220 research outputs found
Acidi grassi omega 3. Dagli alimenti all'uomo: biodisponibilità, conversione metabolica e livelli ematici in popolazioni
No full textIl grande interesse, in rapida crescita, per gli aspetti salutistici degli Acidi
Grassi Omega 3 richiede: a) attenzione verso le “peculiarità” biologiche e
nutrizionali di tali composti, e b) consapevolezza delle problematiche nella
programmazione, sviluppo e valutazione dei risultati in studi epidemiologici
e di intervento controllato sugli effetti degli Omega 3. Infatti: a) Gli
Omega 3 hanno una reperibilità assai limitata negli alimenti convenzionali,
essendo presenti esclusivamente in organismi che nel corso dell’evoluzione
si sono adattati ad ambienti acquatici; in certi alimenti “nicchia” non adeguatamente
esplorati, e, negli organismi superiori, solo in strutture ad alta
specializzazione funzionale (es. sinapsi, strutture contrattili, ecc.) dove
svolgono ruoli essenziali. b) È necessaria la valutazione dello “status” in
AG Omega 3 in qualsiasi popolazione o gruppo (dato non estrapolabile
dall’uso di questionari alimentari), nella programmazione di studi epidemiologici
e di intervento. Tali dati sono esclusivamente ricavabili dall’analisi
della composizione in AG dei lipidi circolanti (plasma, sangue intero),
a sua volta correlabile con i consumi alimentari di Omega 3, e con parametri
fisiopatologici dall’altro. Inoltre tale informazione è utile per valutare la
biodisponibilità di Omega 3 a seconda delle fonti e formulazioni, parametro
tuttora non adeguatamente valutato. Una strategia analitica innovativa
da noi sviluppata per la raccolta di gocce di sangue dal polpastrello e la
preparazione dei derivati per l’analisi gas cromatografica, permette di valutare
lo “status” in Omega 3 in ampie popolazioni, anche difficilmente gestibili,
quali i neonati, donne in gravidanza, in paesi remoti (es. Cambogia
e Tibet), e di affrontare i problemi sopra indicati (dagli alimenti all’uomo
Effects of high-frequency vibration on critical marangoni number
No full textThe influence of vibration on thermocapillary convection and critical Marangoni number in liquid bridge of half floating zone was discussed for the low frequency range 0.4-1.5 Hz and the intermediate frequency range 2.5-15 Hz in our previous papers. This paper extends the study to high frequency range 15-100Hz. This ground based experiment was completed on the deck of an electromagnetic vibration machine. The results of our experiment shows when the frequency of the applied acceleration is high enough, the amplitude of the time varying part of the temperature response is disappear and the shape of the free surface of the liquid bridge exhibits no fluctuations due to inertia. The critical Marangoni number which is defined to describe the transitions from a peroidical convection in response to vibration to an oscillatory convection due to internal instability is nearly the same as the critical Marangoni number for oscillatory flow in the absence of vibration
Suppression of Marangoni Convection of Silicon Melt By A Non-Contaminating Method
No full textA set of numerical simulation of the effect of the gas shearing flow over a silicon melt free surface on Marangoni convection under microgravity condition was conducted by using finite element method. For given gas channel width, Marangoni number and aspect ratio a remarkable reduction of Marangoni convection in silicon liquid bridge can be achieved by choosing the optimal gas velocity in accordance with the correlation proposed in the paper. The effectiveness of the reduction of the gas flow under different conditions shows that, in some cases, Marangoni convection reduction of 99% can be realized by this non-contaminating method. (C) 2003 Elsevier Ltd. All rights reserved
Rayleigh-Marangoni-Benard Instability in Two-Layer Fluid System
No full textRayleigh-Marangoni-Benard instability in a system of two-layer fluids is studied numerically. The convective instabilities in the system of Silicon Oil (10cSt) and Fluorinert (FC70) liquids have been analyzed. The critical parameters at onset of convection are presented in a large range of two-layer depth ratios from 0.2 to 5.0. Numerical results show that the instability of the two-layer system depends strongly on its depth ratio. When the depth ratio increases, the instability mode changes from mechanical coupling to thermal coupling. Between these two typical coupling modes, a time-dependent oscillation is detected. Nevertheless, traveling wave states are found in the case of oscillatory instability. The oscillation mode results from the competition between Rayleigh instability and Marangoni effect
Numerical investigation of thermocapillary migration of the drop for large Marangoni numbers
No full textAn axisymmetric model is adopted to simulate the problem of unsteady drop thermocapillary motion for large Marangoni numbers. Front tracking methods are used in the investigation. It is found that the non-dimensional drop migration velocity will decrease with increasing Marangoni number. This agrees well with the experimental results obtained from the 4th Shen-Zhou space ship. In the meanwhile, this is also the first time for numerical simulations to verify the experimental phenomenon under large Marangoni numbers
Influence Of Free Surface Curvature Of A Liquid Layer On The Critical Marangoni Convection
No full textThe Pearson instability was suggested to discuss the onset of Marangoni convection in a liquid layer of large Prandtl number under an applied temperature difference perpendicular to the free surface in the microgravity environment. In this case, the temperature distribution on the curved free surface is nonuniform, and the thermocapillary convection is induced and coupled with the Marangoni convection. In the present paper the effect of volume ratio of the liquid layer on the critical Marangoni convection and the corresponding spatial variation of the convection structure in zero-gravity condition were numerically investigated by two-dimensional model. (C) 2008 Elsevier Ltd. All rights reserved
Effects of g-jitter on the critical marangoni number
No full textA half floating zone is fixed on a vibrational deck, which supports a periodical applied acceleration to simulate the effect of g-jitter. This paper deals with the effects of g-jitter on the fluid fields and the critical Marangoni number, which describes the transition from a forced oscillation of thermocapillary convection into an instability oscillatory convection in a liquid bridge of half floating zone with top rod heated. The responses of g-jitter field on the temperature profiles and flow pattern in the liquid bridge were obtained experimentally. The results indicated that the critical Marangoni number decreases with the increasing of g-jitter effect and is slightly smaller for higher frequency of g-jitter with fixed strength of applied gravity
Unsteady Model of Drop Marangoni Migration in Microgravity
No full textThe experiments of drop Marangoni migration have been performed by the drop shift facility of short period of 4.5 s, and the drop accelerates gradually to an asymptotic velocity during the free fall. The unsteady and axisymmetric model is developed to study the drop migration for the case of moderate Reynolds number Re = O(1), and the results are compared with the experimental ones in the present paper. Both numerical and experimental results show that the migration velocity for moderate Reynolds number is several times smaller than that given by the linear YGB theory
Marangoni-Benard instability with the exchange of evaporation at liquid-vapour interface
No full textA new two-sided model rather than the one-sided model in previous works is put forward. The linear instability analysis is performed on the Marangoni-Benard convection in the two-layer system with an evaporation interface. We define a new evaporation Biot number which is different from that in the one-sided model, and obtain the curves of critical Marangoni number versus wavenumber. The influence of evaporation velocity and Biot number on the system is discussed and a new phenomenon uninterpreted before is now explained from our numerical results
Thermovibrational Instability Of Rayleigh-Marangoni-Benard Convection In Two-Layer Fluid Systems
No full textThe thermovibrational instability of Rayleigh-Marangoni-Benard convection in a two-layer system under the high-frequency vibration has been investigated by linear instability analysis in the present paper. General equations for the description of the convective flow and within this framework, the generalized Boussinesq approximation are formulated. These equations are dealt with using the averaging method. The theoretical analysis results show that the high-frequency thermovibrations can change the Marangoni-Benard convection instabilities as well as the oscillatory gaps of the Rayleigh-Marangoni-Benard convection in two-layer liquid systems. It is found that vertical high-frequency vibrations can delay convective instability of this system, and damp the convective flow down. (C) 2007 COSPAR. Published by Elsevier Ltd. All rights reserved
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