The behaviour of heavy particles in isotropic, homogeneous, decaying turbulence has been experimentally studied. The settling velocity of the particles has been found to be much larger than in a quiescent fluid. It has been determined that the enhancement of the settling velocity depends on the particle loading, increasing as the volume fraction of particles in the flow increases. The spatial and temporal distribution of the particle concentration field is shown to exhibit large inhomogeneities. As the particles interact with the underlying turbulence they concentrate preferentially in certain regions of the flow. A characteristic dimension of these particle clusters is found to be related to the viscous scales of the flow. Measurements of the settling velocity conditioned on the local concentration of particles in the flow have shown that there is a monotonic increase in the settling velocity with the local concentration (the relation being quasi-linear). A simple phenomenological model is proposed to explain this behaviour.

Turbulent flows laden with inertial particles present multiple open questions and are a subject of great interest in current research. Due to their higher density compared to the carrier fluid, inertial particles tend to form high-concentration regions, i.e., clusters, and low-concentration regions, i.e., voids, due to the interaction with the turbulence. In this work we present an experimental investigation of the clustering phenomenon of heavy sub-Kolmogorov particles in homogeneous isotropic turbulent flows. Three control parameters are varied over significant ranges: ${\text{Re}}_{\lambda }\in [170–450], \text{St}\in [0.1–5]$, and volume fraction ${\varphi }_v\in [(2\times {10}^{-6})–(2\times {10}^{-5})]$. The scaling of clustering characteristics, such as the distribution of Voronoi areas and the dimensions of cluster and void regions, with the three parameters is discussed. In particular, for the polydispersed size distributions considered here, clustering is found to be enhanced strongly (quasilinearly) by ${\text{Re}}_{\lambda }$ and noticeably (with a square-root dependence) with ${\varphi }_v$, while the cluster and void sizes, scaled with the Kolmogorov length scale $\eta$, are driven primarily by ${\text{Re}}_{\lambda }$. Cluster characteristic size $\sqrt{\langle A_c\rangle }$ scales up to $\approx 100\eta$, measured at the highest ${\text{Re}}_{\lambda }$, while void sizes $\sqrt{\langle A_v\rangle }$, scaled also with $\eta$, are typically twice as large as clusters at the same ${\text{Re}}_{\lambda }\left(\approx 200\eta \right)$. The lack of sensitivity of the above characteristics to the Stokes number lends support to the sweep-stick particle accumulation scenario. The non-negligible influence of the volume fraction, however, is not considered by that model and can be connected with collective effects.

This study quantifies thrombogenic potential (TP) of a wide range of left ventricular assist device (LVAD) outflow graft anastomosis angles through state-of-the-art techniques: 3D imaged-based patient-specific models created via virtual surgery and unsteady computational fluid dynamics with Lagrangian particle tracking. This study aims at clarifying the influence of a single parameter (outflow graft angle) on the thrombogenesis associated with flow patterns in the aortic root after LVAD implantation. This is an important and poorly-understood aspect of LVAD therapy, because several studies have shown strong inter and intrapatient thrombogenic variability and current LVAD implantation strategies do not incorporate outflow graft angle optimization. Accurate platelet-level investigation, enabled by statistical treatment of outliers in Lagrangian particle tracking, demonstrates a strong influence of outflow graft anastomoses angle on thrombogenicity (platelet residence times and activation state characterized by shear stress accumulation) with significantly reduced TP for acutely-angled anastomosed outflow grafts. The methodology presented in this study provides a device-neutral platform for conducting comprehensive thrombogenicity evaluation of LVAD surgical configurations, empowering optimal patient-focused surgical strategies for long-term treatment and care for advanced heart failure patients.

Using wind to disperse microfliers that fall like seeds and leaves can help automate large-scale sensor deployments. Here, we present battery-free microfliers that can change shape in mid-air to vary their dispersal distance. We designed origami microfliers using bistable leaf-out structures and uncovered an important property: A simple change in the shape of these origami structures causes two dramatically different falling behaviors. When unfolded and flat, the microfliers exhibit a tumbling behavior that increases lateral displacement in the wind. When folded inward, their orientation is stabilized, resulting in a downward descent that is less influenced by wind. To electronically transition between these two shapes, we designed a low-power electromagnetic actuator that produces peak forces of up to 200 millinewtons within 25 milliseconds while powered by solar cells. We fabricated a circuit directly on the folded origami structure that includes a programmable microcontroller, a Bluetooth radio, a solar power–harvesting circuit, a pressure sensor to estimate altitude, and a temperature sensor. Outdoor evaluations show that our 414-milligram origami microfliers were able to electronically change their shape mid-air, travel up to 98 meters in a light breeze, and wirelessly transmit data via Bluetooth up to 60 meters away, using only power collected from the sun.

An electrohydrodynamic (EHD) flow in a point-to-ring corona configuration is investigated experimentally and via a multiphysics computational model. The model couples the ion transport equation and the Navier-Stokes equations (NSE) to solve for the spatiotemporal distribution of electric field, flow field, and charge density. The numerical simulation results are validated against experimental measurements of the cathode voltage, ion concentration, and velocity profiles. The maximum flow velocity is at the centerline, and it decays rapidly with radial distance due to the viscous and electric forces acting on the partially ionized gas. To understand this coupling, a nondimensional parameter, $X$, is formulated as the ratio of the local electric force to the inertial term in the NSE. In the region of $X\ge 1$, the electric force dominates the flow dynamics, while in the $X\ll 1$ region, the balance of viscous and inertial terms yields traditional pipe flow characteristics. This approach expands on the analytical model of Guan et al. by adding a description of the developing flow region. The approach allows the model to be used for the entire EHD domain, providing insights into the near-field flow in the corona region.