Abstract
A laser beam refraction technique for a measurement of free surface waves on a high-speed water jet has been developed in the present work. The new method employing two pulse lasers makes it possible to determine the cross correlation function of local slope angle fluctuation between two measurement locations separated for several mm on the jet free surface. The free surface wave speed is evaluated from dominant time lag of cross correlation functions. The measurements are conducted for several locations along the jet of 5 m/s. At the immediately downstream the nozzle-exit, the wave speed increases with the distance from the nozzle exit and agree with the most-unstable solutions of the linear stability theory of the velocity shear layer beneath the free surface. On the other hand, as the shear layer relaxes, the wave speed increases close to that of capillary gravity wave propagating on jet free surface. The shape of wave is also evaluated by integrating the local free-surface slope angle data. It is found the wave height increases with the distance from nozzle exit and reaches maximum at downstream location where the shear layer instability ceases from occurring.
