Skin Friction Measurements Using Elastic Films (Invited)

Abstract

A new technology for distributed measurements of skin friction and pressure 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 normal and tangential deformations of the film surface, and then converting these deformations into pressure and skin friction using a finite element model. This technology is currently under development for measurements of skin friction and pressure in wind tunnels, water tunnels, and for contact forces. In this article, we report the results from several experiments that demonstrate the operation of the sensor in both low speed and high speed wind tunnels. Validation of the skin friction measurements is demonstrated using a full developed channel flow and a high Reynolds number boundary layer. In the channel flow, measurements of skin friction using the sensor are compared to experimental and theoretical standards and agreement is better than 5% full scale. Measurements of skin friction are also conducted in a water tunnel with a high Reynolds number boundary layer. Here the results are compared to experimental skin friction measurements conducted using a drag balance and LDV measurements of velocity at the wall. The sensor again compares to the experimental measurements to better than 5% full scale. Finally, a qualitative demonstration is of unsteady flow on a cylinder in cross flow is presented demonstrating the potential for an unsteady skin friction sensor.