Simultaneous free-surface deformation and near-surface velocity measurements

 A newly developed non-intrusive approach has been devised for studying near-surface flows where it is important to be able to construct correlations between small-sloped free-surface deformations and near-surface velocities. This method combines digital particle image velocimetry (DPIV) and the reflective mode of the free-surface gradient detector (FSGD) technique into a single measurement system, providing us with an approach to be able to characterize correlations between elevation and kinematic properties, such as velocity and vorticity, which is essential in understanding near surface turbulence. Furthermore, as the free-surface elevation is directly proportional to the pressure for low Froude number flows, this method will allow for the measurement of pressure near the free surface. This will also be useful in calculating the pressure-velocity term in the turbulent kinetic energy equation for near-surface flows. The approach is explained and demonstrated by measuring these correlations for a vertical shear layer intersecting a free surface. Received: 2 August 1999/Accepted: 23 July 2000