Flow velocity measurements through a fluctuating free surface by means of adaptive optics

The noninvasive investigation of flows, where media of different optical density participate, is often not possible. A measurement through dynamic water-air surfaces will result in higher measurement uncertainties or they will hinder any measurement at all. To overcome this issue we make use of adaptive optics (AO). Dependent on the measurement system, an adaptive correction of the illumination path and the observation path can result in improved measurement properties. AO can therefore help to directly measure noninvasively flow velocities and flow turbulence at places that where inaccessible before. Examples of interest are convection flows at emergency core cooling, where cold water is injected into a pressurized water reactor, forming a fluctuating, free falling jet, as well thin film flows and biomedical flows. Interferometric velocity measurement systems are especially sensitive to wavefront distortions since the straightedge of the system the interference pattern is directly influenced and needs to be stabilized by AO when measuring in multi-phase flows.