246 research outputs found
Viscous heating effects in fluids with temperature-dependent viscosity: triggering of secondary flows
Viscous heating can play an important role in the dynamics of fluids with
strongly temperature-dependent viscosities because of the coupling between the
energy and momentum equations. The heat generated by viscous friction produces
a local temperature increase near the tube walls with a consequent decrease of
the viscosity and a strong stratification in the viscosity profile. The problem
of viscous heating in fluids was investigated and reviewed by Costa & Macedonio
(2003) because of its important implications in the study of magma flows.
Because of the strong coupling between viscosity and temperature, the
temperature rise due to the viscous heating may trigger instabilities in the
velocity field, which cannot be predicted by a simple isothermal Newtonian
model. When viscous heating produces a pronounced peak in the temperature
profile near the walls, a triggering of instabilities and a transition to
secondary flows can occur because of the stratification in the viscosity
profile. In this paper we focus on the thermal and mechanical effects caused by
viscous heating. We will present the linear stability equations and we will
show, as in certain regimes, these effects can trigger and sustain a particular
class of secondary rotational flows which appear organised in coherent
structures similar to roller vortices. This phenomenon can play a very
important role in the dynamics of magma flows in conduits and lava flows in
channels and, to our knowledge, it is the first time that it has been
investigated by a direct numerical simulation.Comment: 18 pages manuscript, 10 figures, to be published in Journal of Fluid
Mechanics (2005
Bigraphical Logics for XML
Bigraphs have been recently proposed as a meta-model for global computing resources; they are built orthogonally on two structures: a hierarchical ‘place’ graph for locations and a ‘link’ (hyper-)graph for connections. XML is now the standard meta-language for the data exchange and storage on the web. In this paper we address the similarities between bigraphs and XML and we propose bigraphs as a rich model for XML (and XML contexts). Building on this idea we proceed by investigating how the recently proposed logic of BiLog can be instantiated to describe, query and reason about web data (and web contexts)
Spatial Logics for Bigraphs
Bigraphs are emerging as an interesting model for concurrent calculi, like CCS, pi-calculus, and Petri nets. Bigraphs are built orthogonally on two structures: a hierarchical place graph for locations and a link (hyper-)graph for connections. With the aim of describing bigraphical structures, we introduce a general framework for logics whose terms represent arrows in monoidal categories. We then instantiate the framework to bigraphical structures and obtain a logic that is a natural composition of a place graph logic and a link graph logic. We explore the concepts of separation and sharing in these logics and we prove that they generalise some known spatial logics for trees, graphs and tree contexts
Viscous heating in fluids with temperature-dependent viscosity: implications for magma flows
International audienceViscous heating plays an important role in the dynamics of fluids with strongly temperature-dependent viscosity because of the coupling between the energy and momentum equations. The heat generated by viscous friction produces a local temperature increase near the tube walls with a consequent decrease of the viscosity which may dramatically change the temperature and velocity profiles. These processes are mainly controlled by the Peclét number, the Nahme number, the flow rate and the thermal boundary conditions. The problem of viscous heating in fluids was investigated in the past for its practical interest in the polymer industry, and was invoked to explain some rheological behaviours of silicate melts, but was not completely applied to study magma flows. In this paper we focus on the thermal and mechanical effects caused by viscous heating in tubes of finite lengths. We find that in magma flows at high Nahme number and typical flow rates, viscous heating is responsible for the evolution from Poiseuille flow, with a uniform temperature distribution at the inlet, to a plug flow with a hotter layer near the walls. When the temperature gradients induced by viscous heating are very pronounced, local instabilities may occur and the triggering of secondary flows is possible. For completeness, this paper also describes magma flow in infinitely long tubes both at steady state and in transient phase
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Brief communication. Rain effect on the load of tephra deposits
Accumulation of tephra fallout produced during explosive eruptions can cause roof collapses in areas near the volcano, when the weight of the deposit exceeds some threshold value that depends on the quality of buildings. The additional loading of water that remains trapped in the tephra deposits due to rainfall can contribute to increasing the loading of the deposits on the roofs. Here we propose a simple approach to estimate an upper bound for the contribution of rain to the load of pyroclastic deposits that is useful for hazard assessment purposes. As case study we present an application of the method in the area of Naples, Italy, for a reference eruption from Vesuvius volcano
Numerical model of gas dispersion emitted from volcanic sources
An Eulerian model for passive gas dispersion based on the K-theory for turbulent diffusion, coupled with a mass
consistent wind model is presented. The procedure can be used to forecast gas concentration over large and complex
terrains. The input to the model includes the topography, wind measurements from meteorological stations,
atmospheric stability information and gas flow rate from the ground sources. Here, this model is applied to study
the distribution of the CO2 discharged from the hot sources of the Solfatara Volcano, Naples, Italy, where the input
data were measured during a 15 day campaign in June 2001 carried out to test an Eddy Covariance (EC) station
by Osservatorio Vesuviano-INGV, Naples
BiLog: Spatial Logics for Bigraphs
Bigraphs are emerging as a (meta-)model for concurrent calculi, like CCS, ambients, -calculus, and Petri nets. They are built orthogonally on two structures: a hierarchical place graph for locations and a link (hyper-)graph for connections. Aiming at describing bigraphical structures, we introduce a general framework, BiLog, whose formulae describe arrows in monoidal categories. We then instantiate the framework to bigraphical structures and we obtain a logic that is a natural composition of a place graph logic and a link graph logic. We explore the concepts of separation and sharing in these logics and we prove that they generalise well known spatial logics for trees, graphs and tree contexts. As an application, we show how XML data with links and web services can be modelled by bigraphs and described by BiLog. The framework can be extended by introducing dynamics in the model and a standard temporal modality in the logic. However, in some cases, temporal modalities can be already expressed in the static framework. To testify this, we show how to encode a minimal spatial logic for CCS in an instance of BiLog
Static BiLog: a Unifying Language for Spatial Structures
Aiming at a unified view of the logics describing spatial structures, we introduce a general framework, BiLog, whose formulae characterise monoidal categories. As a first instance of the framework we consider bigraphs, which are emerging as a an interesting (meta-)model for spatial structures and distributed calculi. Since bigraphs are built orthogonally on two structures, a hierarchical place graph for locations and a link (hyper-)graph for connections, we obtain a logic that is a natural composition of other two instances of BiLog: a Place Graph Logic and a Link Graph Logic. We prove that these instances generalise the spatial logics for trees, for graphs and for tree contexts. We also explore the concepts of separation and sharing in these logics. We note that both the operator * of Separation Logic and the operator | of spatial logics do not completely separate the underlying structures. These two different forms of separation can be naturally derived as instances of BiLog by using the complete separation induced by the tensor product of monoidal categories along with some form of sharing
FPLUME-1.0: An integrated volcanic plume model accounting for ash aggregation
Eruption source parameters (ESP) characterizing
volcanic eruption plumes are crucial inputs for atmospheric
tephra dispersal models, used for hazard assessment and
risk mitigation.We present FPLUME-1.0, a steady-state 1-D
(one-dimensional) cross-section-averaged eruption column
model based on the buoyant plume theory (BPT). The model
accounts for plume bending by wind, entrainment of ambient
moisture, effects of water phase changes, particle fallout and
re-entrainment, a new parameterization for the air entrainment
coefficients and a model for wet aggregation of ash particles
in the presence of liquid water or ice. In the occurrence
of wet aggregation, the model predicts an effective grain size
distribution depleted in fines with respect to that erupted at
the vent. Given a wind profile, the model can be used to determine
the column height from the eruption mass flow rate or
vice versa. The ultimate goal is to improve ash cloud dispersal
forecasts by better constraining the ESP (column height,
eruption rate and vertical distribution of mass) and the effective
particle grain size distribution resulting from eventual
wet aggregation within the plume. As test cases we apply the
model to the eruptive phase-B of the 4 April 1982 El Chichón
volcano eruption (México) and the 6 May 2010 Eyjafjallajökull
eruption phase (Iceland). The modular structure of the
code facilitates the implementation in the future code versions
of more quantitative ash aggregation parameterization
as further observations and experiment data will be available
for better constraining ash aggregation processes
È VIVO: Virtual eruptions at Vesuvius; A multimedia tool to illustrate numerical modeling to a general public
Dissemination of scientific results to the general public has become increasingly important in our society. When science deals with natural hazards, public outreach is even more important: on the one hand, it contributes to hazard perception and it is a
necessary step toward preparedness and risk mitigation; on the other hand, it contributes to establish a positive link of mutual
confidence between scientific community and the population living at risk. The existence of such a link plays a relevant role in
hazard communication, which in turn is essential to mitigate the risk. In this work, we present a tool that we have developed to
illustrate our scientific results on pyroclastic flow propagation at Vesuvius. This tool, a CD-ROM that we developed joining
scientific data with appropriate knowledge in communication sciences is meant to be a first prototype that will be used to test the
validity of this approach to public outreach. The multimedia guide contains figures, images of real volcanoes and computer
animations obtained through numerical modeling of pyroclastic density currents. Explanatory text, kept as short and simple as
possible, illustrates both the process and the methodology applied to study this very dangerous natural phenomenon. In this first
version, the CD-ROM will be distributed among selected categories of end-users together with a short questionnaire that we have
drawn to test its readability. Future releases will include feedback from the users, further advancement of scientific results as well as a higher degree of interactivity
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