light is applied to the oil. The spacing between neighboring fringes changes depending on the gradient of thickness of the oil. The air was sent to a wind tunnel using a blower and passed through several rectifying grids for a uniform flow field to be formed. The wind speed at the centre of the wind tunnel exit, i.e., the mainstream speed, was measured using a pitot tube. A flat plate, onto which a drop of oil was dropped, was placed along the stream at the centre of the wind tunnel exit. The change in fringe spacing was measured and the coefficient of surface frictional resistance was estimated. The relation between the Reynolds number based on the distance from the front edge of the flat plate and the coefficient of surface friction resistance is summarized. The results show good agreement between the experimental and the theoretical trends.