Search results for "Flake"

Isometric embeddings of snowflakes into finite-dimensional Banach spaces

2016

We consider a general notion of snowflake of a metric space by composing the distance by a nontrivial concave function. We prove that a snowflake of a metric space $X$ isometrically embeds into some finite-dimensional normed space if and only if $X$ is finite. In the case of power functions we give a uniform bound on the cardinality of $X$ depending only on the power exponent and the dimension of the vector space.

A Wind Tunnel and Theoretical Study of the Melting Behavior of Atmospheric Ice Particles. IV: Experiment and Theory for Snow Flakes

1990

Abstract An experiment in the Mainz vertical Cloud Tunnel is described in which natural and laboratory-made aggregates of snow crystals (snow flakes) were melted under free fall conditions in the vertical air stream of the tunnel, which was allowed to warm up at the rates experienced by falling snow flakes in the atmosphere. The variation of the fall mode, the fall velocity, and the percentage of ice melted, as a function of percentage of distance travelled for 99% melting was recorded by cinematography. The laboratory results were confirmed by the results of a theoretical heat transfer model which we developed for the melting of a snow flake. In this model a snow flake was idealized by an …

In-line holography of cloud volumes applied to the measurement of raindrops and snowflakes

1998

Abstract The ground-based HODAR (HOlographic Droplet and Aerosol Recording) of the University of Mainz has been operational since 1989 on the Kleiner Feldberg Mountain Observatory near Frankfurt/Main, Germany, in a `small volume' configuration. With this setup an air volume of approximately 1 l can be in situ recorded, which allows it to image all aerosol particles and droplets with diameter larger than 6 μm contained in the sample volume in situ. Recently the beam diameter and optical path length of the inline system have been enlarged such that large cloud volumes (approximately 500 l) can be recorded on single holograms. During the holographic image reconstruction step in the laboratory,…

2017

Abstract. During free fall in clouds, ice hydrometeors such as snowflakes and ice particles grow effectively by riming, i.e., the accretion of supercooled droplets. Volatile atmospheric trace constituents dissolved in the supercooled droplets may remain in ice during freezing or may be released back to the gas phase. This process is quantified by retention coefficients. Once in the ice phase the trace constituents may be vertically redistributed by scavenging and subsequent precipitation or by evaporation of these ice hydrometeors at high altitudes. Retention coefficients of the most dominant carboxylic acids and aldehydes found in cloud water were investigated in the Mainz vertical wind tu…

Single-crystal I h ice surfaces unveil connection between macroscopic and molecular structure

2017

Physics and chemistry of ice surfaces are not only of fundamental interest but also have important impacts on biological and environmental processes. As ice surfaces—particularly the two prism faces—come under greater scrutiny, it is increasingly important to connect the macroscopic faces with the molecular-level structure. The microscopic structure of the ubiquitous ice Ih crystal is well-known. It consists of stacked layers of chair-form hexagonal rings referred to as molecular hexagons. Crystallographic unit cells can be assembled into a regular right hexagonal prism. The bases are labeled crystallographic hexagons. The two hexagons are rotated 30° with respect to each other. The linkage…

Petrographic and spectroscopic (FT-IR) study of Western Mediterranean obsidians geological sources and of a lithic collection from Ustica Island (Sic…

2019

I n this study we applied petrochemical methods (SEM-EDS; FT-IR) in order to characterize a group of obsidian flakes collected at Ustica island (Sicily). Despite the absence of obsidian geological outcrops, a lot of obsidian fragments still emerging from the lands of Ustica testify that the island was a major import center of obsidian during the prehistory. On this island, there are some prehistoric settlements, dated from the Neolithic to the Middle Bronze Age (6000- 1200 BC), in which the use of obsidian continued until the beginning of metals age. Our study includes: i) Macroscopic and microscopic optical observations, which allowed selecting 18 obsidian flakes (starting from 50 obsidian…

2014

Abstract. Dynamical and microphysical processes in pyroconvective clouds in mid-latitude conditions are investigated using idealized three-dimensional simulations with the Active Tracer High resolution Atmospheric Model (ATHAM). A state-of-the-art two-moment microphysical scheme building upon a realistic parameterization of cloud condensation nuclei (CCN) activation has been implemented in order to study the influence of aerosol concentration on cloud development. The results show that aerosol concentration influences the formation of precipitation. For low aerosol concentrations (NCN = 200 cm−3), rain droplets are rapidly formed by autoconversion of cloud droplets. This also triggers the f…

Model Analysis of Hydrometeor Scattering Effects on Free Space Near-Infrared Links

2012

A promising technology for peer-to-peer connections and urban area networks is represented by wireless communications through free space using optical carrier (Free Space Optics, FSO). This technology ensures high data rates, with relatively low error rates, low power consumption and inherent security. Nevertheless FSO links are quite sensitive to atmospheric condition. Fog droplets, but also raindrops and snowflakes, may introduce severe path attenuation which drastically reduces the channel availability. A parametric model to simulate droplets scattering effects over the FSO link in terms of extinction coefficient, albedo factor and asymmetry coefficient as function of the particle water …

Weighted pointwise Hardy inequalities

2009

We introduce the concept of a visual boundary of a domain �¶ �¼ Rn and show that the weighted Hardy inequality  �¶ |u|pd�¶ �A.p  C  �¶ |�Þu|pd�¶ �A, where d�¶(x) = dist(x, �Ý�¶), holds for all u �¸ C �� 0 (�¶) with exponents �A < �A0 when the visual boundary of �¶ is sufficiently large. Here �A0 = �A0(p, n, �¶) is explicit, essentially sharp, and may even be greater than p . 1, which is the known bound for smooth domains. For instance, in the case of the usual von Koch snowflake domain the sharp bound is shown to be �A0 = p . 2 + �E, with �E = log 4/ log 3. These results are based on new pointwise Hardy inequalities.

Alkane dehydrogenation on defective BN quasi-molecular nanoflakes: DFT studies

2020

Lower alkanes are feedstocks readily available but relatively inert. The con- version of low cost alkanes to industrially relevant alkenes is usually carried out on metal-based heterogeneous catalysts. Considering both the cost and the potential harmfulness of the metal involved in the dehydrogenation cat- alysts (typically, platinum or chromium), the study of metal-free processes represents an important challenge for the industrial chemistry in order to address more sustainable protocols and different routes either to activate or transform alkanes. Framed in this context, it was investigated, using a den- sity functional theory approach, the potential dehydrogenation activity of de- fectiv…