Simultaneous Skin Friction and Pressure Sensitive Paint

Currently, the contribution of skin friction to the total drag of a wind tunnel model is estimated by comparing measurements of the total drag to the integrated pressure drag. While this yields an estimate for the average skin friction, the distribution of the skin friction cannot be determined from such measurements. The distribution of skin friction and pressure is available from computational fluids models, however, these models must be validated using experimental data. An experimental tool for distributed measurements of skin friction and pressure would be useful for both aerodynamic configuration development and numerical code validation. We propose the development of an image-based sensor for simultaneous measurements of skin friction and pressure that is based on combining Pressure-Sensitive Paint with a new image-based measurement technique for skin friction, Surface Stress Sensitive Films (S3F). The basis of the S3F technique is an elastic film that distorts under the action of the applied forces. Skin friction is determined by monitoring these distortions and applying a finite element model to the film. The S3F technique can operate over a range of temperatures from cryogenic (160 K) to well above ambient (470 K), thus there is a potential to deploy this system in a variety of wind tunnels. Quantitative measurements of skin friction using S3F have been demonstrated from 10-m/s to Mach 5 and the accuracy of the S3F sensor has been validated to be better than 5% full scale in a fully developed channel and high Reynolds number boundary layer. Several experimental demonstrations of a combined PSP/S3F sensor have been performed in small wind tunnels and bench-top experiments, thus demonstrating that this approach is possible. The key innovations in this proposal are to develop a multi-color data acquisition system that can acquire both pressure and skin friction data simultaneously, and validate the accuracy and stability of the combined sensor.

Surface Stress Sensitive Films are being investigation for a variety of applications in aerodynamics, hydrodynamics, and biomedical research. Skin friction is a quantity of interest in many aerodynamics applications such as validation of CFD, and investigation of supersonic/hypersonic inlets. Biomedical applications include ongoing research with the Cleveland Clinic for identification and correlation of shear on the foot of diabetics. The formation of bed sores is believed to be related to shear stress, and therefore this would be a similar application of the technology. Other biomedical applications include shear stress on surfaces of artificial implants such as stints, hearts, valves, and assist pumps. Finally, ISSI has recently begun investigating S3F as a tactile sensor for artificial limbs and robotic touch sensors. This technology may be of interests for unmanned planetary probes.

Surface Stress Sensitive Films, is under investigation for a variety of applications in aerodynamics, hydrodynamics, and biomedical research. Traditional interest exists in skin friction for CFD validation and drag reduction on supersonic/hypersonic air vehicles and hydrodynamic drag reduction. A miniature point version of the sensor has been produced by ISSI that will sense 2-components of skin friction and provide real time feedback for closed loop flow control. This sensor is of interest for using in Navy applications, and may have applications in MAV's. Biomedical applications include ongoing research with the Cleveland Clinic for identification and correlation of shear on the foot of diabetics. Other biomedical applications include shear stress on surfaces of artificial implants such as stints, hearts, valves, and assist pumps. The S3F sensor has recently been used to detect shear forces on tires and a means of using the system for predictive maintenance of fleet vehicles is underway.

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