Uncertainty analysis on thin-oil-film skin friction measurements in compressible flow

A theoretical uncertainty analysis was performed on two thin-oil-film deformation techniques for determining shear stresses on aerodynamic surfaces. One technique acquires data with a dual-laser-beam (DLB) configuration while the other gathers information with an expanded-laser-beam (ELB) configuration. The major objective was to compare the uncertainties that arise in the DLB and ELB techniques when used in zero-pressure-gradient, constant shear stress, compressible flows of short duration. The results show that when only the minimal data are available, large uncertainties can arise in the DLB measurements of shear stress compared to smaller uncertainties in the ELB measurement. The larger uncertainties in the DLB technique originate from its inability to measure the oil film slope directly. In both techniques, accurate oil viscosity determinations and optimum laser beam alignment are recommended to reduce uncertainties in shear stress measurements