Oil-Film Interferometry Skin-Friction Measurement Under White Light

The interference fringes obtained under white light by a thin oil e lm were used to measure distribution of the skin-friction coefe cient on a e at plate. The model placed in a subsonic e ow was illuminated by a 500-W xenon white light source. The e lm height and variation over time were measured by analyzing the fringe colors that were similar to those of Newton’ s tints. The use of color shows that very high accuracy can be achieved for interferogram analysis using a numerical model of the phenomenon based on a nonconstant stress assumption. Experimental data were compared with those obtained by solving the simplie ed Navier‐ Stokes equations and with the solutions supplied by a boundary-layer code. Experiments showed that the white light interferometry technique was well suited to determine the transition region and that the stress levels varying from a few pascals up to around a hundred pascals were measured with an accuracy of a few percent. The main sources of error were identie ed, and their effects were quantie ed.