Friction Velocity from Active Thermography and Shape Analysis

A novel technique is presented that makes it feasible to measure the viscous shear stress τμ from active thermography. With a CO2 laser, patterns are written to the sea surface. This temperature structure is distorted by the velocity profile in the viscous boundary layer. Due to the non-zero penetration depth of both the laser and the infrared camera, this vertical velocity profile can be resolved. The viscous shear stress is extracted from the recovered velocity profile. The computation relies on a shape analysis of the visualized thermal structures. An analytical function is derived which captures the dynamical processes. The model parameters can then be computed in a standard nonlinear parameter estimation framework. This novel technique is tested both on simulated data and on measurements conducted in a small annular wind-wave flume. The friction velocity computed in this fashion compared favorably to independent measurements. Although not tested yet, this technique should be equally applicable to field measurements.