Simulation of aerosol formation due to rapid cooling of multispecies vapors

\u3cp\u3eAn extended classical nucleation approach is put forward with which aerosol formation from rapidly cooled, supersaturated multispecies vapor mixtures can be predicted. The basis for this extension lies in the treatment of the critical cluster that forms as part of the nucleation burst—a multispecies treatment of the thermodynamically consistent approach is proposed that can be solved efficiently with a Newton iteration. Quantitative agreement with Becker–Döring theory was established in case the equilibrium concentration of the critical clusters is properly normalized. The effects of nucleation, condensation, evaporation, and coalescence are consolidated in the numerical framework consisting of the Navier–Stokes equations with Euler–Euler one-way coupled vapor and liquid phases. We present a complete numerical framework concerning generation and transport of aerosols from oversaturated vapors and focus on numerical results for the aerosol formation. In particular, using adaptive time-stepping to capture the wide range of time scales that lie between the nucleation burst and the slower condensation and coalescence, the aerosol formation of a system of up to five alcohols in a carrier gas is studied. The effects of the temperature levels, the cooling rate, and the composition of the vapor mixture under a constant temperature drop, on the formation and properties of the aerosol are investigated. A striking nonuniform dependence of the asymptotic number concentration of aerosol droplets on temperature levels was found. A decrease of the rate of cooling was shown to reduce the number concentration of aerosol droplets which asymptotically leads to significantly larger droplets. The simplification of the vapor mixture by removing the higher alcohols from the system was found to yield an increase in the asymptotic size of the droplets of about 15%, while the number density was reduced accordingly.\u3c/p\u3

Evaluation of oscillation-free fluid-porous interface treatments for segregated finite volume flow solvers

\u3cp\u3eThe volume-averaged approach to simulate flow in porous media is often used because of its practicality and computational efficiency. Derivation of the volume-averaged porous flow equations introduces additional porous resistance terms to the momentum equation. These porous resistance terms create body force discontinuities at fluid-porous interfaces, which may lead to spurious oscillations if not accounted for properly. A variety of techniques has been proposed to treat this problem, but only recently two approaches were developed by Nordlund et al. [Improved PISO algorithms for modeling density varying flow in conjugate fluid-porous domains. Journal of Computational Physics 2016;306:199–215. http://www.sciencedirect.com/science/article/pii/S0021999115007755. http://dx.doi.org/10.1016/j.jcp.2015.11.035;. Modified Rhie–Chow/PISO algorithm for collocated variable finite porous media flow solvers. No. 40 in ECI Symposium Series; 5th International Conference on Porous Media and Their Applications in Science, Engineering and Industry; 2014], concentrating on the combination of collocated grids and segregated solvers, which are commonly used for industrial applications. In this paper, we compared these two methods for the treatment of fluid-porous interfaces: (i) the Re-Distributed Resistivity (RDR) and (ii) the Face Consistent Pressure (FCP) methods, for a wide range of process conditions and porous media parameters, covering most practical porous media flow applications. The numerical robustness and accuracy of both methods were evaluated by applying a systematic quantitative comparison procedure. Both the RDR and the FCP approaches proved to be suitable for treating discontinuities in the porous resistance terms, yielding smooth solutions around the interface and good convergence with grid refinement. The RDR method was found to be the most robust method, particularly at very low Darcy numbers and on unstructured grids, distinguishing it from the FCP approach.\u3c/p\u3

Decay or collapse:aircraft wake vortices in grid turbulence

Trailing vortices are naturally shed by airplanes and they typically evolve into a counter-rotating vortex pair. Downstream of the aircraft, these vortices can persist for a very long time and extend for several kilometers. This poses a potential hazard to following aircraft, particularly during take-off and landing. Therefore, it is of interest to understand what effects control the decay rate in strength of the trailing vortices. The decay of the aircraft wake vortices is strongly dependent on weather conditions. To simulate an environment of atmospheric turbulence, homogeneous isotropic turbulence is introduced next to localized vortical structure. In this contribution, the effect of external turbulence on the vortex decay will be investigated numerically, using a DNS method, and experimentally. Wind tunnel experiments are carried out using a smoke visualization technique

The flow structure in the wake of a fractal fence and the absence of an "inertial regime"

Recent theoretical work has highlighted the importance of multi-scale forcing of the flow for altering the nature of turbulence energy transfer and dissipation. In particular, fractal types of forcing have been studied. This is potentially of real significance in environmental fluid mechanics where multi-scale forcing is perhaps more common than the excitation of a specific mode. In this paper we report the first results studying the detail of the wake structure behind fences in a boundary layer where, for a constant porosity, we vary the average spacing of the struts and also introduce fractal fences. As expected, to first order, and in the far-wake region, in particular, the response of the fences is governed by their porosity. However, we show that there are some significant differences in the detail of the turbulent structure between the fractal and non-fractal fences and that these override differences in porosity. In the near wake, the structure of the fence dominates porosity effects and a modified wake interaction length seems to have potential for collapsing the data. With regards to the intermittency of the velocities, the fractal fences behave more similarly to homogeneous, isotropic turbulence. In addition, there is a high amount of dissipation for the fractal fences over scales that, based on the energy spectrum, should be dominated by inter-scale transfers. This latter result is consistent with numerical simulations of flow forced at multiple scales and shows that what appears to be an “inertial regime” cannot be as production and dissipation are both high

Review of Underwater and In-Air Sounds Emitted by Australian and Antarctic Marine Mammals

The study of marine soundscapes is a growing field of research. Recording hardware is becoming more accessible; there are a number of off-the-shelf autonomous recorders that can be deployed for months at a time; software analysis tools exist as shareware; rawor preprocessed recordings are freely and publicly available. However, what is missing are catalogues of commonly recorded sounds. Sounds related to geophysical events (e.g. earthquakes) and weather (e.g. wind and precipitation), to human activities (e.g. ships) and to marine animals (e.g. crustaceans, fish and marine mammals) commonly occur. Marine mammals are distributed throughout Australia's oceans and significantly contribute to the underwater soundscape. However, due to a lack of concurrent visual and passive acoustic observations, it is often not known which species produces which sounds. To aid in the analysis of Australian and Antarctic marine soundscape recordings, a literature review of the sounds made by marine mammals was undertaken. Frequency, duration and source level measurements are summarised and tabulated. In addition to the literature review, new marine mammal data are presented and include recordings from Australia of Omura's whales (Balaenoptera omurai), dwarf sperm whales (Kogia sima), common dolphins (Delphinus delphis), short-finned pilot whales (Globicephala macrorhynchus), long-finned pilot whales (G. melas), Fraser's dolphins (Lagenodelphis hosei), false killer whales (Pseudorca crassidens), striped dolphins (Stenella coeruleoalba) and spinner dolphins (S. longirostris), as well as the whistles and burst-pulse sounds of Australian pygmy killer whales (Feresa attenuata). To date, this is the most comprehensive acoustic summary for marine mammal species in Australian waters