Calibration and verification techniques for a multi-axis force balance to measure small changes in viscous drag on riblet surfaces

At laboratory scales, direct measurement of viscous drag reduction by riblet surfaces is a difficult task to perform accurately because of the milli-Newton scale force measurements required. This is particularly the case when comparing the performance characteristics of different riblet geometries and evaluating the effects of slight damage that could be realistically expected in real-world use. To address this challenge, a multi-axis force balance has been developed that incorporates the unique combination of an easily deformable elastomeric structure and precision Hall Effect sensors to measure structure deformation. To interpret the data, an automated calibration stand has been developed to tilt the balance to create force components in the stream-wise and span-wise directions. A multi-variable regression analysis of calibration data allows subsequent balance measurements from wind tunnel experiments to be analyzed in terms of viscous drag. These measurements have been verified against Oil Film Interferometry taken on a flat plate.