Thrust Augmentation of Micro-Resistojets by Steady Micro-Jet Blowing into Planar Micro-Nozzle

The present work investigates the impact of steady micro-jet blowing on the performance of a planar micro-nozzle designed for both liquid micro-thrusters and nitrogen cold-gas micro-resistojets. Two micro-injectors have been placed into the divergent region along the sidewalls, injecting a secondary flow of propellant perpendicularly to the wall where they have been located. The micro-jet actuator configuration is characterized by the dimensionless momentum coefficient cμ. The best performance improvement is retrieved at the maximum cμ for both water vapor (Δ%T,jet = +22.6% and Δ%Isp,Tjet = +2.9% at cμ = 0.168) and nitrogen gaseous flows (Δ%T,jet = +36.1% and Δ%Isp,Tjet = +9.1% at cμ = 0.297). The fields of the Mach number and the Schlieren computations, in combination with the streamline visualization, reveal the formation of two vortical structures in the proximity of secondary jets, which energize the core flow and enhance the expansion process downstream secondary jets. The compressible momentum thickness along the width-wise direction θxy in presence of secondary injection reduces as a function of cμ. In particular, it becomes smaller than the one computed for the baseline configuration at cμ > 0.1, decreasing up to about and -57% for the water vapor flow at cμ = 0.168, and -64% for the nitrogen gaseous flow at cμ = 0.297.

Higher Abundance of Sediment Methanogens and Methanotrophs Do Not Predict the Atmospheric Methane and Carbon Dioxide Flows in Eutrophic Tropical Freshwater Reservoirs

Freshwater reservoirs emit greenhouse gases (GHGs) such as methane (CH4) and carbon dioxide (CO2), contributing to global warming, mainly when impacted by untreated sewage and other anthropogenic sources. These gases can be produced by microbial organic carbon decomposition, but little is known about the microbiota and its participation in GHG production and consumption in these environments. In this paper we analyzed the sediment microbiota of three eutrophic tropical urban freshwater reservoirs, in different seasons and evaluated the correlations between microorganisms and the atmospheric CH4 and CO2 flows, also correlating them to limnological variables. Our results showed that deeper water columns promote high methanogen abundance, with predominance of acetoclastic Methanosaeta spp. and hydrogenotrophs Methanoregula spp. and Methanolinea spp. The aerobic methanotrophic community was affected by dissolved total carbon (DTC) and was dominated by Crenothrix spp. However, both relative abundance of the total methanogenic and aerobic methanotrophic communities in sediments were uncoupled to CH4 and CO2 flows. Network based approach showed that fermentative microbiota, including Leptolinea spp. and Longilinea spp., which produces substrates for methanogenesis, influence CH4 flows and was favored by anthropogenic pollution, such as untreated sewage loads. Additionally, less polluted conditions favored probable anaerobic methanotrophs such as Candidatus Bathyarchaeota, Sva0485, NC10, and MBG-D/DHVEG-1, which promoted lower gaseous flows, confirming the importance of sanitation improvement to reduce these flows in tropical urban freshwater reservoirs and their local and global warming impact.

Effect of Mach number on droplet aerobreakup in shear stripping regime

The present experimental study investigates the shear stripping breakup of single droplets in subsonic and supersonic gaseous flows. In contrast to most research that places emphasis on the Weber number (We), we focus on the individual effects exerted by flow Mach (M∞) and Reynolds numbers (Re). Millimeter-sized droplets made of either ethylene glycol or water are exposed to shock-induced flows. Shadowgraph and schlieren images of the breakup process are recorded by an ultra-high-speed camera. The experimental We is constrained at 1100, while M∞ is varied from 0.3 to 1.19 and Re from 2600 to 24,000. A systematic analysis of the experiment series reveals that the breakup pattern alters with M∞ although a constant We is maintained. The classical stripping behavior with fine mist shed from the peripheral sheet changes to rupture of multiple bags along the periphery at M∞ = 0.63, and further to stretching of ligament structures from the leeward surface at M∞ = 1.19. The corresponding breakup initiation is delayed and the resultant fragments are sized less uniformly and distributed over a narrower spread. In terms of the early-stage deformation, droplets experience less intense flattening and slower sheet growth at higher M∞. The change of Re introduces additional variations, but only to a minor extent. Graphical abstract

The evolution of dust in discs influenced by external photoevaporation

ABSTRACT Protoplanetary discs form and evolve in a wide variety of stellar environments and are accordingly exposed to a wide range of ambient far-ultraviolet (FUV) field strengths. Strong FUV fields are known to drive vigorous gaseous flows from the outer disc. In this paper we conduct the first systematic exploration of the evolution of the solid component of discs subject to external photoevaporation. We find that the main effect of photoevaporation is to reduce the reservoir of dust at large radii and this leads to more efficient subsequent depletion of the disc dust due to radial drift. Efficient radial drift means that photoevaporation causes no significant increase of the dust-to-gas ratio in the disc. We show that the disc lifetime in both dust and gas is strongly dependent on the level of the FUV background and that the relationship between these two lifetimes just depends on the Shakura–Sunyaev α parameter, with the similar lifetimes observed for gas and dust in discs pointing to higher α values (∼10−2). On the other hand, the distribution of observed discs in the plane of disc size versus flux at 850 μm is better reproduced by lower α (∼10−3). We find that photoevaporation does not assist rocky planet formation but need not inhibit mechanisms (such as pebble accretion at the water snow line) which can be effective sufficiently early in the disc’s lifetime (i.e. well within a Myr).

New solid particle seeding generator for laser-based velocity measurements in reacting flows

The design of a new solid particle seeding generator for laser-based velocity measurements in gaseous flows is reported. Performance tests revealed that the new seeder has the ability to control the concentration of seeding particles in a flow independently of the flowrate. It also can provide a steady supply of seeding particles, operate over an extended range of flowrate, and break up agglomerates into smaller particles. These features give this seeder advantages over its counterparts, especially at low volumetric flowrate.