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Fri, 20 Sep 2024 14:31:18 GMT
20240920T14:31:18Z

Flow and airentrainment around partially submerged vertical cylinders
http://hdl.handle.net/10985/17944
Flow and airentrainment around partially submerged vertical cylinders
AGEORGES, Valentin; PEIXINHO, Jorge; PÉRRET, Gaële
In this study, a partially submerged vertical cylinder is moved at constant velocity through water, which is initially at rest. During the motion, the wake behind the cylinder induces freesurface deformation. Eleven cylinders, with diameters from D=1.4 to 16 cm, were tested under two different conditions: (i) constant immersed height h and (ii) constant h/D. The range of translation velocities and diameters are in the regime of turbulent wake with experiments carried out for 4500<Re<240000 and 0.2<Fr<2.4, where Re and Fr are the Reynolds and Froude numbers based on D. The focus here is on dragforce measurements and relatively strong freesurface deformation up to airentrainment. Specifically, two modes of airentraiment have been uncovered: (i) in the cavity along the cylinder wall and (ii) in the wake of the cylinder. A scaling for the critical velocity for airentrainment in the cavity has been observed in agreement with a simple model. Furthermore, for Fr>1.2, the drag force varies linearly with Fr.
Tue, 01 Jan 2019 00:00:00 GMT
http://hdl.handle.net/10985/17944
20190101T00:00:00Z
AGEORGES, Valentin
PEIXINHO, Jorge
PÉRRET, Gaële
In this study, a partially submerged vertical cylinder is moved at constant velocity through water, which is initially at rest. During the motion, the wake behind the cylinder induces freesurface deformation. Eleven cylinders, with diameters from D=1.4 to 16 cm, were tested under two different conditions: (i) constant immersed height h and (ii) constant h/D. The range of translation velocities and diameters are in the regime of turbulent wake with experiments carried out for 4500<Re<240000 and 0.2<Fr<2.4, where Re and Fr are the Reynolds and Froude numbers based on D. The focus here is on dragforce measurements and relatively strong freesurface deformation up to airentrainment. Specifically, two modes of airentraiment have been uncovered: (i) in the cavity along the cylinder wall and (ii) in the wake of the cylinder. A scaling for the critical velocity for airentrainment in the cavity has been observed in agreement with a simple model. Furthermore, for Fr>1.2, the drag force varies linearly with Fr.

Experiments and Simulations of FreeSurface Flow behind a Finite Height Rigid Vertical Cylinder
http://hdl.handle.net/10985/21219
Experiments and Simulations of FreeSurface Flow behind a Finite Height Rigid Vertical Cylinder
AGEORGES, Valentin; PEIXINHO, Jorge; PERRET, Gaële; LARTIGUE, Ghislain; MOUREAU, Vincent
We present the results of a combined experimental and numerical study of the freesurface flow behind a finite height rigid vertical cylinder. The experiments measure the drag and the wake angle on cylinders of different diameters for a range of velocities corresponding to 30,000 <Re< 200,000 and 0.2<Fr<2 where the Reynolds and Froude numbers are based on the diameter. The threedimensional large eddy simulations use a conservative levelset method for the airwater interface, thus predicting the pressure, the vorticity, the freesurface elevation and the onset of air entrainment. The deep flow looks like single phase turbulent flow past a cylinder, but close to the freesurface, the interaction between the wall, the freesurface and the flow is taking place, leading to a reduced cylinder drag and the appearance of Vshaped surface wave patterns. For large velocities, vortex shedding is suppressed in a layer region behind the cylinder below the free surface. The wave patterns mostly follow the capillarygravity theory, which predicts the crest lines cusps. Interestingly, it also indicates the regions of strong elevation fluctuations and the location of air entrainment observed in the experiments. Overall, these new simulation results, drag, wake angle and onset of air entrainment, compare quantitatively with experiments.
Fri, 01 Jan 2021 00:00:00 GMT
http://hdl.handle.net/10985/21219
20210101T00:00:00Z
AGEORGES, Valentin
PEIXINHO, Jorge
PERRET, Gaële
LARTIGUE, Ghislain
MOUREAU, Vincent
We present the results of a combined experimental and numerical study of the freesurface flow behind a finite height rigid vertical cylinder. The experiments measure the drag and the wake angle on cylinders of different diameters for a range of velocities corresponding to 30,000 <Re< 200,000 and 0.2<Fr<2 where the Reynolds and Froude numbers are based on the diameter. The threedimensional large eddy simulations use a conservative levelset method for the airwater interface, thus predicting the pressure, the vorticity, the freesurface elevation and the onset of air entrainment. The deep flow looks like single phase turbulent flow past a cylinder, but close to the freesurface, the interaction between the wall, the freesurface and the flow is taking place, leading to a reduced cylinder drag and the appearance of Vshaped surface wave patterns. For large velocities, vortex shedding is suppressed in a layer region behind the cylinder below the free surface. The wave patterns mostly follow the capillarygravity theory, which predicts the crest lines cusps. Interestingly, it also indicates the regions of strong elevation fluctuations and the location of air entrainment observed in the experiments. Overall, these new simulation results, drag, wake angle and onset of air entrainment, compare quantitatively with experiments.

Dewetting of a thin polymer film under shear
http://hdl.handle.net/10985/21218
Dewetting of a thin polymer film under shear
KADRI, Kheireddin; PEIXINHO, Jorge; SALEZ, Thomas; MIQUELARDGARNIER, Guillaume; SOLLOGOUB, Cyrille
The objective of this work is to give new insight into the stability of thin polymer films under shear, in order to pave the way to a better control of the nanolayer coextrusion process. To do so, a finitedifference numerical scheme for the resolution of the thin film equation was set up taking into account capillary and van der Waals (vdW) forces. This method was validated by comparing the dynamics obtained from an initial harmonic perturbation to established theoretical predictions. With the addition of shear, three regimes have then been evidenced as a function of the shear rate. In the case of low shear rates the rupture is delayed when compared to the noshear problem, while at higher shear rates it is even suppressed: the perturbed interface goes back to its unperturbed state over time. In between these two limiting regimes, a transient one in which shear and vdW forces balance each other, leading to a nonmonotonic temporal evolution of the perturbed interface, has been identified. While a linear analysis is sufficient to describe the rupture time in the absence of shear, the nonlinearities appear to be essential otherwise.
Fri, 01 Jan 2021 00:00:00 GMT
http://hdl.handle.net/10985/21218
20210101T00:00:00Z
KADRI, Kheireddin
PEIXINHO, Jorge
SALEZ, Thomas
MIQUELARDGARNIER, Guillaume
SOLLOGOUB, Cyrille
The objective of this work is to give new insight into the stability of thin polymer films under shear, in order to pave the way to a better control of the nanolayer coextrusion process. To do so, a finitedifference numerical scheme for the resolution of the thin film equation was set up taking into account capillary and van der Waals (vdW) forces. This method was validated by comparing the dynamics obtained from an initial harmonic perturbation to established theoretical predictions. With the addition of shear, three regimes have then been evidenced as a function of the shear rate. In the case of low shear rates the rupture is delayed when compared to the noshear problem, while at higher shear rates it is even suppressed: the perturbed interface goes back to its unperturbed state over time. In between these two limiting regimes, a transient one in which shear and vdW forces balance each other, leading to a nonmonotonic temporal evolution of the perturbed interface, has been identified. While a linear analysis is sufficient to describe the rupture time in the absence of shear, the nonlinearities appear to be essential otherwise.

Direct numerical simulations of laminar and transitional flows in diverging pipes
http://hdl.handle.net/10985/18599
Direct numerical simulations of laminar and transitional flows in diverging pipes
VITTAL SHENOY, Dhanush; SHADLOO, M. S.; PEIXINHO, Jorge; HADJADJ, Abdellah
Purpose: Fluid flows in pipes whose crosssectional area are increasing in the streamwise direction are prone to separation of the recirculation region. This paper aims to investigate such fluid flow in expansion pipe systems using direct numerical simulations. The flow in circular diverging pipes with different diverging half angles, namely, 45, 26, 14, 7.2 and 4.7 degrees, are considered. The flow is fed by a fully developed laminar parabolic velocity profile at its inlet and is connected to a long straight circular pipe at its downstream to characterise recirculation zone and skin friction coefficient in the laminar regime. The flow is considered linearly stable for Reynolds numbers sufficiently below natural transition. A perturbation is added to the inlet fully developed laminar velocity profile to test the flow response to finite amplitude disturbances and to characterise subcritical transition. Design/methodology/approach: Direct numerical simulations of the Navier–Stokes equations have been solved using a spectral element method. Findings: It is found that the onset of disordered motion and the dynamics of the localised turbulence patch are controlled by the Reynolds number, the perturbation amplitude and the half angle of the pipe. Originality/value: The authors clarify different stages of flow behaviour under the finite amplitude perturbations and shed more light to flow physics such as existence of Kelvin–Helmholtz instabilities as well as mechanism of turbulent puff shedding in diverging pipe flows.
Tue, 01 Jan 2019 00:00:00 GMT
http://hdl.handle.net/10985/18599
20190101T00:00:00Z
VITTAL SHENOY, Dhanush
SHADLOO, M. S.
PEIXINHO, Jorge
HADJADJ, Abdellah
Purpose: Fluid flows in pipes whose crosssectional area are increasing in the streamwise direction are prone to separation of the recirculation region. This paper aims to investigate such fluid flow in expansion pipe systems using direct numerical simulations. The flow in circular diverging pipes with different diverging half angles, namely, 45, 26, 14, 7.2 and 4.7 degrees, are considered. The flow is fed by a fully developed laminar parabolic velocity profile at its inlet and is connected to a long straight circular pipe at its downstream to characterise recirculation zone and skin friction coefficient in the laminar regime. The flow is considered linearly stable for Reynolds numbers sufficiently below natural transition. A perturbation is added to the inlet fully developed laminar velocity profile to test the flow response to finite amplitude disturbances and to characterise subcritical transition. Design/methodology/approach: Direct numerical simulations of the Navier–Stokes equations have been solved using a spectral element method. Findings: It is found that the onset of disordered motion and the dynamics of the localised turbulence patch are controlled by the Reynolds number, the perturbation amplitude and the half angle of the pipe. Originality/value: The authors clarify different stages of flow behaviour under the finite amplitude perturbations and shed more light to flow physics such as existence of Kelvin–Helmholtz instabilities as well as mechanism of turbulent puff shedding in diverging pipe flows.

Drop dynamics of viscoelastic filaments
http://hdl.handle.net/10985/18383
Drop dynamics of viscoelastic filaments
PINGULKAR, Hrishikesh; PEIXINHO, Jorge; CRUMEYROLLE, Olivier
The stretching of viscoelastic polymer solutions close to breakup can create attached drops on a filament, whose properties and dynamics are little understood. The stretching of capillary bridges and the consecutive filament, until its breakup, can be quantified using diameterspacetime diagrams, which demonstrate hierarchy, as well as asymmetry of satellite drops around a big central drop. All drops experience migration, oscillation, and merging. In addition, the position of the minimum diameter on the filament is determined, along with the number of drops, their positions, the diameters of drops and the filament breakup time. The maximum number of drops on the filament can be predicted using the Deborah number. The diagrams also quantify the large Hencky strains in the filaments before pinchoff. The obtained minimum diameter is used to measure the extensional viscosity, which indicates the effect of polymer concentration and direction of filament thinning.
Wed, 01 Jan 2020 00:00:00 GMT
http://hdl.handle.net/10985/18383
20200101T00:00:00Z
PINGULKAR, Hrishikesh
PEIXINHO, Jorge
CRUMEYROLLE, Olivier
The stretching of viscoelastic polymer solutions close to breakup can create attached drops on a filament, whose properties and dynamics are little understood. The stretching of capillary bridges and the consecutive filament, until its breakup, can be quantified using diameterspacetime diagrams, which demonstrate hierarchy, as well as asymmetry of satellite drops around a big central drop. All drops experience migration, oscillation, and merging. In addition, the position of the minimum diameter on the filament is determined, along with the number of drops, their positions, the diameters of drops and the filament breakup time. The maximum number of drops on the filament can be predicted using the Deborah number. The diagrams also quantify the large Hencky strains in the filaments before pinchoff. The obtained minimum diameter is used to measure the extensional viscosity, which indicates the effect of polymer concentration and direction of filament thinning.

Dewetting Dynamics of Sheared Thin Polymer Films: An Experimental Study
http://hdl.handle.net/10985/22738
Dewetting Dynamics of Sheared Thin Polymer Films: An Experimental Study
DMOCHOWSKA, Anna; PEIXINHO, Jorge; SOLLOGOUB, Cyrille; MIQUELARDGARNIER, Guillaume
An experimental investigation is reported on the effect of shear on the bursting of molten ultrathin polymer films embedded in an immiscible matrix. By use of an optical microscope coupled with a shearing hot stage, the dewetting dynamics, i.e., the growth of dewetting holes, is monitored over time at various shear rates. It is observed that their circularity is modified by shear and that for all temperatures and thicknesses studied the growth speed of the formed holes rapidly increases with increasing shear rate. A model balancing capillary forces and viscous dissipation while taking into account shear thinning is then proposed and captures the main features of the experimental data, such as the ellipsoid shape of the holes and the faster dynamics in the direction parallel to the shear. This research will help to understand the instabilities occurring during processing of layered polymeric structures, such as multilayer coextrusion.
Fri, 01 Apr 2022 00:00:00 GMT
http://hdl.handle.net/10985/22738
20220401T00:00:00Z
DMOCHOWSKA, Anna
PEIXINHO, Jorge
SOLLOGOUB, Cyrille
MIQUELARDGARNIER, Guillaume
An experimental investigation is reported on the effect of shear on the bursting of molten ultrathin polymer films embedded in an immiscible matrix. By use of an optical microscope coupled with a shearing hot stage, the dewetting dynamics, i.e., the growth of dewetting holes, is monitored over time at various shear rates. It is observed that their circularity is modified by shear and that for all temperatures and thicknesses studied the growth speed of the formed holes rapidly increases with increasing shear rate. A model balancing capillary forces and viscous dissipation while taking into account shear thinning is then proposed and captures the main features of the experimental data, such as the ellipsoid shape of the holes and the faster dynamics in the direction parallel to the shear. This research will help to understand the instabilities occurring during processing of layered polymeric structures, such as multilayer coextrusion.

Liquid Transfer for Viscoelastic Solutions
http://hdl.handle.net/10985/21075
Liquid Transfer for Viscoelastic Solutions
PINGULKAR, Hrishikesh; PEIXINHO, Jorge; CRUMEYROLLE, Olivier
Viscoelastic liquid transfer from one surface to another is a process that finds applications in many technologies, primarily in printing. Here, cylindricalshaped capillary bridges pinned between two parallel disks are considered. Specifically, the effects of polymer mass fraction, solution viscosity, disk diameter, initial aspect ratio, final aspect ratio, stretching velocity, and filling fraction (alike contact angle) are experimentally investigated in uniaxial extensional flow. Both Newtonian and viscoelastic polymer solutions are prepared using polyethylene glycol and polyethylene oxide, with a wide variety of mass fractions. The results show that the increase in polymer mass fraction and solvent viscosity reduces the liquid transfer to the top surface. Moreover, the increase in the initial and final stretching heights of the capillary bridge also decreases the liquid transfer for both Newtonian and viscoelastic solutions. Finally, the shape of the capillary bridge is varied by changing the liquid volume. Now, Newtonian and viscoelastic solutions exhibit opposite behaviors for the liquid transfer. These findings are discussed in terms of interfacial shape instability and gravitational drainage.
Fri, 01 Jan 2021 00:00:00 GMT
http://hdl.handle.net/10985/21075
20210101T00:00:00Z
PINGULKAR, Hrishikesh
PEIXINHO, Jorge
CRUMEYROLLE, Olivier
Viscoelastic liquid transfer from one surface to another is a process that finds applications in many technologies, primarily in printing. Here, cylindricalshaped capillary bridges pinned between two parallel disks are considered. Specifically, the effects of polymer mass fraction, solution viscosity, disk diameter, initial aspect ratio, final aspect ratio, stretching velocity, and filling fraction (alike contact angle) are experimentally investigated in uniaxial extensional flow. Both Newtonian and viscoelastic polymer solutions are prepared using polyethylene glycol and polyethylene oxide, with a wide variety of mass fractions. The results show that the increase in polymer mass fraction and solvent viscosity reduces the liquid transfer to the top surface. Moreover, the increase in the initial and final stretching heights of the capillary bridge also decreases the liquid transfer for both Newtonian and viscoelastic solutions. Finally, the shape of the capillary bridge is varied by changing the liquid volume. Now, Newtonian and viscoelastic solutions exhibit opposite behaviors for the liquid transfer. These findings are discussed in terms of interfacial shape instability and gravitational drainage.

Identification of viscoelastic material properties by ultrasonic angular measurements in double throughtransmission
http://hdl.handle.net/10985/25481
Identification of viscoelastic material properties by ultrasonic angular measurements in double throughtransmission
POUDREL, AnneSophie; GATTIN, Max; ROSI, Giuseppe; RÉBILLAT, Marc; PEIXINHO, Jorge; BOCHUD, Nicolas; MARGERIT, Pierre
Recent advances in additive manufacturing (AM) of viscoelastic materials have paved the way toward the design of increasingly complex structures. In particular, emerging biomedical applications in acoustics involve structures with periodic microarchitectures, which require a precise knowledge of longitudinal and transverse bulk properties of the constituent materials. However, the identification of the transverse properties of highly soft and attenuating materials remains particularly challenging. Thereby, the present work provides a methodological framework to identify the frequencydependent ultrasound characteristics (i.e., phase velocity and attenuation) of viscoelastic materials. The proposed approach relies on an inverse procedure based on angular measurements achieved in double throughtransmission, referred as θscan. Toward this goal, a forward modeling of the double transmitted waves through a homogeneous solid is proposed for any incidence angle based on the global matrix formalism. The experimental validation is conducted by performing ultrasound measurements on two types of photopolymers that are commonly employed for AM purposes: a soft elastomer (ElasticoTM Black) and a glassy polymer (VeroUltraTM White). As a result, the inferred dispersive ultrasound characteristics are of interest for the computational calibration and validation of models involving complex multimaterial structures in the MHz regime.
Mon, 01 Jul 2024 00:00:00 GMT
http://hdl.handle.net/10985/25481
20240701T00:00:00Z
POUDREL, AnneSophie
GATTIN, Max
ROSI, Giuseppe
RÉBILLAT, Marc
PEIXINHO, Jorge
BOCHUD, Nicolas
MARGERIT, Pierre
Recent advances in additive manufacturing (AM) of viscoelastic materials have paved the way toward the design of increasingly complex structures. In particular, emerging biomedical applications in acoustics involve structures with periodic microarchitectures, which require a precise knowledge of longitudinal and transverse bulk properties of the constituent materials. However, the identification of the transverse properties of highly soft and attenuating materials remains particularly challenging. Thereby, the present work provides a methodological framework to identify the frequencydependent ultrasound characteristics (i.e., phase velocity and attenuation) of viscoelastic materials. The proposed approach relies on an inverse procedure based on angular measurements achieved in double throughtransmission, referred as θscan. Toward this goal, a forward modeling of the double transmitted waves through a homogeneous solid is proposed for any incidence angle based on the global matrix formalism. The experimental validation is conducted by performing ultrasound measurements on two types of photopolymers that are commonly employed for AM purposes: a soft elastomer (ElasticoTM Black) and a glassy polymer (VeroUltraTM White). As a result, the inferred dispersive ultrasound characteristics are of interest for the computational calibration and validation of models involving complex multimaterial structures in the MHz regime.

Instability and rupture of sheared viscous liquid nanofilms
http://hdl.handle.net/10985/25544
Instability and rupture of sheared viscous liquid nanofilms
DHALIWAL, Vira; PEDERSEN, Christian; KADRI, Kheireddin; MIQUELARDGARNIER, Guillaume; SOLLOGOUB, Cyrille; PEIXINHO, Jorge; SALEZ, Thomas; CARLSON, Andreas
Liquid nanofilms are ubiquitous in nature and technology, and their equilibrium and outofequilibrium dynamics are key to a multitude of phenomena and processes. We numerically study the evolution and rupture of viscous nanometric films, incorporating the effects of surface tension, van der Waals forces, thermal fluctuations, and viscous shear. We show that thermal fluctuations create perturbations that can trigger film rupture, but they do not significantly affect the growth rate of the perturbations. The film rupture time can be predicted from a linear stability analysis of the governing thin film equation, by considering the most unstable wavelength and the thermal roughness. Furthermore, applying a sufficiently large unidirectional shear can stabilize large perturbations, creating a finiteamplitude traveling wave instead of film rupture. In three dimensions, unidirectional shear does not inhibit rupture, as perturbations are not suppressed in the direction perpendicular to the applied shear. However, if the direction of shear varies in time, then the growth of large perturbations is prevented in all directions, and rupture can be impeded. © 2024 American Physical Society.
Thu, 01 Feb 2024 00:00:00 GMT
http://hdl.handle.net/10985/25544
20240201T00:00:00Z
DHALIWAL, Vira
PEDERSEN, Christian
KADRI, Kheireddin
MIQUELARDGARNIER, Guillaume
SOLLOGOUB, Cyrille
PEIXINHO, Jorge
SALEZ, Thomas
CARLSON, Andreas
Liquid nanofilms are ubiquitous in nature and technology, and their equilibrium and outofequilibrium dynamics are key to a multitude of phenomena and processes. We numerically study the evolution and rupture of viscous nanometric films, incorporating the effects of surface tension, van der Waals forces, thermal fluctuations, and viscous shear. We show that thermal fluctuations create perturbations that can trigger film rupture, but they do not significantly affect the growth rate of the perturbations. The film rupture time can be predicted from a linear stability analysis of the governing thin film equation, by considering the most unstable wavelength and the thermal roughness. Furthermore, applying a sufficiently large unidirectional shear can stabilize large perturbations, creating a finiteamplitude traveling wave instead of film rupture. In three dimensions, unidirectional shear does not inhibit rupture, as perturbations are not suppressed in the direction perpendicular to the applied shear. However, if the direction of shear varies in time, then the growth of large perturbations is prevented in all directions, and rupture can be impeded. © 2024 American Physical Society.

Kinetic, Products and Shrinkage for the Pyrolysis of Flax Fibers
http://hdl.handle.net/10985/25187
Kinetic, Products and Shrinkage for the Pyrolysis of Flax Fibers
DHAHAK, Asma; CÉZARD, Laurent; BAUMBERGER, Stéphanie; PEIXINHO, Jorge
Biomass pyrolysis is a thermochemical process used for renewable products and energies. However, there are still issues that need to be addressed for process modeling and optimization. This study focuses on the relationship between heating rate, shrinkage, and products from flax fibers using thermogravimetric analysis (TGA), microscopic observation, and pyrolysisgas chromatography/mass spectrometry (PyGC/MS). TGA confirms sequential evaporation of water then decomposition of hemicellulose, cellulose, and lignin. Observations from the microreactor show that flax fibers undergo shrinkage within the temperature range of 335 to 370 °C, depending on the heating rate. Pyrolysis products were analyzed using PyGC/MS at four different final temperatures from 350 to 500 °C, revealing the presence of anhydrosugars, furans, ketones, phenols, esters, alcohols, aldehydes, and acids. The results indicate a correlation between temperature and the increase in furans and ketones. The analysis suggests that furans and ketones are associated with shrinkage.
Wed, 01 May 2024 00:00:00 GMT
http://hdl.handle.net/10985/25187
20240501T00:00:00Z
DHAHAK, Asma
CÉZARD, Laurent
BAUMBERGER, Stéphanie
PEIXINHO, Jorge
Biomass pyrolysis is a thermochemical process used for renewable products and energies. However, there are still issues that need to be addressed for process modeling and optimization. This study focuses on the relationship between heating rate, shrinkage, and products from flax fibers using thermogravimetric analysis (TGA), microscopic observation, and pyrolysisgas chromatography/mass spectrometry (PyGC/MS). TGA confirms sequential evaporation of water then decomposition of hemicellulose, cellulose, and lignin. Observations from the microreactor show that flax fibers undergo shrinkage within the temperature range of 335 to 370 °C, depending on the heating rate. Pyrolysis products were analyzed using PyGC/MS at four different final temperatures from 350 to 500 °C, revealing the presence of anhydrosugars, furans, ketones, phenols, esters, alcohols, aldehydes, and acids. The results indicate a correlation between temperature and the increase in furans and ketones. The analysis suggests that furans and ketones are associated with shrinkage.