A single-camera, 3D scanning velocimetry system for quantifying active particle aggregations

Abstract

A three-dimensional (3D) scanning velocimetry system is developed to quantify the 3D configurations of particles and their surrounding volumetric, three-component velocity fields. The approach uses a translating laser sheet to rapidly scan through a volume of interest and sequentially illuminate slices of the flow containing both tracers seeded in the fluid and particles comprising the aggregation of interest. These image slices are captured by a single high-speed camera, encoding information about the third spatial dimension within the image time-series. Where previous implementations of scanning systems have been developed for either volumetric flow quantification or 3D object reconstruction, we evaluate the feasibility of accomplishing these tasks concurrently with a single-camera, which can streamline data collection and analysis. The capability of the system was characterized using a study of induced vertical migrations of millimeter-scale brine shrimp (Artemia salina). Identification and reconstruction of individual swimmer bodies and 3D trajectories within the migrating aggregation were achieved up to the maximum number density studied presently, 8 × 10 5 animals per m 3 . This number density is comparable to the densities of previous depth-averaged 2D measurements of similar migrations. Corresponding velocity measurements of the flow indicate that the technique is capable of resolving the 3D velocity field in and around the swimming aggregation. At these animal number densities, instances of coherent flow induced by the migrations were observed. The accuracy of these flow measurements was confirmed in separate studies of a free jet at Re D = 50.Keywords First keyword • Second keyword • More IntroductionTurbulent flows containing dispersed particles are a common feature in many environmental and industrial processes. The particles within these flows include both passive phases such as solid particles (Balachandar and Eaton, 2010), bubbles (Rensen et al., 2005;Risso, 2018), and droplets (Aliseda and Heindel, 2021), as well as 'active' phases such as swimming zooplankton (Jumars et al., 2009). At sufficiently large particle volume fractions (10 −6 ≤ Φ v ≤ 10 −3 ), the presence of the particles creates unique flow dynamics associated with the Funding for this project was generously provided by the Gordon and Betty Moore Foundation

Magnetic Signature of Vertically Migrating Aggregations in the Ocean

“…To determine magnetic signatures produced by vertical migrations, representative velocity fields are chosen for high and low aspect ratio configurations of migrating aggregations. The high aspect ratio model is analogous to observations in laboratory experiments involving induced vertical migrations of zooplankton aggregations (Fu et al., 2021; Houghton et al., 2018; Wilhelmus & Dabiri, 2014), where the velocity signature is confined to a narrow extent of the domain and configurations have a large vertical extent relative to their width. This velocity field is modeled as a homogeneous, unidirectional flow in the vertical ( z ) direction with Dirac delta distributions in the horizontal plane (i.e., xy ‐plane).…”

“…Using this relationship, two representative models for the biologically induced velocity field are analyzed to predict the behavior of the corresponding flow‐induced magnetic field. The first model is representative of high aspect ratio aggregations, such as those encountered in laboratory experiments (Fu et al., 2021; Houghton & Dabiri, 2019; Houghton et al., 2018; Wilhelmus & Dabiri, 2014). In contrast, the second emulates the wider, low aspect ratio configurations observed in the field (Sato et al., 2013; Wiebe et al., 1979).…”