Marios Avgousti - Academia.edu (original) (raw)

Papers by Marios Avgousti

L'invention concerne des procedes qui permettent d'agglomerer une biomasse microbienne, d... more L'invention concerne des procedes qui permettent d'agglomerer une biomasse microbienne, d'extraire une composition lipidique raffinee a partir de la biomasse agglomeree, dans des conditions supercritiques, et de distiller la composition lipidique raffinee, au moins une fois dans des conditions de distillation directe, pour obtenir une fraction contenant des lipides. L'invention concerne egalement des procedes de fabrication de concentres d'huile contenant des lipides a partir de celle-ci, par la trans-esterification et l'enrichissement de la fraction contenant des lipides.

Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, 1994

Recently, an account of the linear and nonlinear analysis of the viscoelastic Taylor–Couette flow... more Recently, an account of the linear and nonlinear analysis of the viscoelastic Taylor–Couette flow between independently rotating cylinders against axisymmetric disturbances was presented (Avgousti & Beris 1993 a ). However, more recent linear stability analysis has shown that for the range of geometric and kinematic parameters studied and for high enough values of flow elasticity, the critical instabilities are caused by non-axisymmetric, time-dependent disturbances (Avgousti & Beris 1993 b ). In this work, we calculate the bifurcating families corresponding to each one of the two possible non-axisymmetric patterns emerging at the point of criticality, namely the spirals and ribbons and determine their stability. It is shown that for a narrow gap size, for upper convected Maxwell and Oldroyd-B fluids, at least one of the non-axisymmetric families bifurcates subcritically. This result, in conjunction with the theoretical analysis of Hopf bifurcation in presence of symmetries (Golubit...

Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, 1993

The morphogenesis of secondary vortices is investigated for the axisymmetric flow of a viscoelast... more The morphogenesis of secondary vortices is investigated for the axisymmetric flow of a viscoelastic fluid, confined between two independently rotating, infinitely long cylinders, in the region near the onset of instability of the purely azimuthal Couette flow. The Oldroyd-B constitutive equation is used to model viscoelasticity. Three characteristic regions in the parameter space, corresponding to three distinct solution families have been investigated where the onset of instability is due primarily to inertia, both inertia and elasticity, and exclusively elasticity, respectively. The secondary flow corresponds to a steady Taylor vortex in the first case, but to a time-periodic one when elasticity becomes important (Hopf bifurcation). Degenerate Hopf bifurcation theory in the presence of symmetries ( O (2) × S 1 ) has been used to show the existence of two different time-periodic solution families, each following either one of two possible patterns, the rotating wave or the standing...

Journal of Non-Newtonian Fluid Mechanics, 2000

We investigate the influence of eccentricity on linear stability of purely elastic Dean flow of a... more We investigate the influence of eccentricity on linear stability of purely elastic Dean flow of an Upper Convected Maxwell liquid. A pseudo-spectral Chebyshev–Fourier collocation (CFC) technique, that exploits smoothness of the computational domain, periodicity in the azimuthal direction and exponential convergence characteristics of spectral approximations, is employed for the spatial discretization of the governing equations. Arnoldi subspace iteration technique is

Journal of Non-Newtonian Fluid Mechanics, 1993

ABSTRACT In this work, the linear stability analysis of the viscoelastic Taylor-Couette flow agai... more ABSTRACT In this work, the linear stability analysis of the viscoelastic Taylor-Couette flow against non-axisymmetric disturbances is investigated. A pseudospectrally generated, generalized algebraic eigenvalue problem is constructed from the linearized set of the three-dimensional governing equations around the steady-state azimuthal solution. Numerical evaluation of the critical eigenvalues shows that for an upper-convected Maxwell model and for the specific set of geometric and kinematic parameters examined in this work, the azimuthal Couette (base) flow becomes unstable against non-axisymmetric time periodic disturbances before it does so for axisymmetric ones, provided the elasticity number ϵ (De/Re) is larger than some non-zero but small value (ϵ⪢ 0.01). In addition, as ϵ increases, different families of eigensolutions become responsible for the onset of instability. In particular, the azimuthal wavenumber of the critical eigensolution has been found to change from 1 to 2 to 3 and then back to 2 as ϵ increases from 0.01 to infinity (inertialess flow).In an analogous fashion to the axisymmetric viscoelastic Taylor-Couette flow, two possible patterns of time-dependent solutions (limit cycles) can emerge after the onset of instability: ribbons and spirals, corresponding to azimuthal and traveling waves, respectively. These patterns are dictated solely by the symmetry of the primary flow and have already been observed in conjunction with experiments involving Newtonian fluids but with the two cylinders counter-rotatng instead of co-rotating as considered here. Inclusion of a non-zero solvent viscosity (Oldroyd-B model) has been found to affect the results quantitatively but not qualitatively. These theoretical predictions are of particular importance for the interpretation of the experimental data obtained in a Taylor-Couette flow using highly elastic viscoelastic fluids.

Journal of Non-Newtonian Fluid Mechanics, 1992

Spectral solutions of viscoelastic flow equations offer significant advantages emanating from the... more Spectral solutions of viscoelastic flow equations offer significant advantages emanating from their exponentially fast convergence with mesh refinement. The analysis of the behaviour of the Giesekus model is presented in (a) the undulating channel steady-state flow and (b) the evolution of Taylor-Couette flow instabilities. By comparison to the results corresponding to an inelastic shear-thinning analog, it is shown that the presence of elasticity, while it introduces a minor change in the flow resistance for the flow within an undulating channel, significantly affects the stability of the Couette flow. In addition, no purely elastic (inertialess) instability is obtained in the Couette flow, in contrast to the Oldroyd-B fluid calculations. Thus, the stabilizing effect of the negative second normal stress, present in the Giesekus model, is verified.

International Journal for Numerical Methods in Fluids, 1993

The time evolution of finite amplitude axisymmetric perturbations (Taylor cells) to the purely az... more The time evolution of finite amplitude axisymmetric perturbations (Taylor cells) to the purely azimuthal, viscoelastic, cylindrical Couette flow was numerically simulated. Two time integration numerical methods were developed, both based on a pseudospectral spatial approximation of the variables, efficiently implemented using fast Poisson solvers and optimal filtering routines. The first method, applicable for finite Re numbers, is based on a time-splitting integration with the divergence-free condition enforced through an influence matrix technique. The second one, is based on a semi-implicit time integration of the constitutive equation with both the continuity and the momentum equations enforced as constraints. Stability results for an upper convected Maxwell fluid were obtained for the supercritical bifurcations, either steady or time-periodic, developed after the onset of instabilities in the primary flow. At small elasticity values, E = De/Re, the time integration of finite amplitude disturbances confirms the stability of the single branch of steady Taylor cells. At intermediate E values the rotating wave family of time-periodic solutions developed at the onset of instability is stable, whereas the standing wave is found to be unstable. At high E values, and in particular for the limit of creeping flow (E = a) , the present study shows that the rotating wave family is unstable and the standing (radial) wave is stable, in agreement with previous finite-element investigations. It is thus shown that spectral techniques provide a robust and computationally efficient method for the simulation of complex, non-linear, time-dependent viscoelastic flows. KEY WORDS Spectral collocation method Influence matrix Viscoelastic Taylor-Couette flow Hopf bifurcations Rotating and standing wave.

Journal of Non-Newtonian Fluid Mechanics, 1996

ABSTRACT The viscoelastic flow of an Upper Convected Maxwell fluid, confined between two infinite... more ABSTRACT The viscoelastic flow of an Upper Convected Maxwell fluid, confined between two infinitely long eccentric rotating cylinders, is investigated. The two-dimensional steady-state flow as well as the stability of the flow against true three-dimensional (azimuthally and axially periodic and radially non-periodic) disturbances is analyzed numerically using pseudospectral methods. In this numerical scheme, variables are expressed as Fourier series in the periodic direction and as Chebyshev polynomials in the radial direction.The linear stability analysis shows that the critical Reynolds number, corresponding to the onset of flow instability, increases with eccentricity for a Newtonian flow. The critical wavenumber in the axial direction is found to remain nearly constant in the eccentricity range between 0 and 0.5. Addition of small flow elasticity in high Re flows is found to destabilize the system, causing the critical wavenumbers to slightly increase. For a purely elastic flow, it is found that the critical Deborah number decreases with an increase in eccentricity and the critical wavenumber also decreases for the parameters examined in our study.

The morphogenesis of secondary vortices is investigated for the flow of a viscoelastic fluid, con... more The morphogenesis of secondary vortices is investigated for the flow of a viscoelastic fluid, confined between two independently rotating, infinitely long cylinders, in the region near the onset of instability of the purely azimuthal Couette flow. For the Upper Convected Maxwell (UCM) model it was found that the secondary flow corresponds to a steady Taylor vortex in the case of

A method of pelletizing microbial biomass, extracting a refined lipid composition from pelletized... more A method of pelletizing microbial biomass, extracting a refined lipid composition from pelletized biomass under supercritical conditions, and distilling the refined lipid composition at least once under short-path distillation conditions to obtain a lipid-containing fraction Provided. Also disclosed is a method of transesterifying and enriching a lipid-containing fraction to make a lipid-containing oil concentrate therefrom.

ABSTRACT We investigate the stability of two periodically constricted viscoelastic flows: pressur... more ABSTRACT We investigate the stability of two periodically constricted viscoelastic flows: pressure driven flow in the gap between two eccentric cylinders (Eccentric Dean Flow) and pressure driven flow in a periodically constricted channel (PCC Flow). Both inertial (Re>>1, O(1): De) and purely elastic (Re≡ 0, O(1): De) instabilities are investigated using local and/or global linear stability analysis and Digital Particle Imaging Velocimetry (DPIV). In the inertial regime, results will be presented to elucidate the influence of geometric modulation on the stability and (Orr-Sommerfeld) eigenspectrum of plane Poiseuille flow. Subsequently, the influence of elastic effects on the inertial transitions will be discussed. For Re≡ 0, viscoelastic plane Poiseuille flow is linearly stable since the real parts of the most dangerous eigenvalues approach 0 as -1/De. (Gorodtsov and Leonov 1967; Renardy 1986; Sureshkumar and Beris 1995). On the other hand, the interplay between streamline curvature and normal stress distribution leads to a purely elastic instability in the Dean flow for O(1) Deborah number (Joo and Shaqfeh 1992). Our results will show how the stability characteristics of these flows are influenced by periodic geometric modulation.

L'invention concerne des procedes qui permettent d'agglomerer une biomasse microbienne, d... more L'invention concerne des procedes qui permettent d'agglomerer une biomasse microbienne, d'extraire une composition lipidique raffinee a partir de la biomasse agglomeree, dans des conditions supercritiques, et de distiller la composition lipidique raffinee, au moins une fois dans des conditions de distillation directe, pour obtenir une fraction contenant des lipides. L'invention concerne egalement des procedes de fabrication de concentres d'huile contenant des lipides a partir de celle-ci, par la trans-esterification et l'enrichissement de la fraction contenant des lipides.

Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, 1994

Recently, an account of the linear and nonlinear analysis of the viscoelastic Taylor–Couette flow... more Recently, an account of the linear and nonlinear analysis of the viscoelastic Taylor–Couette flow between independently rotating cylinders against axisymmetric disturbances was presented (Avgousti & Beris 1993 a ). However, more recent linear stability analysis has shown that for the range of geometric and kinematic parameters studied and for high enough values of flow elasticity, the critical instabilities are caused by non-axisymmetric, time-dependent disturbances (Avgousti & Beris 1993 b ). In this work, we calculate the bifurcating families corresponding to each one of the two possible non-axisymmetric patterns emerging at the point of criticality, namely the spirals and ribbons and determine their stability. It is shown that for a narrow gap size, for upper convected Maxwell and Oldroyd-B fluids, at least one of the non-axisymmetric families bifurcates subcritically. This result, in conjunction with the theoretical analysis of Hopf bifurcation in presence of symmetries (Golubit...

Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, 1993

The morphogenesis of secondary vortices is investigated for the axisymmetric flow of a viscoelast... more The morphogenesis of secondary vortices is investigated for the axisymmetric flow of a viscoelastic fluid, confined between two independently rotating, infinitely long cylinders, in the region near the onset of instability of the purely azimuthal Couette flow. The Oldroyd-B constitutive equation is used to model viscoelasticity. Three characteristic regions in the parameter space, corresponding to three distinct solution families have been investigated where the onset of instability is due primarily to inertia, both inertia and elasticity, and exclusively elasticity, respectively. The secondary flow corresponds to a steady Taylor vortex in the first case, but to a time-periodic one when elasticity becomes important (Hopf bifurcation). Degenerate Hopf bifurcation theory in the presence of symmetries ( O (2) × S 1 ) has been used to show the existence of two different time-periodic solution families, each following either one of two possible patterns, the rotating wave or the standing...

Journal of Non-Newtonian Fluid Mechanics, 2000

We investigate the influence of eccentricity on linear stability of purely elastic Dean flow of a... more We investigate the influence of eccentricity on linear stability of purely elastic Dean flow of an Upper Convected Maxwell liquid. A pseudo-spectral Chebyshev–Fourier collocation (CFC) technique, that exploits smoothness of the computational domain, periodicity in the azimuthal direction and exponential convergence characteristics of spectral approximations, is employed for the spatial discretization of the governing equations. Arnoldi subspace iteration technique is

Journal of Non-Newtonian Fluid Mechanics, 1993

ABSTRACT In this work, the linear stability analysis of the viscoelastic Taylor-Couette flow agai... more ABSTRACT In this work, the linear stability analysis of the viscoelastic Taylor-Couette flow against non-axisymmetric disturbances is investigated. A pseudospectrally generated, generalized algebraic eigenvalue problem is constructed from the linearized set of the three-dimensional governing equations around the steady-state azimuthal solution. Numerical evaluation of the critical eigenvalues shows that for an upper-convected Maxwell model and for the specific set of geometric and kinematic parameters examined in this work, the azimuthal Couette (base) flow becomes unstable against non-axisymmetric time periodic disturbances before it does so for axisymmetric ones, provided the elasticity number ϵ (De/Re) is larger than some non-zero but small value (ϵ⪢ 0.01). In addition, as ϵ increases, different families of eigensolutions become responsible for the onset of instability. In particular, the azimuthal wavenumber of the critical eigensolution has been found to change from 1 to 2 to 3 and then back to 2 as ϵ increases from 0.01 to infinity (inertialess flow).In an analogous fashion to the axisymmetric viscoelastic Taylor-Couette flow, two possible patterns of time-dependent solutions (limit cycles) can emerge after the onset of instability: ribbons and spirals, corresponding to azimuthal and traveling waves, respectively. These patterns are dictated solely by the symmetry of the primary flow and have already been observed in conjunction with experiments involving Newtonian fluids but with the two cylinders counter-rotatng instead of co-rotating as considered here. Inclusion of a non-zero solvent viscosity (Oldroyd-B model) has been found to affect the results quantitatively but not qualitatively. These theoretical predictions are of particular importance for the interpretation of the experimental data obtained in a Taylor-Couette flow using highly elastic viscoelastic fluids.

Journal of Non-Newtonian Fluid Mechanics, 1992

Spectral solutions of viscoelastic flow equations offer significant advantages emanating from the... more Spectral solutions of viscoelastic flow equations offer significant advantages emanating from their exponentially fast convergence with mesh refinement. The analysis of the behaviour of the Giesekus model is presented in (a) the undulating channel steady-state flow and (b) the evolution of Taylor-Couette flow instabilities. By comparison to the results corresponding to an inelastic shear-thinning analog, it is shown that the presence of elasticity, while it introduces a minor change in the flow resistance for the flow within an undulating channel, significantly affects the stability of the Couette flow. In addition, no purely elastic (inertialess) instability is obtained in the Couette flow, in contrast to the Oldroyd-B fluid calculations. Thus, the stabilizing effect of the negative second normal stress, present in the Giesekus model, is verified.

International Journal for Numerical Methods in Fluids, 1993

The time evolution of finite amplitude axisymmetric perturbations (Taylor cells) to the purely az... more The time evolution of finite amplitude axisymmetric perturbations (Taylor cells) to the purely azimuthal, viscoelastic, cylindrical Couette flow was numerically simulated. Two time integration numerical methods were developed, both based on a pseudospectral spatial approximation of the variables, efficiently implemented using fast Poisson solvers and optimal filtering routines. The first method, applicable for finite Re numbers, is based on a time-splitting integration with the divergence-free condition enforced through an influence matrix technique. The second one, is based on a semi-implicit time integration of the constitutive equation with both the continuity and the momentum equations enforced as constraints. Stability results for an upper convected Maxwell fluid were obtained for the supercritical bifurcations, either steady or time-periodic, developed after the onset of instabilities in the primary flow. At small elasticity values, E = De/Re, the time integration of finite amplitude disturbances confirms the stability of the single branch of steady Taylor cells. At intermediate E values the rotating wave family of time-periodic solutions developed at the onset of instability is stable, whereas the standing wave is found to be unstable. At high E values, and in particular for the limit of creeping flow (E = a) , the present study shows that the rotating wave family is unstable and the standing (radial) wave is stable, in agreement with previous finite-element investigations. It is thus shown that spectral techniques provide a robust and computationally efficient method for the simulation of complex, non-linear, time-dependent viscoelastic flows. KEY WORDS Spectral collocation method Influence matrix Viscoelastic Taylor-Couette flow Hopf bifurcations Rotating and standing wave.

Journal of Non-Newtonian Fluid Mechanics, 1996

ABSTRACT The viscoelastic flow of an Upper Convected Maxwell fluid, confined between two infinite... more ABSTRACT The viscoelastic flow of an Upper Convected Maxwell fluid, confined between two infinitely long eccentric rotating cylinders, is investigated. The two-dimensional steady-state flow as well as the stability of the flow against true three-dimensional (azimuthally and axially periodic and radially non-periodic) disturbances is analyzed numerically using pseudospectral methods. In this numerical scheme, variables are expressed as Fourier series in the periodic direction and as Chebyshev polynomials in the radial direction.The linear stability analysis shows that the critical Reynolds number, corresponding to the onset of flow instability, increases with eccentricity for a Newtonian flow. The critical wavenumber in the axial direction is found to remain nearly constant in the eccentricity range between 0 and 0.5. Addition of small flow elasticity in high Re flows is found to destabilize the system, causing the critical wavenumbers to slightly increase. For a purely elastic flow, it is found that the critical Deborah number decreases with an increase in eccentricity and the critical wavenumber also decreases for the parameters examined in our study.

The morphogenesis of secondary vortices is investigated for the flow of a viscoelastic fluid, con... more The morphogenesis of secondary vortices is investigated for the flow of a viscoelastic fluid, confined between two independently rotating, infinitely long cylinders, in the region near the onset of instability of the purely azimuthal Couette flow. For the Upper Convected Maxwell (UCM) model it was found that the secondary flow corresponds to a steady Taylor vortex in the case of

A method of pelletizing microbial biomass, extracting a refined lipid composition from pelletized... more A method of pelletizing microbial biomass, extracting a refined lipid composition from pelletized biomass under supercritical conditions, and distilling the refined lipid composition at least once under short-path distillation conditions to obtain a lipid-containing fraction Provided. Also disclosed is a method of transesterifying and enriching a lipid-containing fraction to make a lipid-containing oil concentrate therefrom.

ABSTRACT We investigate the stability of two periodically constricted viscoelastic flows: pressur... more ABSTRACT We investigate the stability of two periodically constricted viscoelastic flows: pressure driven flow in the gap between two eccentric cylinders (Eccentric Dean Flow) and pressure driven flow in a periodically constricted channel (PCC Flow). Both inertial (Re>>1, O(1): De) and purely elastic (Re≡ 0, O(1): De) instabilities are investigated using local and/or global linear stability analysis and Digital Particle Imaging Velocimetry (DPIV). In the inertial regime, results will be presented to elucidate the influence of geometric modulation on the stability and (Orr-Sommerfeld) eigenspectrum of plane Poiseuille flow. Subsequently, the influence of elastic effects on the inertial transitions will be discussed. For Re≡ 0, viscoelastic plane Poiseuille flow is linearly stable since the real parts of the most dangerous eigenvalues approach 0 as -1/De. (Gorodtsov and Leonov 1967; Renardy 1986; Sureshkumar and Beris 1995). On the other hand, the interplay between streamline curvature and normal stress distribution leads to a purely elastic instability in the Dean flow for O(1) Deborah number (Joo and Shaqfeh 1992). Our results will show how the stability characteristics of these flows are influenced by periodic geometric modulation.