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

Abstract

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 wellresolved, 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.

Regime transitions and energetics of sustained stratified shear flows

“…• averaged vertical density profile ρ x,y,t (z) and velocity profile u x,y,t (z) ('bottom right' panels f,l ). For more complete visualisations, including horizontal planes and the other velocity components v and w (not shown here), see Partridge et al (2018).…”

“…These measurements relied on a novel technique introduced by Partridge, Lefauve & Dalziel (2019) in which a thin, pulsed vertical laser sheet (in the plane) is scanned rapidly back and forth in the spanwise direction (along ) to span a duct sub-volume of non-dimensional cross-section and non-dimensional length (typically a small fraction of full duct length ). Simultaneous stereo particle image velocimetry (sPIV) and planar laser induced fluorescence (PLIF) are employed to obtain the three-dimensional, three-component velocity and density fields in successive planes at spanwise locations and respective times .…”

The structure and origin of confined Holmboe waves

“…This allowed us to control the successive positions of the pulsed laser sheet independently of the traverse, and thus obtain pairs of images at the same spanwise location while continuously scanning the thin laser sheet across the measurement volume. A detailed description of the scanning system and other aspects of the sPIV/PLIF measurements can be found in Partridge et al (2018)).…”

“…However, this time lag can be made very small by increasing the scanning speed while maintaining the co-location of laser sheet pairs. As discussed in more detail in Partridge et al (2018), this method has three main advantages over existing methods for obtaining three-dimensional experimental diagnostics of a stratified flow across a volume: it is less sensitive to unavoidable residual refractive index variations; it yields a higher spatial resolution; and crucially, it allows simultaneous measurements of the density field.…”

“…The process of capturing still images of the target was repeated by shifting the target in the spanwise direction to three different (x, z) planes inside the duct to obtain a set of approximately 600 points used to fit a least-square polynomial mapping between pixel and (x, y, z) world coordinates. Given the importance of an accurate mapping for sPIV reconstruction, and since the light sheet is not infinitely thin, a further step of coordinate mapping refinement was used, as discussed in Partridge et al (2018).…”