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1. Upstream mobility and swarming of light activated micromotors

   https://doi.org/10.1039/D3MA00814B  

   Wu, Bingzhi; Rivas, David P; Das, Sambeeta (March 2024, Materials Advances)

   Swarms of light-activated micromotors were created and moved against fluid flows in microchannels. 

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2. Slow Shock Formation Upstream of Reconnecting Current Sheets

   https://doi.org/10.3847/1538-4357/ac423b  

   Arnold, H.; Drake, J. F.; Swisdak, M.; Guo, F.; Dahlin, J. T.; Zhang, Q. (February 2022, The Astrophysical Journal)

   Abstract The formation, development, and impact of slow shocks in the upstream regions of reconnecting current layers are explored. Slow shocks have been documented in the upstream regions of magnetohydrodynamic (MHD) simulations of magnetic reconnection as well as in similar simulations with the kglobal kinetic macroscale simulation model. They are therefore a candidate mechanism for preheating the plasma that is injected into the current layers that facilitate magnetic energy release in solar flares. Of particular interest is their potential role in producing the hot thermal component of electrons in flares. During multi-island reconnection, the formation and merging of flux ropes in the reconnecting current layer drives plasma flows and pressure disturbances in the upstream region. These pressure disturbances steepen into slow shocks that propagate along the reconnecting component of the magnetic field and satisfy the expected Rankine–Hugoniot jump conditions. Plasma heating arises from both compression across the shock and the parallel electric field that develops to maintain charge neutrality in a kinetic system. Shocks are weaker at lower plasma β , where shock steepening is slow. While these upstream slow shocks are intrinsic to the dynamics of multi-island reconnection, their contribution to electron heating remains relatively minor compared with that from Fermi reflection and the parallel electric fields that bound the reconnection outflow. 

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3. Upstream swimming and Taylor dispersion of active Brownian particles

   https://doi.org/10.1103/PhysRevFluids.5.073102  

   Peng, Zhiwei; Brady, John F. (July 2020, Physical Review Fluids)
4. Experimental Study of Geysering in an Upstream Vertical Shaft

   https://doi.org/10.3390/w15091740  

   Allasia, Daniel G.; Böck, Liriane Élen; Vasconcelos, Jose G.; Pinto, Leandro C.; Tassi, Rutineia; Minetto, Bruna; Persch, Cristiano G.; Pachaly, Robson L. (May 2023, Water)

   Transient flows in stormwater systems can lead to damaging and dangerous operational conditions, as exemplified by geysering events created by the uncontrolled release of entrapped air pockets. Extreme rain and associated rapid inflows may result in air pocket entrapment, which causes significant changes in flow conditions and potentially geysering. Stormwater geysers have been studied in different experimental and numerical modeling studies, as well as from limited data gathered within real systems. However, there is still no complete understanding of geysering events, as stormwater system geometries vary considerably. Most past studies involved releasing air from an intermediate shaft, in which a significant fraction of the entrapped air may bypass the release. This work advances the understanding of geysering by considering uncontrolled air release through an upstream shaft or manhole. In such cases, the entire air pocket is released upon reaching the shaft, worsening the occurrence of geysering. Pressure and water level measurements were performed for various combinations of initial water pressure, trapped air pocket volume, and vertical shaft geometries, indicating the higher severity of these geysering events. The results obtained also corroborate previous studies in that the measured pressure heads were lower than the grade elevation. Future studies should include larger-scale testing and the representation of this geometry using CFD. 

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5. Investigating Frontal Precipitation Enhancement Upstream of the Olympic Mountains

   Garcia, B G; Eidhammer, T; Hence, D A; Rauber, R M (January 2021, 20th Student Conference, 101st American Meteorological Society Annual Meeting)

   Orographic precipitation enhancement is the tendency of mountains to cause clouds to produce more precipitation than they would otherwise, which greatly affects the total rainfall in mountainous regions and plays a major role in flooding and mudslides. Most past studies have examined this enhancement close to the mountains themselves. The present study examined the orographic enhancement of frontal precipitation upstream of the Olympic Mountains of Washington State, using data from a Weather Research and Forecasting (WRF) regional climate simulation. Using this simulation, we strived to determine how thermodynamic and dynamic conditions affect the enhancement of frontal precipitation upstream of the Olympic Mountains. To do so, the characteristics of frontal passages were analyzed, including frontal type, orientation, velocity, warm-air moisture content, and accumulated precipitation. Of the five fronts analyzed to date, there were two cold fronts, two warm fronts, and one occluded front. We then analyzed each characteristic by comparing each event to each other and as a function of distance from the mountains. We found that all fronts slowed down as they approached the mountains, four of which to almost half of their original speeds, and some even beginning to decelerate upwards of 700 km upstream. Areas of enhanced precipitation were observed along the two cold fronts, as far upstream as 560 km. These results document upstream impacts on frontal passages, but further analysis is needed to examine additional frontal passages and determine if the observed precipitation enhancement and frontal deceleration were caused by orographic effects from the Olympic Mountains. 

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