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Burghelea T.I.,CNRS Nantes Thermocinetique Lab | Stary Z.,Friedrich - Alexander - University, Erlangen - Nuremberg | Munstedt H.,Friedrich - Alexander - University, Erlangen - Nuremberg
Journal of Non-Newtonian Fluid Mechanics

An experimental investigation of the viscosity overshoot phenomenon observed during uniaxial extension of a low density polyethylene is presented. For this purpose, traditional integral viscosity measurements on a Münstedt-type extensional rheometer are combined with local measurements based on the in-situ visualization of the sample under extension. For elongational experiments at constant strain rates within a wide range of Weissenberg numbers (Wi), three distinct deformation regimes are identified. Corresponding to low values of Wi (regime I), the tensile stress displays a broad maximum, but such maximum is observed with various polymeric materials deformed at low rates and it should not be confused with the " viscosity overshoot" phenomenon. Corresponding to intermediate values of Wi (regime II), a local maximum of the integral extensional viscosity is systematically observed. Moreover, within this regime, a strong discrepancy between integral measurements and the space average of the local elongational viscosity is observed which indicates large deviations from an ideal uniaxial deformation process. Images of samples within this regime reinforce this finding by showing that, corresponding to the maximum of the integral viscosity, secondary necks develop along the sample. The emergence of a maximum of the integral elongational viscosity is, thus, related to the distinct inhomogeneity of deformation states and most probably not to the rheological properties of the material. In the fast stretching limit (high Wi, regime III), the overall geometric uniformity of the sample is well preserved, no secondary necks are observed and both the integral and the space averaged transient elongational viscosity show no maximum. A detailed but yet incomplete comparison of the experimental findings with results from the literature is presented and several open questions are stated. © 2011 Elsevier B.V. Source

Perez L.,CNRS Nantes Thermocinetique Lab | Autrique L.,University of Angers
IEEE Transactions on Instrumentation and Measurement

Expertise of innovative materials by nondestructive techniques is a key goal in process engineering development. In this context, if identification of thermal diffusivity of liquid is a crucial requirement to develop a reliable mathematical model of knowledge, it is essential to propose a complete and valid methodology. Based on the analysis of thermal wave propagation (generated by a periodic excitation), an experimentation is developed in order to avoid the implementation of a pyroelectric sensor required in usual photopyroelectric techniques. The proposed approach is investigated in a trilayer system. Theoretical aspects of the identification of thermal parameters in the frequency domain are presented. A feasibility study is discussed in order to justify this approach for liquids. A sensitivity analysis is implemented in a particular case to provide an optimal experimental bench. Finally, experimental results for several liquids are presented and discussed. © 1963-2012 IEEE. Source

Ben-Abdallah P.,CNRS Nantes Thermocinetique Lab | Joulain K.,CNRS Pprime Institute | Pryamikov A.,CNRS Pprime Institute
Applied Physics Letters

We theoretically investigate the nonradiative heat transfer between two photonic crystals separated by a small gap in nonequilibrium thermal situation. We predict that the surface Bloch states coupling supported by these media can make heat exchanges larger than those measured at the same separation distance between two massive homogeneous materials made with the elementary components of photonic crystals. These results could find broad applications in near-field technologies. © 2010 American Institute of Physics. Source

Some S.C.,University | Gaudefroy V.,University | Delaunay D.,CNRS Nantes Thermocinetique Lab
International Journal of Heat and Mass Transfer

Asphalt pavements are obtained after mixing aggregates and bitumen both preheated in a temperature range between 100 °C and 160 °C. The manufacturing temperatures are important to ensure good bitumen and aggregates bonding. However, the accurate evaluation of the bonding quality remains a challenge. A setup has been used to quantify the bonding quality between hot bitumen and granular substrate.The method is based on the determination of the thermal contact resistance (TCR) when hot bitumen is putted into contact with granular substrate. This TCR is assessed from solving inverse heat conduction problem and is interpreted as a bonding quality indicator. Results prove that the increase of component temperatures induces better bonding between bitumen and the substrate. © 2012 Elsevier Ltd. All rights reserved. Source

Ho T.D.,Rennes Institute of Physics | Valance A.,Rennes Institute of Physics | Dupont P.,INSA Rennes | Ould El Moctar A.,CNRS Nantes Thermocinetique Lab
Physical Review Letters

We report on wind tunnel measurements on saltating particles in a turbulent boundary layer and provide evidence that over an erodible bed the particle velocity in the saltation layer and the saltation length are almost invariant with the wind strength, whereas over a nonerodible bed these quantities vary significantly with the air friction speed. It results that the particle transport rate over an erodible bed does not exhibit a cubic dependence with the air friction speed, as predicted by Bagnold, but a quadratic one. This contrasts with saltation over a nonerodible bed where the cubic Bagnold scaling holds. Our findings emphasize the crucial role of the boundary conditions at the bed and may have important practical consequences for aeolian sand transport in a natural environment. © 2011 American Physical Society. Source

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