COVID-19 in health-care workers in three hospitals in the south of the Netherlands: a cross-sectional study

Background: 10 days after the first reported case of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection in the Netherlands (on Feb 27, 2020), 55 (4%) of 1497 health-care workers in nine hospitals located in the south of the Netherlands had tested positive for SARS-CoV-2 RNA. We aimed to gain insight in possible sources of infection in health-care workers. Methods: We did a cross-sectional study at three of the nine hospitals located in the south of the Netherlands. We screened health-care workers at the participating hospitals for SARS-CoV-2 infection, based on clinical symptoms (fever or mild respiratory symptoms) in the 10 days before screening. We obtained epidemiological data through structured interviews with health-care workers and combined this information with data from whole-genome sequencing of SARS-CoV-2 in clinical samples taken from health-care workers and patients. We did an in-depth analysis of sources and modes of transmission of SARS-CoV-2 in health-care workers and patients. Findings: Between March 2 and March 12, 2020, 1796 (15%) of 12 022 health-care workers were screened, of whom 96 (5%) tested positive for SARS-CoV-2. We obtained complete and near-complete genome sequences from 50 health-care workers and ten patients. Most sequences were grouped in three clusters, with two clusters showing local circulation within the region. The noted patterns were consistent with multiple introductions into the hospitals through community-acquired infections and local amplification in the community. Interpretation: Although direct transmission in the hospitals cannot be ruled out, our data do not support widespread nosocomial transmission as the source of infection in patients or health-care workers. Funding: EU Horizon 2020 (RECoVer, VEO, and the European Joint Programme One Health METASTAVA), and the National Institute of Allergy and Infectious Diseases, National Institutes of Health

Three-dimensional flow fields and forces on revolving flat plates

The evolution of three-dimensional flow structures of revolving low-aspect-ratio plates in the Reynolds number range of 10,000 to 20,000 was studied, combining Tomographic Particle Image Velocimetry with force measurements. Two motion kinematics were considered: (1) a revolving surge motion where the wing accelerates to a terminal velocity with a constant acceleration at a fixed angle of attack and then remains to revolve at a constant rate; (2) a revolving pitch motion which is initiated by a constant acceleration from rest to a terminal velocity at zero angle of attack, followed by a pitch-up motion at a constant pitch rate and revolution at a constant rate. In the experiments, the terminal velocity, acceleration, angle of attack and pitch rate were varied to study their effect on the resultant flow fields and forces. In general, a vortex system that consists of a leading edge vortex, a tip vortex and a trailing edge vortex is observed. The vortex system bursts into substructures as the motion progresses, which does not lead to a decrease in the forces. The evolution of spanwise flow and the effects of centrifugal acceleration and spanwise pressure gradient are discussed

Visualization of the structure of vortex breakdown in free swirling jet flow

In this paper we investigate the three dimensional flow structures in a free annular swirling jet flow undergoing vortex breakdown. The flow field is analyzed by means of time-resolved Tomographic Particle Image Velocimetry measurements. Both time-averaged and instantaneous flow structures are discussed, including a detailed analysis of the first and second order statistical moments. A Reynolds decomposition of the flow field shows that the time averaged flow is axisymmetric with regions of high anisotropic Reynolds stresses. Two recirculation zones exist with regions of very intensive mixing around them. Despite the axisymmetric nature of the time-averaged flow, a non-axisymmetric structure of the instantaneous flow is revealed, including a central vortex core which breaks up into a double helix. The winding sense of this double helix is opposite to the swirl direction and it is wrapped around the vortex breakdown bubble. The double helix precesses around the central axis of the flow with a precessing frequency corresponding to a Strouhal number of 0.13. To the authors’ knowledge, this structure of vortex breakdown has not been previously reported in the literature to occur for turbulent jet flow and it suggests that the well-known Precessing Vortex Core (PVC) found in swirling jets corresponds to the helical mode of vortex breakdown

Asymmetric vortex shedding in the wake of an abruptly expanding annular jet

© 2021, The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.Abstract: In this paper, the structure of the turbulent wake behind the inner tube of a suddenly expanding annular jet flow is studied. The flow field is measured using tomographic particle image velocimetry and analyzed using proper orthogonal decomposition (POD). It was found that both the instantaneous and time-averaged central wakes behind the inner pipe are highly asymmetric despite the axisymmetric structure of the geometry. This asymmetry is the result of a bifurcation at low Reynolds numbers which persists up to the turbulent regime. The asymmetry induces a pair of counter-rotating vortices in the jet which are aligned with the main flow direction. Moreover, the asymmetry also induces a highly dynamical flow field. Analyzing the flow structures using POD shows that the wake oscillates around the asymmetric equilibrium position at a very low Strouhal number in the order of 0.01. On top of this motion, the inner shear layer oscillates with Strouhal numbers in the range of 0.1-0.3. This oscillation causes an asymmetric shedding of vortices of the hairpin type in the inner shear layer. As such, a local asymmetric region of very intensive mixing is induced near the stagnation point. Graphical Abstract: [Figure not available: see fulltext.

Three-dimensional flow and load characteristics of flexible revolving wings at low Reynolds number

Increasing interest in the field of micro air vehicles has stimulated research activities regarding biological flapping-wing flight. An important characteristic of biological flapping-wing flight that is not commonly taken into account in mechanical model simulations is the influence of wing flexibility. This experimental study explores the flow field and fluid-dynamic loads generated by revolving low-aspect-ratio wings with different degree of chordwise flexibility at a Reynolds number of 10,000. The experimental campaign consisted of phase-locked tomographic particle image velocimetry (PIV) measurements complemented with simultaneous force measurements. The three-dimensional velocity fields are captured in three measurement volumes positioned side-by-side along the span of the wing for different phases of the revolving motion, generating a time-resolved volumetric velocity field data set. Subsequently, from the velocity data the pressure fields are reconstructed as well as the loads acting on the wing. In this study the link between the temporal evolution of the vortical structures and the associated pressure forces acting on the wing is investigated in detail