Galvanometer-based PIV for liquid flows

A cost-effective Particle Image Velocimetry (PIV) system has been developed that is capable of resolving flow fields at frequencies of up to 200 Hz (covering the typical scales of interest for hydraulic researchers), with recording durations of over 8 min. The system uses a high-speed camera to image flow fields illuminated by a scanning-beam lightsheet, where the lightsheet is generated by a galvanometer-driven mirror (computer controlled) together with a parabolic mirror. The system has several advantages over rotating-polygon type scanning systems, including: that the mirror is always positioned at the focal point of the parabola, and that the lightsheet generation is extremely versatile, with the lightsheet width (for a single parabolic mirror) and beam scan velocities easily and independently adjustable. Additionally, the beam scan velocity, which is typically nonlinear in rotating-polygon systems due to inherent properties of parabolic reflectors, can be constant in a galvanometer-based system (giving a uniform intensity lightsheet) by driving the galvanometer at an unsteady angular velocity. Integrated synchronisation options for the system permit frame-straddling techniques to be used in order to reduce interframe times to below 1 /(camera frame rate). The system also offers additional benefits over equivalent double-pulsed or twin-laser setups that rely on beam expansion by lens systems, and that typically only allow measurement at frequencies up to 50 Hz. Manipulation of the beam diameter using lens systems is outlined. The system has been implemented and used to obtain PIV measurements in flows of water and oil.