The evolution of a short injection-hole jet issuing into a crossflow at low blowing ratios is presented. Particle image velocimetry (PIV) is used to determine structural features of the jet/crossflow interaction throughout its development from within the jet supply channel (which feeds the holes), through the injection hole, and into the crossflow. The effect of supply channel feed orientations, i.e. counter to, or in the same direction as the crossflow is emphasized. Feed orientation profoundly affects such jet characteristics as trajectory and lateral spreading, as well as its structural features. Fluid within the high-speed supply channel exhibits swirling motions similar to the flow induced by a pair of counter-rotating vortices. The sense of rotation of the swirling fluid depends upon the orientation of the supply channel flow with respect to the crossflow, and in turn impacts the in-hole velocity fields. In the coflow supply channel geometry (channel flow is in the same direction as the free stream), a pair of vortices exists within the hole with the same sense of rotation as the primary jet counter-rotating vortex pair (CRVP). In contrast, the counterflow supply channel configuration has in-hole vortices of opposite rotational sense to that of the CRVP. The in-hole vortices interact constructively or destructively with the CRVP, thus affecting the strength and coherence of the CRVP. The counterflow configuration has a weakened CRVP because of destructive interference with the in-hole vortices. The weaker CRVP has a lower trajectory and increased spanwise spreading. External to the injection hole, a pair of vortices exists immediately downstream of the jet. They are initially perpendicular to the boundary-layer plate near the wall and are outboard of the streamwise hole centreline. These vortices, denoted ‘downstream spiral separation node’ (DSSN) vortices, are affected by both the supply channel feed direction and the blowing ratio. They appear to form by free-stream fluid wrapping around the jet and interacting with the CRVP. The coflow supply channel geometry is associated with the largest and most well-defined DSSN vortices, and their size is inversely proportional to the blowing ratio. At low blowing ratio, these vortices induce a large recirculating flow region downstream of the injection hole at the wall.

The ongoing COVID-19 pandemic has highlighted the importance of aerosol dispersion in disease transmission in indoor environments. The present study experimentally investigates the dispersion and build-up of an exhaled aerosol modeled with polydisperse microscopic particles (approximately 1 μm mean diameter) by a seated manikin in a relatively large indoor environment. The aims are to offer quantitative insight into the effect of common face masks and ventilation/air purification, and to provide relevant experimental metrics for modeling and risk assessment. Measurements demonstrate that all tested masks provide protection in the immediate vicinity of the host primarily through the redirection and reduction of expiratory momentum. However, leakages are observed to result in notable decreases in mask efficiency relative to the ideal filtration efficiency of the mask material, even in the case of high-efficiency masks, such as the R95 or KN95. Tests conducted in the far field ($2\hspace{0.17em}\mathrm{m}$ distance from the subject) capture significant aerosol build-up in the indoor space over a long duration ($10\hspace{0.17em}\mathrm{h}$ ). A quantitative measure of apparent exhalation filtration efficiency is provided based on experimental data assimilation to a simplified model. The results demonstrate that the apparent exhalation filtration efficiency is significantly lower than the ideal filtration efficiency of the mask material. Nevertheless, high-efficiency masks, such as the KN95, still offer substantially higher apparent filtration efficiencies (60% and 46% for R95 and KN95 masks, respectively) than the more commonly used cloth (10%) and surgical masks (12%), and therefore are still the recommended choice in mitigating airborne disease transmission indoors. The results also suggest that, while higher ventilation capacities are required to fully mitigate aerosol build-up, even relatively low air-change rates ($2\hspace{0.17em}{\mathrm{h}}^{-1}$ ) lead to lower aerosol build-up compared to the best performing mask in an unventilated space.

Low power consumption and activation voltage combined with high flexibility and minimal weight make ionic polymer metal composites (IPMCs) well-suited for miniaturized underwater propulsion systems. In the present study, we investigate the flow field generated by an IPMC strip vibrating in a quiescent aqueous environment using planar particle image velocimetry. We use the time-averaged flow field to compute the momentum transfer to the fluid and estimate the mean thrust generated by the vibrating actuator. We find that the mean thrust produced by the vibrating IPMC increases with the Reynolds number, defined by the maximum tip speed and IPMC width, and is only marginally affected by the relative vibration amplitude. The results of this study can guide the optimization of IPMC-based propulsion systems for miniature biomimetic robotic swimmers.

In this letter, we study the transient response of a cantilevered ionic polymer metal composite impacted by a self-propagating vortex ring in an otherwise quiescent fluid. Experiments are performed using time-resolved particle image velocimetry to elucidate the flow physics during the vortex ring propagation and subsequent interaction with the cantilever. Images from these experiments are analyzed to extract the vibration of the structure, which is used to estimate the energy transferred from the vortex ring. A small fraction of this energy is further transduced into an electrical signal by the chemoelectromechanical behavior of the ionic polymer metal composite.