A versatile scanning method for volumetric measurements of velocity and density fields

Understanding turbulence in a stratified environment requires a detailed picture of both the velocity field and the density field. Experimentally, this represents a significant measurement challenge, especially when full three-dimensional data is needed to accurately characterise the turbulent fields. This paper presents a new approach to obtaining such data through well-resolved, near-instantaneous volume-spanning measurements. This is accomplished by rapidly scanning the volume with a light sheet so that, at each scan location, planar two-dimensional measurements of three velocity components via stereo particle image velocimetry (PIV) and simultaneous density information via planar laser induced fluorescence (PLIF) can be made. Ultimately, this rapid scanning technique permits the measurement of all three components of velocity in a volume as well as simultaneously capturing the three-dimensional density field. The scanning of the light sheet is accomplished by mounting the optics producing the light sheet on a linear traverse that is capable of rapidly scanning the volume in a continuous manner. The key and novel aspect enabling high scan rates is the addition of two mirrors on galvanometers to make small adjustments to the position of the light sheet and ensure a precise overlap between pairs of image frames. This new technique means the light sheet does not have to be excessively thick, the scanning speed too slow, or that inappropriately small interframe times have to be used, while ensuring overlapping particle patterns between pairs of images that are required by the PIV algorithm. The technique is illustrated with some preliminary results from the buoyancy-driven exchange flow through an inclined duct connecting two reservoirs containing different density fluids.