Publications

12/03/2024

Experimental investigation and tomography analysis of Darcy-Forchheimer flows in thermal protection systems

Authors : LIU, Shaolin AHMADI, Azita SCANDELLI, Hermes LACHAUD, Jean
Publisher : Elsevier BV
n thermal protection systems (TPS), Darcy’s law or Darcy-Forchheimer’s law is employed to model the pyrolysis gas flow within the anisotropic porous ablator depending on the flow regime considered. A key challenge with using these laws is first, the knowledge of the validity domain of each flow regime in terms of a critical Reynolds number (������ ). Secondly, the lack of data on macroscopic properties, namely, the permeability and Forchheimer tensors is particularly challenging for the relevance of the models. The objective of this work is to contribute to overcoming these challenges by performing experimental and X-ray tomographic image-based characterization of Calcarb, a commercial carbon preform used for manufacturing TPS. For this purpose, fluid flow within Calcarb was studied experimentally in the Through-Thickness (TT) and the In-Plane (IP) directions for Reynolds numbers of 0.05 to 10.46 -representative of the TPS application. Tomography image-based micro-scale simulations, involving the direct resolution of the Navier–Stokes equations under both flow regimes, were also performed. Experimental results exhibit the anisotropic nature of Calcarb, namely through ������ values, corresponding to the Darcy flow regime limit, slightly different in the two directions (������ of 0.31 and 0.43) with measured permeability values of 1.248 × 10−10 m2 and 1.615 × 10−10 m2 for TT and IP directions respectively. In the Forchheimer regime, experimental Forchheimer coefficients �� were 2.0010 × 105 m−1 (TT) and 1.4948 × 105 m−1 (IP). During the simulation process, a numerical strategy was defined to obtain the permeability tensor yielding values within 8% of the experimental ones. The comparison of experimental results vs simulation results in the Forchheimer regime was performed through the analysis of the pressure gradients as functions of ���� in the ��, ��, and �� directions. The numerical estimations were compared successfully with experimental measurements, with a discrepancy of 5.2%, for ���� values up to 2.4
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11/03/2024

Spin crossover molecular ceramics by Cool-SPS: consequences on switching features beyond the sole microstructural effect

Authors : EL-KHOURY, Liza DARO, Nathalie CHASTANET, Guillaume ROSA, Patrick DENUX, Dominique ETIENNE, Laetitia MAZEL, Vincent JOSSE, Michaël MARCHIVIE, Mathieu
Publisher : Royal Society of Chemistry
The sintering of spin crossover material using Spark Plasma Sintering at low temperature (Cool-SPS) lead to a new way of shaping such compounds into functional molecular ceramics. These ceramics reach a high relative density of 95%, what may address several issues for using spin crossover materials into barocaloric devices. Starting from the reference complex [Fe(Htrz)2(trz)]BF4, we first investigated the magnetic, structural, microstructural properties as well as the fatigability behavior of the starting powder using multiple magnetic measurements, X-ray diffraction and calorimetry to compare them to the elaborated ceramics. The best conditions of pressure and temperature during the SPS process to obtain reproductible molecular ceramics with high relative density where found to be between 250 and 300 °C, and 300 and 400 MPa. The same complete set of characterizations made on a molecular ceramic of 95% of relative density reveal that crystal structure as well as the abrupt hysteretic SCO of [Fe(Htrz)2(trz)]BF4 are perfectly conserved after sintering. However, ceramic presents a faster stabilization of their microstructural and magnetic properties upon cycling and a higher cooperativity at the macroscopic level was observed compared to the starting powder.
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11/03/2024

On the dynamic performance of additively manufactured visco-elastic meta-materials

Authors : LE BARBENCHON, Louise LISSNER, Maria
Publisher : Elsevier BV
Additive manufacturing (AM) has revolutionized the production of structures with tailored material properties, including elastomer polyurethanes (EPU) which exhibit exceptional mechanical performance. EPU possesses unique characteristics, such as high elongation at break, efficient energy dissipation, and superior specific strength, making it well-suited for applications requiring resilience to dynamic loadings. By combining the advantages of AM and EPU, enhanced and customized meta-materials can be created, surpassing the mechanical performance of traditional bulk materials. However, because of the non-linear stress–strain response of both the constitutive material and the structure, designing such meta-materials for high strain-rates can be challenging. In this work, therefore, quasi-static and dynamic experiments were conducted to evaluate a meta-material architecture. The investigation revealed a strong positive rate dependency. The mechanical performances, including strength, and dissipated energy, increased with increasing loading rate. The EPU meta-materials demonstrate their suitability for dynamic applications where high energy dissipation is crucial for reducing transmitted loads.
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08/03/2024

Turbulent transition in a channel with superhydrophobic walls: anisotropic slip and shear misalignment effects

Authors : JOUIN, Antoine CHERUBINI, Stefania ROBINET, Jean-Christophe
Publisher : Cambridge University Press (CUP)
Superhydrophobic surfaces dramatically reduce skin friction of overlying liquid flows. These surfaces are complex and numerical simulations usually rely on models to reduce this complexity. One of the simplest consists of finding an equivalent boundary condition through a homogenisation procedure, which in the case of channel flow over oriented riblets, leads to the presence of a small spanwise component in the homogenised base flow velocity. This work aims at investigating the influence of such a three-dimensionality of the base flow on stability and transition in a channel with walls covered by oriented riblets. Linear stability for this base flow is investigated: a new instability region, linked to cross-flow effects, is observed. Tollmien–Schlichting waves are also retrieved but the most unstable are three-dimensional. Transient growth is also affected as oblique streaks with non-zero streamwise wavenumber become the most amplified perturbations. When transition is induced by Tollmien–Schlichting waves, after an initial exponential growth regime, streaky structures with large spanwise wavenumber rapidly arise. Modal mechanisms appear to play a leading role in the development of these structures and a secondary stability analysis is performed to retrieve successfully some of their characteristics. The second scenario, initiated with cross-flow vortices, displays a strong influence of nonlinearities. The flow develops into large quasi-spanwise-invariant structures before breaking down to turbulence. Secondary stability on the saturated cross-flow vortices sheds light on this stage of transition. In both cases, cross-flow effects dominate the flow dynamics, suggesting the need to consider the anisotropicity of the wall condition when modelling superhydrophobic surfaces.
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08/03/2024

Gravity-driven remediation of DNAPL polluted aquifers using densified biopolymer brine solution

Authors : ALAMOOTI, Amir COLOMBANO, Stéfan DAVARZANI, Dorian LION, Fabien AHMADI, Azita
Publisher : Elsevier BV
Polymer solutions aid DNAPL (Dense Non Aqueous Phase Liquid)-contaminated soil remediation but are impacted by gravity and viscous forces. This study assesses the interplay between buoyancy and viscous forces in influencing the distribution of DNAPL and the invading phase, by introducing a densified brine (NaI) biopolymer (xanthan) solution as remediation fluid. A matrix of experiments was conducted, encompassing rheological measurements, multiphase flow tests in 1D-columns and 2D-tanks. Numerical modeling was used to assess polymer and DNAPL propagation under different conditions. NaI addition maintains xanthan's shear-thinning yet lowers mid-range shear viscosity 2.6 times. Confined column tests show similar 89 % performance for viscous polymer solutions regardless of density. Unconfined tests mimicking real sites reveal non-densified viscous polymer solution yield mere 0.09 recovery due to density-driven flow. Densified polymer attains radial invasion, boosting recovery to 0.46 with 1.21 aspect ratio. Numerical simulations aligned with experiments, suggesting a near-zero gravity number is necessary to prevent density-driven flow problems. The multiphase flow experiments in confined multilayer system are performed and using the numerical modeling the effects of the permeability contrast and dimensions of the layers on the shape of front are analyzed.
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08/03/2024

Influence of the microstructure on the compressive behaviour of porous aluminas: From microstructural characterisation to fracture mechanisms

Authors : HENRY, Quentin VIOT, Philippe LE BARBENCHON, Louise COSCULLUELA, Antonio KOPP, Jean-Benoit
Publisher : Elsevier BV
The mechanical response of porous aluminas under compressive loading was studied and compared with the fracture mechanisms. Aluminas with a wide range of pore sizes and porosity rates (1–60%) were produced to deconvolve the effects of pore rate, size and morphology on mechanical response. A transition from brittle to quasi-brittle behaviour appears when the porosity rate reaches 60% and a decrease in mechanical properties as the porosity rate increased. At a porosity rate of 20%, a first alumina was produced with micrometric, interconnected pores, while a second was produced with isolated spherical and mesometric pores. The Young’s modulus is little affected by pore size and morphology, while failure stress decreases with increasing pore size. At iso-density, different grain arrangements lead to different fracture mechanisms despite a similar mechanical response: the more compact the grain arrangement, the more transgranular the crack propagation.
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06/03/2024

Love waves propagation in layered viscoelastic waveguides characterized by a Zener model

Authors : EL BAROUDI, Adil LE POMMELLEC, Jean Yves COUANET, Vincent
Publisher : Elsevier
This paper describes a theory of surface Love waves propagating in lossy waveguides consisting of a viscoelastic layer deposited on a semi-infinite elastic substrate. The Zener model to describe the viscoelastic behavior of a medium is used. This simple model captures both the relaxation and retardation. A new form of the unsteady momentum equation for viscoelastic waveguides has been established. By using appropriate boundary conditions, an analytical expression for the complex dispersion equation of Love waves has been deduced. The influence of the loss factor and the ratio of shear moduli of the surface layer on the dispersion curves of Love waves velocity and attenuation is analyzed numerically. The numerical solutions show the dependence of the velocity change and the wave attenuation in terms of the loss factor and the ratio of shear moduli. The obtained results show that the change in the ratio of shear moduli can represent a hardening or softening effect of the surface layer. These effects depend on the loss factor value of the surface layer. In addition, these results are novel, fundamental and can be applied in the characterization of the viscoelastic properties of soft biomaterials and tissues, in nondestructive testing of materials, in geophysics and seismology. Thus, the obtained complex dispersion equation can be very useful to interpret the experimental measurements of Love waves properties in viscoelastic waveguides.
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06/03/2024

Modal and nonmodal stability analysis of turbulent stratified channel flows

Authors : VARIALE, Donato PARENTE, Enza ROBINET, Jean-Christophe CHERUBINI, Stefania
Publisher : Purpose-Led Publishing (AIP Publishing, American Physical Society - APS - and IOP Publishing)
Unstable or optimally growing perturbations of turbulent flows are often representative of the energy-containing coherent structures populating the flow, as for streaks in a turbulent channel. Within this framework, this work aims at studying the modal and nonmodal stability of stably stratified turbulent channel flow, assessing the influence of stratification while changing the friction Richardson number, Riτ, at fixed friction Reynolds number, Reτ. When increasing the stratification of the flow, the energy gain for streamwise independent perturbations at the outer peak increases by two orders of magnitude, and the spanwise wavenumber for which the energy gain peaks reaches values comparable to those reported in the direct numerical simulations of Garcia-Villalba and Del Alamo. At the same time, the value of the optimal gain for the inner peak slightly changes, corroborating the observations made through direct numerical simulation (DNS) about the fact that the wall cycle is not altered by the presence of stratification. Moreover, for nonzero values of the streamwise and spanwise wavenumbers, α and β, the energy gain curve has two peaks, one for shorter target times and α > β, leading to a center-channel temperature peak, and another occurring for α < β at larger target times. In the former case, energy production is mostly linked to velocity production, whereas, in the latter case, the strongest term is that of temperature production, indicating that this mechanism is driven by the increase of the potential energy rather than the kinetic one, and it is intimately linked to the presence of stratification. For strong stratification, the optimal energy gain considerably extends towards higher values of α, leading to energy amplifications reaching three orders of magnitudes for values of α up to 2. The associated optimal perturbations are characterized by temperature patches at the center channel, phase lagged by π/2 with the wall-normal velocity, similarly to gravity waves recovered in the DNS for sufficiently large stratification. However, for large values of β, we observe an increasing asymmetry in the optimal perturbations, probably due to the shielding effect of the core of the channel, as also observed in the DNS of Garcia-Villalba and Del Alamo.
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05/03/2024

Development and validation of a local thermal non-equilibrium model for high-temperature thermal energy storage in packed beds

Authors : LIU, Shaolin AHMADI, Azita LEVET, Cyril LACHAUD, Jean
Publisher : Elsevier BV
High-temperature thermal energy storage (TES) in packed beds is gaining interest for industrial energy recovery. The wide range of temperature distributions causes significant variations in thermophysical properties of the fluid and solid phases, leading to inaccuracies of classical TES models and heat transfer correlations. The objective of this work is to develop and validate a detailed but pragmatic model accounting for high-temperature effects. Based on a literature survey spanning over several communities interested in high- temperature porous media, we propose a generic local thermal non-equilibrium model for granulate porous media accounting for conservation of mass, momentum and energy (two-equation temperature model). The effective parameters needed to inform the model are the effective thermal conductivities of the different phases and the heat transfer coefficient. An experimental-numerical inverse analysis method is employed to determine these parameters. A dedicated experimental facility has been designed and built to study a model granulate made of glass bead of 16 mm diameter. Experiments are conducted using the Transient Single-Blow Technique (TSBT) by passing hot air (ranging from 293 K to 630 K) through cold particles at various mass flow rates, covering a Reynolds number range of 58 to 252. The new model was implemented in the Porous material Analysis Toolbox based on OpenFoam (PATO) used to compute the transient temperature fields. Two optimization algorithms were employed to determine the parameters by minimizing the error between experimental and simulated temperatures: a Latin Hypercube Sampling (LHS) method, and a local optimization method Adaptive nonlinear least-squares algorithm (NL2SOL). The results indicate that the value of heat transfer coefficient ℎ�� in the two-equation model falls in the range of 1.0 × 104 ∼ 1.93× 104 W/(m3 K) under the given conditions. The axial dispersion gas thermal conductivity was found to be around 5.9 and 67.1 times higher than the gas thermal conductivity at Peclet numbers of around 55 and 165, respectively. Furthermore, two improved correlations of Nusselt number (���� = 2+1.54����(�� )0.6�� ��(�� )1∕3) and of axial dispersion gas thermal conductivity (��������,∥ = 0.00053����(�� )2.21�� ��(�� ) ⋅ ���� ) are proposed and validated for a range of Reynolds number from 58 to 252. The overall approach is therefore validated for the model granulate of the study, opening new perspectives towards more precise design and monitoring of high-temperature TES systems.
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04/03/2024

Effect of cutting tool geometry on hole quality in orbital drilling

Authors : REY, Pierre-André SENATORE, Johanna LANDON, Yann
Publisher : Springer Science and Business Media LLC
The orbital drilling process is a very complex machining operation. Due to the helical path of the tool in the material and the tool geometry that can be very complex, the geometry of the chip is very variable along the cutting edge and during a revolution of the tool. This complexity leads to variable cutting forces during drilling and makes it difficult to model and estimate for different tool geometries. The aim of this study is, therefore, to use a modelling of the orbital drilling process in order to study the influence of the geometry of the tool and the cutting conditions on the cutting forces. The final objective is to identify their impact on the quality of the drilled hole and, thus, to control the final quality. First, the chip geometry is modelled from the cutting parameters and the macro-geometry of the tool. It needs to determine the tooth trajectory into the material for each point of the cutting edge. Cutting force models, based on the instantaneous chip thickness, are then implemented. An experimental study validates the modelling by cutting force measurements carried out during orbital drilling tests. From this modelling, it is now possible to study the influence of the geometry of the cutting tool on the forces in order to control the loading on the tool and therefore the final quality of the drilled hole.
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