Validation of an Image-Based Skin friction Sensor in a Fully Developed Channel Flow

A new technology for distributed measurements of pressure and skin friction has been developed. The active element for this sensor is a thin film made of an elastomer with known thickness and shear modulus. The film deforms under load but does not compress or yield. The measurement is accomplished by determining the surface normal and tangential deformations of the film and then converting these deformations into pressure and skin friction. This technology is currently being utilized in wind tunnels for investigation of a variety of flows including laminar separation bubbles on low speed airfoils, shock-boundary layer interactions, and plasma flow control. While reasonable skin friction measurements are being obtained, it is necessary to validate the sensor by conducting measurements in a well characterized shear field. A validation test has been conducted in a fully developed channel flow built at Purdue University. Several quantitative and qualitative experiments were conducted including measurements of skin friction on the wall of the channel and measurements of pressure and skin friction on the wall near a strut end-wall junction. Qualitative analysis of the data near the strut-endwall junction indicates that the film is responding to the direction and magnitude of the local skin friction. A quantitative comparison of the skin friction measurements from the film with those obtained using fluorescent oil agree to within 15 percent.